With coal-fired power plants achieving an average 33% efficiency, it's crucial to build advanced HELE plants to reduce global carbon emissions.
Renewable predictions
With low carbon a priority in energy generation, it is imperative to develop a more efficient, lower-emissions technology for coal. Notwithstanding rapid advances in renewables and energy storage, fossil fuels will continue to be essential in the global power mix. The International Energy Agency predicts that coal will generate more electricity in 2040 than all new renewable technologies (excluding hydro) combined. The IEA World Energy Outlook published in late 2016 forecasts that 730 gigawatts (GW) of new, higher-efficiency, lower-emissions (HELE) coal plants will be built by 2040, much of this in developing countries.
HELE
Figures from the World Coal Association, a keen advocate of HELE, show that the average efficiency of coal-fired power plants around the world today is 33%. Modern state-of-the-art plants can achieve rates of 45%, while "off-the-shelf" rates are around 40%. Increasing the efficiency of coal-fired power plants by just 1% reduces CO2 emissions by between 2–3%.
These technologies make up a diverse group that is improving combustion rates and reducing emissions. Techniques include supercritical and ultracritical technology, integrated gasification combined cycle (IGCC), and fluidized bed combustion. By reducing the volume of CO2 produced, HELE technologies are an important step on the road toward carbon capture and sequestration (CCS), which will be a key technology if global climate change objectives are to be achieved.
New pulverized coal combustion systems—utilizing supercritical and ultra-supercritical technology—operate at increasingly higher temperatures and pressures, achieve higher efficiencies than conventional units and offer significant CO2 reductions. Supercritical steam cycle technology has been in use for a while. Germany and Japan are driving development of the next generation of ultra-supercritical units, capable of potentially operating at up to 50% efficiency.
IGCC
Integrated gasification combined cycle (IGCC) is a technology that uses a high-pressure gasifier to convert coal or other carbon-based fuels into pressurized gas. The synthesis gas (syngas), a mixture of carbon monoxide, carbon dioxide, and hydrogen, can be used to drive a combined cycle turbine, a cleaner way of burning coal. The technology is attracting interest in a number of countries including the US, China, and Germany. In the US, Mississippi Power's IGCC plant in Kemper County can operate using either syngas (produced from locally mined lignite) or natural gas. The 582-MW plant is designed to capture 65% of its carbon emissions and transport it via pipeline for use in enhanced oil recovery. One of the biggest IGCC plants planned is China's 800-MW Dongguan Taiyangzhou IGCC.
FBC
Another more efficient, lower-emissions technique is fluidized bed combustion (FBC). FBC evolved from efforts to find a combustion process able to control pollutant emissions without external emissions controls. It is a flexible method of electricity production using combustible material including coal, biomass, and general waste. FBC systems improve the environmental impact of coal-based electricity, reducing SOx and NOx emissions by 90%.
Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. This leads to more effective chemical reactions and heat transfer. Combustion takes place at temperatures of 1,400 to 1,700 degrees F—below the threshold at which NOx gases form. Sulfur-absorbing chemicals, such as limestone or dolomite, are used to capture pollutants inside the boiler.
There are two main FBC systems: atmospheric systems (FBC) and pressurized systems (PFBC). FBC in boilers can be particularly useful for high-ash coals. In PFBC systems, steam generated from the heat in the fluidized bed is sent to a steam turbine, while the flue gasses are used in a gas turbine. Second-generation FBCs can include an integrated coal gasifier to produce syngas, which, when added to the energy entering the gas turbine, increases efficiency.
TAKEAWAY
Coal is set to remain a major player in the global power market, and increasing the deployment of higher-efficiency, lower-emissions technologies and CCS will be essential to achieving global carbon emissions targets.
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