Study On The Utilization Of Fly Ashes From Biomass Grate-Fired Power Plants

Nearly half of biomass-fired power plants apply grate-fired technology. The ashes produced include bottom ash and fly ash. Similar to the bottom ash, fly ash also contains nutrient elements. However, hazardous and toxic heavy metal elements may enrich in the ash because their volatility and fine ash forced during biomass combustion process. In particular, it is common practice that boilers designed to fire agricultural straws burn mixed fuel of the straws and woody biomass including wood barks and used wood materials to tackle the problems of fuel supply, boiler slagging, fouling and corrosion, which results high contents of heavy metals in the fly ashes. The potential impact of heavy metals on the environment may limit its applications for agricultural and forestry purposes. It therefore necessitates comprehensively characterizing and assessing the properties of fly ash, and exploring the treatment approaches of fly ash, which is the prerequisite for realizing their utilizations. The present work, aimed at the reality of biomass direct combustion for electricity generation using grate firing technology in China and particularly the feature of mixedly burning wood fuels, employed various advanced experimental and analytical methods to systematically investigate the physic-chemical and leaching characteristics of fly ashes from 6 grate-fired units. Based on these, systematically analysis and evaluation of the utilizations of fly ashes were carried out and a comprehensive treatment method of the fly ashes was proposed.Investigation on physical and chemical properties of fly ashes mainly include basic properties, particle size distribution, elemental and mineralogical composition. Analyses on the basic properties indicated that the fly ashes are often high in TOC content and the TOC contents of cyclone ashes are lower than those of filter ashes; both the cyclone and filter ashes are strongly alkaline but with pH values less than 13, which meets the requirement of pH value for agricultural and forestry utilizations. Particle size distribution showed that the fly ash is mainly dominated by small particles, which may be not beneficial for the utilization of the fly ashes. The elemental analysis showed that fly ashes not only are rich in nutrient elements (K, Ca, Mg, P) but also contain various heavy metal elements (As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, V, Zn). The mineralogical analysis indicated that fly ashes were mainly composed of SiO2, CaCO3, CaSO4, KCl, feldspars, etc.Study on the leaching behavior of fly ashes mainly included Cl leaching process, the leaching properties of major and heavy elements, elemental and mineralogical composition of the leached fly ashes, and the mineralogical composition of the leached liquid samples. Analysis on Cl leaching process indicated that Cl could be easily washed out and pre-removing unburned carbon from the fly ashes accelerated the Cl leaching. For nutrient elements, the leaching percentages of K and Na were higher, while those of Ca, Mg and P were relatively lower. For the heavy elements, the leaching percentages of the elements of all ashes were very low, much less than 1% except for Mo and Sb. It was found from the analyses of the washed fly ashes that the contents of K and Cl could be significantly reduced through washing. The mineralogical composition of the leaching liquor samples showed that most K leached as KCl with a small amount as K2SO4, Na leached as NaCl and Ca leached as CaSO4.Evaluation on the potential utilization of the fly ashes was performed based on the characterization of the physical and chemical properties and the leaching behaviors. It showed that cyclone ashes from burning agricultural and forestry residues are applicable for agricultural and forestry utilization purposes. The filter ashes and the cyclone ashes from burning mixed waste woods cannot be used in agriculture and forest land because of high contents of heavy metals. Cyclone and filter ashes have high contents of TOC, alkali metals, chlorine and sulfur, which limit their direct uses as building materials. Although washing reduced the contents of alkali metals, Cl, and S, it still could not solve the problem of the high TOC content. In view of the above situation, a comprehensive pre-treatment method was proposed to develop the value-added applications of the fly ashes.