Although alternative energy sources are desirable and technology has advanced, coal will continue to have centre-stage of our energy scenario for many years to come, notes Prof. Mukesh Sharma, Department of Civil Engineering, IIT Kanpur.
Coal is the most significant and abundant fossil fuel for our industrial growth with estimated reserves of 290 billion tonnes (as estimated by the Geological Survey of India). Coal consumption and demand have grown immensely in our country, largely dominated by steel and power plants.
The power sector accounts for over 80 per cent of the total coal consumption (around 300 million tpa: Annual Report 2012-13, Central Electricity Authority) producing over 1,30,000 MW (out of 2,23,626 MW installed capacity). The Planning Commission has set a target of adding over 88,000 MW of power generation capacity in the 12th Plan period (2012-2017). Although alternative energy sources are desirable and technology has advanced, coal will continue to have centre-stage of our energy scenario for many years to come.
There are several issues that need to be addressed, if we have to sustain ascendancy of coal-based power generation in the country. Most notable environmental issues include air emissions and thermal discharges into surface water. Air emissions pose serious environmental concerns, and if not addressed, will cause widespread degradation and can take a toll on public and ecological health. While environmental laws partly remediate the problem, at times, laws are inadequate but their stringency in coming time is obvious.
The main emissions from coal combustion at thermal power plants are carbon dioxide (CO2), nitrogen oxides (NOx), sulfur dioxide (SO2), and air-borne inorganic particles such as fly ash, soot, and other trace metals, especially mercury. Oxides of nitrogen and sulfur play an important role in atmospheric chemistry and form very fine particles along with ammonia adding to already existing high burden of particulate pollution in the country.
Estimated emissions of various pollutants from coal-based power plants are: 0.6 million tonnes of particulate matter, 1.5 million tonnes of NOx and 3.0 million tonnes of SO2 every year. These are huge quantities. If one does not recognise this humongous environmental issue and not invest in technology, either we pay through increased human morbidity or mortality or face serious energy crises. As seen, the coal-based power generation produces massive amounts of sulfur and nitrogen dioxide, the convenient alibi not to control these gaseous emissions is, sulfur dioxide levels in air is low. A little the authority may know that sulfur dioxide is converted into sulfates posing greater health problem than precursor sulfur dioxide itself.
Technology can answer the environmental issues, which should ensure long-term sustenance of coal-based power generation without compromising on environmental degradation and public health. There are several advanced emissions control technologies applicable in power plants. These technologies include selective catalytic reduction (SCR) of NOx to nitrogen, electrostatic precipitators (ESP) and fabric filters for particulate matter, and flue gas desulphurization (FGD) for SO2 control. Unfortunately, neither the power plants nor the Indian regulations subscribe to control of SO2 and NOx but situation may change quickly and old power plants may be forced to have the retrofits of these technologies, which may be more challenging and costly.
The common technologies for particulate control include electrostatic precipitators (ESP), fabric filters, mechanical collectors and venturi scrubbers. Each uses a different principle and collection process with varying collection performance. The high efficiency technologies (particulate control efficiency exceeding 99.9 per cent) are ESP and fabric filters. An ESP electrically charges the ash particles (mostly negative) in the flue gas stream and particle migrates towards grounded plate where it is collected. The ESP consists of a series of parallel vertical plates through which the flue gas passes. The charging electrode is between the collecting plates and charges the ash particles. The resistivity of the ash is an important consideration in ESP design. The benefits of an ESP are high collection efficiency, high reliability, low pressure drop and low maintenance.
SO2 removal technologies include wet FGD, dry FGD utilising a spray dryer absorber and dry sorbent (lime and lime stone) injection. Most SO2 removal processes include an engineered oxidation system which transforms calcium sulfite (CaSO3) formed by the SO2 removal process to calcium sulfate (CaSO4: gypsum).
In a De-NOx-ing (removal of NOx) system, NOx is reduced by ammonia (NH3) or urea to nitrogen and water. Based on economic considerations, a suitable reducing agent can be selected out of ammonia like materials. This process is called selective catalytic reduction (SCR). SCR De-NOx-ing system consists of reactor, injection system and catalyst.
The cost of pollution control needs to be internalised in the installation cost power plant and pollution control devices should be integral parts of the power plant. A broad cost break (can vary significantly) is for every MW, the installation cost is about Rs.7 crore (including particulate control) and for SO2 and NOx control, an additional cost could be about Rs.0.7 crore per MW. In other words about 10 per cent of the cost could be attributed to pollution control system. The cost of pollution control device may appear high but this outweighs the cost of environmental degradation and cost of public health in terms of increased morbidity and mortality. No alibi is acceptable for not controlling the emissions from power plants. Currents and future technologies can provide effective solutions to arrest large air emissions. Adequate environmental protect can only sustain coal based power generation in future.