Study On The Alkali Transformation Behavior Of Straw Combustion

The utilization of biomass is very helpful for alleviating pressure from energy shortageand pollution in the world. Combustion is one important way of the biomass utilization. Theinfluence factors of the combustion are attracting more and more attention. Among thesefactors, as the important contents of the alkali metals affects the combustion of biomass verymuch. Potassium, always brings agglomeration, corrosion and deposition on the heatingsurfaces in combustion system, and becomes the main reason which limits the utilization ofstraw in large scale. Consequently, to better understand the alkali behaviors during thecombustion of agricultural residue, and seize the law of alkali emission and transformation isof great importance to solve the alkali problems.This paper pays some attention on the transformation behavior of alkali in biomasscombustion process. And base on the comparative analysis of extensive literatures, issuesrelated to alkali metal during the energy utilization process of biomass in the world, also beenmade a comprehensive discussion. The equilibrium analysis software FactSage, is used todetermine the transformation behavior of alkali, and combined with the combustionexperiments, the alkali behavior characteristics of alkali are be revealed. In order to provide atheoretical basis for reducing the alkali metal problem, the characteristic of straw catalyticcombustion also been studied.The catalytic combustion characteristics of biomass straw under conditions of differentatmosphere, heating rate and catalyst are studied on the thermal analyzer. Combustionparameters such as ignition temperature, peak temperature of maximum weight loss rate,burnout temperature, maximum rate of combustion, and combustibility index were analyzed.The result shows that: high-heating rate contributes to the thermal hysteresis. And theoxygen-enriched atmosphere also can enhance the fixed-carbon combustion, while thecontribution of it is limited.Combustion rate increased by adding catalyst, and ignition temperature reduced, soadding catalyst is conducive to reduce ignition. The kinetic parameters are obtained bynon-linear dynamic model. The activation energy reduced mainly below α <0.6when catalystis added, adding CuO is better than other ones, the maximum reduction is66.41Kj/mol, socatalysts reduce the activation energy is mainly due to the volatile combustion catalysis. Thecontribution of catalyst to fixed-carbon combustion is limited, while the fixed-carboncombustion begins earlier during straw combustion because of the catalyst, it means there isoverlapping on time among volatilization and fixed-carbon combustion process. By combustion experiments in fixed-bed reactor, the influence of combustiontemperature and time on devolatilization, char combustion, potassium transformation andevolution behaviors were investigated. The result shows that: volatile components of strawhave rapid devolatilization and good combustion features, while the residual char burns outrelatively slowly. At lower temperatures there is a greater backward delay for char combustion.Therefore, the gas phase combustion and char combustion of the straw biomass can beseparated by controlling the reactor temperature.Under low combustion conditions or during the initial burning stage, the evolution ofpotassium is associated with decomposition of the organic structure, while the emissionsunder high temperature conditions correspond to the evaporation of inorganic potassium saltas a result of the increased vapor pressure. The ash residue analysis shows the molar ratio of（K+Na）/Cl increased with temperature, indicating that chlorine in the straw has highervolatility than alkali metals under the combustion conditions. XRD analysis revealed that KCland K2SO4were the main compounds associated with potassium in combustion residuals, andKCl was shown to play a dominant role in potassium evolution, while the contribution ofK2SO4is limited.The equilibrium analysis software FactSage, is used to simulate thermodynamics state ofthe transformation behavior of alkali during the straw combustion. The result shows that: thewater soluble K+may react with anions that have low thermal stability, forming inorganicsalts such as KCl（s）、K₂SO₄（s）、K₂Si₄O₉（s） at low temperature. The amounts of KCl（s）decreased with increasing temperature. The rapid evolution of KCl（s） began at about600oC,and it was transformed into KCl（g）,（KCl）₂（g）, KCl（salat）, and K₂O（slagd）, which indicatesthat combustion should be conducted at less than600oC to retain potassium within theresidual straw. At the same time, high silica content in biomass would restrain the release ofgaseous K. However, more agglomeration of straw ash may occur in such cases, and theeutectic phenomenon appears between KCl and K₂SO₄leading to formation of a eutecticmixture, which also contributes to the slagging problem that was observed at themid-temperature range （670～800oC）.Variable conditions simulation result shows that: the evolution of alkali element wouldbe reduced mid-temperature range （below750oC） with the increasing of the oxygenconcentration. And, it also able to reduce the mount of the corrosive gas HCl, that would beconducive to reduce the problem of deposition, fouling, slagging and agglomeration.Improving the oxygen concentration for straw combustion had no particular importance for solving corrosion problems, at high-temperature range, but would help alleviate thedeposition problem on heating surface.