NOx Treatment Equipment

— NOx Treatment Equipment: Nitrogen Oxide Chemistry and Route Selection

NOx treatment equipment removes nitrogen oxide emissions from industrial exhaust in three families: selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and wet scrubbers with low-temperature oxidation (LOTOX), the choice following from the gas temperature and its split between nitric oxide and nitrogen dioxide. Wet scrubbers and oxidation systems use oxidants or ozone to convert insoluble nitric oxide into soluble nitrogen dioxide and dinitrogen pentoxide, captured in water or alkali. SNCR injects ammonia or urea into the 850 to 1,100 degrees Celsius combustion zone, breaking them down into nitrogen and water with no catalyst bed, no internals and no pressure drop, and tolerating a dust-laden flow, at sixty to seventy per cent. SCR passes the flue gas through a catalyst bed alongside ammonia at lower operating temperatures than SNCR and achieves high removal above ninety per cent, at the cost of a catalyst temperature band, dust removed beforehand and a permit-limited ammonia slip.

Product Overview

A nitrogen oxide treatment package is not a scrubber with a different label on it, because the substance it treats is a mixture whose constituents do not behave alike and whose proportions are decided by the process upstream. Nitric oxide is very nearly insoluble, so a water wash and an alkaline wash both pass it through almost untouched; nitrogen dioxide is soluble and hydrolyses into nitric and nitrous acid; nitrous oxide passes through wet and catalytic treatment alike. Published treatment literature states the constraint plainly — because the nitric oxide fraction makes up more than ninety per cent of the nitrogen oxides in a typical combustion flue gas and is almost completely insoluble, wet scrubbing succeeds only after that fraction has been oxidised — and the absorption reaction shows the same thing from the other side, where 3NO₂ + 2NaOH → 2NaNO₃ + NO↑ + H₂O returns one nitrogen atom in three to the gas phase. A single alkaline stage therefore has a ceiling built into its chemistry rather than into its packing, and the documented movement from about seventy per cent removal to about ninety-five per cent on a train with an oxidation stage is the measure of what that stage is for.

The origin of the nitrogen oxide is legible in the gas and decides the route. Thermal fixation needs a flame temperature above roughly 1,200 °C and produces a strongly nitric-oxide-dominated, dry, dust-bearing gas; fuel nitrogen is released at whatever temperature the fuel burns; chemical liberation by nitric acid or a nitrate at liquor temperature is strongest in nitrogen dioxide, saturated with water, and comes in batches that follow the reaction. Each route has published upstream countermeasures — lower working temperature, shorter residence time in the hottest zone, less excess oxygen — and the chemical route has more: a thiosulfate leaching route in place of aqua regia cutting chlorine generation by about ninety-eight per cent, a pressure-oxidation route dissolving gold without an acid system, and an acid-mist suppressant at about 0.05 per cent reducing mist escape by about forty per cent. Source reduction is the first line, and a package that does not say so is selling downstream capacity that upstream discipline would have made unnecessary.

What remains is a decision made from a temperature map rather than from preference. Where the gas passes through 850–1,100 °C in a place where an injection lance can be installed, non-catalytic reduction is available at the source with no catalyst, no reactor internals, no additional pressure drop, a small footprint and tolerance of a dust-laden flow, at a published forty to seventy per cent. Where it passes through 320–400 °C with the particulate already removed, catalytic reduction is available at ninety to ninety-five per cent and above, up to a stated ninety-eight per cent, bringing an ammonia feed-ratio problem, a slip limit that is a permit number and a bisulfate deposit chemistry below about 250 °C. Where neither band exists — the usual case for a wet dissolution or parting vessel whose gas leaves under 100 °C — the routes are oxidative wet absorption, with ozone at about 86 per cent conversion at an ozone-to-nitric-oxide ratio of 0.9 or with hydrogen peroxide or sodium chlorite, and acid recovery, which absorbs into sulphuric acid as nitrosylsulfuric acid at around 2 atm and 35 °C and reverses at about 180 °C and 0.5 atm to release nitric acid, published as a route for low loadings with corrosive reagents and a substantial footprint. The sections below state that discipline for each origin, for the four routes, for the nine stages of one implementation, and for the boundary at which the honest answer is a change upstream rather than another stage downstream.

Application Scenarios

Aqua regia dissolution in a precious-metal refinery. The duty this equipment was developed for: gold-bearing feed dissolved in hydrochloric and nitric acid in a heated kettle, evolving a dense nitrogen dioxide stream with a nitric oxide fraction, a chlorine and nitrosyl chloride content, and a batch shape that follows the dissolution. Hot, wet, acidic, oxidising, and captured at the kettle head under negative pressure.
Nitric acid parting of silver and of silver-bearing anode material. The reaction 3Ag + 4HNO₃ → 3AgNO₃ + NO↑ + 2H₂O is a chemical nitrogen oxide source at liquor temperature rather than a combustion one, strongly nitric-oxide-dominated, hot and wet, and shaped by the batch.
Platinum-group metal dissolution and chloride chemistry. Hydrochloric-acid dissolution with an oxidant, and the chlorination steps that follow, carry chlorine and hydrogen chloride alongside the nitrogen oxides — a mixed acid-and-halogen gas whose nitrogen oxide load is lower than the aqua regia case but whose materials duty is harder.
Copper, nickel and other electrolytic tankhouse acid mist. Electrolytic refining from a sulphate electrolyte produces acid mist and, where nitrate-bearing feed has entered the circuit, nitrogen oxides at low concentration in a large volumetric flow. The cell-side duty belongs to the high current density copper electrolytic cell and the tankhouse around it; the nitrogen oxide train sits downstream of that ventilation rather than instead of it.
Silver electrorefining with nitric-acid pretreatment. Residual nitrates carried in from a nitric-acid pretreatment report at the cells and at the pretreatment vessel. Published practice for this duty pairs closed-loop bottom exhaust on the cells with a wet absorption stage, which is the arrangement this train completes on a silver electrolysis equipment.
Gold chloride make-up and electrolyte preparation. Dissolving gold and preparing a chloride electrolyte generates chlorine, hydrogen chloride and nitrogen oxides at the make-up vessel rather than at the cells, so the capture has to follow the vessel as well as the tankhouse extraction system.
Smelting, roasting and converting off-gas with a hot window. Where the gas leaves a furnace an accessible zone at 850–1,100 °C, non-catalytic reduction is available at the furnace itself — the case a side-blown or bottom-blown oxygen-enriched bottom blowing furnace creates, and the duty a lead-copper anode slime converter presents on the same logic. Where a 320–400 °C band also exists behind a waste-heat boiler, the catalytic route joins the menu.
Incineration, spent-catalyst and electronic-residue thermal treatment. Nitrogen oxides arrive from both the thermal route and the nitrogen content of the feed, alongside acid gases, particulate and metal fume, so the nitrogen oxide stage is one branch of a train and the route is chosen after the dust and acid gases are dealt with.
Laboratory and pilot benches using aqua regia. A bench dissolving a gram or a kilogram produces the same gas as a plant at a thousand-fold smaller flow, with the same corrosion and capture problems; published practice relies on a corrosion-lined fume cupboard at about 0.5 m/s, a curtain above the vessel and a compact integrated unit of the kind served by laboratory waste gas treatment equipment. This page’s train begins where the flow stops being a bench problem.
Retrofit where a stack figure has tightened, and continuous combustion sources. Existing plants whose vessels and capture already exist but whose train was sized against an older limit are re-specified rather than rebuilt; steady sources such as boilers, engines and kilns invert the sizing logic and are sized on the maximum continuous flow rather than on a batch peak. Duties where the gas has never been speciated, where the reagents cannot be legally stored, or where the requested figure lies beyond what the temperature map and the available stage count can deliver are refused with the reason written.

