New power generation systems are beginning to emerge, which, if developed into practical systems, could ultimately have a significant impact on coal-based power generation, writes Dr Qian Zhu
Almost all coal-fired power plants are steam power generating units based on the traditional Rankine cycle.
The generation efficiency of this system has improved steadily due to a range of measures. Today’s state-of-the-art ultra-supercritical (USC) coal-fired power generating units can achieve a net energy efficiency of around 45 per cent (LHV, lower heating value), compared to traditional subcritical plants which may operate at less than 30 per cent efficiency.
An A-USC plant operating at 48 per cent efficiency would emit up to 28 per cent less CO2 than a subcritical plant. Thus, the efficiency of a coal-fired power plant has a major influence on the environmental impact of coal combustion.
However, the maximum efficiency of a Rankine cycle is restricted by the second law of thermodynamics. So any further substantial increase in efficiency will be difficult and expensive. A number of innovative power cycle concepts are being investigated as alternatives.
The goal of the R&D activities is to ensure a more efficient, more environmentally benign, economic and flexible use of coal.
Novel power generation systems are beginning to emerge, which, if developed into practical systems, could ultimately have a significant impact on coal-based power generation.
These novel systems include fuel cells, chemical looping combustion, solar-coal hybrid power stations, magneto-hydrodynamics, indirect coal-firing, and alternatives to the steam Rankine cycle.
Fuel cells are electrochemical devices that convert chemical energy in fuels into electrical energy (and heat) directly and so can produce power with high efficiency and low environmental impact.
Solid oxide (SOFC) and molten carbonate (MCFC) fuel cells operate at high temperatures, which offer the best opportunity for thermal integration with coal gasification systems.
Significant advances have been made in fuel cell technology in terms of cell and component design, materials, performance, reduced costs and so on.
Recently, progress has been made in the development of fuel cells that use solid fuel (carbon) and convert the chemical energy in carbon directly into electricity without the need for gasification.
Fuel cells are still under development but they are beginning to emerge in the commercial market.
When coupled with coal gasifiers, SOFCs and MCFCs have the best attributes to compete for the large, base-load power market. It seems that IGFC systems with carbon capture could potentially achieve high net plant efficiencies in the range of 40- 56 per cent, with CO2 capture rates up to 99 per cent.
Gas-fired fuel cell simple-cycle power plants are in commercial operation. As experience from operating these plants leads to further technological advances/improvements and cost reduction, it is possible that fuel cell combined cycle and maybe coal-fuelled IGFC power systems will one day emerge as alternative power generation technologies to traditional coal-fired systems.
Chemical looping combustion (CLC) is an indirect form of combustion in which an oxygen-containing solid material, such as a metal oxide, supplies the oxygen to a fuel, and the spent oxygen ‘carrier’ is separately regenerated by air at high temperature.
As there is no direct contact between air and fuel, CLC produces a stream of CO2 and water vapour from which the CO2 can be readily recovered eliminating the need for additional energy intensive CO2 separation. CLC also minimizes NOx formation.
Several CLC processes fuelled by coal or coal-derived syngas are under development. In addition, several chemical looping coal gasification processes are being developed that provide flexibility to produce electricity, hydrogen or syngas and to integrate with alternative power cycles such as fuel cells.
Various chemical looping-based power plant configurations have been proposed and studied. Recent studies all indicate that, compared with conventional coal-fired plants with carbon capture, chemical looping-based power plants could achieve higher efficiency and a high carbon capture rate with considerably lower energy penalty and costs for carbon capture.
If CLC can be successfully developed into practical systems and commercialized, they may revolutionize coal-based power generation.
However, there are still a number of issues that require further investigation. For example, development of oxygen carriers with high reactivity and stability remains a challenge for CLC.
Solar-coal hybrid power plants
There have been some projects recently that integrate solar energy with coal-fired power systems. Solar thermal energy can be used to produce high-pressure, high-temperature steam, which can then supplement the coal power steam cycle to reduce the consumption of coal in the production of electric power at the plant.
The solar generated steam can be directly used to drive the steam turbine, or to replace the steam extracted from turbine for feedwater heating.
Alternatively, the solar thermal energy is used to preheat the combustion air.
Off-the-shelf technologies are used in solar-coal hybrid power systems so no technology development work is needed. This approach can help utility companies to generate more renewable power with significant cost savings.
The emissions of CO2 and other air pollutants are reduced as a result of reduced coal use. Solar-coal hybrid plants can be new constructions, or solar fields can be retrofitted to existing coal plants. More work is needed to find the most effective way to integrate the solar energy with coal power.
Currently, coal provides about 40 per cent of the energy for global power generation. It will continue to play an important role for many years.
If the technologies for alternative power generation systems can be developed into practical systems, they could ultimately have a significant impact on coal-based power generation and greatly reduce its environmental impact.
Dr Qian Zhu is an author and analyst at the IEA Clean Coal Centre