Study On The Migration,Transformation And Release Control Of Potassium During Combustion Of Torrefied Biomass

The biomass torrefaction technology developed in recent years is a low-temperature pyrolysis technology that can improve the characteristics of biomass storage and transportation and energy quality,and is an important research and development direction for biomass energy utilization.But torrefaction technology can’t relieve the problem of high alkali metal content of biomass itself.During biomass combustion,the high concentration of alkali metal compounds gas will cause ash accumulation,slagging and corrosion on the heating surface of the boiler.K is the main alkali metal of biomass.In this paper,by the tube furnace experiment and HSC Chemistry chemical thermodynamic equilibrium calculation software,the release and migration and control of the main alkali metal K in the torrefied biomass during the combustion process,as well as the influence of silico-aluminum and phosphoric acid additives on the release and migration of K,were studied.Provided a theoretical basis for the industrial application of roasted biomass.Using cotton stalks,rice stalks and wheat stalks as experimental materials,a tube furnace was used to study the combustion of biomass before and after torrefaction.The results showed that torrefaction mainly reduces the release of K by increasing the amount of water-soluble K and char-K in the biomass combustion ash at 600°C.At 700-900°C,torrefaction also promotes K silicate or aluminosilicate to reduce the release of K.During the combustion of roasted biomass,as the temperature increases,water-soluble K and ammonium acetate K are continuously converted and released.At 600°C,water-soluble K and ammonium acetate K were mainly converted to char-K,and at 700-900℃,water-soluble K and ammonium acetate K were also converted into K silicate or KAl Si O₄.The increase of O₂concentration increased the release rate of K during the combustion process of roasted biomass by promoting the release of water-soluble and ammonium acetate K.The CO₂ atmosphere mainly reduced the release rate of K by inhibiting the release of water-soluble and ammonium acetate K,and the negative effect of CO₂atmosphere is stronger than the positive effect O₂.Adding NH₄H₂PO₄,Ca3（PO4）2,diatomite and calcined kaolin to the torrefied wheat straw.The effect of additives on the K release and migration during the combustion of torrefied wheat straw was studied in a tube furnace.The following conclusions were obtained.All four additives can reduce the release of K during the combustion of torrefied wheat straw.The effect of phosphoric acid additives was better than that of silicon-aluminum additives.NH₄H₂PO₄had the best effect,while diatomaceous earth had the worst effect.Considering economy and K retention,effect of phosphoric acid additives is best when the addition amount was 10%,while the optimal addition amount of silicon and aluminum was 5%.In air atmosphere,at 700-900°C,under the condition of the optimal amount of addition,the best K retention temperature of phosphoric acid and silicon-aluminum additives are 800°C and900°C,respectively.Due to the formation of high melting point K-Al-Si and K-Ca-P d salts,both phosphoric acid additives and calcined kaolin can alleviate the ash fusion of torrefied wheat straw and the effect of Ca₃（PO₄）₂and calcined kaolin is better.Due to the formation of low melting point K₂Si₄O₉,diatomite intensified the melting of torrefied wheat straw burning ash.For torrefied wheat straw mixed with NH₄H₂PO₄,CO₂and O₂can affect the release of K during the combustion process by inhibiting and promoting the formation of KPO₃and K₃PO₄,respectively.For mixed with other additives,the influence mechanism of atmosphere on K release and transformation is the same as not added.Chemical thermodynamic balance calculation results showed that during the combustion of roasted biomass,K was released in the form of KCl,but also in the form of KOH.During the combustion process of the torrefied biomass,part of K was released in the form of K₂SO₄for the torrefied cotton stalks.In addition,torrefaction inhibits the release of K during biomass combustion and promotes the formation of K₂CO₃and K silicate,which was consistent with the experimental results.The three types of additives can effectively inhibit the release of K during the burning of roasted wheat straw.Among them,phosphoric acid additives had the strongest effect,and silicon-aluminum additives had the weakest effect.The atmosphere not only directly affected K₂CO₃,K₂SO₄,K-P salt,K silicate and aluminosilicate through the amount of O₂and CO₂,but also affected the conversion of K₂CO₃to K-P salt,K silicate and K aluminosilicate conversion by influencing the amount of K₂CO₃.