Coal gasification is a process that converts coal from a solid to a gaseous fuel through partial oxidation. Once the fuel is in the gaseous state, undesirable substances, such as sulfur compounds and coal ash, may be removed from the gas by established techniques. The net result is a clean, transportable gaseous energy source.

In contrast to combustion process which works with excess air, gasification process works on partial combustion of coal with the oxygen supply controlled (generally 20 to 70% of the amount of O₂ theoretically required for complete combustion) such that both heat and a new gaseous fuel are produced as the coal is consumed.

C + ¹/₂ O₂ gasificationà CO

C + H₂O gasificationà CO + H₂

The IGCC process is a two-stage combustion with cleanup between the stages. The first stage employs the gasifier where partial oxidation of the solid/liquid fuel occurs by limiting the oxidant supply. The second stage utilizes the gas turbine combustor to complete the combustion thus optimizing the gas turbine/combined cycle (GT/CC) technology with various gasification systems. The SynGas produced by the Gasifiers however, needs to be cleaned to remove the particulate, as well as wash away sulphur compounds and NOx compounds before it is used in the Gas Turbine. It is the Integration of the entire system components which is extremely important in an IGCC Plant.

Various sub-systems of an IGCC Plant thus are:

i) Gasification Plant

ii) Power Block

iii) Gas Clean-up System

Recent advances in the Gas Turbine technologies have presented great potential towards much higher gas turbine efficiencies. Increasing the firing temperatures and utilizing materials that withstand higher temperatures can increase the efficiency of gas turbine. Continuous developments have been taking place in the newer materials of construction thus consequent higher gas turbine performance. At present the efficiency of gas turbines is in the range of 45-50% which is projected to go upto 60% with the development of H-technology by GE. The advances in gas turbines would improve the overall efficiency of IGCC plant.

EXPECTED IMPROVEMENTS OF IGCC POWER PLANT EFFICIENCY

IGCC is an environmentally benign technology. The emission levels in terms of NOx, SOx and particulate from an IGCC plant have been demonstrated to be much lower when compared to the emission levels from a conventional PC fired steam plant. In fact, no additional equipment is required to meet the environment standards.

The Coal Gasification requires the presence of an oxidant in the process. Air or Oxygen may be used as an oxidant and the gasifiers are accordingly known as either Air-Blown or Oxygen-Blown Gasifiers.

Sulfur from the hot fuel gas is captured by reducing it to H₂S, COS, CS₂ etc. The current sulfur removal systems employ zinc based regenerative sorbents (zinc ferrite, zinc titanate etc.) Such zinc based sorbents have been demonstrated at temperatures upto 650 ⁰C.

Sulfur is also removed by addition of limestone in the gasifier. This is commonly adopted in air-blown fluidised bed gasifiers.

In fact, in the case of Air Blown Gasifiers, sulfur is captured in the gasifier bed itself (above 90%) because of addition of limestone. The sulfur captured in the bed is removed with ash.

The Power Block in the IGCC Plant is essentially a Gas Turbine Unit that operates on SynGas. This Gas Turbine Unit is basically the same as used for Natural Gas with certain modifications. The areas that are modified and also which need to be critically evaluated for use with SynGas are:

• Modification of Fuel Supply System
• Modification in the Burners -- Special burners are required when using SynGas because of its higher flame propagation velocity.
• Checking for Surge Conditions and suitability of Gas Turbine Units because of excess flow in case of SynGas on account of it being a lean gas.

The gas turbine combined cycle technology has been proven for use with natural gas as well as with syngas.

Typical problems that have been encountered in various projects relate to the following areas:

The costs for the IGCC based plants as reported are noted to be somewhat variable, depending on economy of scale, local labor costs, and applicable engineering standards. Further, gasification costs usually are estimated in combination with the downstream processing equipment necessary for delivery of a syngas suitable for conversion to the designed end product. Accordingly, gasification investment costs are best addressed on a project specific basis. The typical project costs as reported for different demonstration/commercial projects are as below: