An Investigation On Transformation And Deposition Behavior Of Alkali Metals In Biomass Combustion

As a renewable energy source, biomass energy has large reserves and little pollutionand CO2emission are produced in the use of biomass. Using biomass in large-scale willhelp alleviate the energy shortage, environmental pollution and greenhouse gas emissionsissues nowadays. However, due to the high content of alkali metal in biomass, the use ofbiomass in the combustion process can easily cause severe ash deposition and corrosionproblems. Therefore, research on the transformation and deposition of alkali metal inbiomass combustion is of importance to solve the ash deposition and corrosion problems inbiomass boilers and enlarge the scale of biomass utilization.In this paper, the Gibbs free energy criterion was used to analyze the possible reactionsof alkali species in biomass combustion and the transformation pathways of alkali metalwas discussed. In addition, the behavior of alkali metals have been predicted bythermodynamic equilibrium calculations based on Gibbs free energy minimization model.The results show that when temperature is below600°C, alkali metals are in solid species,such as K2O2SiO2, KCl, K2SO4, Na2CO3, Na2SO4, Na2SiO3, etc. When temperature isabove600°C, alkali species appear in gas phrase in the form of alkali chloride. Above1000°C, alkali hydroxide may also appear in gas phrase.The effect of Cl、S、Si、Al on the behavior of alkali metals in biomass combustion wasalso investigated by thermodynamic equilibrium calculations. It shows that Cl can promotethe release of alkali metals while S, Si, Al probably inhibit the release of alkali metals. Theoptimal S/(K+Na) molar ratio is about1and optimal Al/(K+Na) molar ratio isapproximately equal to1.5. Although the additional Si can reduce the volatilization ofalkali metals, the amount of silicate with low melting point will also increase. Thus, Si haslittle effect on the alleviating ash deposition problem.Ash samples taken from high and low temperature superheaters of a straw boiler wasanalyzed combined with the results of the thermodynamic calculations. The results show that the main reason of ash deposition on superheaters of the boiler is that gaseous alka lispecies condense on superheater tubes and absorb SO2in flue gas. By comparing the ashsamples from high and low temperature superheaters, it is found that the sulfation of alkalichloride is more likely to happen in high temperature superheater region.