Experimental Study On Biomass Torrefaction And Co-gasification With Coal

As China's sustained and rapid economic development, energy demand is increasing. But extensive use of fossil fuels caused serious environmental problems, and its reserves are limited. Biomass is CO2 neutral and renewable clean energy, the annual products is very huge. However agricultural biomass have some drawbacks such as high moisture content, low energy density and widely distributed, as a result the cost of transport and store are high. Moreover, the raw biomass is poor grindability so that it is difficult to be used in pulverised boiler or entrained flow gasifier. Torrefaction is a mild pyrolysis process carried out at temperature ranging from 200℃to 300℃to dealing with those problems. Co-gasification of biomass and coal is not only beneficial to large-scale use of biomass, but also to improve the characteristics of coal gasification.Four samples were heated in a fix-bed at moderate temperature (200,250 and 300℃) under a N2 atmosphere for 30 minutes. The biomass chars after torrefied have a higher energy density and an improvement in the grindability characteristics compared with raw biomass and even have hydrophobic characteristics. The volatiles consist of a condensable fraction and a non-condensable fraction. The former mainly contain water and tar (organic products mainly acetic acid ). The non-condensable products are typically comprised of CO2, CO and a small amount of CH4, even trace of H2. The volatiles increased with the increasing of the torrefaction temperature, on the contrary the solid yield and energy yield all decreased. However, the grindability and energy density of the biomass char have a great improvement. A kinetic study on the generation of main non-condensable gases was accomplished.Then the feasibility analysis of torrefaction preprocessing was studied. The heating value of semi-char and energy yields of torrefaction were measured. Although the heating value of semi-char rises with torrefaction temperature increased, but the energy yield decline. Heat required of torretied biomass is very small, can be providing by a small part of biomass combustion. From the solid energy yields, the transport and storage costs, grinding energy consumption, untreated biomass and the solid products under the torrefaction conditions of 250℃, 30min were compared. Found that the pretreatment technology has competitive advantage.Finally using TGA,reactivity of biomass semi-char,coal and their blends in CO2 atmosphere was processed. The results showed that, for four biomass chars,the reactivity in CO2 atmosphere has same trend,the reactivity increases with carbon conversion. Different from reactivity of coal in CO2 atmosphere. Co-gasification reactivity is better than separate fuel gasification. Gasification reactivity related with the types of biomass and torrefaction temperature. Note the synergy makes the mixed fuel gasification occurs at a lower temperature, and the gasification rate of increase makes gasification process shorten. This result may be due to higher alkali metal content in biomass char than that in coal.