This analysis suggests that, because of the incorporation of CO2 capture processes, the need to run at higher temperatures, and the temperature changes resulting from intermittency, current designs of plant may not be ideal. The need to find a more promising alternative is more pressing for coal. One such option is a modification of the Integrated Gasification Combined Cycle (IGCC), a gasification-based process that can easily be adapted to capture carbon dioxide, while at the same time generating hydrogen as a fuel gas.
The IGCC is a collective name for a variety of processes in which coal is gasified to produce a fuel gas, and which, after purification, is burned in the gas turbine in a conventional CCGT, built as part of the gasifier complex. Because the gasification process produces a large amount of waste heat, which can only be used for raising steam, the steam systems in the combined-cycle HRSG and gasification system are 'integrated' - hence the appellation IGCC.
IGCCs intended to capture carbon dioxide and produce hydrogen are likely to use gasifiers of the entrained flow type. Here, oxygen plus steam or water is reacted with coal at about 1300° C to 1600° C to produce a raw 'syngas' gas (which mainly consists of carbon monoxide and hydrogen, and is often used to synthesize chemicals, hence the name). After purification of the syngas to remove sulphur compounds, the mixture of CO and H2 is used in a conventional IGCC as the fuel gas for the gas turbine. But to raise the hydro gen level and to enable the carbon in the coal to be captured as CO2, the CO in the syngas is catalytically reacted with steam in a shift converter, the reaction being:
After the 'shift reaction', the CO2 would be absorbed using alkaline solutions, compressed and sent to a geological storage site. The hydrogen that remains could be used as fuel gas in the CCGT section of the plant. The overall reaction to produce hydrogen using coal as a fuel can be represented as:
The problem with IGCCs is that they not at all suitable as plants that have to be started up and shut down frequently. The IGCC gasifier and process train contains potentially explosive gases, and the necessary precautions on start-up could be lengthy and complex. Furthermore, an IGCC needs a great deal of ancillary plant for it to operate. These comprise units for removing hydrogen sulphide (H2S) from the gas stream and oxidizing this gas to produce sulphur, a cryogenic air separation unit for producing oxygen, and the CO2 capture plant itself. The conclusion would appear to be that a CO2 capture-type IGCC has to be a base-load-generating system.
This is would be true if the only output from the plant was to be electricity. But by the time such plants are required, a hydrogen economy may be well developed in some countries, with a network of hydrogen pipelines. As a result, the Institute for Energy has proposed that at times when there is no demand for electricity, the hydrogen that the IGCC is producing should be diverted to the pipeline network. In this manner, the gasifier section of the IGCC could be kept at full output at all times.
Although fully supportive of this approach, it is the view of the author that it might be more practical for the UK to adopt a compromise situation, also based on the IGCC. This is recognizing that the UK economy is very dependent upon natural gas, accounting for over 70 per cent of the non-transport energy use. In keeping with this, the UK has a vast natural gas network, all of which has been renovated and expanded over the past 30 years. In this proposal, the purified syngas would be used to produce methane as a substitute natural gas (SNG). The methane from the gasifier stream could either be used as fuel gas in the gas turbine or, when electricity is not needed, could be put into the natural gas system as SNG. This should be a very attractive option for the UK as the SNG will supplement its declining gas reserves (see Figure 6.6).
To make SNG, a modified syngas mixture containing hydrogen, carbon monoxide and carbon dioxide are reacted together to produce methane:
But the overall reaction for the complete process of coal gasification, shift conversion and 'methanation' can be represented as:
It is apparent from Equations 2 and 5 that the main argument against an IGCC-electricity-methane process is that the rate of carbon capture is only half that when such a process makes hydrogen. On the other hand, such a system makes use of the existing UK natural gas infrastructure, which, of course, comprises not only the pipeline and distribution network, but also the Rough storage field in the North Sea, the liquid natural gas (LNG) storage sites, and also, at the consumer level, the burners (which will only work using a methane rich gas), in central heating systems, gas cookers, industrial furnaces, and natural gas-fired CCGT plants.
Whether the fuel gas is hydrogen or methane, the combined-cycle section of the IGCC would still be subject to start and stop operation; but it would have considerable advantages over the natural gas-fired CCGTs of today. The HRSG section of the plant could be kept hot by bleeding off steam from the gasifier steam system. This will eliminate much of the temperature cycling to which a normal HRSG is subject. It also gives the option of an extremely fast start-up since the HRSG is kept hot.
There are other advantages. In principle, if liquid oxygen were stored, it should be possible to run the cryogenic plant at a reduced output, saving some power at times when there was a high demand for electricity. This would help to overcome one of the problems of CCGTs - that is, reduced power output on hot days. More important is the prospect of constructing the gasifier to be undersized in relation to the declared electrical output from the plant. The CCGT would utilize some of the hydrogen or methane that was stored in the pipeline network. This ability is shown schematically in Figure 6.6. Both of these ideas would have a significant effect on the capital costs.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.