Division 1.7.1.3 SO2/TRS emissions prevention and control
United Kingdom
| Technique | Description |
|---|---|
| High dry solid black liquor | With a higher dry solid content of the black liquor, the combustion temperature increases. This vaporises more sodium (Na), which can bind the SO2 forming Na2SO4 thus reducing SO2 emissions from the recovery boiler. A drawback to the higher temperature is that emissions of NOx may increase |
| Fuel selection/low-S fuel | The use of low-sulphur content fuels with a sulphur content of about 0,02 – 0,05 % by weight (e.g. forest biomass, bark, low-sulphur oil, gas) reduces SO2 emissions generated by the oxidation of sulphur in the fuel during combustion |
| Optimised firing | Techniques such as efficient firing rate control system (air-fuel, temperature, residence time), control of excess oxygen or good mixing of air and fuel |
| Control of Na2S content in lime mud feed | Efficient washing and filtration of the lime mud reduces the concentration of Na2S, thus reducing the formation of hydrogen sulphide in the kiln during the re-burning process |
| Collection and recovery of SO2 emissions | Highly concentrated SO2-gas streams from acid liquor production, digesters, diffusers or blow tanks are collected. SO2 is recovered in absorption tanks with different pressure levels, both for economic and environmental reasons |
| Incineration of odorous gases and TRS | Collected strong gases can be destroyed by burning them in the recovery boiler, in dedicated TRS burners, or in the lime kiln. Collected weak gases are suitable for burning in the recovery boiler, lime kiln, power boiler or in the TRS burner. Dissolving tank vent gases can be burnt in modern recovery boilers |
| Collection and incineration of weak gases in a recovery boiler | Combustion of weak gases (large volume, low SO2 concentrations) combined with a back-up system. Weak gases and other odorous components are simultaneously collected to be burnt in the recovery boiler. From the exhaust gas of the recovery boiler, the sulphur dioxide is then recovered by counter-current multistage scrubbers and reused as a cooking chemical. As a back-up system, scrubbers are used. |
| Wet scrubber | Gaseous compounds are dissolved in a suitable liquid (water or alkaline solution). Simultaneous removal of solid and gaseous compounds may be achieved. Downstream of the wet scrubber, the flue-gases are saturated with water and a separation of the droplets is required before discharging the flue-gases. The resulting liquid has to be treated by a waste water process and the insoluble matter is collected by sedimentation or filtration |
| ESP or multicyclones with multistage venturi scrubbers or multistage double inlet downstream scrubbers | The separation of dust is carried out in an electrostatic precipitator or multistage cyclone. For the magnesium sulphite process, the dust retained in the ESP consists mainly of MgO but also to a minor extent, K, Na or Ca compounds. The recovered MgO ash is suspended with water and cleaned by washing and slaking to form Mg(OH)2 which is then used as an alkaline scrubbing solution in the multistage scrubbers in order to recover the sulphur component of the cooking chemicals. For the ammonium sulphite process, the ammonia base (NH3) is not recovered, as it is decomposed in the combustion process in nitrogen. After the removal of dust, the flue-gas is cooled down by passing through a cooling scrubber operated with water and it then enters a three or more staged scrubber of the flue-gas where the SO2 emissions are scrubbed with the Mg(OH)2 alkaline solution in the case of the magnesium sulphite process, and with a 100 % fresh NH3 solution in the case of the ammonium sulphite process. |