Applicable Raw Materials

The feed to this equipment is a gas, and specifying a gas means stating five things and not one: its speciation, its temperature, its moisture, its other pollutants and its shape in time. Every stream below is described as it arrives at the capture point. Output in each case is the condition in which the stream is handed to the treatment stages.

Aqua regia dissolution off-gas. From a heated hydrochloric-and-nitric-acid dissolution of gold-bearing feed — the front end of a precious metal hydrometallurgical refining system — with nitrogen dioxide dominant, a nitric oxide fraction, hydrogen chloride, chlorine, nitrosyl chloride and saturated water vapour. Output: a hot, wet, mixed-acid oxidising gas with a batch-shaped evolution that peaks in minutes, captured at the vessel head under negative pressure.
Nitric acid parting off-gas. From the nitric acid attack on silver or silver-bearing anode material, strongly nitric-oxide-dominated by the stoichiometry of the reaction, hot and wet, with entrained acid droplets and a nitrogen dioxide fraction that grows as the gas cools in the duct. Output: a hot wet nitrogen-oxide gas whose speciation continues to change downstream, sampled after stabilisation rather than at the nozzle.
Nitrate liquor contact off-gas and platinum-group dissolution gas. Pickling, bright dipping, stripping and residue conditioning contact nitrate-bearing liquor with metal at liquor temperature and produce a low-particulate, nitric-oxide-dominated stream; the hydrochloric-acid dissolution of platinum-group feed with an oxidant in an enameled reaction kettle adds chlorine and hydrogen chloride to the nitrogen oxides. Output: a cool-to-warm nitric-oxide-dominated stream with no dust load, and a warm mixed acid-and-halogen stream that sets the hardest materials duty in this list.
Silver and copper cellhouse gas. Gas drawn from a high current density silver electrolytic cell or a copper tankhouse, carrying acid mist, a nitric oxide and nitrogen dioxide mixture and hydrogen evolved at the cathode, already mixed with a large volume of ventilation air. Output: a dilute, cold, moist acid-mist-bearing stream whose nitrogen oxide load is carried by volume rather than by concentration.
Smelting, roasting and converting off-gas. Gas leaving a smelting furnace, roaster or converter at 850–1,100 °C with nitrogen oxides from the thermal route, sulphur dioxide, particulate and sometimes metal fume. Output: a hot, dust-laden, multi-pollutant gas, and the only feed class in this list for which a non-catalytic route is available at the source.
Catalytic-window duct gas at 320–400 °C. Gas taken from a duct or from behind a waste-heat boiler at the band in which the commercial catalyst works, with the particulate already removed to the level the catalyst tolerates. Output: a hot, clean-enough gas that can be handed to a catalytic reactor instead of to a wet stage, with its dust specification set by the catalyst rather than by the stack.
Incinerator, spent-catalyst and electronic-residue thermal off-gas. From the incineration, roasting or smelting of waste, catalyst material and circuit-board concentrates, with nitrogen oxides from burner operation and from the feed’s own nitrogen content, plus acid gases, metal fume and particulate. Output: a mixed-pollutant gas whose dust must be removed before it meets either a catalyst or a wet absorber.
Mixed acid-gas streams and reagent-bearing streams presented for co-treatment. Streams that are primarily hydrogen chloride, chlorine, sulphur dioxide or ammonia but carry a nitrogen oxide fraction, and streams carrying the vapour of a solvent, reductant or precipitator used in the refining circuit, arriving at the same train because the plant has one extraction system rather than three. Output: a mixed stream whose nitrogen oxide stage is designed for the nitrogen oxide fraction, with compatibility of the reagent and the materials checked rather than assumed.
Feed refused at the gate. A gas with no measured speciation and no measured load; a stream hot enough to soften the polymer selected with no quench or dilution available; a particulate load that would make the wet stage a sludge handler; a stream carrying mercury, phosphine, silane, a pyrophoric or an explosive fraction; an undeclared concentrated-acid condensate; a site with no legal route for the nitrate and nitrite blowdown; and any duty whose real objective is a stack figure the available temperature map cannot reach. Output: a written refusal with the physical or chemical mechanism, the upstream alternative where one exists, and the change in process or capture that would make the stream treatable.

Finished Products

What leaves a nitrogen oxide train is a gas, a liquor and a record: the gas is what the permit is written about, the liquor is the nitrogen that was removed and has to be dealt with somewhere, and the record is what makes both of them defensible.

A stack gas that meets a stated limit at a stated basis. Treated gas released with its concentration declared against the oxygen reference and the dry or wet basis the applicable standard names, together with the outlet ammonia figure where a reducing-agent route is used. Output: a measured, recorded outlet concentration with its reference basis and sampling point stated.
A separated nitrogen oxide load in kilograms per hour. Inlet load, load converted in the oxidation stage, load absorbed in the liquor and load released, reconciled per campaign so the guaranteed removal can be recomputed from the plant’s own numbers. Output: a per-campaign nitrogen oxide balance closed within a stated tolerance.
A nitrogen-bearing salt in the circulating liquor. Nitrate and nitrite accumulate and leave as blowdown at the point of highest concentration — a real product stream with a destination, and a feedstock rather than a waste where a fertiliser-grade or biological denitrification route exists. Output: blowdown at a specified volume and concentration, neutralised as required and consigned to a named destination.
Recovered nitric acid where the acid-recovery route is used. In the sulphuric-acid route the nitrogen dioxide is held as nitrosylsulfuric acid and released as nitric acid on reversal, so the nitrogen leaves as a reusable acid rather than a salt. Output: recovered nitric acid returned to the process, with its strength and residual sulphuric acid content stated.
Recovered condensate and a reagent consumption statement. Condensate from quenching or cooling is a dilute acid that has to be collected rather than drained, and caustic or lime, oxidant, urea or ammonia and make-up water are each accounted against the nitrogen oxide removed. Output: a condensate stream with its acid strength and destination fixed, and a consumption figure per kilogram of nitrogen oxide removed for each reagent.
A cooled, dedusted gas where the train includes those stages. The train’s first stages deliver gas at the temperature and cleanliness the absorption stage and the materials of construction require. Output: gas at a stated temperature and particulate figure at the tower inlet, held inside the limits of the polymer and the packing in use.
A capture verification and oxidation conversion record. Face or curtain velocities and the pressure at the vessel boundary measured with the vessel in operation; and the interstage speciation that proves the nitric oxide fraction was actually converted rather than that oxidant was dosed. Output: a measured capture record for the operating condition, and an inlet-and-interstage speciation pair taken at commissioning and repeated on the routine schedule.
A consumable schedule, a monitoring record and a compliance file. Catalyst layers where a reactor is installed, packing and eliminator elements, spray nozzles, seals and gaskets, reagent filters, analyser sensors and calibration gases, each with a campaign expectation and a replacement trigger; the analyser’s range, calibration interval and drift history; the interlock test record and the retained data set; and the applicable standard’s limit, reference oxygen or reference exhaust volume and reporting period with the measured performance mapped onto that instrument. Output: a consumable schedule priced as a running cost, a calibration and interlock record, a retained data set and a compliance file naming the standard, its limit and the measured performance against it.
A written handover to the stages on either side, and not destruction of the nitrogen. The nitrogen is either released as nitrogen gas on a reduction route or held as a nitrate, a nitrite or a recovered acid on an absorption route, and in every case its destination is written down. Gas leaves with its residual composition and load declared, with the acid-gas and dust fractions handed to the acidic and alkaline gas treatment equipment and the dust stages. Output: a residual load statement to the stack and to the neighbouring stages, and a package that names where the nitrogen went instead of claiming elimination.

