Study on Characteristics of Ash Deposition and Release of Potassium/Chlorine during Biomass Co-fired with Coal
|Course||Thermal Power Engineering|
|Keywords||ash deposition alkali metal Cl ash characteristics DLA model|
The energy shortage and environmental pollution problems become more and more serious recent years. People have paid more and more attention to biomass due to its renewable and zero emission of CO2, and the bioenergy utilization has become a hot research in the energy field. Biomass and coal co-combustion technology in power generation can use the infrastructure in coal-fired power plant to realize the effective use of biomass fuels. Combustion characteristics, pollutant emission, ash fusion temperature and so on during biomass and coal co-combustion have been theoretical and experimental studied at home and abroad, so in this paper, the ash deposition characteristics during biomass and coal co-combustion are experimental investigated in a one-dimensional drop-tube furnace, the release characteristics of alkali metal and Ci under different condition are analyzed and the chemical mechanisms during ash deposition process are studied through analysis the ash phase composition, which have great significance to widely use this technology in power plant and largely promote the use of biomass.First of all, the effect of different biomass mixing ratio on topography and phase composition of the deposition are studied in the drop-tube furnace. the higher biomass mixing ratio, the denser structure of ash deposits and more difficult to clean; the mass percentage of alkali metal oxides K2O、Na2O and Cl in ash deposits increase, while the content of Al2O3and SiO2decrease,the existence of alkali aluminosilicate with low melting point in ash is the main reason for ash deposition aggravating; the increasing ratio of S/Ca makes the fusion temperature lower, which lead to fouling and slagging aggravating too.Secondly, the release characteristics of K and Cl during biomass and coal co-combustiom are studied in a tube furnace. Biomass has higher exhalation rate of alkali metal and lower exhalation rate of Cl than K. The alkali metal K is mainly released as KC1, while Cl is released as Cl2and HC1except KC1. Biomass and coal co-combustion can inhibit the release of alkali metal K,and promote the release of Cl. The release of alkali metal K and Cl increase with the increasing of K and Cl content,, but not obvious linear relationship. With the increasing of biomass mixing ratio, the exhalation rate of alkali metal K increase at first and then decrease, while the exhalation rate of Cl kept between60%and80%. Fe2O3in fuels can inhibit the release of alkali metal, CaO can promote the release of alkali metal, S can inhibit the release of alkali metal and increase the release of Cl by sulfuration reaction. Combustion temperature has obvious effect on the release of alkali metal and the release amount of K increase at first and then decrease with the temperature increasing. This is mainly due to the reactivity of fuel component. The effect of additive to the release of K and Cl during biomass and coal co-combustion are studied. Results showed that Al2O3、Kaolinite can react with KC1to generate alkali aluminum silicate, and Marialite and losite, which inhibit the release of K and C1.Thirdly, energy distributions of ash generated during biomass and coal co-combustion are analyzed with MDI Jade5.0software. Results showed that SiO2is the main phase in ash, and with the biomass mixing ratio increasing, the diffraction peak of SiO2shape becomes lower, while the diffraction peak of alkali aluminum silicate becomes higher. Due to the higher S content and greater potassium fixation capacity of lean coal, lean coal co-fired with40%corn stalk has higher diffraction peak of CaSO4and alkali aluminum silicate comparison than bituminous coal co-fired with corn stalk. With the combustion temperature increasing, the diffraction peak of alkali metal compound increases, and the phase of Ca(Al2Si2O8)、Mg3Al2(SiO4)3emerged gradually, the fusion temperature decreased, the viscous of deposition increase, which aggravated the ash deposition.Finally, the condensation process of ash particles with the biomass mixing ratio increasing during biomass and coal co-combustion was simulated with DLA model of fractal, Under the same restrictive radius condition, with the population increasing, the branches of condensates increase, Self-similarity and fractal dimension increase. It is similar to the biomass and coal co-combustion process. With the biomass mixing ratio increasing, the release amount of alkali metal increased, the fusion temperature decreased, the structure of ash deposits become denser, which proved that the simulation has a good conformance, and it is feasible.