Is there such a thing as “clean coal” as a fuel for large-scale power generation? It is a question that, until recently, would probably have evinced an emphatic “no” as an answer – at least from the average person in the street.
Instead, renewable energy sources – wind or wave – or nuclear power would most likely have been cited as the way to keep the lights on in the developed world in the 21st century. But that scenario has problems – some renewable sources, wind power certainly, are intermittent, while many people are hostile to nuclear technology.
So back to coal, for which the answer to that question is increasingly moving towards the affirmative, and not merely at a theoretical level. Real installations are being built in which coal will not only be the basis for highly efficient energy generation but which may also point the way to an industrial-scale means by which atmospheric carbon dioxide levels may not just be stabilised but even reduced.
One such scheme is the RDK8 power station, now taking shape at a total project cost of €1 billion by the Rhine in the German city of Karlsruhe for EnBW Kraftwerke, the regional electricity supply company. When it comes on-stream early in 2014, the station will have an output of 912MW of electricity to the grid and the potential for a further 220MW contribution to local
district heating.
Although the installation’s output is not exceptional in scale, the levels of efficiency it is intended to achieve will be. Its net efficiency – the basic rate at which it converts the thermal energy generated by coal combustion into electricity – will exceed 46%, a figure that will rise as high as 58% when the district heating element is factored in.
The plant will have CO2 emission levels of less than 740g/kWh – roughly 40% less than the norm for a coal-powered station built three decades ago. In addition, the corresponding figures for CO and NOx emissions will be less than 100mg/m3 at 0°C and 1.013bar.
The plant is an example of a small but growing number of installations worldwide described as “ultra-supercritical” steam power plants, meaning that the combination of the temperatures and pressures achieved is well in excess of the “critical point” (374.14°C and 221.2bar) at which the- separate liquid and vapour states of steam become indistinguishable.
The term is used, for instance, by Charles Soothill, senior vice-president – technology for Alstom Thermal Power, a division of the French-owned engineering conglomerate which has supplied the core boiler, turbines and generator package for the RDK8 project. What he means is represented by these figures – 600°C, 620°C
and 275bar. The first two are, respectively, the temperatures at which steam enters the first – high-pressure – turbine in the plant and then that at which after a reheating process it enters the second – intermediate-pressure – turbine in a sequence of five turbines in total, the subsequent three being low-pressure devices. The last figure is the initial “live steam pressure” at which steam enters the high-pressure turbine.
Soothill says the concept of “ultra-supercriticality” is generally accepted as being valid when the initial steam input temperature is at least 600°C. But what gives RDK8 its edge is the temperature of the reheated steam – a good 10° higher than has previously been achieved, certainly by Alstom and as far as he is aware in any other similar power plant so far.
Construction of the plant started in spring 2009 and is largely completed, with “first fire” scheduled for next April. That fact allows for some appreciation of its overall scale. The complete boiler structure, for example, stands 118.5m in height and comprises 35,000 tonnes of steel, including 700km of piping with 80,000 separate welds. It is fired by imported bituminous “hard” coal that is pulverised on-site to a grain size of a maximum of 90µm before being mixed with air and blown through a system of corner-mounted, tangential, tilting burners on four levels, which introduce a degree of swirl into the combustion process to enhance its efficiency.
The fan that drives the air into the boiler is itself rated at 7MW of power and rotates at 750rpm. Meanwhile the feedwater tank for the boiler, 43m in length and weighing 184 tonnes, was delivered in a single piece from its manufacturing site in Spain – a logistical feat aided by the plant’s riverside location.
That location will also be an important element in at least two aspects of the plant’s day-to-day operations. One will be the delivery of the coal that fuels it. At full load, the plant will consume 240 tonnes of coal an hour. Although it will have an on-site storage capacity of 5,000 tonnes, the river will facilitate a regular supply schedule. The other aspect is that the plant will draw water from the river into its condenser for cooling purposes.
Again the figures are impressive: a 27MW pump will draw water from the river at the rate of 22 tonnes per second. The efficiency of the plant’s operations will be subject to some seasonal variation, with performance being slightly better in the winter when the river water is colder and more effective as a condensing agent.
The turbine and generator sequence comprises the five turbines that make up one of Alstom’s STF100 systems, followed by a two-pole Gigatop turbogenerator that uses hydrogen and water for cooling. The combination covers a length of 60m. One of the more intriguing facts about the installation is the arduous nature of the welding involved. Each join in the high-pressure steam pipes took a week of continuous welding and annealing to achieve.
If RDK8 represents the state of the art in environmentally friendly and energy-efficient coal-fired power generation, is there scope for significant further development in the foreseeable future? Soothill certainly thinks there is.
He says there will be a move to achieve even higher initial and reheated steam temperatures – 700°C and 720°C respectively could well become a reality within a few years. Indeed, he confirms that Alstom has already carried out the development work – mainly formulating the materials that could withstand the stresses involved – that would enable it to collaborate with a prospective client on an appropriate project. “We have been in a position to develop a first project for a 700°C installation for a little while,” he says.
Beyond this, the prospect of “advanced ultra-supercritical” coal-fired power plants – in which initial and reheated steam temperatures may reach as high as 700°C and 760°C – is also beginning to be a subject of serious discussion within the industry. A workshop on the subject organised by the International Energy Agency took place in Vienna in September. A paper presented by a team from Alstom posited that such an installation could achieve a net efficiency gain over an ultra-supercritical plant such as RDK8 of a further 7%.
But perhaps the most interesting aspect of the paper was its implicit message that pushing on to achieve these performance levels would be a matter of evolutionary development of existing capabilities rather than revolutionary change. Indeed, the paper explicitly stated that current conventional steam generator designs “can be configured for 760°C steam temperatures”.
We will also see an increase in the use of co-firing of a primary fuel such as coal with biomass materials – whether the waste from forestry activities or specially grown “fuel crops”. This is a reality already, but the prospect that Soothill holds out is much more fundamental than anything in current practice. He says that in combination with carbon capture and storage (CCS) techniques it is feasible that a power station firing a mix of coal and biomass could act to reduce atmospheric CO2 levels.
A possible scenario, says Soothill, might be one in which CCS obviates 90% of the CO2 emissions from a plant in which 20% of the heat input to the power generation process was produced by the biomass element of the fuel mix. Provided the station was part of a wider system where the supply of the biomass material was guaranteed through continuous replanting and fresh growth, then the net effect would be the absorption by the new growth not just of the 10% of the plant’s CO2 emissions that reach the atmosphere but of the equivalent of a further 10% as well.
Soothill’s verdict on the potential of this approach is worth pondering: “It is the only industrial-scale process I know of that could have that effect.”