Technology Type

Reactions

Gasification is a partial oxidation process in which carbonaceous solids react with oxygen, enriched air or air to produce a CO/H₂ mixture (syngas) according to the overall reaction:

C_nH_m + n_/2 O₂ → nCO + m_/2 H₂

The overall reaction can be summarized in several basic chemical reactions such as:

• Gasification with O₂:

C + ½ O₂ → CO     ΔH = – 123.1 kJ/mol (1)

• Combustion with O₂:

C + O₂ → CO₂     ΔH = – 123.1 kJ/mol (2)

• Gasification with CO₂:

C + CO₂ → 2 CO     ΔH = 159.7 kJ/mol (3)

• Gasification with Steam:

C + H₂O → CO + H₂     ΔH = 118.9 kJ/mol (4)

In overall, Gasification is a Controlled Combustion in O₂ depleted Atmosphere. In this case, most of the O₂ fed to the Gasifier is consumed in Reactions (1) and (2). These Reactions generate heat to increase temperatures at which Chemical Bonds are broken and Gasification Reactions (3) and (4) become favorable. If the Gas is considered for a subsequent Synthesis, the Water-Gas Shift Reaction (5), i.e.:

CO + H₂O → H₂ + CO₂     ΔH = -40.9 kJ/mol (5)

also becomes important for adjusting the H₂/CO ratio.

Otherwise, the primary objective is to maximize content of Combustibles such as CO and H₂. CH₄ can also be formed at low Gasification temperatures. Sulphur in the feed is converted mainly to H₂S and small amount of COS. Traces of S₂ and CS₂ can also be formed. Most of the Nitrogen in the Feed is converted to N₂. However, small amounts of HCN and NH₃ are also formed. HCl is the main Cl-containing Product formed during Gasification.^[1]

Feedstock

There are a large number of different Feedstock types for use in a Gasifier, each with different characteristics, including size, shape, bulk density, moisture content, energy content, chemical composition, ash fusion characteristics, and homogeneity of all these properties. Coal and Petroleum Coke are used as primary Feedstocks for many large Gasification Plants worldwide. Additionally, a variety of Biomass and Waste-derived Feedstocks can be gasified, with Wood Pellets and Chips, Waste Wood, Plastics and Aluminium, Municipal Solid Waste (MSW), Refuse-derived Fuel (RDF), Agricultural and Industrial Wastes, Sewage Sludge, Switch Grass, discarded Seed Corn, Corn Stover and other Crop Residues all being used.^[2]

Processes

Several Types of Gasifiers are currently available for commercial use: Counter-Current Fixed Bed, Co-Current Fixed Bed, Fluidized Bed, Entrained Flow, Plasma, and Free Radical.^[2]

A good example of Coal Gasification used to produce Chemicals from Coal is the Eastman Chemical Plant in Kingsport, Tennessee. At the Kingsport plant, Coal or a Mixture of Coal and Petcoke is gasified and Particulate Material is removed from the resulting Syngas Stream. Next, the Syngas is purified through the Removal of Mercury (Hg), Sulfuric Acids (H₂S), and Carbon Dioxide (CO₂). The Sulfuric Acids are treated to form Raw Sulfur (S). Next, the cleaned Syngas is separated into Carbon Monoxide (CO) and Hydrogen (H₂), which by themselves, are Chemical Products. Further Treatment results in Methanol (CH₃OH), which is the Source Material for many important Chemicals. One of those Chemicals is Methyl Acetate, which is used in the production of Acetic Acid and Acetic Anhydride. The main Function of the Kingsport Plant is the Production of Acetic Anhydride, which is used to make Photographic Film, Synthetic Textiles, and other Products.^[3]

Mass Balance

Examples of Mass Balances from Gasification Processes is provided for Coal and Coke Gasification respectively.^[1]

1. Furimsky, Edward. (1999). Gasification in Petroleum Refinery of 21st Century. Oil & Gas Science and Technology-revue De L Institut Francais Du Petrole - OIL GAS SCI TECHNOL. 54. 597-618. 10.2516/ogst:1999051.

2. Gasification, Wikipedia

3. Chemicals from Coal Gasification, University of Kentucky