2.3.6 Solid Waste
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; John and Singh, 2011; Arena, 2012; 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.
Municipal solid waste is a readily available, low-cost fuel, with a high organic content when processed to suit the particular gasification process being used. Several plants in Japan and Europe already employ gasification technology for treatment of municipal solid waste. Metal and glass must be removed from the municipal solid waste as it is preprocessed into refuse-derived fuel in order to increase the heating value of the feedstock and avoid gasifier operational problems. In communities with recycling programs, costs associated with removing these materials will be minimized, giving waste gasification the greatest opportunity for success. The systems used for the production of refuse-derived fuel usually use a combination of size reduction, screening, magnetic separation and density separation to remove the non-combustible materials (such as metal and glass) from the municipal solid waste.
The principle behind waste gasification and the production of gaseous fuels is that waste contains carbon and it is this carbon that is converted to gaseous products via gasification chemistry. Thus when waste is fed to a gasifier, water, and volatile matter are released and a char residue is left to react further. 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 feedstock alongside these fuels overcomes some of these difficulties and risks. However, the issues associated with gasification of municipal solid waste include, like the gasification of any mixed feedstock, feedstock homogeneity, for many gasifiers, feedstock heterogeneity and process scale up can lead to a number of mechanical problems, shutdowns, sintering and hot spots leading to corrosion and failure of the reactor wall (most of the processes proposed for waste gasification do not include a separation process).
Furthermore, the disposal of municipal and industrial waste has become an important problem because the traditional means of disposal, landfill, are much less environmentally acceptable than previously. Much stricter regulation of these disposal methods will make the economics of waste processing for resource recovery much more favorable. One method of processing waste streams is to convert the energy value of the combustible waste into a fuel. One type of fuel attainable from waste is a low heating value gas, usually 100 to 150 Btu/scf, which can be used to generate process steam or to generate electricity. Co-processing such waste with coal is also an option (Speight, 2008, 2013a, 2014b). However, co-gasification technology varies, being usually site specific and high feedstock dependent (Ricketts et al., 2002).
One of the major challenges to the gasification process of landfill waste is that such waste has high moisture content and is heterogeneous in nature. Particle size and the presence of a number of components in the waste, such as sulphur, chlorides or metal vary considerably. The interconnected properties of heating value and moisture content play an important role. Hence, pre-preparation must be carefully considered in any waste gasification process. There are a number of different approaches to pre-preparation. Most of these involve mechanical shredding and metals removal using magnetic and electric devices.
Analyses of the composition of municipal solid waste indicate that plastics do make up measurable amounts (5 to 10% w/w or more) of solid waste streams. Many of these plastics are worth recovering as energy. In fact, many plastics, particularly the poly-olefin derivatives, have high calorific values and simple chemical constitutions of primarily carbon and hydrogen. As a result, waste plastics are ideal candidates for the gasification process. Because of the myriad of sizes and shapes of plastic products size reduction is necessary to create a feed material of a size less than 2 inches in diameter. Some forms of waste plastics such as thin films may require a simple agglomeration step to produce a particle of higher bulk density to facilitate ease of feeding. A plastic, such as high-density polyethylene, processed through a gasifier is converted to carbon monoxide and hydrogen and these materials in turn may be used to form other chemicals including ethylene from which the polyethylene is produced – closed the loop recycling.
Recovering energy from municipal solid waste in waste-to-energy (WTE) plants reduces the space required for land filling and offsets the use of fossil fuels for electrical production. When compared to combustion for processing of municipal solid waste, gasification decreases air/water emissions. Within this context, gasification uses oxygen and water vapor to produce a combustible synthesis gas from organic compounds in the municipal solid waste, which can be used to generate electricity, produce chemicals, liquid fuels, hydrogen (H2), etc. The synthesis gas produced from municipal solid waste by a gasifier is cleaned up more economically and using simpler systems compared to combustion exhaust gases due to the synthesis gas being more condensed. The conversion of energy in gasification is also much more efficient than the thermal conversion offered by combustion. Challenges to the commercialization of the gasification of municipal solid waste include the processing costs of converting municipal solid waste to refuse-derived fuel (RDF) and the formation of tars in the high temperature and pressure environment of the gasifier. Tars can make downstream processing of the synthesis gas more difficult and may result in excessive process train downtime.
The heat content of the refuse-derived fuel depends on the amount of moisture and combustible organic material. Refuse-derived fuel typically has less variability than municipal solid waste which can vary greatly when looking at a small sample. This is important for gasification due to the need to optimize gasifier conditions for specific fuel compositions. To reduce the residence time in the gasifier, the refuse-derived fuel is shredded to a smaller size – the shredding process also serves the purpose of uniformly distributing the various materials, giving the refuse derived a more stable composition, in addition to decreasing the moisture content of the refuse-derived fuel.
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.
Co-utilization of waste and biomass with coal may provide economies of scale that help achieve the above identified policy objectives at an affordable cost. In some countries, governments propose co-gasification processes as being well suited for community-sized developments suggesting that waste should be dealt with in smaller plants serving towns and cities, rather than moved to large, central plants (satisfying the so-called proximity principle).
In fact, neither biomass nor wastes are currently produced, or naturally gathered at sites in sufficient quantities to fuel a modern large and efficient power plant. Disruption, transport issues, fuel use, and public opinion all act against gathering hundreds of megawatts (MWe) at a single location. Biomass or waste-fired power plants are therefore inherently limited in size and hence in efficiency (labor costs per unit electricity produced) and in other economies of scale. The production rates of municipal refuse follow reasonably predictable patterns over time periods of a few years. Recent experience with the very