Gasification Principles

Once a carbonaceous solid or liquid material is converted to a gaseous state, undesirable substances such as sulfur compounds and ash may be removed from the gas. In contrast to combustion processes, which work with excess air, gasification processes operate at substoichiometric conditions with the oxygen supply controlled (generally 35 percent of the 

Some gasification processes also use indirect heating, avoiding combustion of the feed material in the gasification reactor and avoiding the dilution of the product gas with nitrogen and excess C02. 

---

The three principal reactor types employed in coal gasifier design are the moving packed bed, the entrained flow, and the fluidized bed reactor. In the discussion of gasification principles the moving packed bed (Fig. 3) was used to illustrate steam-oxygen or steam-air gasification of coal. 

Partial oxidation of heavy Hquid hydrocarbons requires somewhat simpler environmental controls. The principal source of particulates is carbon, or soot, formed by the high temperature of the oxidation step. The soot is scmbbed from the raw synthesis gas and either recycled back to the gasifier, or recovered as soHd peUetized fuel. Sulfur and condensate treatment is similar in principle to that required for coal gasification, although the amounts of potential poUutants generated are usually less. 

Arcus gasification combuster is described and the principles upon which it works are explained. This combuster combines solid fuel gasification with the burning of the lean gases produced on a small capacity scale. The types of solid fuels which can be used are listed and these include segregated municipal waste and industrial waste such as rigid PU foam and plastics mixed with other materials. Uses of the gas produced are included. 

In oxygen deficient expls an appreciable fraction of the carbon product is in elemental form even though sufficient oxygen is present for gasification. This is a consequence of Le Chateliefs Principle that with increasing pressure the number of molecules of gaseous products tends to be minimized by a shift to condensed reaction products... 

In another test method (ASTM D-5515), a dilatometer is used to measure the swelling of bituminous coal. The test method is limited in applicability to coal that has a free swelling index < 1 (ASTM D-720). The principle of this test method is that the final volume of char obtained at the conclusion of a standard dilatation test is dependent on the mass of coal in the coal pencil and on the radius of the retort tube. This test method incorporates a procedure that determines the mass of air-dried coal in the coal pencil, provides a means to measure the average retort tube radii and employs a means to report coal expansion on an air-dried coal weight basis. The value of the dilatation properties of coals may be used to predict or explain the behavior of a coal or blends during carbonization or in other processes, such as gasification and combustion. 

The quasi-thermodynamic approach outlined above will obviously remain applicable to future coal conversion technologies as well, because the underlying thermodynamic principles are universal and invariant. Figure 2 is illuminating in this context it shows that by-product waters of liquefaction processes can be differentiated from their gasification counterparts by their redox potential and pH characteristics (13). 

When ethane is cracked to ethylene and hydrogen, there is a volume expansion and thus, by the Le Chatelier s Principle, the pyrolysis is favoured by lowering the pressure. In practice this is achieved by adding large volumes of steam so as to lower the partial pressure of the hydrocarbons. Steam addition also has the advantage of removing coke by steam gasification ... 

---