Gasification

HISTORICAL DEVELOPMENT OF GASIFICATION

The development of human history is closely related to fire and therefore also to fuels. This relationship between humankind, fire, and earth was already documented in the myth of Prometheus, who stole fire from the gods to give it to man. Prometheus was condemned for his revelation of divine secrets and bound to earth as a punishment. When we add to fire and earth the air that we need to make fire and the water to keep it under control, we have the four Greek elements that play such an important role in the technology of fuels and for that matter in gasification.

The first fuel used by humans was wood, and this fuel is still used today by mil­lions of people to cook their meals and to heat their homes. But wood was and is also used for building and, in the form of charcoal, for industrial processes such as ore reduction. In densely populated areas of the world this led to a shortage of wood with sometimes dramatic results. It was such a shortage of wood that caused iron production in England to drop from 180,000 to 18,000 tons per year in the period of 1620 to 1720. The solution—which in hindsight is obvious—was coal.

Although the production of coal had already been known for a long time, it was only in the second half of the eighteenth century that coal production really took hold, not surprisingly starting in the home of the industrial revolution, England. The coke oven was developed initially for the metallurgical industry to provide coke as a substitute for charcoal. Only towards the end of the eighteenth century was gas pro­duced from coal by pyrolysis on a somewhat larger scale. With the foundation in 1812 of the London Gas, Light, and Coke Company, gas production finally became a commercial process. Ever since, it has played a major role in industrial development.

The most important gaseous fuel used in the first century of industrial development was town gas. This was produced by two processes: pyrolysis, in which discontinuously operating ovens produce coke and a gas with a relatively high heating value (20,000-23,000 kJ/m3), and the water gas process, in which coke is converted into a mixture of hydrogen and carbon monoxide by another discontinuous method (approx. 12,000 kJ/m3 or medium Btu gas).

The first application of industrial gas was illumination. This was followed by heating, then as a raw material for the chemical industry, and more recently for power generation. Initially, the town gas produced by gasification was expensive, so most people used it only for lighting and cooking. In these applications it had the clearest advantages over the alternatives: candles and coal. But around 1900 electric bulbs replaced gas as a source of light. Only later, with increasing prosperity in the twentieth century, did gas gain a significant place in the market for space heating. The use of coal, and town gas generated from coal, for space heating only came to an end—often after a short intermezzo where heating oil was used—with the advent of cheap natural gas. But one should note that town gas had paved the way to the success of the latter in domestic use, since people were already used to gas in their homes. Otherwise there might have been considerable concern about safety, such as the danger of explosions.

A drawback of town gas was that the heating value was relatively low, and it could not, therefore, be transported over large distances economically. In relation to this problem it is observed that the development of the steam engine and many industrial processes such as gasification would not have been possible without the parallel development of metal tubes and steam drums. This stresses the importance of suitable equipment for the development of both physical and chemical processes. Problems with producing gas-tight equipment were the main reason why the production processes, coke ovens, and water gas reactors as well as the transport and storage were carried out at low pressures of less than 2 bar. This resulted in relatively voluminous equipment, to which the gasholders that were required to cope with variations in demand still bear witness in many of the cities of the industrialized world.

Until the end of the 1920s the only gases that could be produced in a continuous process were blast furnace gas and producer gas. Producer gas was obtained by partial oxidation of coke with humidified air. However, both gases have a low heat­ing value (3500-6000 kJ/m3, or low Btu gas) and could therefore only be used in the immediate vicinity of their production.

The success of the production of gases by partial oxidation cannot only be attrib­uted to the fact that gas is easier to handle than a solid fuel. There is also a more basic chemical reason that can best be illustrated by the following reactions:

These reactions show that by “investing” 28% of the heating value of pure carbon in the conversion of the solid carbon into the gas CO, 72% of the heating value of the carbon is conserved in the gas. In practice, the fuel will contain not only carbon but also some hydrogen, and the percentage of the heat in the original fuel, which becomes available in the gas, is, in modern processes, generally between 75 and 88%. Were this value only 50% or lower, gasification would probably never have become such a commercially successful process.

Although gasification started as a source for lighting and heating, from 1900 onwards the water gas process, which produced a gas consisting of about equal amounts of hydrogen and carbon monoxide, also started to become important for the chemical industry. The endothermic water gas reaction can be written as:

C + H2O^CO+H2 +131 MJ/kmol (1-4)

By converting part or all of the carbon monoxide into hydrogen following the CO shift reaction,

C0 + H20^H2+C02 -41 MJ/kmol (1-5)

it became possible to convert the water gas into hydrogen or synthesis gas (a mixture of H2 and CO) for ammonia and methanol synthesis, respectively. Other applications of synthesis gas are for Fischer-Tropsch synthesis of hydrocarbons and for the synthesis of acetic acid anhydride.

