As the supplies of fossil fuel feedstocks decrease or as environmental legislation makes the fossil fuel feedstocks completely undesirable/unacceptable (and this can become a reality!), 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 the costs of natural gas will increase, allowing coal gasification 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 coal gasification.
The most likely option for the integration of alternate feedstock into the refinery is the installation of an on-site gasifier. Thus, such a refinery (often referred to as a gasification refinery) w'ould have, as the center piece, gasification technology as is the case of 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.
Synthesis gas (syngas) is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen generated by the gasification of a carbon-containing fuel to a gaseous product with a heating value. Examples include the gasification of coal or crude oil residua (Speight, 2013,2014,2020a). Synthesis gas is used as a source of hydrogen or as an intermediate in producing hydrocarbon derivatives via the Fischer-Tropsch synthesis (Speight, 2019, 2020b).
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 used as a fertilizer. Biomass gasification is therefore one of the most technically and economically convincing energy possibilities for a potentially carbon-neutral economy.
The manufacturing of gas mixtures of carbon monoxide and hydrogen has been an important part of chemical technology for about a century. Originally, such mixtures were obtained by the reaction of steam w'ith 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 a wide application for synthesis gas manufacturing, with the special feature that they could utilize low-value feedstocks such as crude oil residua. In recent years, catalytic partial oxidation employing very short reaction times (milliseconds) at high temperatures (850°C-1,000°C) is providing still another approach to synthesis gas manufacturing (Speight, 2020b).
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. 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 the manufacturing of chemical or fertilizer.
Thus, in terms of the adaptability of the gasification refinery to a variety of carbonaceous feedstocks, the gasification refinery could well be the refinery of the future. The typical gasification system incorporated into the refinery consists of several process plants including (i) feedstock preparation, (ii) the gasifier, (iii) an air separation unit, (iv) the synthesis gas cleanup train, (v) the sulfur recovery unit, and (vi) a series of downstream process options depending on the desired products. In fact, the gasification of carbonaceous feedstock can provide high purity hydrogen for a variety of uses within the refinery (Speight, 2014, 2017). Hydrogen is used in the refinery to remove sulfur, nitrogen, and other impurities from intermediate to finished product streams and in hydrocracking operations for the conversion of high-boiling distillates into low-boiling products, such as naphtha, kerosene, and atmospheric gas oil. Hydrocracking and severe hydrotreating require hydrogen which is at least 99% v/v pure, while less severe hydrotreating can use 90% v/v pure hydrogen and above and a current refinery typically requires continuous hydrogen availability (Speight, 2014, 2017).