NOx Treatment Equipment Process Flow

The nine stages below are the working sequence of a nitrogen oxide treatment package, written at the level of the decisions the chemistry forces. A nitrogen oxide train is not commissioned by switching it on and reading the stack: it is commissioned by proving that the load was measured rather than assumed, that the capture holds the vessel boundary negative through the peak of a batch, that the oxidation stage converted what it was supposed to convert, that the nitrogen balance closes, and that the analyser is measuring the species the permit names. Load in kilograms per hour, peak-to-average ratio, speciation, capture velocity, inlet temperature, oxidant molar ratio, reagent strength, liquor pH and oxidation-reduction potential, tower pressure drop and analyser calibration are all fixed during design against your own process, your own standard and your own site services.

Nitrogen oxide treatment working sequence — nine stages from the load accounting that fixes the design case through the speciation that fixes the oxidation stage, the temperature map that selects the route, the capture that fixes the flow, the reagent system that fixes the operating cost, the tower internals that fix the mass transfer, the oxidation stage that makes the absorption arithmetic close, the blowdown that carries the nitrogen out of the circuit, to a measured, interlocked and retained outlet figure.

Steps 1 — Gas Source Survey and NOx Load Accounting. Every source of nitrogen oxide in the plant is listed, its origin named — a flame, nitrogen in a fuel, or a chemical reaction at liquor temperature — and its shape in time recorded, because a plant never has one source and the design case is the sum of the sources that can run together. For the batch sources the peak evolution rate in kilograms per hour, the duration of the peak and the peak-to-average ratio are established, since those three numbers and not the shift average size the fan, the duct, the tower cross-section and the dosing pump. Setting and utility input: the vessel and equipment list, batch sheets with reaction times and charge sizes, the combustion devices with their ratings, the operating schedule and any existing emission measurements. Output: a source schedule with the origin named per source, a governing design load in kilograms per hour with its peak-to-average ratio, and a written statement of the simultaneous case.

Steps 2 — Sampling, Speciation and Emission Target Setting. A concentration without a speciation, an oxygen reference and a basis is not a specification. The gas is sampled where it is representative rather than convenient, and the nitric oxide and nitrogen dioxide fractions are measured separately, because the insoluble fraction is the fraction that has to be converted and a total cannot tell the designer how much. The target is fixed against the instrument that governs the site: the integrated standard sets nitrogen oxide limits with separate provisions for the nitric acid and nitrogenous fertiliser industries, and the industry-specific standard for nitric acid production sets 200 mg/m³ against a reference exhaust volume of 3,400 m³ per tonne of product. Setting and utility input: the sampling points and their access, the sampling train used for speciation, the gas temperature, moisture and oxygen content, the applicable standard with its limit and reference basis, and the permit’s monitoring requirements. Output: a measured speciation, the concentration expressed both in parts per million and in milligrams per cubic metre with the conversion basis stated, and a target outlet concentration in the units the governing standard uses.

Steps 3 — Route Selection: Reduction, Absorption, Oxidation. The route is decided by the temperature map the site actually has, in four questions. Does the gas pass through 850–1,100 °C where a lance can be installed? Then non-catalytic reduction is available at the source, needing no catalyst, no reactor internals, no additional pressure drop and little footprint, tolerating a dusty flow, at a published forty to seventy per cent. Does it pass through 320–400 °C with the dust already removed to a catalyst’s tolerance? Then catalytic reduction is available at ninety to ninety-five per cent and above, up to a stated ninety-eight per cent, bringing a feed-ratio optimisation, a slip limit and a bisulfate deposit chemistry below about 250 °C. If neither band exists, the route is oxidative wet absorption and the design question becomes which oxidant, at what molar ratio and with what by-product destination. And is the nitrogen value high enough to justify acid recovery, which absorbs into sulphuric acid as nitrosylsulfuric acid at around 2 atm and 35 °C and reverses at about 180 °C and 0.5 atm to release nitric acid? Setting and utility input: the speciation and load from Step 2, the temperature profile of every duct and vessel, the space available, the particulate load and its removal stage, the reagent storage and licensing constraints, and the by-product destination options. Output: a selected route with its temperature basis, reagent, expected removal and slip, by-product, space and utility demand, and a written record of the rejected alternatives.

Steps 4 — Ducting, Capture and Negative-Pressure Containment. This is where the real inlet flow is set, so it is treated as part of the treatment train rather than as a building-services item. The capture is arranged at the vessel opening so the pressure boundary sits at the vessel head, with a velocity specified for the duty: published practice gives a fume-cupboard face velocity of about 0.5 m/s with capture above ninety-five per cent and a curtain velocity above a reactor of roughly 0.3 m/s. Duct velocities are fixed to carry droplets without settling and without carrying mist into the fan, material and slope are fixed against a wet acid stream with drainage along the run, and the total inlet flow is calculated as the captured gas plus the dilution air the arrangement necessarily admits — because that dilution sizes the fan, the tower cross-section, the packing area and the reagent pump. The vessel head is verified under negative pressure at the peak of a batch, which is the only condition in which the answer is meaningful. Setting and utility input: the vessel geometry and head arrangement, the operator access the duty requires, the duct route, the material selection, the fan curve, and the peak evolution rate and duration from Step 1. Output: a capture design with its velocities and pressure boundary stated, a total inlet flow including dilution air, a duct specification, and a verified negative pressure at the vessel head.

Steps 5 — Reagent Selection, Dosing and Storage Safety. What is dosed follows from the route and how it is dosed follows from what has to be controlled. On the reduction routes the reagent is urea or ammonia solution, the control variable is the feed ratio against the measured load, and the excess above the optimum appears as unreacted ammonia downstream and as a bisulfate deposit. On the absorption routes the reagent is caustic soda, lime or a mixture, controlled on circulating strength, pH and oxidation-reduction potential, with published practice for this duty holding a first alkaline stage at about ten per cent caustic and a pH between twelve and thirteen. On the oxidation stage the reagent is ozone, hydrogen peroxide or sodium chlorite, controlled on the molar ratio to the nitrogen oxide to be converted. Storage and handling are designed in the same stage: ammonia solution is an alkaline hazard with a vapour pressure, an oxidiser must be segregated from anything it can oxidise, and the dosing area needs secondary containment, ventilation, eyewash and a written procedure. Setting and utility input: the selected route from Step 3, the reagents available with their delivered strength and cost, the storage space with its segregation and bunding, and the licensing constraints on ammonia and oxidisers. Output: a reagent specification with its strength and consumption basis per kilogram of nitrogen oxide removed, a dosing arrangement with its control variable and measuring point, and a storage design with containment, ventilation and a written emergency procedure.

Steps 6 — Tower Internals: Packing, Spray, Mist Elimination. The tower is where the mass transfer happens, and a nitrogen oxide train asks more of its internals than a simple acid wash does. The packing is selected for the service temperature and chemistry and arranged in two or more sections with a redistribution device between them, because liquid that has run down the wall of a single deep bed starves the middle and loses the transfer efficiency the height was bought for. Spray distribution is designed for coverage of the bed rather than throughput, with nozzle count, pattern, angle, material and clean-out fixed against a saturated gas and a scaling liquor. Mist elimination is placed after the last spray stage as a staged baffle-plus-mesh arrangement, with published practice in this class claiming removal above roughly five micrometres, because the outlet carries droplets that would otherwise leave as a visible plume and as liquid carryover to the fan. Pressure drop is a design figure rather than a start-up discovery, and the inlet temperature is checked against the polymer’s softening point with a quench or dilution step where the gas arrives too hot. Setting and utility input: the gas flow, temperature, moisture and particulate figures from Step 4, the character of the liquor, the packing and eliminator materials available, the fan’s available pressure, the polymer’s temperature limit and the tower geometry the space permits. Output: a tower specification with its packing type and sectioning, redistribution, spray arrangement, mist-elimination stages and outlet droplet figure, with its designed pressure drop and verified inlet temperature limit.

