Synthesis Gas. James G. Speight

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СКАЧАТЬ unstable area. Incidents in the region led to the three energy crisis in 1973, 1979, and the two Gulf Wars. While a smaller share of imported oil from the Gulf producers means lesser vulnerability, it may increase vulnerability by having to rely on other sources, such as Venezuela. Whichever measure is used to assess energy dependence, the United States remains susceptible to an energy crisis because of the high dependence on imported oil. The United States has not made a significant reduction in dependence on imports since the mid-1980s and continues to import almost 70% of the total crude oil consumption.

      The crude oil share in the total energy supply also reflects the dependence on crude oil by the United States. In recent years, however, this share has increased in the United States. Because of this, the United States is highly vulnerable to oil supply disruptions. Indeed, the possibility of energy crisis in the foreseeable future is greater than in previous years. Furthermore, the use of the Strategic Petroleum Reserve or government-controlled stocks to lessen the impact of an energy crisis is subject to debate. In fact, the premature release of oil stocks from the Strategic Petroleum Reserve may exacerbate an energy crisis as it depletes the stocks while shortages still exist since it can lead to stabilized (or even lower) prices and increased consumption (Alhajji and Williams, 2003).

      Dependency and vulnerability to oil imports in the United States and, for that matter, in other oil-importing countries, can be reduced by diversification of suppliers and by energy diversification. In addition, diversification of suppliers has the potential to lower the relative impact of supply disruption on most countries. The political instability that swings back and forth in countries such as Venezuela, Nigeria, and Iraq emphasizes the need for diversification of suppliers and so removing the reliance on a small number of oil-producing countries.

      The projections for the continued use of fossil fuels indicate that there will be at least another five decades of fossil fuel use (especially natural gas, crude oil, and coal) before biomass and other forms of alternate energy take hold. Furthermore, estimations that the era of fossil fuels (natural gas, crude oil, and coal) will be almost over when the cumulative production of the fossil resources reaches 85% of their initial total reserves may or may not have some merit. In fact, the relative scarcity (compared to a few decades ago) of crude oil was real but it seems that the remaining reserves make it likely that there will be an adequate supply of energy for several decades. The environmental issues are very real and require serious and continuous attention.

Synthesis gas (synthesis gas) fuel gas mixture consisting predominantly of carbon monoxide and hydrogen and is typically a product of a gasification. The gasification process is applicable to many carbonaceous feedstocks including natural gas, crude oil resids, coal, biomass, by reaction of the feedstock with steam (steam reforming), carbon dioxide (dry reforming) or oxygen (partial oxidation). Synthesis gas is a crucial intermediate resource for production of hydrogen, ammonia, methanol, a variety of chemicals, as well as synthetic hydrocarbon fuels.

      Thus, as the reserves of natural gas, crude oil, and other forms of conventional energy are depleted, there will be the need to seek other sources, some of which are outlined in the previous sections.

      Energy production such as electricity production or combined electricity and heat production remain the most likely area for the application of gasification or co-gasification. The lowest investment cost per unit of electricity generated is the use of the gas in an existing large power station. This has been achieved in several large utility boilers, often with the gas fired alongside the main fuel. This option allows a comparatively small thermal output of gas to be used with the same efficiency as the main fuel in the boiler as a large, efficient steam turbine can be used. It is anticipated that addition of gas from a biomass or wood gasifier into the natural gas feed to a gas turbine will be technically possible but there will be concerns as to the balance of commercial risks to a large power plant and the benefits of using the gas from the gasifier.

      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, 2013, 2014b).

      Co-gasification technology varies, being usually site specific and high feedstock dependent. At the largest scale, the plant may include the well-proven fixed-bed and entrained-flow gasification processes. At smaller scales, emphasis is placed on technologies which appear closest to commercial operation. Pyrolysis and other advanced thermal conversion processes are included where power generation is practical using the on-site feedstock produced. However, the needs to be addressed are (i) core fuel handling and gasification/ pyrolysis technologies, (ii) fuel gas clean-up, and (iii) conversion of fuel gas to electric power (Ricketts et al., 2002).

      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. 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 limited current biomass for energy harvesting has shown unpredictable variations in harvesting capability with long periods of zero production over large areas during wet weather.

      The potential unreliability of biomass, longer-term changes in refuse and the size limitation of a power plant using only waste and/or biomass can be overcome combining biomass, refuse and coal. It also allows benefit from a premium electricity price for electricity from biomass and the gate fee associated with waste. If the power plant is gasification-based, rather than direct combustion, further benefits may be available. These include a premium price for the electricity from waste, the range of technologies available for the gas to electricity part of the process, gas cleaning prior to the main combustion stage instead of after combustion and public image, which is currently generally better for gasification as compared to combustion. These considerations lead to current studies of co-gasification of wastes/biomass with coal (Speight, 2008).

      For large-scale power generation (>50 MWe), the gasification field is dominated by plant based on the pressurized, oxygen-blown, entrained-flow or fixed-bed gasification of fossil fuels. Entrained gasifier operational experience to date has largely been with well-controlled fuel feedstocks СКАЧАТЬ