Thermal gasification of municipal solid waste (MSW) is a chemical process that generates a gaseous, fuel-rich product. This product can then be combusted in a boiler, producing steam for power generation. Just as with combustion of MSW, thermal MSW gasification does not necessarily compete with recycling programmes, but should be considered complementary in any generically constructed MSW plan.
This procedure differs from waste-to-energy (WTE, see Combustion of MSW for district heat or electricity), which is based on waste combustion. Another MSW gasification option is bio-gasification (without heating of the waste), but this is considered less efficient then thermal MSW gasification and is therefore not analysed in this chapter.
Within MSW gasification, two processes must take place in order to produce a useable fuel gas (Klein, 2002). First, through pyrolysis the volatile components of the fuel are released at temperatures below 600°C. As a side benefit from this process, char is produced which consists mainly of fixed carbon and ash. Second, the carbon remaining after pyrolysis is either reacted with steam or hydrogen or combusted with air or pure oxygen at temperatures between 760 and 1,650° C under high pressure. Gasification with air results in a nitrogen-rich, low-Btu fuel gas. Gasification with pure oxygen results in a higher quality mixture of CO and hydrogen and virtually no nitrogen. Gasification with steam is generally called ‘reforming’ and results in a hydrogen- and CO2-rich ‘synthetic’ gas (syngas). Cleaned from contaminants, the syngas can be combusted in a boiler, producing steam for power generation (Jenkins, 2007). Figure 1 illustrates the process of MSW gasification.
Gasification is a more complex process than waste incineration, but the reactors used for both processes are quite similar. However, in contrast with waste incineration, the gasification technology reduces MSW into simpler molecules and substances like dioxins, and furans are generally destroyed (AES, 2004). The main reactor types are fixed beds and fluidised beds. Fixed-bed reactors typically have a grate to support the feed material and maintain a stationary reaction zone. They are relatively easy to design and operate and are therefore useful for small- and medium-scale power and thermal energy use. However, it is difficult to keep operating temperatures at constant levels and to ensure adequate gas mixing in the reaction zone. As a result, gas yields can be unpredictable and are not optimal for large-scale power purposes (i.e. over 1 MW) (Klein, 2002). Therefore, larger capacity gasifiers are preferable for treatment of MSW because they allow for variable fuel feed, uniform process temperatures, good interaction between gases and solids, and high levels of carbon conversion.
Fluidised beds offer the best design for the (larger-scale) gasification of MSW. In a fluidised bed boiler, inert material and solid fuel are fluidised by means of air distributed below the bed. A stream of gas (typically air or steam) is passed upward through a bed of solid fuel and material (such as coarse sand or limestone). The gas acts as the fluidising medium and also provides the oxidant for combustion and tar cracking. The fluidised bed behaves like a boiling liquid and has some of the physical characteristics of a fluid. Waste is introduced either on top of the bed through a feed chute or into the bed through a so-called auger. Fluidised-beds have the advantage of extremely good mixing and high heat transfer, resulting in very uniform bed conditions and efficient reactions. Fluidised bed technology is more suitable for generators with capacities greater than 10 MW because it can be used with different fuels, requires relatively compact combustion chambers and allows for good operational control.
Although the technology of gasification has been in use for over 200 years, gasification of MSW is still in its early stages of development. Despite the fact that gasification technology is, in potential more energy efficient than and can be financially competitive with other waste management options, such as WTE, implementation of thermal MSW gasification technologies has only recently started to gain momentum. Instead, coal gasification is being applied worldwide to produce ‘town gas’ for heating, cooking and lighting (Jenkins, 2007). Although MSW gasification can be considered a viable technology, i.e. the individual processes described have been proven to work well, combining the steps needed for electricity generation is rather new and not yet mature yet (AES, 2004). Only recently, MSW gasification has been further paid attention to by implementing facilities that produce either steam or electricity. With rising costs of landfills in Europe due – among others – to higher taxes for landfilling, the ‘gasification’ option has become more interesting and several plants are operational in various European countries already. These are mostly fluidized bed type facilities built over the last ten years (Jenkins, 2007).
