As more was understood of the behaviour of different coals under different heating regimes and, in particular the interaction of competing chemical processes of decomposition and recombination was studied in the laboratory, the production of coke was put on a firmer base, allowing increased flexibility in the blending of the feed material and more control over the properties of the resulting coke.
At the same time, the chemistry of the blast furnace was being better understood, so that the desirable properties of the coke and other ingredients could be better defined. This better understanding has led to great improvements in the overall performance of blast furnaces. A hundred years ago the production of a ton of iron in a blast furnace required the consumption of four tons of coal. In most recent practice this has been reduced to about half a ton of coal per ton of iron (Figure 1).
These developments and the improvement of materials of construction have enabled the use of higher steam temperatures. The most recent plant, using superheated steam, can have overall efficiencies approaching 50%, compared with the 38% of plant with standard steam conditions. Further developments will take efficiencies even higher. Figure 2 is illustrative.
However, if CO2 is captured for transport and storage, the overall efficiency of a pf (pulverised fuel) plant using supercritical steam would be reduced to some 25%. This drastic reduction in efficiency has increased the interest in alternative coal-fired systems. There are two main contenders Oxyburn – the use of oxygen instead of air for combustion in an otherwise conventional design – and systems based on gasification with oxygen using gas turbines and steam turbines in combined cycles. In both these routes CO2 does not have to be separated from nitrogen and in the gasification system it is captured at high pressure, ready for transport and storage.
In considering the merits of the different options for future power plant, the need to cater for the capture of CO2, has become one of the most important criteria. Many studies have been made of the merits of the options for power generation, pre-combustion with its large efficiency penalty or post combustion with high capital costs. These studies indicate that in general there is little to choose between the overall economics, so the final choice will depend on the different conditions at different sites which may favour a particular option.
Although all the components of the various options for CO2 capture and storage have been operated on the large scale separately, there is the need to demonstrate their robustness in power generation. These issues are the heart of the Government’s competitions, which are intended to demonstrate carbon capture and storage in four projects using different systems. In order to reduce the duplication of CO2 handling facilities, it would it be desirable for the Government to promote a national scheme for transport and long term storage, taking into account problems of ownership and responsibility for health and safety aspects.