Coal Gasification

Water Gas Reaction

There are several steps in coal gasification. Let's focus on step 1: conversion of the solid coal to a burnable gas.

In the gasifier, crushed coal is combined with steam at high pressure and temperature. The quantity of oxygen is limited to prevent complete combustion of the coal. Instead, the carbon reacts with the steam to form a mixture of CO, H2, and other gases.

There is always some combustion, so some CO2 is formed. At these temperatures, the sulfur compounds from the coal react with some of the hydrogen gas to produce H2S and some COS. The nitrogen-containing material is converted to ammonia and HCN. Mercury compounds are also in the gas phase. The remaining minerals mostly fall to the bottom of the gasifier, where they can be removed.

The gas can be scrubbed of acidic compound (hydrogen sulfide and carbon dioxide) by passing it through a basic material, such as CaCO3. Water, mercury, and ammonia are condensable compounds and can be removed by cooling the gas stream. The remaining material is a relatively pure mixture of CO/H2 called syn gas.

Syn gas, previously called town gas, was used as a fuel for gas lights in the 19th and early 20th centuries.

People's Energy Corporation, based in Chicago, began as Peoples Gas Light & Coke Company in 1855 to supply gas for street lighting and home heating and provide coal and coke for both industrial and residential uses.

There was a town gas plant in our town that supplied gas for street lights and homes in Urbana.

Water Gas Shift Reaction

The mixture of CO and hydrogen is a burnable gas but combustion of the CO produces carbon dioxide. Treating the mixture with water vapor over a catalyst converts the CO to CO2 and produces more hydrogen.

There are many transition metal catalysts for the water gas shift reaction, both heterogeneous and homogeneous. In industry, it is typical to use a transition metal or a metal oxide on a solid support for this reaction. We understand the mechanism of the reaction through studies of homogeneous metal complexes. The catalytic cycle for the WGS reaction catalyzed by Fe(CO)5 is shown below. This reactions involves:
  1. Nucleophilic attack by hydroxide on the complexes CO ligand
  2. Loss of CO2 from the Fe-C(O)OH moiety
  3. Protonation of the anionic Fe-H complex by water
  4. Reductive elimination of H2
  5. Complexation of CO to the Fe center

Purification of the Gas

  1. Precipitators
      The gas can be treated with water. This removes the particulates as a slurry. Dry precipitators are heat exchangers and the particulates are removed as a solid from these.

  2. Hydrolysis
      A fixed bed, alumina hydrolysis reactor converts HCN and COS to NH3, CO2, and H2S.

  3. Adsorbtion
      Those gases with acidic properties (H2S and CO2) can be separated from the rest of the gas stream through absorption chromatography.

  4. Sulfur recovery: Claus process
      In the Claus sulfur recovery unit, the gas containing H2S and CO2 is treated with oxygen at high temperature over an alumina catalyst.

  5. Membranes
      The gas stream contains mostly CO and H2 after the purification treatment but there are trace impurities remaining. To purify the gas for use in a hydrogen fuel cell, these impurities and the CO must be removed.

      Research is underway on membranes that would separate hydrogen from all other gases. Membranes containing platinum metals are most promising.

Example: Polk Power Plant

A power plant in Polk county, Florida has been using the gas from coal as a fuel for electricity. It went online in 1999 and produces 250 megawatts of electricity. More than 94% of the sulfur is removed prior to combustion and the effluent is also lower in nitrogen oxides.

Professor Shapley, University of Illinois, 2011