Spray Layer Desulfurization Efficiency Analysis: Performance Evaluation, Testing, and Optimization

Spray layer desulfurization efficiency analysis is a core performance evaluation for wet flue gas desulfurization (WFGD) absorbers, quantifying how effectively each spray bank removes sulfur dioxide from industrial flue gas. This article explains the absorption mechanism, the key parameters that govern removal performance, and the standardized sampling and testing procedures used by accredited third-party laboratories to verify efficiency guarantees in power plants, steelworks, and chemical facilities.

Technical Overview

Spray layer desulfurization refers to the absorption of sulfur dioxide (SO2) from flue gas by atomized limestone slurry droplets generated on successive spray levels (spray banks) inside a wet absorber tower. In the limestone–gypsum process, which dominates the flue gas desulfurization (FGD) market, flue gas rises counter-current to descending slurry sprays, and each spray layer provides an intimate gas–liquid contact stage. A typical absorber is equipped with three to six spray layers, each fed by dedicated recycle pumps and fitted with silicon carbide spiral or dual-lateral nozzles arranged on a network of headers above the sump.

The desulfurization chemistry proceeds through three coupled steps that determine overall spray layer efficiency:

• Absorption: SO2 dissolves into the alkaline slurry droplet and dissociates into bisulfite and sulfite ions (SO2 + H2O → HSO3− → SO32−).
• Neutralization: dissolved carbonate from limestone reacts with bisulfite to form calcium sulfite (CaCO3 + HSO3− → CaSO3 + CO2 + H2O).
• Oxidation and crystallization: forced oxidation air converts calcium sulfite to gypsum (CaSO3 + 1/2 O2 + 2H2O → CaSO4·2H2O), which crystallizes in the absorber sump and is recovered as a byproduct.

Because SO2 absorption is largely liquid-film controlled, spray layer efficiency is governed by interfacial contact area, droplet residence time, slurry alkalinity reserve, and gas distribution uniformity. Well-designed single-loop spray systems routinely achieve 95–98% SO2 removal, while absorbers combining spray layers with alloy trays or additional polishing layers exceed 99%, meeting ultra-low emission limits of 35 mg/Nm3 or less specified in GB 13223.

Testing Samples

Flue Gas Samples

Samples are extracted from the absorber inlet and outlet ducts using multi-point sampling grids established under GB/T 16157 or ISO 10396. These samples support determination of SO2 concentration, O2 content, moisture, temperature, static pressure, and volumetric flow, all of which feed into the mass-balance-based efficiency calculation.

Limestone Slurry and Feedstock Samples

Representative grabs are taken from the absorber recycle stream, the limestone slurry feed tank, and the mill product. Testing covers slurry density, solid content, and pH, together with limestone quality parameters such as CaCO3 content, acid-insoluble residue, MgO, and particle size distribution.

Gypsum and Absorber Liquor Samples

Gypsum slurry from the hydrocyclone underflow and clarified absorber liquor from the overflow are sampled to assess gypsum purity, free moisture, residual CaCO3, oxidation completeness, and chloride enrichment—each of which reflects back on spray layer absorption performance.

Nozzle and Spray System Components

Nozzles, swirl inserts, and pump components removed during planned outages are examined for wear, scaling, and plugging, and are subjected to laboratory flow–pressure and spray pattern verification on calibrated test rigs.

Testing Items

A comprehensive spray layer desulfurization efficiency analysis evaluates the following parameters:

• Inlet and outlet SO2 concentration (mg/Nm3, dry, corrected to reference O2 content)
• Overall desulfurization efficiency and the efficiency contribution of individual spray layers under different recycle pump operating combinations
• Liquid-to-gas (L/G) ratio, circulation flow rate, and spray header pressure
• Absorber slurry pH, density, and suspended solids concentration
• Gas velocity and flow distribution uniformity at the absorber inlet and within the spray zone
• Droplet size distribution, spray coverage overlap, and nozzle discharge coefficients
• Mist eliminator carryover, expressed as liquid droplet concentration in outlet gas
• Limestone reactivity, fineness, and purity; gypsum purity, residual carbonate, and moisture
• Chloride, fluoride, and trace element enrichment in absorber liquor
• Pressure drop across the absorber and the spray zone

Testing Methods

SO2 Concentration Measurement

Instrumental methods dominate modern efficiency testing. Non-dispersive infrared absorption per HJ 629, ultraviolet fluorescence, and fixed-potential electrolysis per HJ 57 are used for portable measurements, while ISO 7934 (thorin spectrophotometry), ISO 11632 (ion chromatography), and EN 14791 serve as wet-chemical reference methods. Gas velocity and volume flow are measured with S-type pitot tubes in accordance with ISO 10780 or GB/T 16157. All analyzers are calibrated with certified reference gases before and after each test run, and results are cross-checked against the plant continuous emission monitoring system (CEMS).

