| Biomass, as a renewable energy, is the fourth largest energy after coal, petroleum and natural gas in the world total energy consumption. Biomass resources is extensive, carbonization is one of the important technologies for the efficient use of biomass resources. Carbonization is the process of the biomass material occurring physical and chemical changes when heated to a high temperature in the case of isolation of air. Biomass carbonization has fewer requirements for raw material. Biomass carbonization process also is simple and has the advantages of low energy consumption. Compared with biomass the energy density of biomass charcoal increases significantly, and the storage and transportation of biomass chaecoal is convenient. Compared with coal, the ash, nitrogen and sulfur content of biomass charcoal is less, the emission characteristics is better. So the biomass charcoal is a kind of good alternative fuel. The typical biomass carbonization performance, the characteristics of biomass charcoal, combustion and emission characteristics and the influence of additive to the transport of alkali metal elements in the combustion and carbonization process was studied.(1) The tube-type furnace system was utilized to study the carbonization of biomass material. The experiment simulated three-step carbonization equipment. The effects of temperature on carbon yield, calorific value and energy yield of the biomass charcoal were studied, and the characteristic of biomass charcoal was analyzed. The results showed that the productivity of charcoal and energy decreased with the increasing of carbonization temperature. When the tempreture was300℃, the carbon yields of corn stalk, cotton stalk and poplar wood were respectively64.5%,66.4%,60.4%, the energy yields were74.7%,82.9%and81.3%. When the temperature exceeded400℃, the charcoal yield and energy yield decreased obviously. The energy productivities of three samples were between40%and50%at800℃. In addition, the additives had little influence on the charcoal yield. The energy yield of torrefaction at 230℃was approximate the energy yield of carbonization at300℃, and the charcoal yield of torrefaction was higher. However, the calorific value was lower.(2) With carbonization temperature increased, the calorific value of biomass charcoal increased.The content of fixed carbon and ash increased with the increasing of carbonization temperature, and the volatile content decreased. The carbon content increased with the increasing of temperature, hydrogen and oxygen element content decreased. The calorific values of three kinds of biomass charcoal at carbonization temperature300℃were respectively21.5MJ/kg,21.9MJ/kg and22.4MJ/kg. When the carbonization temperature was800℃, the calorific values were between24.1MJ/kg and29.8MJ/kg. According to the proximate analysis and ultimate analysis of biomass charcoal fitted linear formula of heating value of biomass charcoal by the method of least squares, and the error of biomass charcoal prediction heating value was analyzed, the mean absolute error of predicted value was0.36MJ/kg, the average absolute error was1.5769%, the average bias error was0.0289%. The formula could be used to predict calorific value of biomass charcoal.The alkali metal content of biomass and biomass charcoal was measured. The results showed that with the increasing of carbonization temperature the content of K and Na of corn stalk and charcoal continued to increase until the carbonization temperature reached600℃, then the content began to decrease. In the carbonization process, the additives of (NH4)H2PO4and CaCO3had little effect on the content of K and Na, the additives of CaO could promote the evolution of alkali metal. In this paper, biomass and charcoal ash fusion points were measured by ash fusion temperature pyrometer. This showed that with the increasing of carbonization temperature the biomass and charcoal ash fusion points continued to decrease until the carbonization temperature reached600℃, then the ash fusion points began to increase. When the carbonization temperature was600℃, ash fusion points of cotton stalk was the lowest, the temperature was1100℃.(3) The thermal gravimetric analysis method was applied to research the combustion characteristics of biomass charcoal in air and O2/CO2atmosphere. Similar to biomass combustion process, the combustion process of biomass charcoal could be divided to dehydration, weight loss and burnout three stages. The combustion modes of biomass charcoal were different, the combustion mode of charcoal which was obtained at low carbonization temperature was pyrolysis and volatile firstly, then volatile matter and fixed carbon combustion. The combustion mode of charcoal which was obtained at higher carbonization temperature was volatile matter and fixed carbon combustion at the same time. With the increase of carbonization temperature, the comprehensive index of the combustion characteristics of biomass charcoal decreased. This showed that the combustion characteristics of biomass charcoal were worse. In this paper, the Coats-Redfern method was utilized to compute the experimental data, solving the kinetic parameters of its combustion. As the carbonization temperature increased, the activation energy and frequency factor were gradually raised.(4) Then the tube-type furnace combustion system was used to research the emission characteristics of biomass charcoal and the influence of CaO,(NH4)H2PO4and CaCO3to pollutant emission. In the corn stalk charcoal combustion process SO2emission was unimodal, similar to the original corn straw.The emission is mainly organic sulfur release. With the increasing of carbonization temperature the emissions and conversion rate of biomass charcoal continued to increase until the carbonization temperature reached500℃, then the emissions and conversion rate began to decrease. When the carbonization temperature was500℃, emissions and conversion rate was respectively0.98mg/g and18.91%. The emissions and conversion rate of biomass charcoal which was obtained at400℃,700℃and800℃was less and the values were respectively0.25mg/g-0.33mg/g and5.174%-5.854%. The additives of CaO,(NH4)H2PO4and CaCO3were able to reduce the emissions and conversion rate of SO2. The emissions and conversion rate were0.0028mg/g-0.01mg/g,0.05%～1.97%. The release of NO in the corn stalk combustion process focused mainly in the volatile combustion process. While the release of NO in the corn stalk charcoal combustion process was gentle. The additives of CaO and CaCO3had little effect on NO emission, however the additives of (NH4)H2PO4would cause the increase of NO emission of the corn stalk and corn stalk charcoal.The results enrich basic theories of biomass carbonization process, fuel characteristics of biomass charcoal, combustion and pollutant emissions characteristics of biomass charcoal, which has reference value and practical significance to the development of biomass carbonization process and biomass charcoal combustion technology. |
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