Rice Straw Thermal Physical Experiment Fluidized Bed Gasification And Ash

Biomass is a kind of environment-friendly renewable energy resources that can reduce the amount of CO2emissions in the process of energy utilization and has the properties of low sulphur and low nitrogen. The utilization of crops straw for energy had exceeded500million tons annually, of which rice straw and rice husk had240million tons and40million tons respectively. With the expanding commercial application scale of biomass gasification energy utilization in China, it paid more and more attention to utilizing biomass ash residues that are made into building materials and heat-resisting materials. In this thesis, four kinds of crops straw (rice straw, wheat straw, corn stalk and rice husk) were chosen as raw materials for the study of thermophysical properties, the preparation of ashes, cold and hot test of rice straw fluidized-bed gasification, physicochemical characteristics of biomass ash and rice straw gasification ash residues and ash melting characteristics of experimental research. The results were as follows:(1) The ash content in four kinds of biomass had greater differences, values of which was3.27%-16.25%; the VM content was60.23%-71.93%; the FC content was14.65%-18.85%. Campare with coal, the sulphur content was very low, but the nitrogen content was on the high side in four kinds of biomass selected in the thesis.(2) In order to improve the conversion rate of biomass fluidized-bed gasification, it needed the crushing treatment. After the pretreatment of rice straw, it showed short fibrous or short rods and some of the powder particles. It’s more accurate to characterize the dimension of rice straw with the length and standard screen size. The experimental study on the cold test of rice straw fluidized-bed gasification determined its related parameters. In the hot test, when the reaction temperatures of gasification were600℃,700℃,750℃and800℃respectively, the relative proportion of furnace bottom slag, fly ash of cyclone separator and spray scrubber of rice straw gasification ash residues were24.53%-93.85%,3.52%-74.75%and0.72%-2.63%respectively. With the temperature rising, rice straw gasification fly ash that were collected in the cyclone separator decreased gradually, but bottom slags and spray scrubber fly ash increased gradually. Quartz sand had a minor effect on rice straw fly ash; however, furnace bottom slags were strongly affected by quartz sand. Through the proximate analysis of rice straw gasification ash residues, the content of water, VM and FC varied widely between2.18%-21.18%,0.72%-5.16%and0.04%-10.44%respectively. (3) The apparent morphology of biomass ashes had some differences. From ash particles size distribution, shape, agglomeration, pore structure and so on, ash particles were formed by many smaller particles which sintered and aggregated. Rice straw gasification ash residues also varied from one another in micro-morphology. The particles of furnace bottom slags showed irregular block structure and smooth sections; the particles of spray scrubber fly ash showed tiny graininess and homogeneous distribution; cyclone separator fly ash overall showed block, flake, short stick and so on.(4) By EDS analysis of ash residues samples, rice straw ash and wheat straw ash had higher content of K, Na, Cl and Fe elements; corn stalks ash and wheat straw ash had higher content of Ca and Mg elements. The element distribution of rice straw gasification ash residues had the following two features:one is that K, Na, Ca, Mg and Cl elements mainly enriched in the cyclone and spray tower; another is that the large grain size and hardness of elements mainly enriched in the bottom, such as Si, Al and Fe etc. By XRD analysis, species of phases closely related to results of EDS analysis. Biomass residues all contained the chemical component of SiO2crystal phase, rice straw ash and its gasification fly ash and wheat straw ash also contained KC1or K2SO4, and the main crystal phase of corn stalk ash were MgO and CaO; cristobalite in the furnace bottom slag of rice straw gasification might result in blending with bed material, during the test, opal and SiO2partly changed into quartz or vitreosity with alkali-metal.(5) The ash fusion point of biomass ash residues related to the melting point and relative content of components. The ash fusion point of rice husk ash and furnace bottom slag of rice straw gasification were higher, and the ash fusion point of rice straw ash and wheat straw ash were lower, which DT were between1220℃-1265℃and1085℃-1145℃respectively; In the strongly reducing atmosphere, wheat straw ash, rice straw and its gasification fly ash had low temperatures of ash fusion point. Adding alumina could be effective to improve the ash fusion point of biomass ash residues. When the alumina ratio was greater than64.9%,50%and55%respectively, ash fusion point of mixtures were greater than1500℃which could satisfy their basic requirements of refractability. Biomass ash residues in the higher content of alkali-metal could be improved their ash fusion point by water-washing.(6) The ash melt type of rice husk ash, rice straw ash and its gasification ash residues were vitreous slag, and the ash melt type of corn stalk and wheat straw were crystallized slags of low melting point. These types couldn’t be changed by water-washing, but could be changed vitreous slag into crystallized slag of high melting point. Using of the viscosity and slagging index of coal ash, it had larger errors on the calculations of critical viscosity-temperature, ratio of acid to alkali, silica ratio and contamination factor etc. It should carry out a large number of experimental research and theoretical analysis; establish the evaluation system that is suitable for the characteristics and ashes melt index of biomass ash residues.The results of this thesis had laid a certain technological base and accumulated reliable data for resource utilization of biomass gasification ash residues such as rice straw, wheat straw, corn stalk and rice husk etc.