Efficiency Calculation

Desulfurization efficiency is computed on a mass-concentration basis as η = (1 − Cout/Cin) × 100%, where inlet and outlet SO2 concentrations are converted to a common dry, standard-state basis and corrected to the reference oxygen content (typically 6% O2 for coal-fired boilers). To isolate the contribution of each spray layer, tests are repeated with defined pump combinations while unit load, coal sulfur content, and slurry chemistry are held constant; the marginal efficiency of a layer equals the incremental removal gained when that layer is brought into service. Formal acceptance testing follows the procedures of GB/T 21508 for coal-fired FGD equipment performance verification.

Liquid-Side and Material Characterization

Slurry pH is measured per ISO 10523 or ASTM D1293; density and solids content by gravimetric methods; limestone and gypsum particle size distribution by laser diffraction per ISO 13320; chloride and other dissolved anions by ion chromatography per ISO 10304 or ASTM D4327; and limestone chemical composition in accordance with EN 12485, with gypsum chemical analysis following GB/T 5484.

Spray Performance Verification

Removed nozzles are bench-tested to confirm flow–pressure curves, spray angle, droplet Sauter mean diameter (typically 1500–3000 μm for spiral nozzles), and pattern uniformity. Coverage overlap of each spray layer—generally designed at 150–250% of the absorber cross-section—is verified geometrically and reconciled against computational fluid dynamics (CFD) predictions using in-situ velocity traverses.

Applications

Spray layer desulfurization efficiency analysis supports design, acceptance, and optimization programs across a wide range of industries:

• Coal-fired power plants verifying ultra-low emission performance and FGD acceptance guarantees
• Steel and iron plants desulfurizing sinter machine and pelletizing furnace off-gas
• Petroleum refineries and petrochemical units treating fluid catalytic cracking (FCC) and boiler flue gas
• Waste-to-energy and biomass plants controlling acid gas emissions
• Cement, glass, and non-ferrous metallurgical furnaces applying wet scrubbing for SO2 and acid mist control
• Marine exhaust gas cleaning systems certified under international marine emission regulations
• Retrofit projects that add spray layers or trays to upgrade removal efficiency from 95% to above 99%

FAQ

What spray layer desulfurization efficiency is typically achieved?

Conventional single-loop limestone–gypsum absorbers with three to five spray layers achieve 95–98% SO2 removal. Systems augmented with trays, additional polishing spray layers, or dual-loop designs reach 99–99.7%, sufficient to meet ultra-low emission limits of 35 mg/Nm3 even when firing high-sulfur coal.

How many spray layers does an absorber need?

The number of layers depends on inlet SO2 concentration, the required outlet limit, and the selected L/G ratio. Typical designs use three operating layers plus one spare; high-sulfur applications may require five to six layers. Efficiency analysis during commissioning verifies whether the installed configuration meets the guarantee across the full load range.

Which factors most commonly reduce spray layer efficiency?

The leading causes are nozzle plugging or wear, non-uniform gas distribution, slurry pH drift, insufficient limestone fineness or reactivity, excessive chloride accumulation, scaling on spray headers and mist eliminators, and recycle pump degradation. Periodic efficiency testing identifies the root cause so corrective maintenance can be scheduled before emission limits are breached.

How is the L/G ratio related to efficiency?

The liquid-to-gas ratio expresses the total slurry circulation rate per unit of flue gas flow (L/m3). Raising the L/G ratio increases contact surface area and absorption driving force, so efficiency climbs steeply up to an economic optimum—typically 10–25 L/m3 for limestone systems—beyond which additional pumping power yields diminishing returns.

How often should spray layer efficiency be evaluated?

A full performance test is performed at commissioning and after major retrofits in line with GB/T 21508, while routine efficiency monitoring relies on calibrated CEMS data reviewed monthly. Annual spray system inspections, including nozzle sampling and laboratory verification, are recommended for units firing high-sulfur or high-ash coal.

Can spray layer testing be performed without a unit shutdown?

Yes. Inlet and outlet gas sampling, slurry chemistry monitoring, and marginal layer tests using different pump combinations are all executed online at stable load. Nozzle removal, spray rig testing, and internal coverage surveys require a planned outage.