Waste can either be thermally treated by adding sufficient amounts of air, whereby the waste is simply combusted resulting in completely burned out bottom ash and flue gas, or by maintaining an air deficiency, whereby the waste is pyrolised or gasified. In the latter case, the result is a partly burned gas, which can be sent on to a separate incineration plant, e.g., a gas engine or, preferably, a gas turbine for energy production. Potentially, the quantity of power produced per tonne of waste would be larger than when applying the direct WTE incineration method. However, depending on the specific circumstances, the gas can be polluted by tar and heavy metals. Hence, it has to be washed prior to incineration – whereby the energy applied in the pyrolysis/gasification process is cooled away (Kleis and Dalager, 2004). Then, it might be possible to burn the gas retrieved in a combined cycle gas turbine – a so-called biomass integrated gasification combined cycle (BIGCC) – which further increases efficiency.
So-called ‘plasma gasification’ has the potential to be more efficient in terms of electricity production than conventional gasification. This technology has been in use in steelmaking and is used to melt WTE ash to meet limits on dioxin/furan content. It has been installed on a commercial scale in Japan for treating MSW and auto shredder residue. Interesting opportunities may also arise from integration of conventional gasification technology with pyrolysis. Then, more syngas can be produced as the carbon char left over from the pyrolysis process can be refed into the adjacent gasification chamber. Thus, this would result in higher conversion efficiencies. Another option with the syngas produced using a conversion technology is its further processing to produce methanol or ethanol, e.g., for use in cookstoves in developing countries (see: Cookstoves on ethanol/methanol and biomass gasification). Accordingly, synergies can be reaped between different technology conversion processes where the one technology produces the feedstock for the other.
In terms of barriers, municipalities appear to be hesitant to seriously consider MSW gasification option (Jenkins, 2007). Detrimental in feasibility calculations is the MSW throughput and subsequent economies of scale that can be reaped. Also, institutional changes might be required. A specific case in the USA has shown which instituational problems could inhibit the further uptake of MSW gasification technology. In 2002, legislation was passed in California recognising thermal MSW gasification as a renewable resource and distinct from incineration. However, it was also decided that the definition of MSW gasification needed to be revised and it was not defined how gasification fits into the pre-existing hierarchy of methods for handling MSW. This requirement stalled the further development of MSW gasification in California.
Similar to WTE technology, the waste gasification technology has the benefit that it reduces the volume of waste diverted to landfills (possibly the amount of MSW can be reduced by 95%). Consequently, methane emissions from landfills are reduced. In addition, the electricity produced through the MSW gasification technology could lead to reduction of CO2 emissions as it could replace fossil-fuel based electricity production capacity.
The by-products in the process may have an economical value as well. For example, the inert, glassy slag recovered from high-temperature gasification is similar to that produced from steel mills and coal-fired power plants and can be used for making roofing tiles and as sandblasting grit or asphalt filler (Jenkins, 2007). Finally, when compared to landfilling, advantages of gasification of MSW for electricity and/or heat production is that digesters can be close to urban areas, thereby reducing transportation costs, and much less land would be required for the gasification technology.
Through pre-processing subsystems a more homogeneous feedstock can be produced, which is also referred to as refuse-derived fuel (RDF). This provides the opportunity to recover chlorine-containing plastic (for recycling), which could otherwise contribute to the formation of organic compounds or trace contaminants. Moreover, syngas produced by thermal conversion technologies is a much more homogeneous and cleaner burning fuel than MSW, since the conversion system is closed.
Negative aspects of MSW gasification could be the following. The gas resulting from waste gasification contains various tars, particulates, halogens, heavy metals and alkaline compounds depending on the fuel composition and the particular gasification process. This can result in agglomeration in the gasification vessel, which can lead to clogging of fluidised beds and increased tar formation. In general, no slagging occurs with fuels having an ash content below 5%. MSW has a relatively high ash content of 10-12%.
