Synthesis Gas. James G. Speight
* The feedstock material is subjected to the action of steam and high‐pressure carbon dioxide before being discharged through a nozzle.
Typically, the fundamental steps in the pretreatment of biomass involve processes such as (i) washing/separation of inorganic matter such as stones/pebbles, (ii) size reduction which involves grinding, milling, and crushing, and (iii) separation of soluble matter (Table 1.4). Also, the pretreatment process that is selected is, depending upon the character of the biomass and the process, likely to be different for different raw materials and desired products.
One aspect of feedstock preparation in the light of the processes in which the biomass is to be used and converted, is the concept of torrefaction which is used as a pretreatment step for biomass conversion techniques, such as gasification and cofiring. The thermal treatment not only destructs the fibrous structure and tenacity of biomass, but is also known to increase the calorific value of the biomass (Prins et al., 2006a, 2006b, 2006c). Typically, torrefaction commences when the temperature reaches 200ºC (390ºF) and ends when the process is again cooled from the specific temperature to 200ºC (390ºF). During the process the biomass partly devolatilizes, leading to a decrease in mass, but the initial energy content of the torrefied biomass is biomass which makes it more attractive for, for example, transportation (to a conversion site). After torrefaction at, say, temperatures up to 300ºC (570ºF) the grindability of raw biomass shows an improvement in grindability.
A wide range of biomass feedstocks can be used in pyrolysis processes. The pyrolysis process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only produces dust instead of oil. High-moisture waste streams, such as sludge and processing wastes, require drying before subjecting to pyrolysis. Thus, the efficiency and nature of the pyrolysis process is dependent on the particle size of feedstocks. Most of the pyrolysis technologies can only process small particles to a maximum of 2 mm keeping in view the need for rapid heat transfer through the particle. The demand for small particle size means that the feedstock has to be size-reduced before being used for pyrolysis.
Moisture in the biomass is another consideration for feedstock preparation because moisture in the feedstock will simply vaporize during the process and then recondense with the bio-oil product which has an adverse impact on the resulting quality of the bio-oil. It should also be noted that water is formed as part of the thermochemical reactions occurring during pyrolysis. Thus, if dry biomass is subjected to the thermal requirements for fast pyrolysis the resulting bio-oil will still contain water (as much as 12 to 15% w/w). This water is process-originated water that is the result of the dehydration of carbohydrate derivatives in the feedstocks as well as the result of reactions occurring between the hydrogen and oxygen at the high temperature (500°C, 930°F) of the process environment.
The moisture in the feedstock acts as heat sink and competes directly with the heat available for pyrolysis. Ideally it would be desirable to have little or no moisture in the feedstock but practical considerations make this unrealistic. Moisture levels on the order of 5 to 10% w/w are generally considered acceptable for the pyrolysis process technologies currently in use. As with the particle size, the moisture levels in the feedstock biomass are a trade-off between the cost of drying and the heating value penalty paid by having moisture in the feedstock. If the moisture content in biomass feedstock is too high, the bio-oil may be produced with high moisture content which eventually reduces its calorific value. Therefore biomass should undergo a pretreatment (drying) process to reduce the water content before pyrolysis is carried out (Dobele et al., 2007). In contrast, high temperature during the drying process could be a critical issue for the possibility of producing thermal-oxidative reactions, causing a cross-linked condensed system of the components and higher thermal stability of the biomass complex.
To achieve high yields of the products (gases, liquids, and solids), it is also necessary to prepare the solid biomass feedstock in such a manner that it can facilitate the required heat transfer rates in the pyrolysis process. There are three primary heat transfer mechanisms available to engineers in designing reaction vessels: (i) convection, (ii) conduction, and (iii) radiation. To adequately exploit one or more of these heat transfer mechanisms as applied to biomass pyrolysis, it is necessary to have a relatively small particle for introduction to the reaction vessel. This ensures a high surface area per unit volume of particle and, as a result of the small particle size the whole particle achieves the desired temperature in a very short residence time. Another reason for the conversion of the feedstock to small particles is the physical transition of biomass as it undergoes pyrolysis when char develops at the surface of the particle. The char can act as an insulator that impedes the transfer of heat into the center of the particle and therefore runs counter to the requirements needed for pyrolysis. The smaller the particle the less of an affect this has on heat transfer (Bridgwater et al., 2001).
1.3.4 Solid Waste
Energy generation utilizing biomass and municipal solid wastes (MSW) are also promising in regions where landfill space is very limited. Technological advances in the fields have made this option efficient and environmentally safe, possibility even supplementing refinery feedstocks as sources of energy through the installation of gasification units (Speight, 2008, 2011a, 2011b, 2011c, 2013b).
Waste may be municipal solid waste (MSW) which had minimal presorting, or refuse-derived fuel (RDF) with significant pretreatment, usually mechanical screening and shredding. Other more specific waste sources (excluding hazardous waste) and possibly including crude oil coke may provide niche opportunities for co-utilization (Bridgwater, 2003; Arena, 2012; Basu, 2013; Speight, 2013, 2014b). The traditional waste-to-energy plant, based on mass-burn combustion on an inclined grate, has a low public acceptability despite the very low emissions achieved over the last decade with modern flue gas clean-up equipment. This has led to difficulty in obtaining planning permissions to construct needed new waste to energy plants. After much debate, various governments have allowed options for advanced waste conversion technologies (gasification, pyrolysis and anaerobic digestion), but will only give credit to the proportion of electricity generated from non-fossil waste.
Use of waste materials as co-gasification feedstocks may attract significant disposal credits (Ricketts et al., 2002). Cleaner biomass materials are renewable fuels and may attract premium prices for the electricity generated. Availability of sufficient fuel locally for an economic plant size is often a major issue, as is the reliability of the fuel supply. Use of more-predictably available coal alongside these fuels overcomes some of these difficulties and risks. Coal could be regarded as the base feedstock which keeps the plant running when the fuels producing the better revenue streams are not available in sufficient quantities.
Wood fuels are fuels derived from natural forests, natural woodlands and forestry plantations, namely fuelwood and charcoal from these sources. These fuels include sawdust and other residues from forestry and wood processing activities. Over 50% of all wood used in the world is fuelwood. Most of the fuelwood is used in developing countries. In developing countries wood makes up about 80% of all wood used.
Size of the wood waste resource depends upon how much wood is harvested for lumber, pulp and paper. Finally, fuelwood can be grown in plantations like a crop. Fast-growing species such as poplar, willow or eucalyptus can be harvested every few years. With short-rotation poplar coppices grown in three 7-year rotations, it is now possible to obtain 10 to 13 tons of dry matter per hectare annually on soil of average or good quality. Waste wood from the forest products industry such as bark, sawdust, board ends, etc., are widely used for energy production. This industry, in many cases, is now a net exporter of electricity generated by the combustion of wastes.
Overall, wood wastes of all types make excellent biomass fuels and can be used in a wide variety of biomass technologies. Combustion of woody fuels to generate steam or electricity is a proven technology and is