Soda Recovery - Process info
Soda Recovery Boiler - Black Liquor Combustion Introduction and background
Concentrated black liquor from the evaporation plant is burned in the Recovery Boiler.
A recovery boiler serves three critical functions in Kraft Pulp Mill:
1. It acts as a chemical reactor to produce sodium carbonate (Na2CO3) and sodium sulphide (Na2S).
2. It is a fuel combustor and steam generator. Together with a power boiler it generates all steam required by a kraft pulp mill. A steam turbine is utilized to produce electrical power for the mill.
3. It destroys the dissolved organic matter and thus eliminates an environmental discharge.
The chemical reactor function consists of producing a molten smelt of Na2CO3 and Na2S from the sodium and sulphur compounds distributed among many different organic and inorganic compounds in the black liquor. In the lower furnace the combustion is carried out in a reducing atmosphere to produce the final sulpher compound sulphide which is an active kraft pulping agent. The measure of success for the chemical recovery function is the reduction efficiency (defined as: R = Na2S/(Na2S+Na2SO4)*100 mol-%). The smelt is tapped from the furnace through smelt spouts into the dissolving tank and mixed with water to form green liquor.
Combustion in furnace converts the organic material into gaseous products in a series of processes involving drying, pyrolysing (devolatilising), char gasification and finally homogeneous combustion.
Burning of the solid char, the residue after pyrolysis, occurs on the char bed which covers the floor of thef urnace. As the fix carbon in the char is gasified, the inorganic compounds in the char are released and formthe above mentioned molten salt mixture called smelt and flows to the bottom of the char bed.
During the pyrolisis a certain amount of the sulphur forms TRS gases which are burned to SO2 in the upper furnace. In the lower furnace some of the Na is vaporised and reacts with SO2 in the upper furnace to form fume (dust, Na2SO4) particles. The dust is captured in the electrostatic precipitators and recycled to black liquor via a mixing tank.
A small portion of the sulphur leaves the process as flue gas SO2 emission. If the air emissions regulation is very stringent, the recovery boiler can be equipt with a flue gas scrubber to remove SO2. The flue gas scrubber has also often a heat recovery section to produce hot water.