Study On Combustion Characteristics Of Herb Residue In Fluidized Bed

Chinese herb residue has high water content and it is perishable, the herb residuewould cause serious environmental pollution if it is not handled peoperly. The Chineseherb residue is a kind of ideal biomass fuels and recovering the heat by means ofcombustion can achieve good environmental and economic benefits. The residue ofLiuwei Dihuang Pill, Qiju Dihuang Pill, Xiangsha Yangwei Pill and Xiaoyao Pill werestudied in this paper. Experiments were carried out to obtain the fuel characteristics, ashcomposition and combustion characteristics in fluidized bed.The research shows that the dried basis volatile content of these four herb residues isall above75.55%, the ash content is less than5.58%and the high calorific values rangefrom18MJ/kg to19MJ/kg. Alkali metal, Al and Si could generate low-melting eutecticsduring combustion, which would decrease the melting point significantly, while theacidic oxide would increase the melting temperature. The ash softening temperature ofthe four herb residues which were obtained under600℃and815℃are both above1194℃. Thermal gravimetric analysis under air atmosphere shows that the maximumweight loss rate of the four herb residues appears at about305℃, termination reactiontemperatures are lower than820℃, the ultimate weight loss rate are all above95%.By analysing the ash under different temperaturs(600℃,700℃,815℃,900℃)through XRF and XRD, the results show that the alkali metal mainly exists in the form ofhydrogen chlorides, sulfates and oxides and a small amount exists in the form of sulfides,chlorates and silicates, moreover the alkali metal may form low-melting eutectics with Aland Si. The four herb residues have high content of organic alkali metal, the alkali metaland Cl in the ash reduce gradually with the temperature increasing. The organic alkalimetal separates out with volatile rapidly below500℃, Cl separates out mainly as theform of HCl in this period. When the temperature is higher than500℃, the inorganicalkali metal mainly separates out as the form of gaseous KCl and NaCl, Cl is easier toescape than K and Na, the precipitation of the alkali metal is controlled by Cl.The experiments carried out in fluidized bed show that the four herb residues couldbe fluidized and burned well under700~880℃when quartz sand is used as the bedmaterial. The study of flue gas emission characteristics shows that NOxemissionconcentration increases rapidly with the temperature increasing from700℃to850℃,while the concentration increases slowly between850℃and880℃. The NOxemissionconcentration of four herb residues will increase when mixed with urea directly.