Synthesis Gas. James G. Speight

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СКАЧАТЬ style="font-size:15px;">      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 the most common biomass-to-energy process. Different types of woody fuels can typically be mixed together as a common fuel, although differing moisture content and chemical makeup can affect the overall conversion rate or efficiency of a biomass project.

There are at least six subgroups of woody fuels: (i) forestry residues which include in-forest woody debris and slash from logging and forest management activities, (ii) mill residues which include byproducts such as sawdust, hog fuel, and wood chips from lumber mills, plywood manufacturing, and other wood processing facilities, (iii) agricultural residues which includes byproducts of agricultural activities including crop wastes, waste from vineyard and orchard pruning, and rejected agricultural products, (iv) urban wood and yard wastes which includes residential organics collected by municipal programs or recycling centers and construction wood wastes, (v) dedicated biomass crops which includes trees, corn, oilseed rape, and other crops grown as dedicated feedstocks for a biomass project, and (vi) chemical recovery fuels – sometimes known as black liquor) – which includes woody residues recovered out of the chemicals used to separate fiber for the pulp and paper industry. Mill residues are a much more economically attractive fuel than forestry residues, since the in-forest collection and chipping are already included as part of the commercial mill operations. Biomass facilities collocated with and integral to the mill operation have the advantage of eliminating transportation altogether and thus truly achieve a no-cost fuel.

      Softwood residues are generally in high demand as feedstocks for paper production, but hardwood timber residues have less demand and fewer competing uses. In the past, as much as 50% of the tree was left on site at the time of harvest. Whole tree harvest systems for pulp chips recover a much larger fraction of the wood. Wood harvests for timber production often generate residues which may be left on the site or recovered for pulp production. Economics of wood recovery depend greatly on accessibility and local demand. Underutilized wood species include Southern red oak, poplar, and various small-diameter hardwood species. Unharvested dead and diseased trees can comprise a major resource in some regions. When such timber has accumulated in abundance, it comprises a fire hazard and must be removed. Such low-grade wood generally has little value and is often removed by prescribed burns in order to reduce the risk of wildfires.

      Agricultural residues are basically biomass materials that are byproducts of agriculture. This includes materials such as cotton stalks, wheat and rice straw, coconut shells, maize and jowar cobs, jute sticks, and rice husks. Many developing countries have a wide variety of agricultural residues in ample quantities. Large quantities of agricultural plant residues are produced annually worldwide and are vastly underutilized. The most common agricultural residue is the rice husk, which makes up approximately 25% w/w of the rice.

      Corn stalks and wheat straws are the two agricultural residues produced in the largest quantities. However, many other residues such as potato and beet waste may be prevalent in some regions. In addition to quantity it is necessary to consider density and water content (which may restrict the feasibility of transportation) and seasonality which may restrict the ability of the conversion plant to operate on a year-round basis. Facilities designed to use seasonal crops will need adequate storage space and should also be flexible enough to accommodate alternative feedstocks such as wood residues or other wastes in order to operate year-around. Some agricultural residues need to be left in the field in order to increase tilth (the state of aggregation of soil and its condition for supporting plant growth and to reduce erosion) but some residues such as corncobs can be removed and converted without much difficultly.

Agricultural residues can provide a substantial amount of biomass fuel. Similar to the way mill residues provide a significant portion of the overall biomass consumption in areas that are copiously forested, agricultural residues from sugar cane harvesting and processing provide a significant portion of the total biomass consumption in other parts of the world. One significant issue with agricultural residues is the seasonal variation of the supply. Large residue volumes follow СКАЧАТЬ