It was only after Carl von Linde commercialized the cryogenic separation of air during the 1920s that fully continuous gasification processes using an oxygen blast became available for the production of synthesis gas and hydrogen. This was the time of the development of some of the important processes that were the forerunners of many of today’s units: the Winkler fluid-bed process (1926), the Lurgi moving-bed pressurized gasification process (1931), and the Koppers-Totzek entrained-flow process (1940s).

With the establishment of these processes little further technological progress in the gasification of solid fuels took place over the following forty years. Nonetheless, capacity with these new technologies expanded steadily, playing their role partly in Germany’s wartime synthetic fuels program and on a wider basis in the worldwide development of the ammonia industry.

This period, however, also saw the foundation of the South African Coal Oil and Gas Corporation, known today as Sasol. This plant uses coal gasification and Fischer-Tropsch synthesis as the basis of its synfuels complex and an extensive petrochemical industry. With the extensions made in the late 1970s, Sasol is the largest gasification center in the world.

With the advent of plentiful quantities of natural gas and naphtha in the 1950s, the importance of coal gasification declined. The need for synthesis gas, however, did not. On the contrary, the demand for ammonia as a nitrogenous fertilizer grew exponentially, a development that could only be satisfied by the wide-scale introduction of steam reforming of natural gas and naphtha. The scale of this development, both in total capacity as well as in plant size, can be judged by the figures in Table 1-1. Similar, if not quite so spectacular, developments took place in hydrogen and methanol production.

Steam reforming is not usually considered to come under the heading of gasifica­tion. The reforming reaction (allowing for the difference in fuel) is similar to the water gas reaction.

CH4 + H20 53H2 + C0 +206 M J/kmol (1-6)

The heat for this endothermic reaction is obtained by the combustion of additional natural gas:

CH4 + 202 = C02 + 2H20 -803 MJ/kmol (1-7)

Unlike gasification processes, these two reactions take place in spaces physically separ­ated by the reformer tube.

An important part of the ammonia story was the development of the secondary reformer in which unconverted methane is processed into synthesis gas by partial oxidation over a reforming catalyst.

СН4 + ^02 = С0 + 2Н2 -36MJ/kmol (1-8)

The use of air as an oxidant brought the necessary nitrogen into the system for the ammonia synthesis. A number of such plants were also built with pure oxygen as oxidant. These technologies have usually gone under the name of autothermal reform­ing or catalytic partial oxidation.

The 1950s was also the time in which both the Texaco and the Shell oil gasification processes were developed. Though far less widely used than steam reforming for ammonia production, these were also able to satisfy a demand where natural gas or naphtha were in short supply.

Then, in the early 1970s, the first oil crisis came and, together with a perceived potential shortage of natural gas, served to revive interest in coal gasification as an important process for the production of liquid and gaseous fuels. Considerable investment was made in the development of new technologies. Much of this effort went into coal hydrogenation both for direct liquefaction and also for so-called hydro-gasification. The latter aimed at hydrogenating coal directly to methane as a substitute natural gas (SNG). Although a number of processes reached the dem­onstration plant stage (Speich 1981), the thermodynamics of the process dictate a high-pressure operation, and this contributed to the lack of commercial success of hydro-gasification processes. In fact, the only SNG plant to be built in these years was based on classical oxygen-blown fixed-bed gasification technology to provide synthesis gas for a subsequent methanation step (Dittus and Johnson 2001).

The general investment climate in fuels technology did lead to further development of the older processes. Lurgi developed a slagging version of its existing technology in a partnership with British Gas (BGL) (Brooks, Stroud, and Tart 1984). Koppers and Shell joined forces to produce a pressurized version of the Koppers-Totzek gasifier (for a time marketed separately as Prenflo and Shell coal gasification process, or SCGP, respectively) (van der Burgt 1978). Rheinbraun developed the high-temperature Winkler (HTW) fluid-bed process (Speich 1981), and Texaco extended its oil gasification process to accept a slurried coal feed (Schlinger 1984).

However, the 1980s then saw a renewed glut of oil that reduced the interest in coal gasification and liquefaction; as a result, most of these developments had to wait a further decade or so before getting past the demonstration plant stage.