Steps 7 — Oxidation Stage: NO Conversion Ahead of Absorption. This stage decides whether the removal arithmetic closes, because the nitric oxide fraction passes through a wet stage unabsorbed and the absorption reaction itself returns one nitrogen atom in three to the gas phase. The stage therefore converts the insoluble fraction before the gas meets the liquor and is engineered to a molar ratio rather than a dose. With ozone, published work reports about 86 per cent denitrification at an ozone-to-nitric-oxide molar ratio of 0.9 and shows the products moving on to nitrogen dioxide, dinitrogen pentoxide and nitric acid above a molar ratio of one — an effect to be exploited or avoided deliberately according to oxidant cost and the by-product wanted. Hydrogen peroxide oxidises in the liquid phase and sodium chlorite in the liquor, with published practice for a multi-stage laboratory train using an alkaline first stage followed by a sodium chlorite stage specifically to convert the remaining nitric oxide and remove the yellow plume. Whichever oxidant is used, the stage is proved by measuring the species across it, since a dosing-pump reading proves only that reagent was delivered. Setting and utility input: the speciation from Step 2 with the nitric oxide fraction, the selected oxidant and its delivered concentration and cost, the gas temperature and moisture at the injection point, the removal target, and the residence time and mixing length between injection and absorption. Output: an oxidation arrangement with its oxidant, injection point and design molar ratio, a measured interstage speciation showing the conversion achieved, and a consumption curve per kilogram of nitrogen oxide removed.

Steps 8 — Absorption, By-Product Recovery and Blowdown. The absorption stages bring the converted gas into counter-current contact with the liquor, and the stage’s task is to hold the removal while keeping the by-product stream under control. A first alkaline stage neutralises the bulk of the acid gas and begins the absorption, subsequent stages carry the remaining work, and a final low-pH or water stage removes residual mist. The circulating liquor is controlled on strength, pH and oxidation-reduction potential, and because the neutralisation products are soluble nitrate and nitrite that accumulate, the blowdown is specified as a stream with a volume, a concentration and a destination — neutralised where the receiving route requires it, routed to recovery where one exists, and never drained to a surface-water route. The reagent balance closes the stage: caustic or lime in, nitrate and nitrite out, and the difference accounted against the nitrogen oxide removed. Setting and utility input: the converted gas from Step 7, the selected reagent and its strength, the stage count and internals from Step 6, the by-product destination and its acceptance limits, the water and drainage available, and the blowdown handling equipment on site. Output: an absorption section with its stage count and liquor control points, a blowdown specification with its volume, concentration, neutralisation and destination, and a reagent balance reconciled against the nitrogen oxide removed.

Steps 9 — Stack Monitoring, Compliance Proof and Reset. The outlet figure is a measurement with a basis, an instrument and a record. The sampling point is placed where the gas is representative and reaches the analyser without losing the species being measured, the analyser is selected for the species and the moisture actually present — an instrument calibrated for nitrogen dioxide reads a wet nitric-oxide-bearing stream wrongly unless its behaviour is known — and its range, calibration gas, calibration interval and drift check are stated with the machine. The interlock chain is then defined: what the plant does when the analyser reads above the limit, when the oxidant supply fails, when the liquor strength or pH leaves its band, when the circulating pump stops, when the fan trips and when the tower pressure drop moves outside its envelope. The data record is specified with its retention period against what the permit or a customer audit requires, and commissioning closes on a measured acceptance test under the plant’s own load — a load equal to the design case, a negative-pressure check at the vessel head, a conversion check across the oxidation stage, a removal check on the standard’s own basis and a nitrogen balance that closes. Setting and utility input: the standard and permit requirements for measurement, reporting and retention, the sampling point and the analyser’s process conditions, the interlock philosophy and the plant’s shutdown logic, and the acceptance-test load and duration. Output: a monitoring arrangement with its sampling point, instrument, range and calibration interval, an interlock schedule with the action for each trip, a retained data record, a measured acceptance test and a closed nitrogen balance.

Product Structure (Main Equipment)

The package is assembled from five groups of equipment, delivered as separate assemblies so that shipment is straightforward, a failed module is replaced rather than rebuilt, and each module can be proved on site against its own acceptance criterion.

Capture, ducting and the pressure boundary

Vessel-head capture and enclosure. An enclosure around a kettle head, a curtain or ring of air above a vessel, a lined fume cupboard over a bench, or an enclosure with a curtain where the operator has to reach the vessel. Published practice gives a fume-cupboard face velocity of about 0.5 m/s with capture above ninety-five per cent and a curtain velocity above a reactor of roughly 0.3 m/s, and the arrangement is selected against the operation — how often the vessel is opened, how large the opening is, whether a probe has to enter — because the capture that cannot be used is the capture that gets defeated. The design criterion is a negative pressure at the vessel opening measured with the vessel running and at the peak of a batch. Documented case experience in this market includes a closed-loop bottom exhaust arrangement on a silver electrolytic cell installation, the same principle applied to a tankhouse rather than a kettle.

Ductwork, dampers and conveying velocity. The duct carries a hot, wet, acidic gas that carries droplets, so its specification is a materials and a velocity problem at once: velocity high enough to keep droplets entrained and low enough not to put liquid into the fan, material chosen for a wet acid oxidising service with joints, gaskets and fasteners in the same grade rather than a cheaper one, slope and drainage points so condensate has somewhere to go other than the tower and the fan, and dampers so the capture can be re-balanced after commissioning without rebuilding the duct. Where several sources gather into one train, the ducting is where the simultaneous-case flow is established.

Fan or blower, and the pressure budget. The fan pays continuously for every pressure drop in the train — capture losses, duct loss, tower packing, mist eliminator, spray and stack — and is usually the largest continuous electricity consumer in the package. It is specified on the peak flow including dilution air against the sum of those drops, with its material and bearing arrangement chosen for a wet acid gas and its control arranged so the flow can be held as the capture is adjusted and the packing loads up over a campaign.

Reagent and dosing system

Reagent storage, bunding, transfer and dosing. Caustic soda or lime, urea or ammonia solution, an oxidiser and make-up water, each with its own tank, secondary containment, material of construction and filling arrangement, and with the segregation the reagents demand rather than a bank of tanks — because an oxidiser has to be kept from anything it can oxidise and the licensing position on liquid ammonia is materially harder than on the aqueous solution. The dosing system is specified by what it controls: a feed ratio against the measured load on the reduction routes, a circulating strength, a pH and an oxidation-reduction potential on the absorption route, and a molar ratio on the oxidation stage, with published practice holding a first alkaline stage at about ten per cent caustic and a pH between twelve and thirteen. That band is a control target rather than a recipe, because the strength drifts as nitrate and nitrite accumulate and as condensate dilutes the liquor, so the loop has to correct for both.

