1.2.1.1 Thermochemical Processes

Biomass conversion technologies can be broadly classified into primary conver­sion technologies and secondary conversion technologies. The primary conversion technologies such as combustion, gasification and pyrolysis involve the conversion of biomass either directly into heat, or into a more convenient form which can serve as an energy carrier such as gases like methane and hydrogen, liquid fuels like methanol and ethanol, and solids like char. The secondary technologies convert these products of primary conversion into the desired form which may be an energy product such as transportation fuel or a form of energy such as elec­tricity. The different thermochemical conversion processes are given in Fig. 1.5.

These processes involve high temperature and sometimes high pressure pro­cessing of biomass. The combustion process for generation of heat and/or power involves heating the biomass in the presence of excess oxygen. It is responsible for over 97% of the world’s bioenergy production [1]. The other processes such as torrefaction, pyrolysis and gasification involve heating in the presence of restricted or controlled oxygen to produce liquid fuels, heat, and power.

The thermochemical processing of biomass produces gas, liquid, and solid. The gas produced primarily comprises carbon monoxide, carbon dioxide, methane, hydrogen, and some impurities such as nitrogen. This gas is called synthesis gas which can be used as fuel, or can be upgraded or converted to more valuable and/or useful products such as methanol or methane. The liquid product contains mainly noxious and a highly complex mixture of oxygenated organic chemicals consisting of volatile components and non-volatile tars. The solid contains ash and carbon or char.

The suitability of biomass for thermal/thermochemical conversion processes, and the products obtained as a result of these biomass conversion processes, depend greatly on the composition and properties of the biomass used. Physicochemical characterization of biomass is therefore an important step in biomass conversion. This involves the determination of particle size and bulk density; proximate anal­yses such as determination of moisture content, volatile matter, fixed carbon, ash content; ultimate analysis such as determination of carbon, hydrogen and oxygen content; determination of ash deformation and fusion temperature; calorific value; biomass composition; equilibrium and saturation moisture content; and biomass pyrolysis characteristics. There have been a number of projects undertaken the world over, wherein a systematic characterization of different varieties of biomass and species has been undertaken. The output of these systematic studies has, in many cases, resulted in a database on biomass fuel characteristics. Biobank is a set of three databases giving the chemical composition of biomass fuels, ashes, and condensates from flue gas condensers from actual installations. The data set was originally compiled by Biosenergiesysteme GmbH, Graz, Austria. It is continu­ously expanding, using data inputs from other member countries of IEA Bioenergy Task 32. It currently contains approximately 1,000 biomass samples, 560 ash samples, and 30 condensate samples [2]. Another database—BIOBIB has been developed by the Institute of Chemical Engineering, Fuel and Environmental Technology, Vienna, Austria, which gives similar data for European plants. This database covers different types of biomass such as energy crops, straw, wood, wood waste from wood processing industries, pulp and paper industry, and other cellu — losic waste such as that from the food industry. It currently has 331 different biomass fuels listed [3]. Phyllis is yet another database which is designed and maintained by the Netherlands Energy Research Foundation containing informa­tion about composition of biomass and waste fuels [4]. Over 250 biomass species from different parts of India have been characterized with respect to the above properties under the MNES sponsored Gasifier Action Research Project at the Biomass Conversion Laboratory of the Chemical Engineering Department at the Indian Institute of Technology Delhi [5]. An overview of the different thermal and thermochemical conversion processes is given in the following sections.