Experimental Research And Numerical Simulation Of Biomass Pyrolysis And Tar Thermal Cracking

Among biomass energy utilization technologies, biomass pyrolysis and gasification technology is a process which is to convert solid biomass into gaseous fuel through thermal chemical reaction whose gas products of biomass pyrolysis can be used for power generation, gas supply and central heating. But there are some problems for traditional gasification technology, such as high tar content in biomass fuel gas and waste water difficult to treat. In particular, high tar content in fuel gas is the main problem for biomass pyrolysis and gasification technology. At present, one of the most effective methods to thoroughly solve the problem is high-temperature thermal cracking which is simple and feasible and has the engineering application promise. This dissertation aiming at the high tar content in biomass utilization, proposed a technique route of biomass pyrolysis and tar thermal cracking, and experimentally and numerically investigated biomass pyrolysis characteristics and analyzed thermal cracking characteristics of bio-oil and tar. The main results are as follows:(1) Pyrolysis process of typical biomass, such as rice husk, corn straw and birch is investigated by thermogravimertric analytical method. The general rule of biomass pyrolysis is analyzed. Comparing slow heating rate with fast heating rate for biomass pyrolysis process, thermogravimetry may be used to quantitatively predict biomass pyrolysis process of small particles whose diameters are less than 0.2mm, and qualitatively describe biomass pyrolysis process of larger particles whose diameters are less than 1mm.(2) The experimental system of biomass pyrolysis and tar high-temperature thermal cracking was developed, and biomass pyrolysis and tar high temperature cracking were separated by changing heat output of combustor and cross-section area of regenerative ceramic cracker to adjust temperature environment and residence time for tar cracking. Results show that experiment system not only can supply the required high temperature field for tar cracking, but also accurately measure the residence time of tar in the temperature field.(3) The effect of cracking temperature on species of tar compounds, pyrolysis products, pyrolysis products composition and substance in tar, and the effect of residence time on ratio of tar cracking were studied. Results show that the liquid product yield experiences from an increasing process to a decreasing one. Liquid product yield reaches maximum when pyrolysis temperature is 500℃. Liquid product yield gradually decreases as temperature increasing; tar content of biomass fuel gas reaches 11.7mg/Nm~3 when temperature reaches 1200℃; tar composition is the main aromatic compounds with 3-rings and 4-rings, and tar species reduce from 129 to 18 when pyrolysis temperature reaches 1200℃. Tar content in biomass gas is obviously reduced, but when pyrolysis temperature is 1200℃, tar content of biomass gas sharply declines as residence time is 0.5s. Further increasing residence time, tar content will not change significantly.(4) A chemical reaction kinetics model of biomass pyrolysis with liquid product high-temperature thermal cracking was established. Based on characteristic of liquid products, the condensable volatile is divided into two parts: tar and biomass oil. The effects of temperature, residence time, particle size, velocity, pressure and other parameters on biomass pyrolysis and tar high-temperature cracking were numerically investigated. Results show that the effect of endothermic of pyrolysis reaction on temperature field and reaction process is great; the pyrolysis reaction zone appears a constant temperature period in several layers near the center of large biomass particles; before or after the period, pure physical process of material heated is observed according to the index curve of temperature; the simulation results agree with the experimental well.