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Integrated technologies deliver improvements to power production and plant efficiency

Operators of fossil-fuelled power plants are concerned that introducing carbon dioxide capture schemes could impact on plant efficiency and its economics. However, new technologies and optimised heat and power integration encourage reconsideration of the case for carbon dioxide capture.

Foster Wheeler and Shell Global Solutions have collaborated in a study to demonstrate that the efficiency of integrated gasification combined-cycle (IGCC) technology can be substantially improved from “typical” configurations by using the latest technology improvements and to provide an update on the achievable IGCC efficiency, taking into account the available commercialised technology improvements.

Suzanne Ferguson, Carbon Capture Technical Lead, Foster Wheeler, explains, “We have found that applying new technologies and improved process– heat integration to coal-based IGCC with carbon capture could achieve a cumulative efficiency improvement of more than 6% when compared with current designs. To achieve these savings, the industry will have to use innovative solutions, for example, a modified carbon monoxide shift system, THIOPAQ O&G sulphur removal technology1 and a novel carbon dioxide wash system.”

The current typical base case for an IGCC plant with carbon capture contains two gasification lines supplied by a common air separation unit, gas turbines with heat-recovery steam generators and a common steam turbine. Coal is milled, dried and fed to an entrained-flow gasifier. The gas flows through a heatrecovery unit, a quench scrubber and a syngas cooler to maximise heat efficiency. Particulates are removed using a filtration process. A sour carbon monoxide shift, which also performs carbonyl sulphide hydrolysis, converts carbon monoxide into syngas (carbon dioxide and hydrogen).

A selective polyethylene glycol dimethyl ether unit then removes hydrogen sulphide and carbon dioxide. The hydrogensulphide- rich stream is treated in a Claus sulphur removal unit with a tail-gas treating unit. The carbon-dioxide-rich stream is dehydrated and compressed for export to storage.

The improved flow scheme proposed by Foster Wheeler and Shell Global Solutions involves several significant changes from this base-case system.

Taken together, these improvements increase the power output of the plant by more than 10% and increase its overall efficiency by more than 11%.

The Improvements

For the carbon monoxide shift unit, heat efficiency is improved through changing the sour shift catalyst to a two-stage configuration instead of a three-stage one; utilisation of the heat recovered from the process to enhance the saturation of the feed gas; and simplification of the heat recovery system.

The sulphur removal process is simplified from a conventional amine treating and sulphur recovery unit to a single biodesulphurisation process, which is a key step. Ferguson explains: “The THIOPAQ O&G process was originally marketed by Paques for the treatment of biogas produced by the anaerobic digestion of waste water. This process, which was developed in collaboration with Shell Global Solutions, can be applied at high and low pressure in the natural gas and petrochemical environments at an economic scale for projects of up to 50-tonnes-per-day sulphur removal with the potential to achieve a higher load.

“In this IGCC application, the THIOPAQ O&G process replaces the polyethyleneglycol- dimethyl-ether-based solvent hydrogen sulphide removal, the Claus sulphur removal unit and the SCOT tail gas-treating unit of the base case. The biodesulphurisation process has three distinct sections: absorption, reaction and sulphur recovery. The THIOPAQ O&G system provides advantages in capital and operational expenditure along with less equipment handling, easier maintenance and better safety,” Ferguson says.

For the carbon dioxide removal scheme, a different approach is taken by replacing the conventional method with a chilled methanol wash on the feed gas into the removal unit. This simplifies the process so that it uses less power for solvent pumping and eliminates the need for a carbon dioxide dryer downstream. The improved design enables the construction to use carbon steel.

For carbon dioxide compression and pumping, six stages of compression without dehydration are applied instead of seven stages of compression with molecular sieves for dehydration. The sieves are no longer necessary because of the water removal in the methanol wash upstream.

Conclusions

Ferguson concludes that the results are highly significant for operators. “Taken together, these improvements increase the power output of the plant by more than 10% and increase its overall (lower heating value) efficiency by more than 11%.”

The case for these efficiency-enhancing improvements could be made most effectively by applying them to one or more of the IGCC projects now under consideration for the EU NER300 initiative. Ferguson says, “This finding would help to counter a marked tendency for these projects to be limited to technologies already demonstrated at existing IGCC plants owing to the perceived risks of new technology.”

The Benefits

  • The updated flow scheme delivers the following performance improvements:a reduced demand for medium-pressure steam in the carbon monoxide shift and carbon dioxide removal processes, which leaves more steam available for generation of power in the steam turbine; and
  • a significantly lower total parasitic power load for the sulphur removal, carbon dioxide removal and carbon dioxide compression processes.

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