Flue-gas treatment: DeSOx and DeNOx systems

Flue-gas treatment (FGT) is the set of systems that remove acid gases — chiefly SO₂, plus HCl and SO₃ — and nitrogen oxides from combustion gas: DeSOx by alkaline reagent, DeNOx by selective reduction with ammonia or urea. Unlike particulate collection, which is one unit operation, FGT is a chain: reagent preparation and injection, a reaction zone, and a particulate collector that must now also capture the spent sorbent. The particulate train and the FGT system are therefore one design problem, not two purchases.

A plain statement of position: flue-gas treatment is Arrow Energy Co., Ltd.'s development line. We offer it as engineering capability and system integration — process selection, reagent-injection design, reaction-zone engineering and resizing of the downstream collector — built on the company's established gas-cleaning technology: electrostatic precipitators, the proprietary Electrocyclone and bag filters. It is not presented as an installed FGD fleet, and we will not imply one. Where a project needs a full wet FGD absorber island, we scope and integrate it with partners and state that scope explicitly in the offer. Guaranteed emission figures are stated per project after the technical assessment, on a defined basis — mg/Nm³, reference O₂, dry or wet, load range.

The pages on the wet scrubber, electrostatic precipitator and bag filter cover the individual vessels; this page covers how the chemistry is chosen and what it does to the rest of the train.

SNCR (selective non-catalytic reduction) injects urea solution or ammonia into the furnace where the gas passes through 850–1,050 °C. In that window the reagent reduces NO to N₂ without a catalyst; below it the reaction stalls and unreacted ammonia slips through, above it the reagent oxidizes and can generate NOx. Achievable reduction is 30–60 %, set less by chemistry than by mixing: the injection lances must cover the furnace cross-section at the plane where the window sits, and that plane moves with load and fouling. Practical SNCR on a bagasse boiler therefore means multiple injection levels selected automatically from furnace-exit-temperature measurement, droplet sizing matched to penetration depth, and ammonia-slip control — holding slip to low single-digit ppmv by zone selection and feedback trim, because slip is both a stack emission and, with any SO₃ present, a source of sticky ammonium salts on downstream surfaces.

SCR (selective catalytic reduction) performs the same reaction over a vanadia–titania catalyst at 180–420 °C depending on placement, reaching 80–95 % reduction with near-complete reagent utilization. The risks concentrate in the catalyst. It oxidizes a fraction of SO₂ to SO₃, and SO₃ plus ammonia slip forms ammonium bisulfate (ABS) — a sticky, acidic salt that condenses in catalyst pores and on air-preheater surfaces, blinding both. On biomass the exposure is worse: potassium and sodium in the fly ash are direct chemical poisons to vanadia catalysts, and high-dust placement on a fuel like bagasse or rice husk, with its abrasive high-silica ash, erodes and deactivates catalyst layers quickly. Biomass SCR is therefore usually placed low-dust — after the ESP or bag filter — which in turn may require reheat, an energy cost that belongs in the comparison.

Does SNCR or SCR make sense on a bagasse boiler?

SNCR first. Bagasse units usually face moderate NOx limits, and a well-mixed multi-level SNCR system delivers its 30–60 % cut with no catalyst to poison. SCR's 80–95 % is available but must be engineered around potassium-laden ash and ammonium-bisulfate fouling — normally low-dust placement after the collector, with the reheat penalty counted honestly.

FGT selection starts from the applicable limit read precisely: the numeric value, the reference oxygen level, dry or wet basis, and the averaging period. A limit quoted without its basis is not usable for design. The table below lists the regimes most relevant to Arrow Energy's markets; numeric limits are deliberately left as verification items rather than quoted from memory, because limit tables are revised and fuel- and capacity-class-dependent.

Two practical notes. First, particulate limits are always evaluated together with the acid-gas system, because DSI raises the collector's inlet load — the compliance margin on PM can shrink at the moment SO₂ compliance is achieved. Second, continuous-monitoring requirements (CEMS) and averaging periods often decide between a technology that is excellent on average and one that is robust minute-to-minute; SNCR's load-following behaviour, for instance, must be assessed against short averaging windows. Comparable gas-cleaning installations are listed under project references.

What information is needed to size a DeSOx or DeNOx system?

Six items: fuel analysis including sulfur, chlorine and alkali content; gas flow and temperature profile through the train; measured inlet SO₂ and NOx at stated O₂; the applicable limit with reference basis and averaging period; the existing collector's design margins; and the ash-disposal route. With these, route screening and a budget stoichiometry take days, not months.