Читать книгу Synthesis Gas - James Speight G., James G. Speight - Страница 29

Synthesis gas (also frequently referred to by the abbreviated name syngas) is a fuel gas consisting primarily of carbon monoxide and hydrogen with, on occasion depending upon the feedstock and the production process, a smaller amount of carbon dioxide. Synthesis gas has been, for decades, a product of coal gasification and the main application has been, and continues to be, the generation of electricity.

In addition to coal as the (formerly) primary feedstock, synthesis gas can be produced from many sources, including natural gas, coal, biomass, or virtually any hydrocarbon feedstock, by reaction with steam (the steam reforming process), with carbon dioxide (the dry reforming process), or with oxygen (the partial oxidation process). Synthesis gas has become a crucial intermediate resource for production of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels. It is also used as an intermediate in the production of (i) synthetic crude oil, (ii) hydrocarbon fuels and lubricants by way of the Fischer-Tropsch process, and (iii) the methanol-to-gasoline process. In each case, the production methods include (i) steam refoming of natural gas or liquid hydrocarbon derivatives to produce hydrogen and (ii) the gasification of carbonaceous feedstocks such as coal (Chapter 4), crude oil resid (Chapter 5), biomass (Chapter 6), and waste (Chapter 7).

The gasification process is a process that converts organic (carbonaceous) feedstocks into carbon monoxide, carbon dioxide and hydrogen by reacting the feedstock at high temperatures (>700°C, 1290^oF), without combustion, with a controlled amount of oxygen and/or steam (Lee et al., 2007; Speight, 2008, 2013). The resulting gas mixture (synthesis gas, also commonly referred to as syngas) – which is a mixture of carbon monoxide, CO, and hydrogen H₂ – is itself a fuel as well as a source of a wide variety of chemicals (Table 2.1).

The power derived from carbonaceous feedstocks and gasification followed by the combustion of the product gas(es) is considered to be a source of renewable energy if the derived gaseous products are generated from a source (e.g., biomass) other than a fossil fuel (Speight, 2008). The gasification of a carbonaceous feedstock or a derivative (i.e., char produced from the carbonaceous material) is the conversion of a carbonaceous feedstock, such as coal, by any one of a variety of processes to produce gaseous products that are combustible or can be used for the production of a range of chemicals (Figure 2.1).

The advantage of gasification is that the use of synthesis gas is potentially more efficient as compared to direct combustion of the original fuel because it can be (i) combusted at higher temperatures, (ii) used in fuel cells, (iii) used to produce methanol and hydrogen, (iv) converted via the Fischer-Tropsch process into a range of synthesis liquid fuels suitable for use in gasoline engines or diesel engines. The gasification process can also utilize carbonaceous feedstocks which would otherwise have been disposed of (e.g., biodegradable waste). In addition, the high-temperature process causes corrosive ash elements including metal chlorides and potassium salts which allow clean gas production from otherwise problematic fuels.

Table 2.1 Example of chemicals produced from synthesis gas.

Synthesis gas	Fuel gas
	Town gas
	Hydrogen	Ammonia
		Urea
	Fischer-Tropsch liquids	Synthetic natural gas
		Naphtha
		Kerosene
		Waxes
	Methanol	Dimethyl ether	Ethylene	Polyolefins
			Propylene	Polyolefins
		Acetic acid	Methyl acetate
			Acetate esters
			Polyvinyl acetate
			Acetic anhydride

Figure 2.1 Potential products from synthesis gas produced from gasification of carbonaceous feedstocks.

Coal has been the primary feedstock for gasification units for many decades. However, there is a move to carbonaceous feedstocks other than coal for gasification processes with the concern on the issue of environmental pollutants and the potential shortage for coal in some areas (except in the United States) as well as concerns related to the effect of increased coal use on the environment. Nevertheless, coal use still prevails and will continue to do so for at least several decades into the future, if not well into the next century (Speight, 2013).

In fact, gasification plants are cleaner with respect to standard pulverized coal combustion facilities, producing fewer sulfur and nitrogen byproducts, which contribute to smog and acid rain. For this reason, gasification appeals as a way to utilize relatively inexpensive and expansive coal reserves, while reducing the environmental impact. Indeed, the mounting interest in coal gasification technology reflects a convergence of two changes in the electricity generation marketplace: (i) the maturity of gasification technology, and (ii) the extremely low emissions from integrated gasification combined cycle (IGCC) plants, especially air emissions, and the potential for lower cost control of greenhouse gases than other coal-based systems. Fluctuations in the costs associated with natural gas-based power, which is viewed as a major competitor to coal-based power, can also play a role. Furthermore, gasification permits the utilization of a range of carbonaceous feedstocks (such as crude oil resids, coal, biomass, and carbonaceous domestic and industrial wastes) to their fullest potential. Thus, power developers would be well advised to consider gasification as a means of converting a carbonaceous feedstock to gas.

