Study On The Alkali And Chlorine Transformation Behavior During Biomass Combustion

With the large reserves and low pollution, the utilization of biomass is very helpful foralleviating pressure from energy shortage and pollution in the world. However, biomass isrich in alkali/alkaline earth metal, which is easy to cause some problems, such asagglomeration, corrosion and deposition on the heating surfaces. Moreover, chlorine has greatinfluence on the alkali migration. So it’s very necessary to understand the alkali behavior andmaster the rules of alkali emission and transformation.In this paper, the migration behavior of alkali during the biomass combustion processand the effect of different additives on its behaviour are studied by using the equilibriumanalysis software FactSage, fixed-bed reactor and35.5kW circulating fluidized bed. Theresults obtained from the experimentations could provide a theoretical basis for reducing thealkali metal problem.The equilibrium analysis software FactSage is used to determine combustion productsand the distribution of alkali. The effect of different additives has also been studied. Theresults showed that potassium was present in KCl（s） at low temperatures, then KCl（s） wouldbe transformed into KCl（g）,（KCl）₂（g） and KCl（salat） with increasing the temperature. Atmiddle or high temperature, biomass contained high silicon content tended to form theKCl（salta） and K₂SO₄（salta）. Conversely, KAlSi₂O₆（s） with high melting point would beformed. And the migration behaviour of potassium could be judged by the value of（K+Cl）/（Si+Al）. The additives could cause the inhibition of the K₂O （slagd） formation, whichpromoted the formation of the potassium aluminosilicate with high melting point.By combustion experiments in a fixed-bed reactor, the HCl emission curve of the barkand the bagasse taked on a single peak under constant temperature, which the peak valuedecreased first and then increased, and decreased at last. The HCl emission could be dividedinto two steps by500℃. HCl emission was controlled by the content of organic chlorine atlow temperature, which was closely related to the chlorine content at high temperature.Similarly, the potassium emission also could be divided into two steps by500℃, which wascontrolled by the content of organic potassium at low temperature and caused by the releaseof the KCl vapor at high temperature. Moreover, the potassium migration at high temperaturecould be judged by the value of （K+Cl）/（Si+Al）, which had no relation to the value at lowtemperature.After adding calcium oxide, alumina, kaolin, decoking agent and coal in the eucalyptus bark, all the samples were burned in a fixed-bed reactor. The results show that CaO had greatinfluence on the removal of HCl, but its influence became weak at the temperature over800℃. Adding kaolin and decoking agent made the concentration of HCl emission increase.Adding coal could reduce the HCl emission at low temperature, but increased HClconcentration at high temperature. The kaolin and decoking agent was very capable to catchthe potassium to stay in the ash, and CaO had greater capacity to catch chlorine.In order to look for the rule of the potassium and chloride content in the fly ash, and HClemissions, eucalyptus bark was used as the study object in the35.5kW biomass circulatingfluidized bed. The results show that the temperature had little influence on the HCl emission.However, the potassium and chloride content in the fly ash decreased with increasingtemperature. The potassium content of the fly ash increased after adding calcium oxide, andall other additives could reduce the amount of potassium in the fly ash. The removalefficiency sorted by size: decoking agent> kaolin> alumina> coal. All additives could reducethe amount of chlorine in the fly ash, and their dechlorination ability was sorted by size:calcium oxide> decoking agent> alumina> kaolin> coal.