Torrefaction of beechwood: A parametric study including heat of reaction and grindability

Torrefaction describes a thermal treatment of biomass in order to obtain a solid product with more or less coal-like properties. Therefore a mild pyrolysis in an oxygen-free atmosphere is applied. In this work beechwood was used as feedstock because of its consistent quality in commercially available chips. A parametric study was conducted with a continuously working, indirectly heated reactor having a throughput on the order of 1 kgproduct/h, wherein each parameter was varied independently. The reactor temperature ranged from 270 to 300 °C, residence time from 20 to 60 min, and feed moisture content from 0% to 20%. Ultimate and proximate analysis of the products as well as grindability tests have been performed. Additionally, the integral heat of reaction was determined for each test run by measurement of the heat consumption of the reactor. Mass loss of the dry solid is shown to be a good indicator of the degree of torrefaction and ultimate as well as proximate analysis show good agreement with existing literature data. Grindability in terms of HGI depends on the degree of torrefaction and reaches from difficult to grind to very easy to grind. In terms of beater wheel mills used for combined drying and milling in lignite fired power plants, grindability is poor and in general worse than that of Rhenish lignite. The heat of reaction is found to be close to the border between endo- and exothermic with a trend to more exothermic behaviour for increasing degree of torrefaction. Despite the uncertainty of the heat capacity of wood and torrefied wood above a temperature of 420 K, the determined heat of reaction is found to be in the range of −199 J/g (exothermic) to 148 J/g (endothermic) for all tests with spans of about ±70 J/g for each single test run. Besides severe torrefaction, caused by a high reactor temperature, an increased residence time is found to cause a remarkable exothermic heat of reaction.