Liquid fuels, including gasoline, diesel, naphtha and jet fuel, are usually processed via refining of crude oil (Speight, 2014a, 2017). Due to the direct distillation, crude oil is the most suited raw material for liquid fuel production. However, with fluctuating and rising prices of crude oil, coal-to-liquids (CTL) and biomass-to-liquids (BTL) processes are currently starting to be considered as alternative routes used for liquid fuels production. Both feedstocks are converted to synthesis gas which is subsequently converted into a mixture of liquid products by Fischer-Tropsch (FT) processes. The liquid fuel obtained after FT synthesis is eventually upgraded using known crude oil refinery technologies to produce gasoline, naphtha, diesel fuel and jet fuel (Dry, 1976; Chadeesingh, 2011; Speight, 2014a, 2017). Gasification processes can accept a variety of feedstocks but the reactor must be selected on the basis of feedstock properties and behavior in the process. The future depends very much on the effect of gasification processes on the surrounding environment. It is these environmental effects and issues that will direct the success of gasification.

Clean Coal Technologies (CCTs) are a new generation of advanced coal utilization processes that are designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation and use (Speight, 2013). These technologies reduce emissions, reduce waste, and increase the amount of energy gained from coal. The goal of the program was to foster development of the most promising clean coal technologies such as improved methods of cleaning coal, fluidized bed combustion, integrated gasification combined cycle, furnace sorbent injection, and advanced flue-gas desulfurization.

In fact, there is the distinct possibility that within the foreseeable future the gasification process will increase in popularity in crude oil refineries – some refineries may even be known as gasification refineries (Speight, 2011b). A gasification refinery would have, as the center piece, gasification technology as is the case with the Sasol refinery in South Africa (Couvaras, 1997). The refinery would produce synthesis gas (from the carbonaceous feedstock) from which liquid fuels would be manufactured using the Fischer-Tropsch synthesis technology.

In fact, gasification to produce synthesis gas can proceed from any carbonaceous material, including biomass. Inorganic components of the feedstock, such as metals and minerals, are trapped in an inert and environmentally safe form as char, which may have use as a fertilizer. Biomass gasification is therefore one of the most technically and economically convincing energy possibilities for a potentially carbon neutral economy.

The manufacture of gas mixtures of carbon monoxide and hydrogen has been an important part of chemical technology for approximately a century. Originally, such mixtures were obtained by the reaction of steam with incandescent coke and were known as water gas. Eventually, steam reforming processes, in which steam is reacted with natural gas (methane) or crude oil naphtha over a nickel catalyst, found wide application for the production of synthesis gas.

A modified version of steam reforming known as autothermal reforming, which is a combination of partial oxidation near the reactor inlet with conventional steam reforming further along the reactor, improves the overall reactor efficiency and increases the flexibility of the process. Partial oxidation processes using oxygen instead of steam also found wide application for synthesis gas manufacture, with the special feature that they could utilize low-value feedstocks such as heavy crude oil residues. In recent years, catalytic partial oxidation employing very short reaction times (milliseconds) at high temperatures (850 to 1000^oC) is providing still another approach to synthesis gas manufacture (Hickman and Schmidt, 1993).

In a gasifier, the carbonaceous material undergoes several different processes: (i) pyrolysis of carbonaceous fuels, (ii) combustion, and (iii) gasification of the remaining char. The process is very dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.

As crude oil supplies decrease, the desirability of producing gas from other carbonaceous feedstocks will increase, especially in those areas where natural gas is in short supply. It is also anticipated that costs of natural gas will increase, allowing gasification of other carbonaceous feedstocks to compete as an economically viable process. Research in progress on a laboratory and pilot-plant scale should lead to the invention of new process technology by the end of the century, thus accelerating the industrial use of gasification processes.

The conversion of the gaseous products of gasification processes to synthesis gas, a mixture of hydrogen (H₂) and carbon monoxide (CO), in a ratio appropriate to the application, needs additional steps, after purification. The product gases – carbon monoxide, carbon dioxide, hydrogen, methane, and nitrogen – can be used as fuels or as raw materials for chemical or fertilizer manufacture.