Study On Low Temperature Pyrolysis And Carbonization Of Biomass And Development Of Neutralizing Carbonization Furnace

Biomass low-temperature pyrolysis (torrefaction) is a thermo-chemical treatment at 200-300℃, low particle heating rate (<50℃/min) and in the absence of oxygen. The remaining solid product through torrefaction is often referred to as torrefied biomass (or biochar), which has characteristics of low moisture content, high heating value and energy density, good grindability and hydrophobicity, and suitable for long-distance transport and long-term storage. The technology shows great development and application prospects in the fields of combustion, pyrolysis and gasification. Current researches of torrefaction have been focused on the low-temperature pyrolysis properties of different biomass, but fewer study on research and development of the low-temperature pyrolysis facilities, which limits the industrial application of the technology. In this paper, pilot-scale facility was developed for biomass torrefaction based on experimental study of biomass low-temperature pyrolysis, analysis of torrefied products and energy balance calculation. The main work and conclusions are listed as follows:Biomass low-temperature pyrolysis was carried out in a fixed-bed facility, and properties of biochars were characterized through proximate analysis, ultimate analysis, and measuring calorific value and grindability. The effects of pyrolysis temperature and residence time on low-temperature pyrolysis characteristics were obtained. The results indicated that operating parameters has a significant effect on the biomass morphology. With increasing pyrolysis temperature and residence time, the apparent volume of biochars is shrunk, and the color is gradually deepened to brown. With increasing pyrolysis temperature and residence time, the fixed carbon content and calorific value of biochars increase significantly, correspondingly volatile matters content, molar ratios of H/C and O/C decrease. The grindability of biochars were improved compared to the raw biomass. For example, the particle size larger than 420 μm of grinding the PB250/30 biochar is only 2.58%, while the raw biomass is 58.57%. With increasing pyrolysis temperature, solid yield decreases first and then gradually stabilized, corresponding gas yield increases gradually, while the liquid yield increase significantly first and then decreases slowly. With the increasing residence time, the solid yield decreases gradually, the liquid yield increases gradually, however, the variation of gas yield is very little. The mass and energy yields of biochars decreases with increasing pyrolysis temperature. The mass and energy yields of poplar branch char are 64.0% and 80.68%, respectively, at pyrolysis temperature of 225 ℃ and residence time of 30 min.Based on the results of Differential Scanning Calorimetry (DSC) and the pyrolysis products of poplar branch, calculations were taken to obtain the heat absorption of biomass pyrolysis process and the heating values of gas and liquid products. Effects of pyrolysis conditions on biomass low-temperature pyrolysis were discussed based on the energy balance calculation. The results indicated that the heating values of the pyrolysis products (gas and liquid) increase significantly with increasing pyrolysis temperature. The mass yield of poplar branch char is higher than 60% at pyrolysis temperature of lower than 225 ℃ and residence time of 30 min, and the heating value based on combustion of gas and liquid products could provide energy requirement for poplar branch torrefaction.A pilot-scale moving-bed scraper carbonization stove was put forward. The basic dimensions of key components, such as the drying chamber, carbonizing chamber and combustion device for low-quality pyrolysis gas, were designed. A pilot-scale low-temperature pyrolysis carbonization stove was established. An economic analysis was carried out based on the results of pilot-scale test. The results indicated that the pilot carbonization stove, which integrates drying, carbonization, combustion of pyrolysis gas and high efficient heat transfer and many other features, could operation efficiently and produce biochars continuously and stably. The mass yield of biochar is up to 63.67% at pyrolysis temperature of 220 ℃. The expected economic benefits of the carbonization stove were considerable, and the full investment could be recovered in less than two years. It provides a foundation for industrial application of biomass low-temperature pyrolysis carbonization technology.