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Biochar production technology

TitleBiochar production technology
Publication TypeBook Chapter
Year of Publication2015
AuthorsBoateng, Akwasi A., Garcia-Perez Manuel, Masek Ondrej, Brown Robert, and del Campo Bernardo
Book TitleBiochar for Environmental Management: Science and Technology and Implementation

Charcoal and biochar are carbonaceous solid products derived from the thermochemical decomposition of wood or other organic matter in the absence or restricted amounts of oxygen (O2). Charcoal-derived products have been associated with civilization as long as there has been fire. The visible flame associated with fire occurs only after the fuel source (biomass) has been sufficiently heated to release volatiles and gases, a process known as pyrolysis, followed by their rapid oxidation, a process known as combustion. The extent to which pyrolysis products burn depends on the equivalence ratio (ratio between the number of moles of O2 admitted into the reactor) and the stoichiometric ratio (moles of O2­ required for complete combustion of the biomass). In the complete absence of O2­ (equivalence ratio equal to zero) the process is referred to as pyrolysis. Pyrolusis can be an endothermic or exothermic reaction depending on the temperature of the reactants, becoming increasingly exothermic as the reaction temperature decreases (Spokas et al, 2012). For equivalence ratios less than 0.15 the process is known as either pyrolytic  gasification or falminf pyrolytic gasification. For equivalence ratios of 0.15 to 0.3 the process is characterized as gasification, whereby some of the volatile gases and soils are oxidized to carbon monoxide (CO), carbon dioxide (CO2) and water (H2O), with the exothermic energy released by partial oxidation supporting the endothermic pyrolysis reactions at higher temperatures. Pyrolysis is a promising technology to producing stable carbon (C) for sequestration and the production of an energy carrier known as bio-oil or pyrolysis oil, suitable as feedstock for producing second-generation transportation fuels (Brigwater and Peacocke, 2000; Huber, 2008; Granastaein et al, 2009; Mason et al, 2009; Woolf et al, 2010). If the process focuses on maximizing charcoal or biochar, it is known as carbonization. This chapter focuses on some of the techniques important to biochar production. Carbonization of wood for the purpose of manufacturing charcoal has been practiced for centuries (Brown, 1917; Klark and Rule, 1925; Emrich, 1985); however, production of charcoal was not the only ntent. It appears that ancient peoples were equally acquainted with the method of recovering the associated liquid product or tars. Pyrolysis can be adjusted to favour production of tar/liquid or biochar/solid products with one or the other always as a co-product. Present day production of pyrogenic carbonaceous material (PCM) is quite different compared to early societies, when most of the world’s population depended on charcoal as an industrial source of energy, with very polluting carbonization production facilities moving away from industrialized countries to countries with relatively abundant sources of biomass and cheap labour. In 2005, the world production of charcoal was estimated to be more than 44 Mt (Energy Statistics, 2005) and most of it was destined for iron smelting and domestic use (cooking, barbecue). Because current production technologies typically yield on average only 20 per cent w/w of the original biomass as charcoal, it can be estimated that more than 220Mt of dry biomass is processed to produce the world’s supply of charcoal annually. This presents an inefficient use of biomass, as the vast majority of the energy contained in the biomass is wasted. Therefore, enhanced charring and pyrolysis technologies that offer an optimal compromise between high yield and good quality biochar with the capability of producing heat from the combustion of pyrolysis vapours or recovering gaseous and liquid co-products are required for biochar production to yield the most environmental and economic benefits. As of today, Brazil is by far the largest charcoal producer in the world with some production estimated at 9.9Mt yr-1. Other important charcoal producing countries are: Thailand (3.9Mt yr-1), Ethiopia (3.2Mt yr-1), Tanzania (2.5Mt yr-1), India (1.7Mt yr-1), and the Democratic Republic of Congo (1.7Mt yr-1). With production of 0.9Mt yr-1, the United States is the tenth largest charcoal producer in the world (Energy Statistics, 2005). Today, southeastern Missouri produces approximately three-quarters of all the charcoal used for barbecue in the United Stated. For this supply sawmill wastes are the main feedstock of choice (Yronwode, 2000).