As per July 2010, four biomass gasification projects have been registered as CDM projects by the CDM executive board. AS mentioned, MSW gasification reduces the amount of waste directed to landfills. This in turn prevents methane emissions from the decaying MSW. In addition, the electricity generated by the gasification of the MSW substitutes the other fuels used to generate electricity, which are often fossil fuels.
For the methane reduction effect, the GHG accounting methodology "Avoidance of methane production from decay of biomass through controlled combustion, gasification or mechanical/thermal treatment - version 16" (AMS-III.E.) approved by the CDM executive board can be used. For larger installations "Avoided emissions from organic waste through alternative waste treatment processes- version 11" (AM0025) can be used.
For the substitution of fossil fuels on the grid through the production of electricity by MSW combustion the GHG accounting methodology "Grid connected renewable electricity generation - version 16" (AMS-1.D) can be used or the "Consolidated methodology grid-connected electricity generation from renewable sources - version 11" (ACM0002) can be used. Which methodology is appropriate depends on the size of the installation. Both are approved by the CDM executive board.These methodologies help to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored.
General information about how to apply CDM methodologies for GHG accounting, as well as how to calculate GHG emission reductions from transportation or industrial use projects, can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html
The economic feasibility of MSW gasification would, as with many ‘new’, environmentally benign technologies, increase if the technology’s contribution to environmental protection would somehow be valued. For instance, if the negative effects of landfilling of MSW were properly included in cost calculations, the relative competitiveness of thermal gasification of MSW would increase. Figure 2 presents an overview of costs of different technologies. It also shows the difference between WTE and pyrolysis technology from a tender procedure in Los Angeles (USA) based on a 272,100-362,800 tonnes/year throughput.
Cost for alternative base load power;
Cost of disposing of MSW in landfills;
Imposition or non-imposition of fees associated with disposal of MSW into landfills.
Klein (2002) argues that gasification systems are less expensive per kW than combustion plant configurations because of their higher chemical to electrical efficiencies. Moreover, it is argued that overall capital costs (per daily tonne) are lower for gasification plants, which is due the lower amount of product gas from the process, which minimises the need for (expensive) gas cleaning equipment. RDF processing constitutes the largest chunk of investment, accounting for nearly 40% of total capital costs required. Klein (2002) estimates the costs for an Integrated Biomass Gasification system to vary between USD 1,200 and USD 2,000 per kW installed capacity. However, operating costs per tonne of waste processed are higher for gasification plants. Gasification is generally considered a more complex technology which requires more labour and maintenance. Overall, per tonne of waste treated, gasification generates more electricity, has a lower up-front capital cost and is more effective at reducing pollutants in the flue gas. However, relatively high operating costs associated with maintaining gasification systems result in a slightly higher overall costs per tonne of waste treated in comparison to WTE facilities (Klein, 2002: 45).
Financing opportunities of this energy technology is mainly confined to the revenues from the sale of electricity and/or heat produced by it. Hence, the technology should be fully self-financed. The financial viability of biomass gasification on a large scale is, however, far from established, which is in line with the development status of the technology (Boyle, 2004). Under particular circumstances, MSW can be classified as a renewable fuel and may benefit from associated benefits with respect hereto. An example is provided by the US Renewable Portfolio Standards programmes that assures a premium price for using renewable energy in some states (Jenkins, 2007). In regions where electricity prices are high (regardless of whether additional financial incentives exist), gasification has a competitive advantage over WTE as the potential for higher generation of electricity per unit of MSW processed.
AES, 2004. Investigation into Municipal Solid Waste Gasification for Power Generation, Advanced Energy Strategies.
Boyle, G., 2004. Renewable Energy Power for a Sustainable Future, Oxford University Press, Oxford, United Kingdom.
Jenkins, S.D., 2007. Conversion technologies: A new alternative for MSW management, Earthscan.
Klein, A., 2002. Gasification: An Alternative Process for Energy Recovery and Disposal of Municipal Solid Wastes, Earth Engineering Center, Colombia University.
Kleis, H. and Dalager, S., 2004. 100 Years of Waste Incineration in Denmark: From Refure Destruction Plants to High-technology Energy Works, Babcock & Wilcox Vølund/Ramboll, Denmark.