CFB Coal Boiler features high combustion efficiency and strong fuel adaptability, and has built-in advantage in low-pollution emission. Due to low combustion temperature and uniform furnace temperature distribution, in-furnace flue gas recirculation denitration and desulfuration technology is suitable for CFB coal boiler.
The key to inhibit the generation of NOx is the furnace temperature and reducing atmosphere of combustion reaction. Flue gas recirculation (hereinafter referred to as FGR) technology circulates some flue gas with low oxygen content into the furnace, lowering the temperature in the combustion area and forming oxygen-poor combustion environment, and thus inhabiting the generation of NOx. FGR denitration technology may reduce the initial emission of NOx by 20-40%.
In-furnace desulphurization is to add calcium-based sorbent at proper temperature interval, transforming the sulfur content in the coal to sulfate. The sulfate will be discharged with the slag. The chemical reaction is: CaCO3→CaO+CO2; CaO+SO2→CaSO3; CaSO3+0.5O2→CaSO4. The key to in-furnace desulphurization is the performance (particle, porosity and reactivity), proportion (Ca/S ratio) and working temperature of desulfurizer. The desulphurization efficiency can reach 90%.
I. CFB coal boiler specification
The coal CFB boiler model is TG75-3.82/450-M, and is singe drum full membrane wall and semi-outdoor arrangement. Daily operation load is around 45t/h; the oxygen content of flue gas at furnace outlet is around 6%; furnace temperature is around 890-930℃. Detailed design parameter is as follows:
Boiler maximum continuous rating (BMCR): 75t/h
Rated steam pressure: 3.82MPa
Rated steam temperature: 450℃
Feed water temperature: 150℃
Cold air temperature: 30℃
Flue gas temperature: 157℃
Design fuel: coal gangue
Fuel heating value: 6980KJ/kg
Fuel consumption: 38t/h
Thermal efficiency: 85.9%
II. Denitration and Desulphurization reconstruction scheme
1. Denitration reconstruction scheme
Desulphurization reconstruction mainly includes FGR and SNCR reconstruction. FGR can lower the oxygen concentration in the main combustion zone, form reducing atmosphere and reduce the generation of NOx; SNCR reconstruction can improve the reaction efficiency. After starting FGR, recirculating flue gas with low oxygen content (around 10%) enters the coal CFB boiler, the recirculating flue gas amount is nearly 32%. The oxygen content at furnace outlet is around 2.8%, the oxygen content at chimney outlet is around 9.5%, and NOx emission is nearly 75mg/m3.
2. Desulphurization reconstruction scheme
In-furnace desulphurization plus wet desulphurization device can meet the emission requirement. After FGR and desulphurization reconstruction, bed temperature reduces to around 900℃, initial emission reduces to 700mg/m3, and final SO2 emission is nearly 30mg/m3.
