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.
Figure 1: Process of MSW gasification
Feasibility of technology and operational necessities
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.
Status of the technology and its future market potential
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.
How the technology could contribute to socio-economic development and environmental protection
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%.
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
Financial requirements and costs
The following items determine the economics of thermal MSW gasification technology (AES, 2004):
- 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.