Can coal be clean?

Currently, over a quarter of the world's primary energy supply comes from coal. Since the rich world has already created its coal infrastructure, the increased use of coal in China and India is now under scrutiny. In 1980, rich countries used over 65 per cent of the world's coal; in 2000 their share was roughly 50 per cent. By 2005, this figure fell to 38 per cent as consumption in China and India grew.

'China is building one power station a day', newspaper headlines scream. Western green groups want a moratorium on coal-based power stations funded by multilateral agencies like the World Bank. But they overlook the fact that the use of coal in the us and Australia has continued to grow, even after the Kyoto Protocol--150 new coal projects will be built in the us alone. Even the uk, which earned huge benefits by moving to natural gas, is revisiting its options. The price of oil, controlled by 'unstable' governments, is making coal attractive again. And not just in China.

Coal is dirty. It emits co₂, particulates and oxides of sulphur and nitrogen; it generates huge waste in the form of flyash. In addition, power plants consume large quantities of water and discharge equally large quantities of wastewater. These impacts were already driving 'green' technology for coal. The imperative of climate change is pushing proponents of coal to reinventing technologies for 'clean' and 'new' coal.

Clean, new and buried coal
There are three technology approaches available. One is to increase the thermal efficiency of current technology. Higher efficiency reduces coal consumption in electricity generation, thus reducing co₂ emissions. Technologies to increase efficiency are clean-coal technologies.

One of the ways to increase efficiency is to increase the pressure and temperature at which steam is fed to turbines (see table Technology, costs and greenhouse gas emissions). Subcritical power plants (the bulk globally) operate at 163 bar pressure and 538c. The best of these plants have an efficiency of 36 per cent. For the best in India this goes down to 32 per cent.

At the next level are supercritical plants in which the pressure is over 244 bar and temperature over 550c. With 170-plus plants of this type in the world, this technology is getting established. By 2006, in China, 6 per cent of coal-fired plants were based on this technology.

Finally, there are ultra-supercritical plants, in which pressure is over 350 bar and temperature exceeds 600 c. This gives them an efficiency of 45 per cent.

Then there are technologies called fluidized bed combustion (fbc) and pressurized fbc cycle in which crushed coal is suspended on upward blowing jets of air during combustion. The result is a turbulent mixing of gas and solids, providing more effective chemical reactions and heat transfer and burning of coal with greater efficiency.

The other promising technology is the integrated gasification combined cycle (igcc), which can give 50 per cent efficiency. The challenge is to lower costs and make it work with different kinds of coal. In this technology, coal is gasified under pressure with air/oxygen to produce gas, which is burnt in a gas turbine to produce power. There is a double gain as exhaust from the turbine can pass through a heat-recovery steam boiler, which generates more power. Only four plants using this technology have been built successfully-in Europe and the us.


The second approach is 'new coal' technologies. Instead of burning coal in conventional fashion, this approach will either convert coal into gas or recover gas trapped in coal seams underground. These gases can be burnt cleanly.

The third option is simpler-we continue to use coal, but the co₂ that is emitted is captured, compressed and transported for underground storage. This technology, carbon capture and storage, is being touted as the most promising option, allowing us to keep the coal-based energy economy going (see box Quick fix Bury and forget).

Way for India
Planners see no option but to invest in coal because it is cheap and domestically available. In 2005, coal generated roughly 70 per cent of electricity. Currently known coal reserves-250 billion tonnes-are expected to last another 50 years. The current annual consumption of 400 million tonnes is expected to increase to 2 billion tonnes by 2030. There are 81 coal-based power stations and many more are planned.

The issue for India, which will invest some us $1.25 trillion in energy infrastructure between now and 2030, is to see what these technologies-clean coal and otherwise-are, so it can invest in the best options and 'avoid' emissions.

Currently, most Indian power plants are subcritical. But it is now beginning to invest in supercritical plants. The National Thermal Power Corporation (ntpc) is setting up two plants, in Seepat, Chhattisgarh, and Barh, Bihar. Other agencies, too, are setting up projects based on this technology in Sasan, Madhya Pradesh; Akaltara, Chhattisgarh; Mundra, Gujarat; Girye, Maharashtra; and Tadri, Karnataka.

But there is a concern that clean coal technology does not factor in the quality of Indian coal, which has high ash content and low calorific value, about 3,500 kcal/kg, which may not give big efficiency gains with supercritical technology.

Similarly, fluidized bed combustion has been in India for some time, but mostly for smaller captive plants. It is popular due to the technology's fuel flexibility-almost any combustible material, from coal to municipal waste, can be burnt-and its ability to reduce so₂ and no emissions is significant. This technology gives slightly lower efficiency gains than supercritical plants, but it is seen to be more suitable for Indian coal. Its key disadvantage is size. Globally, the largest plant of this kind generates 320 mw. In India, most plants are in the range of 100-250 mw. However, given high losses in transmission and distribution of electricity over distances, this technology could be used for distributed power generation in urban centres.

igcc is the technology of the future. It has an efficiency of 45-50 per cent and there are hopes that this will reach 60 per cent soon. Analysis of Indian coal reveals that the first generation of this technology has the potential to reduce co₂ emissions by 10 per cent more than supercritical technology and 20 per cent more than subcritical technology.

This technology is still at an early stage of development and costs almost double the conventional. A pilot plant of 6.2 mw has been developed by the Bharat Heavy Electrical Limited at Tiruchirapalli, Tamil Nadu. This uses coal with over 40 per cent ash. ntpc is building a similar plant with a capacity of 100-125 mw, in Aurya, Uttar Pradesh.

Historically, most of the industrialized world first invested in low-efficiency technology, which was highly polluting, and then upgraded as it developed. Countries like India and China have the option to move to better technology but the biggest barrier is cost.

From the climate perspective, igcc is the only high-end technology which can substantially cut emissions. Clearly, the options for India would be invest in refurbishing existing plants to increase efficiency by investing in supercritical technology; invest in supercritical for new projects; and build igcc-based plants. But all this will need a global bargain, so India can invest in the best technology and not invest in a half-way measure that will not avoid emissions.