Circulation, liquor circuit and blowdown take-off. The circulating pump moves liquor from the sump to the spray distribution at the rate the liquid-to-gas ratio requires, and its failure is the most expensive in the package because a tower running dry floods hot gas into the packing. The circuit is therefore specified with duty-standby, a strainer and filtration step, level control tied to make-up water, and a blowdown take-off at the point of highest nitrate and nitrite concentration. Where the duty includes neutralising the blowdown before it leaves the boundary, the reaction vessel for it is part of this module — either a dedicated unit or an existing plant vessel, with the neutralization reaction kettle serving that duty where the plant already runs one in a lined grade.

Tower and internals

Tower shell, sump and stage arrangement. Polypropylene or polypropylene homopolymer with an anti-ageing stabiliser, or glass-reinforced plastic, at a diameter set by the gas velocity through the cross-section and a height set by the stage count, the packing depth, the spray-to-bed distance and the mist-elimination space. Published equipment in this class describes a vertical cylindrical polypropylene tower with a supply tank, supply pump, inlet section, spray layers, packing layers, a demisting section, the liquor circuit and inspection openings, and the tower may be supplied as a single body or as a two-body arrangement. The sump volume is set so the circulating liquor has residence time for the neutralisation and for the oxidation-reduction control to act, and the shell is specified with access to every stage, a sight arrangement and a drain — because a tower with no access is a tower whose interior becomes an unknown after the first campaign. The tower considered as a component in its own right — packing support and retaining plates, redistribution device, liquid-to-gas ratio, wall-flow behaviour and operating precautions — is documented on the flue gas absorption tower page; here it is specified as one module of a nitrogen oxide train.

Packing, redistribution, spray distribution and mist elimination. The packing is the mass-transfer surface, selected for service temperature and chemistry and arranged in a two-section bed with a redistribution device between the sections, the published remedy for the wall-flow that starves the centre of a deep bed. The bed sits on a support plate sized for the packing weight and the gas flow, with a retaining arrangement above it, in a grade that will not be consumed by the acid or soften at the tower’s inlet temperature; packing selection is a pressure-drop decision as much as a transfer decision, because a finer packing buys area and charges the fan for it for the whole campaign. Spray distribution is designed for coverage rather than throughput, with nozzle count, pattern, angle, material and clean-out fixed against a saturated acid gas and a scaling liquor. After the last spray a staged baffle-plus-mesh eliminator removes droplets down to the size the site’s plume and liquid-carryover limits require, with published practice in this class claiming removal above roughly five micrometres, and it is specified with its own pressure drop, material, cleaning arrangement and access — because a blinded eliminator raises fan load and carries liquid into the duct, and the visible plume at a stack is more often an eliminator problem than a chemistry problem.

Oxidation and absorption section

Oxidation injection, mixing and the interstage sampling point. The conversion stage is an injection point with the mixing length and residence time the reaction needs: an oxidant injection grid or a liquid-phase oxidant feed, a static or dynamic mixer, an injection point placed where the gas temperature and moisture allow the reaction to proceed, and a sampling point downstream of the mixing length where the conversion can be measured. The sampling point is not an accessory — without it the only evidence that the oxidation happened is a dosing-pump reading.

Absorption stages and the by-product take-off. The absorption section is the tower’s stage arrangement seen from the nitrogen side: a first alkaline stage neutralising the bulk of the acid gas and beginning the absorption, subsequent stages carrying the remaining work, and the oxidation stage placed relative to them according to the route chosen. The by-product is taken off as blowdown at the point of highest nitrate and nitrite concentration, with the take-off rate controlled against the concentration rather than against a level, so the circuit neither accumulates salts to the point of scaling nor wastes reagent by blowing down too early.

Instrumentation, control, interlocks and materials

Analysers, the monitoring chain and the control system. A nitrogen oxide analyser selected for the species and the moisture present, with its sampling point, sample conditioning, range, calibration gas, calibration interval and drift check; pH and oxidation-reduction potential in the circulating liquor; circulating flow; tower pressure drop; and tower inlet temperature against the polymer’s limit. A PLC runs the dosing from the measured load, holds the liquor inside its band, keeps the fan and the circulating pump in the right order, and acts on the interlock schedule — high outlet reading, oxidant failure, strength or pH out of band, circulating pump stopped, fan tripped, pressure drop outside its envelope, high inlet temperature and low liquor level — stating for each condition whether the plant alarms, ramps down or trips. Published laboratory-scale engineering in this class pairs continuous measurement with an interlock that starts a standby absorber and shuts the process down on a rising reading, and retains the record for several years.

Materials, consumables and documentation. Stabilised polypropylene and polypropylene homopolymer, and glass-reinforced plastic, with seals, gaskets, fasteners, nozzles, packing, eliminator elements and analyser wetted parts selected in compatible grades, and stainless steels avoided in the wet acid phase. Consumables are the packing and eliminator elements, spray nozzles, seals and gaskets, reagent filters and strainers, catalyst layers where a reactor is installed, and analyser sensors and calibration gases, each with a campaign expectation and a replacement trigger. Documentation is the source schedule, the speciation and load calculation, the capture verification, the selected route with its rejected alternatives, the reagent and blowdown specification, the interlock schedule, the acceptance-test record, the nitrogen balance and the consumable campaigns, issued as one technical file — which is what lets a plant defend its outlet figure and a customer audit the claim.

Product Features

Specified from the speciation, not from the total. The package is quoted against a measured split between nitric oxide and nitrogen dioxide, because the insoluble fraction is the fraction that has to be converted and no total can tell a designer how much of it there is. A quotation naming one inlet number and one outlet number has assumed the split rather than measured it.
All four routes from one engineering team, arbitrated on the site’s temperature map. Non-catalytic reduction, catalytic reduction, oxidative wet absorption and acid recovery are specified from the same load calculation, so the route follows from the gas and the plant rather than from a single product line — including the honest answer that a site with an 850–1,100 °C zone is better served at the furnace and a site with a 320–400 °C band is better served by a catalytic reactor than by a wet train.
The oxidation stage is engineered to a molar ratio and proved by measurement. Consumption in this class approaches a stoichiometric ratio to the nitrogen oxide actually removed at high removal figures, so the design is a curve of consumption against removal target rather than a fixed dose, and the conversion is verified by an interstage speciation rather than by an oxidant flow rate. The cost of tightening the stack figure by a further increment is visible before the permit is written.
The ceiling of single-stage alkaline washing is in the specification rather than discovered in operation. Because absorption of nitrogen dioxide in caustic returns one nitrogen atom in three to the gas phase, the stage count is a design decision and not a cost option, and the documented movement from about seventy per cent to about ninety-five per cent with an oxidation stage is the evidence for it.
Capture is delivered as a verified pressure boundary. The negative pressure at the vessel head is measured with the vessel in operation at the peak of a batch, the velocities that produce it are recorded against the operating condition, and the dilution air the capture admits is counted in the inlet flow — because the chemistry downstream cannot compensate for a gas that never arrives, and because that dilution is what sizes the entire train.
Sized on the peak of a batch with the peak-to-average ratio declared. Fan, duct, tower cross-section, packing area, sump volume and dosing turndown are engineered against a stated peak in kilograms per hour and a stated ratio, and the customer can check the design case against their own batch sheet rather than trusting an average that was never true at any instant.
The nitrogen’s destination is part of the specification. Every route is described with where the nitrogen ends up — nitrogen gas, nitrate and nitrite in the blowdown, or recovered nitric acid — and the blowdown is specified with its volume, concentration, neutralisation and receiving route, including the fact that the calcium route produces a soluble nitrate that cannot be landfilled as a solid.
Monitoring, materials and consumables are delivered as sets rather than as items. The sampling point, sample conditioning, analyser, range, calibration interval, interlock schedule and retained record are specified and proved together; stabilised polypropylene, glass-reinforced plastic and compatible wetted parts are selected for a wet, acidic and oxidising service with the inlet temperature limited against the polymer’s softening point; and packing and eliminator elements, nozzles, seals, filters, catalyst layers and analyser sensors are each quoted with a campaign expectation and a replacement trigger, so the running cost is a figure rather than a surprise.

