Clean Coal Technologies

.Mario Bartucca
Gaytha A. Langlois

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CleanCoalTechnologies

Modern Clean Coal Technologies reduce CO₂ emissions per unit of electricity by up to 30%.

Clean Coal Technologies (CCTs) are defined as 'technologies designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation and use'. These technologies reduce emissions, reduce waste, and increase the amount of energy gained from each ton of coal.

CCT programs are being vigorously pursued by many countries, with many billions of US dollars equivalent being spent annually on developments in utilization techniques. These technologies will enable coal use to be increasingly efficient and environmentally acceptable as a vital world energy source throughout the next century.

Most CCTs concentrate on power generation from coal, as more than 50% of coal produced is used to generate electricity. An impressive array of technologies is already commercially viable, and a large number of others will become available in the near future.
 
 

Coal Extraction and Preparation

CCTs for the extraction of coal are readily available. Improved exploration methods, such as geophysics and seismic techniques, minimize any environmental impact, while improving mine planning by reducing geological uncertainties. Improved mining technologies aim to maximize extraction efficiencies while minimizing energy usage. Measures to reduce noise and dust levels are standard practice, thus minimizing risks to the operators. Mining can release methane gas from coal, which can be a potential hazard. Various means to drain the gas are used, and in some cases it is being developed as an energy source. Mine plans include provisions to avoid the risk of ground water contamination.

CCTs for the preparation of coals can reduce their ash content, and clean them of impurities such as dirt and sulfur. New technologies are being developed to improve the efficiency and cost of these operations, while improving the quality of any wastewater.
 
 

Stack Gas Treatment - applied to gaseous emissions from Pulverized Fuel (PF) Combustion

Pulverized Fuel (PF) combustion is the most widely used method for burning coal for power generation. In PF combustion, coal is milled to a powder and blown into the boiler with air. As a powder, the coal has a large surface area and is easily combusted in burners. This provides the heat that is used to produce superheated steam to drive turbines and hence generate electricity. At present, nearly all of the world's coal-fired electricity is produced using PF combustion systems.

Emissions from PF combustion can be reduced by post-combustion CCTs. Electrostatic precipitators and/or fabric filters can remove more than 99% of fly ash from flue gases. Flue gas desulfurization (FGD) methods can remove 90-97% of the oxides of sulfur (SO_x) from flue gases, and can convert it into gypsum for use in the building trade.

FGD removes 90-97% of the oxides of sulfur from flue gases.

Among the key CCTs for PF combustion that reduce emissions of nitrogen oxides (NO_x) are low-NO_x burners, which modify emissions by up to 40%, and reburning techniques. Together these modify the combustion process to reduce NO_x emissions by up to 70%, and are being widely adopted as they can be installed into existing plant. Selective catalytic NO_x reduction, a post-combustion technique, can achieve reductions of 80-90%.
 
 

Advanced Pulverized Fuel (PF) Combustion

Industry has continuously striven to increase efficiencies of such conventional plant; for example, the average thermal efficiency of US power stations has increased from 5% in 1900, to around 35% currently. New conventional PF power plants achieve above 40% efficiency. Advanced modern plants use specially developed high strength alloy steels, which enable the use of supercritical and ultra-supercritical steam (pressures greater than 248 bar and temperatures greater than 566°C) and can achieve, depending on location, close to 45% efficiency. Application of new advanced materials to PF power plant should enable efficiencies of 55% to be achieved in the future. These results in corresponding reductions in CO₂ emissions as less fuel is used per unit of electricity generated.

Fluidized Bed Combustion (FBC)

Fluidized bed combustion is a method of burning coal in a bed of heated particles suspended in a gas flow. At sufficient flow rates, the bed acts as a fluid resulting in rapid mixing of the particles. Coal is added to the bed and the continuous mixing encourages complete combustion and a lower temperature than that of PF combustion. The advantages of fluidized beds are they produce less NO_x in the outlet gas, because of lower combustion temperatures, and they produce fewer Socks when limestone is continuously added with the coal. They can also use a wider range of fuels than PF combustion. Atmospheric pressure fluidized beds are commercially available now as two types, bubbling-bed (known as Atmospheric Fluidized Bed Combustion - AFBCs) and circulating-bed (CFBCs). The efficiency of most fluidized beds used for power generation is similar to that of conventional plant. However, use of this technology has been stimulated by its better environmental performance. Pressurized fluidized beds, which can achieve efficiencies of 45%, are at advanced stages of demonstration. As with PF plants, employing higher steam conditions would further boost efficiency.
 
 

Hybrid Systems

Hybrids combined cycles are also under development. These combine the best features of both gasification and combustion technologies, using coal in a two-stage process. The first stage gasifies the majority of the coal and runs a gas turbine; the second stage combusts the residual 'char' to produce steam. Efficiencies greater than 50% are possible.

In addition to these CCTs, a development, which can apply to all of the generating systems, is the co-firing with coal of biomes or wastes. This involves burning or gasifying such materials together with coal. Benefits can include reductions in CO₂, SO_x and NO_x emissions relative to coal-only fired plants, and recovery of useful energy from biomes and wastes at high efficiencies can be achieved, without the need for building dedicated plant. Hence, the coal-fired power industry can support the renewable energy and waste industries.

Rehabilitation

With current technology it is now possible to restore mined land to its original condition and often to a more productive use.

Several years before a mine opens, exhaustive studies of the immediate environment are carried out to define the existing conditions and to identify sensitivities and potential problems. The studies address the impact of mining on factors such as surface and ground water, soils, local land use, and native vegetation and wildlife populations. The findings are then reviewed as part of the process leading to the award of a mining permit by the relevant government authorities. A detailed rehabilitation or reclamation plan is designed and approved for each mine, covering the period from the start of operations until well after mining has finished.

Reclamation activities are progressive - with the shaping and contouring of spoil piles, replacement of topsoil, seeding with grasses and planting of trees taking place on the mined-out areas. Prior to and during mining, care is taken to relocate streams, wildlife, historic sites and other valuable resources. During extraction, the mines are operated in a way to minimize dust and noise pollution, and contamination of water. After mining, some reclamation may make use of residual pits to construct reservoirs, water recreation and amenity areas. The uses for reclaimed land are varied: - agriculture, forestry, recreation, construction for industry or housing, and wildlife habitats. Many mines worldwide have been awarded prizes in recognition of the excellence of their restoration activities.

Today, the coal industry is committed to the protection of the environment and land reclamation is an integral part of most mining operations, whether legally required or not. The majority of coal mining companies worldwide are investing a considerable amount of skill and money in returning land to a state comparable to, or better than, that that existed before mining began.
 
 

The Future

 Analyses prepared for a Programmatic Environmental Impact Statement for the CCT Program found that widespread implementation of clean coal technologies by the year 2010 could lead to significant reductions in emissions of sulfur dioxide and oxides of nitrogen. When compared to a no-action alternative, which assumed those conventional coal-fired technologies with conventional pollution controls would continue to be used.