Popular description of flue gas cleaning
The EU places restrictions on the disposal of combustible waste (i.e. dumping at landfills). The reasons are many. Incineration can be used for the generation of district heating and electricity; waste volumes are reduced, the risk of landfill fires eliminated and infectious agents and pollutants (POPs) are destroyed.
Waste can be incinerated in a safe and effective manner in so-called grate-fired furnaces, where the waste is incinerated on a kind of grating. The hot flue gases are cooled in the boiler whose walls consist of steel tubes containing water. The energy from the waste boils the water and the steam generated is used to drive a steam turbine, which in turn drives a generator to produce electricity for the national grid. On leaving the turbine the steam can be condensed back into water while transferring its energy to the district heating system to provide domestic heating and hot water.
The picture shows a DynaGrate® grate-fired furnace from B&W Vølund for waste incineration.
About acids and bases in waste incineration
Man discovered ash to be alkaline a long time ago. It consists largely of potash (potassium carbonate). If a base is mixed with an acid they react to form a neutral salt and give off heat. Mixing e.g. hydrochloric acid with lye produces common salt and creates heat. Another example is sulfuric acid and limestone, which form gypsum. If on the other hand salt is heated sufficiently it will break down into acid and base again. This happens if e.g. common salt or gypsum is thrown in the trash and burned in a waste incineration facility. Because the acidic substances are volatile, they evaporate and end up in the flue gases, while the bases end up in the ash. By adding alkaline substances during flue gas cleaning the acidic substances can be removed and release through the stack prevented.
Flue gas cleaning
The smoke that comes from the furnace contains many environmentally harmful substances such as acidic gases and fly ash (dust) containing heavy metals and dioxins. Waste incineration installations have very strict emission limits, which place great demands on Flue gas cleaning.
Dust can be removed by means of high voltage in electrostatic precipitators or filtration in so-called baghouse filters. Adding powdered carbon to baghouse filters removes dioxin. If limestone is added to the baghouse filter, acidic gases can also be removed there.
Hydrochloric acid is a powerful acid that is extremely soluble in water. It's enough to drench the gas with water for it to dissolve in it, and this is exploited in wet flue gas cleaning. Sulfur dioxide forms a weak acid that must be helped along by the addition of an alkali to the water such as liquor, which helps the sulfur dioxide dissolve. Wet flue gas cleaning systems are known for their low emissions, but it's also possible to recover even more energy from the flue gases by means of flue gas condensation.