An eco-friendly technology for coal-based units

Carbon dioxide emissions from power generation are influenced by the efficiency with which fossil fuels are converted into electricity. In a typical power plant, about one-third of the energy contained in the fuel is converted into electricity while the remainder is emitted as waste heat. Today, across the globe, around 30% of electricity is generated through coal-based thermal power plants, while in India the ratio is more than 50%.

The installed generation capacity of India is 132,329 MW as of March 2007, yet there are peaking shortages of 12.2% and energy deficit of 8.8%. The government has set a target of 225,029 MW power generation capacities by March 2012, requiring a capacity addition of 92,700 MW in the next five years. The future capacity additions are expected to be largely in the thermal sector, with coal being the predominant and most cost-effective fossil fuel in the country. This being the case, the emission of greenhouse gases (GHGs) from the power sector could increase significantly in the future.

Any strategy for mitigation of GHG emissions should centre on the following:
  • Improvement in efficiency of the fossil fuel-based power plants through technology upgradation, both for existing and future plants;
  • Reduction in losses in the process of supplying power from generating units to consumers;
  • Efficiency improvement in the end-use application;
  • Adoption and promotion of non-fossil fuel-based generation—use of hydro and other renewable sources.

The primary factors that alter carbon dioxide emissions from electricity generation from year to year are the growth in demand for electricity, the type of fuels or energy sources used for generation, and the thermal efficiencies of the power plants.
A number of contributory factors can also be identified: economic growth, the price of electricity, the volume of imported electricity, weather, fuel prices, and electricity available from hydroelectric, renewable and nuclear plants.

Other contributing factors include demand-side management programmes that encourage energy efficiency, strategies to control other air emissions, and installation of new capacity using advanced technologies to increase plant efficiency, such as combined-cycle plants and combined heat and power projects.

Carbon dioxide emission from combustion of coal depends on the quantity of coal consumed, the average carbon content of the coal and a small per cent of carbon that remains unoxidised, largely as particulate matter. Marland and Rotty have estimated that about 1% escapes unoxidised.

Power plants also use small quantities of diesel oil and furnace oil (FO) as supplemental fuels to boost the combustion and heat content. Generally, this supplementary fuel combustion is 0.2-0.3 ml/unit of power. The supplementary fuel consumption in old thermal power plants may range from 1% to 4% of the fuel.

It is estimated that carbon dioxide emissions may increase at an annual growth rate of 3% between 2001 and 2025. This has been exacerbated by the low energy efficiency of coal-fired power stations in the country. There is and will be continuous increases in carbon dioxide emissions in India and the world. The per capita CO2 emission of India is about five times lower than the global per capita carbon dioxide emissions. Currently, the per capita carbon dioxide emissions of India have reduced to three times less the global per capita emissions and the total CO2 emissions have reduced to two times less.

Emissions of carbon dioxide can be reduced by:
  • Supply-side management;
  • Clean coal technologies;
  • Pressurised fluidised-bed combustion;
  • Integrated gassification combined cycle power plant;
  • Renovation and modernisation of power plants;
  • Efficiency improvement in power generating stations;
  • Transmission and distribution loss reduction;
  • Demand-side management; and Renewable energy options.

A very promising technology on the supply side of power generation is pressurised fluidised-bed combustion (PFBC), which is a clean and efficient technology for coal-based power generation capable of increasing plant efficiency by up to 43% in a combined cycle arrangement. PFBC technology has the ability to burn low quality fuels and it offers fuel flexibility, It can remove more than 90% SO2 (sulphur dioxide) and lead to NOx (oxides of nitrogen) emissions of between 100 and 200 parts per million.

The CO2 reduction from per unit of power supplied at bus-bar using PFBC, over the conventional technology, is estimated to be 0.18 kg. Replacing a conventional plant of 500 MW capacity with a PFBC plant will result in estimated CO2 reduction of 0.58 million tonne a year.

PFBC technology uses gas turbine combined cycle technology in combination with coal-fired equipment. PFBC allows the gas turbine to operate free of corrosion. While a conventional combined cycle uses natural gas, PFBC operates at almost the same high levels of energy efficiency, but on less costly coal. PFBC has wide tolerance for differing coal types and can use opportunity fuels, so the owner can take advantage of the lowest energy price.

PFBC can be used either as all-new “green field” site applications, or as a technology to upgrade the capability of an existing steam plant.

With PFBC, coal consumption is 30% less per kilowatt than an existing unit. With its high efficiency, PFBC will have one-third lower emissions of CO2 per kWh than the existing unit and comparatively lower emission of pollutants. PFBC will enhance efficiency of existing power plant by 7%. The cost of setting up a PFBC plant is typically close to $63.2 per KW. The life of the plant is about 30 years. The fuel type can be switched easily throughout the plant’s life. Power companies like NTPC can adopt the PFBC technology at its coal-fired power plants to improve their efficiency and reduce GHG emissions from them.

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