Technical Parameters And Specifications

Site-dependent values — load, flow, temperature, space and services — are given as engineered quantities with the basis on which they are calculated. Chemistry-dependent values — reaction stoichiometry, temperature bands, molar ratios, efficiencies and deposit thresholds — are given with their published basis so they can be checked independently.

Parameter	Specification
Equipment type	Integrated nitrogen oxide abatement package: source capture, ducting and fan, reagent system, oxidation stage, absorption tower with staged internals, mist elimination, instrumentation and control
Routes available	Selective non-catalytic reduction, selective catalytic reduction, oxidative wet absorption, absorption with acid recovery
Primary pollutant	Nitrogen oxides, speciated into nitric oxide, nitrogen dioxide and the residual oxides
Co-treated pollutants	Hydrogen chloride, chlorine, sulphur dioxide, acid mist and ammonia where the plant’s extraction system gathers them
Capture arrangement and velocity	Vessel-head enclosure, air curtain or lined fume cupboard; fume-cupboard face velocity about 0.5 m/s with capture above 95 per cent, curtain velocity above a reactor about 0.3 m/s
Duct conveying velocity	Above the droplet-entrainment threshold and below the mist-carrying threshold, with slope and drainage along the run
Fan duty	Sized on the total inlet flow including capture dilution air against the sum of capture, duct, packing, eliminator and stack pressure drops
Inlet temperature limit	Set by the softening point of the polymer selected, with a quench or dilution stage where the source gas arrives hotter
Reagent for absorption	Caustic soda solution, hydrated lime or a mixture; published working point about 10 per cent caustic with the pH held between 12 and 13, corrected as nitrate, nitrite and condensate accumulate
Reagent for reduction	Urea solution or aqueous ammonia, dosed on a feed ratio against the measured nitrogen oxide load
Oxidant for the conversion stage	Ozone, hydrogen peroxide or sodium chlorite, dosed to a molar ratio against the nitrogen oxide to be converted
Tower internals	Packing in two or more sections with a redistribution device between them, spray distribution designed for bed coverage, staged baffle and mesh mist elimination with removal above about 5 µm
Tower construction	Polypropylene or polypropylene homopolymer with an anti-ageing stabiliser; glass-reinforced plastic where a larger shell or a higher temperature is required
Control variables	Feed ratio and slip on the reduction routes; circulating strength, pH and oxidation-reduction potential on the absorption route; molar ratio on the oxidation stage
Monitoring and interlocks	Nitrogen oxide analyser on the species and moisture present, liquor pH and oxidation-reduction potential, circulating flow, tower pressure drop and inlet temperature; trip or alarm on high outlet reading, oxidant failure, strength or pH out of band, circulating pump stopped, fan tripped, pressure drop outside envelope, high inlet temperature and low liquor level
By-product stream	Nitrate and nitrite in the circulating liquor, taken off as blowdown at the point of highest concentration, neutralised and consigned to a named destination
Capacity	Engineered per project against the measured peak nitrogen oxide evolution rate in kilograms per hour, the peak-to-average ratio, the capture dilution and the required outlet figure; no fixed treatment volume is quoted in advance
Utilities and footprint	Electrical supply for fan, circulating pump, dosing pumps and control system; process water; drainage for condensate and blowdown; reagent storage with secondary containment; footprint engineered against tower diameter and height, reagent area, blowdown holding volume and maintenance access

Technical Advantages

The design starts where the problem starts — at the nitrogen oxide’s origin. Thermal fixation, fuel nitrogen and chemical liberation produce three different gases and are open to three different treatment routes; a quotation that begins with a concentration rather than an origin has begun one step too late. The source schedule is the first deliverable, and it names the governing combination and the sources that must not run together.
The insoluble fraction is designed for explicitly rather than assumed away. Because nitric oxide makes up more than ninety per cent of the nitrogen oxides in a typical combustion gas and is almost insoluble, and because absorption of nitrogen dioxide in caustic returns one nitrogen atom in three to the gas phase, a train without a designed and measured conversion stage has a ceiling built into its chemistry. The oxidation stage is engineered to a molar ratio and proved by an interstage speciation.
Published efficiency is quoted with the assumptions that produced it. The spread from about eighty to about ninety-eight per cent across this class reflects the inlet speciation, the space velocity, the feed ratio and the outlet ammonia allowance rather than the quality of the hardware, so the guarantee travels with those four assumptions and two quotations can be compared on the same duty.
The route menu is complete and the route follows the temperature map. A site with an accessible 850–1,100 °C zone is offered the route that uses it, a site with a 320–400 °C band and clean gas is offered the highest-removal option, and a site with neither is offered the wet routes with their by-product accounted for. No site is sold the route the catalogue carries.
Capture is treated as part of the treatment and verified as a pressure boundary. The negative pressure at the vessel head is measured with the vessel running at the peak of a batch, the velocities are recorded, and the dilution air the capture admits is counted in the inlet flow — because the chemistry downstream cannot compensate for a gas that never arrives.
The nitrogen’s destination is engineered, not asserted. Every route is described with where the nitrogen ends up, and the blowdown is specified with its volume, concentration, neutralisation and receiving route, including the fact that the calcium route produces a soluble nitrate that cannot be landfilled as a solid. No package claims zero discharge.
The deposit chemistry that follows a catalytic installation is designed as one problem. The feed-ratio optimisation, the slip measurement, the bisulfate formation below about 250 °C, the catalyst campaign and the interaction by which a following alkaline scrubber can release ammonia back out of the captured salts are specified together, because an installation commissioned without them meets its stack figure for one catalyst campaign and then plugs its air preheater.
Source reduction is offered first and the smallest compliant package is the recommended one. Published process work gives a thiosulfate leaching route in place of aqua regia cutting chlorine generation by about ninety-eight per cent, a pressure-oxidation route dissolving gold without an acid system, and an acid-mist suppressant at about 0.05 per cent reducing mist escape by about forty per cent, under a published control principle of source minimisation, process containment and end-of-pipe treatment. Where one of those fits, we recommend it and quote the smaller package.

Service Process

The service process is built around one fact: a nitrogen oxide specification cannot be written from a catalogue, so the first work is measurement and the last work is a measured acceptance test.

1. Enquiry review and boundary check. The enquiry is read for the four things that decide whether a train is possible at all: the source or sources of the nitrogen oxide, the process producing each of them, the site’s applicable emission standard, and the site’s reagent storage, licensing and drainage constraints. Combustion sources and chemical sources are separated immediately because the two paths diverge. What leaves this step: a written read-back of the duty and a decision on whether to proceed to measurement, with the reason stated either way.

2. Sampling, speciation and load accounting. Where the plant has no measured speciation, we support the measurement: sampling points and access, the sampling train, the conditions at which the sample must be taken, and a reporting format that separates nitric oxide from nitrogen dioxide and states the oxygen and moisture at the same time. The source schedule is then built, the sources that can run simultaneously are identified, and the design case is written as a peak load in kilograms per hour with a peak-to-average ratio and a duration. What leaves this step: a speciation with both fractions, and a design case document with the peak load, the ratio, the duration and the total inlet flow including capture dilution.

3. Route arbitration and written recommendation. The temperature map is established from the plant’s own ducts and vessels and the four routes are arbitrated against it — non-catalytic reduction where an 850–1,100 °C zone is accessible, catalytic reduction where a 320–400 °C band exists with the dust already removed, oxidative absorption where neither exists, and acid recovery where the nitrogen value and the load justify the circuit. Source reduction is considered alongside all four and offered first where it fits. What leaves this step: a recommendation naming the selected route, its reagent, its expected removal with its assumptions, its by-product, its space and utility demand, and the rejected alternatives with the reason for each.

4. Proposal, specification and drawing approval. The proposal states the capture arrangement and its velocities, the total inlet flow, the tower and internals specification, the oxidation stage with its molar ratio and sampling point, the reagent system with its storage and segregation, the blowdown specification with its destination, the materials with their temperature limits, the monitoring chain with its analyser and calibration interval, the interlock schedule, the consumables with their campaigns, the utilities the site must provide and the acceptance criteria. Removal is guaranteed against the stated inlet speciation, space velocity, feed ratio and slip allowance rather than as a bare percentage. Detailed design follows the accepted proposal and the drawings are issued for approval before manufacture. What leaves this step: a specification package that can be checked line by line against the plant’s own numbers, and an approved drawing set with a bill of materials frozen for manufacture.

5. Manufacture, factory test, shipment and installation. The tower, internals, ducting, reagent system and control panel are manufactured and, where size permits, assembled and tested at the works: a dimensional check, a leak test on the shell and wet sections, a spray-pattern check on the distribution, a rotation and alignment check on the rotating equipment, and a function test on the dosing and control logic. The package ships as modules with the internals crated and identified per stage. Installation begins with a site survey checking the utilities actually available against the load list, the foundation and access route, the duct route as built against the drawing, and the drainage and reagent storage arrangements. What leaves this step: a factory test record, a release-for-shipment note, and an installation record with the as-built deviations listed.

6. Commissioning, acceptance test, training and handover. Commissioning proves the four things a nitrogen oxide train can get wrong: that the capture holds the vessel head negative at the peak of a batch, that the oxidation stage converts the insoluble fraction measured across it, that the absorption section reaches the outlet figure on the governing standard’s own basis, and that the nitrogen balance closes between the inlet load, the outlet gas, the blowdown, the condensate and any recovered acid. The analyser is calibrated, the interlocks are proved one by one, and the first full campaign is run on the plant’s own material. Operators are then trained on the control variables that matter — liquor strength, pH and oxidation-reduction potential, oxidant molar ratio, circulation, pressure drop, capture balance and the meaning of each interlock — on the routine, the sample and calibration schedule, and the actions that follow an alarm. What leaves this step: a commissioning report with the measured figures, an interlock test record, the first nitrogen balance, a trained operating team, a complete technical file, a routine and calibration schedule, and a consumable list with campaigns and triggers.

The sequence is deliberately front-loaded with measurement. Steps 1 to 3 exist because the expensive mistake in this class of equipment is not a badly built tower — it is a well-built tower sized on an unmeasured load, capturing a gas that never reached it, absorbing a fraction that was never converted.

Warranty And After-Sales Service

A nitrogen oxide train is a chemical plant in miniature and it is judged over campaigns rather than over days, so the warranty is written against what the equipment is expected to hold and against what the site is expected to do.

What the warranty covers

Materials and workmanship. The tower shell, internals, ducting, reagent system, oxidation injection arrangement, fan, circulating pump, dosing pumps, control panel and instrumentation are warranted against defects in material and workmanship for the agreed period from the acceptance date, with the period stated per assembly where the assemblies differ — a rotating machine and a moulded polymer shell are not the same proposition and are not warranted as if they were.

The specified performance, against the assumptions it was specified on. The outlet nitrogen oxide concentration is warranted against the inlet speciation, the load in kilograms per hour, the peak-to-average ratio, the space velocity, the reagent and oxidant quality and the outlet ammonia allowance stated in the specification. Where those conditions are held, the specified removal is the committed figure; where the site’s gas arrives outside them, the removal is re-derived rather than disputed.

Capture performance and the nitrogen balance. The negative pressure at the vessel head and the capture velocities recorded at commissioning are warranted for the operating condition they were measured at, which is why they are measured with the vessel running at the peak of a batch rather than at rest. The train is warranted to account for the inlet nitrogen across the outlet gas, the blowdown nitrate and nitrite, the condensate and any recovered acid within the stated tolerance, because the balance is the only check that distinguishes removal from relocation.

What is excluded

Reagent and consumable consumption beyond the stated basis. Consumption figures are supplied as a curve against the removal target and the inlet load; variation caused by the plant’s own load, by reagent quality or by operating practice outside the stated band is the site’s operating cost rather than a warranty item.

Damage caused by an undeclared stream or by operating outside the stated conditions. A halide above the declared level, an undeclared solvent vapour, mercury or a pyrophoric fraction, a particulate load above the declared figure, a tower inlet temperature above the polymer’s limit where the quench was bypassed, liquor outside its strength and pH band, oxidant dosed without circulation running, a capture defeated by an open vessel head and an overridden interlock are all outside the cover. Failure of the site’s electrical supply, process water or drainage, and the site’s regulatory position on the blowdown destination, likewise remain the site’s responsibility.

Spare parts and consumables

A recommended spares list is issued with the package, divided into commissioning spares, one-year spares and campaign spares: packing sections, mist-eliminator elements, spray nozzles, seals, gaskets and fasteners, reagent filters and strainers, analyser sensors and calibration gases, a spare set of dosing-pump components and the rotating-equipment wearing parts. Campaign spares — nozzles, seals and analyser sensors above all — are the items expected to be replaced within a campaign, and holding them on site is what turns a shutdown into a maintenance task.

Remote and field service

Remote support covers the operating questions that decide whether the train holds its figure: liquor strength, pH and oxidation-reduction potential against the trend, oxidant molar ratio against the measured interstage speciation, the capture balance as the plant changes its operations, and the fan and pressure-drop trend as the packing and the eliminator load up. Field service is offered for commissioning and acceptance testing, for the first full campaign on the plant’s own material, for interlock re-testing after any change, for the annual analyser and instrument calibration round, and for the internal inspection and repacking that every tower in this service needs at an interval set by its own duty — the inspection that most often pays for itself, because a redistribution device that has shifted, a worn nozzle and packing that has softened are all visible from inside the tower and none of them is visible from the control room.

Maintenance support and upgrade

A routine and a calibration schedule are issued with the machine and reviewed against the first campaigns: daily checks on the circulating liquor and reagent levels, the capture balance, the pressure-drop and fan-load trend, the analyser zero and span check at the stated interval, and the periodic internal inspection with its criteria. Operating support covers batch-scheduling advice where the capture or the tower is the constraint, because spreading two simultaneous sources apart in time is sometimes the cheapest way to hold a stack figure. Where the standard tightens, the process adds a source, or the measured load turns out higher than the assumed one, the package is upgraded rather than replaced: an additional stage, a second oxidation injection point, additional packing depth, a larger fan, an extra mist-elimination element, a larger reagent system, or a catalytic reactor added where a suitable temperature band is available — an assembly change and a control re-tune rather than a new plant.

Frequently Asked Questions

What is NOx treatment equipment?

NOx treatment equipment is an engineered package that reduces the nitrogen oxide concentration of an industrial gas stream to a figure a permit requires, and it consists of a capture arrangement at the source, a ducting and fan section, one or more treatment stages, a reagent and dosing system, and the instrumentation that proves the outlet figure. The treatment stages are chosen from four families — selective non-catalytic reduction, selective catalytic reduction, oxidative wet absorption and absorption with acid recovery — and which of the four is available to a given plant is decided by whether the gas passes through a usable temperature band and by what the gas contains.

Why can a nitrogen oxide concentration not be treated as a single number?

Because NOx is a convention for a mixture rather than a substance. Nitric oxide is very nearly insoluble in water and passes through a water or alkaline wash almost untouched, nitrogen dioxide is soluble and hydrolyses into nitric and nitrous acid, and nitrous oxide is neither. Published treatment literature states the constraint directly: because nitric oxide makes up more than ninety per cent of the nitrogen oxides in a typical combustion gas and is almost completely insoluble, a wet scrubber works only when the nitric oxide has first been oxidised. A design written against a total without a split has assumed the one variable that decides the whole train.

Why does a single alkaline scrubber stop at around seventy per cent?

Because the absorption reaction itself returns nitrogen to the gas phase. Nitrogen dioxide absorbing in caustic soda proceeds as 3NO₂ + 2NaOH → 2NaNO₃ + NO↑ + H₂O: two nitrogen atoms report as sodium nitrate in the liquor and the third leaves the tower as nitric oxide gas, which is insoluble and goes up the stack. On a nitric-oxide-dominated inlet the fraction that never dissolves at all is added to the fraction returned by the reaction, and published process comparisons put a single-stage alkaline wash at roughly seventy per cent nitrogen oxide removal against about ninety-five per cent for a multi-stage train that includes an oxidation stage on the same duty.

What does the oxidation stage do, and how much oxidant does it consume?

It converts the insoluble fraction into soluble forms before the gas meets the liquor. Ozone does it in the gas phase as O₃ + NO → NO₂ + O₂; hydrogen peroxide and sodium chlorite do it in the liquid phase, with published practice for a multi-stage laboratory train using an alkaline first stage followed by a sodium chlorite stage specifically to convert the remaining nitric oxide and to remove the yellow plume. Published work reports about 86 per cent denitrification at an ozone-to-nitric-oxide molar ratio of 0.9 and shows the products moving on to nitrogen dioxide, dinitrogen pentoxide and nitric acid above a molar ratio of one, with the ozone consumed at removal above ninety per cent approaching the stoichiometric ratio to the nitrogen oxide removed. Consumption therefore scales with the removal target rather than the gas volume, which is why the design output is a consumption curve rather than a fixed dose.

Selective catalytic reduction or selective non-catalytic reduction for my plant?

The temperature map decides. Non-catalytic reduction injects urea or ammonia into gas already at 850 to 1,100 degrees Celsius, so it needs an accessible zone in that band inside a furnace, a kiln or the duct immediately behind it; it needs no catalyst, no reactor internals and no extra pressure drop, it tolerates a dust-laden flow and takes little space, and its published efficiency is roughly forty to seventy per cent. Catalytic reduction needs a gas band of about 320 to 400 degrees Celsius for the commercial vanadium-tungsten-on-titania catalyst with the particulate already removed to the catalyst’s tolerance; it reaches about ninety to ninety-five per cent and above, with one published system stating up to ninety-eight per cent, and it brings an ammonia feed-ratio optimisation, a slip limit and a deposit chemistry with it. Where neither band exists, as in a wet dissolution kettle, both are unavailable and the wet routes are the answer.

What is ammonia slip and why does it matter?

Ammonia slip is the unreacted ammonia that leaves the reactor because the feed ratio was higher than the reaction could consume. It is limited in most jurisdictions, it rises as the catalyst ages, and it reacts downstream with sulphur trioxide and water to form ammonium bisulfate and ammonium sulfate, which deposit on and foul the catalyst, the ductwork and the air preheater, with published practice putting the onset of that fouling below about 250 degrees Celsius. It matters for two further reasons: removal rises with the feed ratio, so slip and removal are the same knob turned in opposite directions, and a following alkaline scrubber can release the ammonia back out of the captured ammonium salts, which means the two stages interact.

Why is the by-product of wet treatment a problem?

Because the nitrogen does not disappear. Alkaline absorption converts the pollutant into nitrate and nitrite in the circulating liquor, and the nitrogen leaves the tower in the blowdown as a soluble nitrogen salt with its own disposal or recovery route; the calcium route in particular produces a nitrate that is soluble and cannot be landfilled as a solid. A treatment train that reaches its stack figure while creating an unmanaged nitrate stream has moved the problem rather than solved it, which is why the blowdown volume, concentration, neutralisation and destination are part of the specification rather than an afterthought.

How is the equipment sized for a batch process?

On the peak of the batch rather than the average of the shift. A dissolution or parting kettle releases its nitrogen oxides in a burst that ends when the reaction ends, so the design case is a peak evolution rate in kilograms per hour, the duration of that peak and a peak-to-average ratio. Those three numbers size the fan, the duct, the tower cross-section, the packing area, the sump volume and the dosing turndown, and the ratio is stated in the specification so that the customer can check the design case against their own batch sheet instead of trusting an average that was never true at any instant.

Why is capture treated as part of the treatment train?

Because the reagent’s stoichiometry is irrelevant to a gas that never reaches the tower, and because capture is the only stage that cannot be verified after the fact. Published practice for this duty gives a fume-cupboard face velocity of about 0.5 m/s with a capture efficiency above ninety-five per cent and a curtain velocity above a reactor of roughly 0.3 m/s, and the design criterion is not a velocity at a grille but a negative pressure at the vessel opening measured with the vessel running at the peak of a batch. The dilution air the capture admits is also part of the flow that sizes everything downstream, so the capture arrangement has to be fixed before the train is drawn.

What materials are used and why not stainless steel?

Because the gas is wet, acidic and oxidising at the same time. Published equipment in this class is built in polypropylene and polypropylene homopolymer with an anti-ageing stabiliser, whose purpose is to stop the shell embrittling under thermal and ultraviolet ageing, and in glass-reinforced plastic where a larger shell or a higher temperature is required. Stainless steels are attacked in the wet acid phase of this service, so the joints, gaskets, fasteners, nozzles, packing and wetted analyser parts are selected in compatible grades rather than in whatever is nearest, and the tower inlet temperature is limited against the polymer’s own softening point with a quench or dilution stage where the source gas arrives hot.

Which emission standard applies to my site, and how do I convert the figures?

The standard that governs the source category the regulator assigns, which is not always the obvious one. The integrated national standard for air pollutants sets nitrogen oxide limits with separate provisions for the nitric acid and nitrogenous fertiliser industries, and the industry-specific standard for nitric acid production sets a nitrogen oxide limit of 200 mg/m³ at the stack against a reference exhaust volume of 3,400 m³ per tonne of product; combustion-derived standards apply an oxygen reference that a chemical source does not use in the same way. Conversion between parts per million and milligrams per cubic metre goes through the molar mass of the species and the molar volume at the reference conditions, so the species and the basis must be stated with every figure — a conversion quoted without them is not a conversion but a recollection.

What does commissioning and the acceptance test involve?

Four measurements taken with the plant running its own material rather than a test gas. The capture is verified by holding the vessel head at a negative pressure at the peak of a batch, with the velocities recorded. The conversion is verified by an interstage speciation showing that the nitric oxide fraction was actually converted. The outlet is measured at the stack on the governing standard’s own basis, with the oxygen or reference-volume correction stated. And the nitrogen balance is closed between the inlet load, the outlet gas, the blowdown nitrate and nitrite, the condensate and any recovered acid within the stated tolerance. The interlocks are proved one by one, the analyser is calibrated, and the consumable campaigns and the routine are issued with the machine.