In 2014 coal provided 36% of global power production. Switching from coal to gas is often considered one of the main ways to reduce greenhouse gas (GHG) emissions from the energy sector.
Coal combustion releases large amounts of CO2, the main greenhouse gas, while natural gas (mostly methane) releases less CO2 on combustion. However, methane is a more powerful GHG than CO2, so if there is much leakage before the gas is used, the benefits of fuel switching will be diminished.
Methane is an important GHG, with a global warming potential (GWP) ~25 times that of CO2 (over 100 years). However, the climate impacts of methane have been considered less harmful than CO2 as less methane is emitted from combustion. It also has a relatively short residence time in the atmosphere (~12 y) compared to CO2. However, recent research is challenging this perception.
Emissions from gas
The IEA Clean Coal Centre (IEA CCC) report, Climate implications of coal to-gas substitution in power generation, by Herminé Nalbandian discusses coal bed methane (CBM) and shale gas, mainly. CBM is trapped within pores and fractures in underground coal deposits. Due to high pressure underground, the gas is usually found in a semi-liquid state, lining the inside surfaces of the coal matrix. CBM is chemically similar to conventional natural gas. CBM occurs, in hard coal, at depths of 700-2000 meters. It is extracted through wells drilled directly into coal seams. Methane emissions occur at several stages during the production, supply and use of CBM. Methane recovered from working or abandoned mines is usually referred to as coal mine methane (CMM). Traditionally, methane was extracted from coals to reduce mining hazards, and the gas was generally vented to the atmosphere. Today, CMM is used for energy production. There are currently 355 coal mine projects in operation or under development. Total emissions avoided by the use of CMM have been calculated as 73.6 MtCO2-eq.
Shale gas occurs at depths of ~1000-5000 meters. There are nearly 700 known shales worldwide in more than 150 basins. In 2013, only a few dozen of these shales had undergone proper assessment for production potential, mostly in North America. The potential volume of shale gas is large and is likely to reshape gas markets worldwide. Shale gas production is most active in the USA. Production there almost doubled from 11% of overall US gas production in 2008 to more than 20% in 2010. It more than doubled again between 2010 and 2013 and some consider that it may approach 50% by 2035.
It is uncertain how much methane is emitted over the lifetime of a natural gas well. It is accepted that, in general, methane emissions from natural gas production are substantial and occur at every stage of the natural gas life cycle, from pre-production through production, processing, transmission and distribution. The US Environmental Protection Agency (EPA) estimates that more than 6 Mt of methane leaked from natural gas systems in the USA in 2011. Measured as CO2 equivalent over a 100-year timeframe, that is more GHG emissions than those emitted by all US iron and steel, cement, and aluminum manufacturing facilities combined. Comparisons depend on the emission factors used for each process. The 1996 Intergovernmental Panel on Climate Change (IPCC) emission factors have been widely used but are often considered to be too low. Newer factors have been used more recently, but may still be too low.
Shale gas has been reported to have a GHG footprint 8–11% higher than conventional gas, where methane emissions from the upstream portion of the natural gas production are unmitigated. If correct, this shows how important it is to implement existing control technologies and best practices to minimize methane emissions of the natural gas system. A clearer picture is expected to emerge as more data is analyzed. However, it may not be possible to obtain a complete picture of the amount of methane emitted through natural gas systems as there are hundreds of thousands of existing natural gas wells, thousands of miles of pipeline, and a growing interest in natural gas development throughout the world.
Emissions from coal
CO2, methane and nitrous oxide are GHG produced during coal combustion. Nearly 99% of the fuel carbon in coal is converted to CO2 during the combustion process. The greater the efficiency of the combustion process, the less coal is consumed and therefore total emissions to the atmosphere are reduced. The contribution of stationary coal combustion to total methane emissions is minor.
Life-cycle emissions for natural gas are reported as ~35% lower than coal on a heat-content basis. In terms of electricity production, natural gas has about 50–60% lower GHG emissions than those of a coal-fired plant. But a current state-of-the-art coal-fired plant operating with a high efficiency ultra-supercritical steam cycle emits almost 20% less CO2 than a subcritical unit operating under similar duty. Developments in advanced ultra-supercritical steam cycles promise to continue this trend. A plant operating at 48% efficiency would emit up to 28% less CO2 than a subcritical plant. Ultra-supercritical plants have been constructed and operated in Europe, Japan and the USA and more recently, in China.
The USA has a major program of substituting coal-fired plants with gas-fired facilities, mainly due to the increase in natural gas production and the subsequent reduction in gas prices. At the end of 2012, there were 1308 coal-fired electric generating units accounting for 310 GW of electricity in the USA. Projections indicate that 60 GW will be retired by 2020.
It is unlikely that the on-going shift from coal to gas in power generation in the USA will provide the 50% reduction in GHG emissions typically attributed to it, over the next three to four decades. It will only be achieved if gas leakage is maintained at the lowest estimated rates of 1 1.5% and the coal replacement rate is maintained at >5% per year. Although the coal replacement program may be achievable, according to the many scientists and researchers reviewed in the IEA CCC report, the 1 1.5% estimates contain significant uncertainty, and lack sufficient, actual, real-time measurements to guide policy decision-making at national level. Numerous studies conclude that if methane emissions exceed 3% of total gas production, the climate advantage of natural gas firing over coal disappears over the 20-year time horizon. Ultimately, switching fossil fuels does not solve the GHG emissions challenge.
Herminé commented: “There is a need to reduce GHG emissions, short- and long-term. Methane and CO2 operate over very different timeframes. Because of that, the potential benefits of replacing coal with natural gas will depend on the time horizon under consideration. If the effects, of climate change in the near term rather than in the long term, are paramount, then, the focus would be on immediate reduction in methane emissions. However, if concern about the impact of climate change in 50 to 100 years is greater than for the next 10 to 20 years; then the focus should be on reducing CO2 emissions and not worry so much about methane, but then again that depends on the percentage of the fugitive methane emissions. The higher the percentage of fugitive methane, the greater the impact on climate change.”
Carbon capture and storage (CCS) will be necessary to mitigate the climate implications of GHG emissions if both fuels, coal and gas, continue to be used in power generation.
The report, Climate implications of coal to-gas substitution in power generation , by Herminé Nalbandian, CCC/248 April 2015 is available for download from the IEA Clean Coal Centre Bookshop. Residents of member countries and employees of sponsoring organizations can download the report at no charge after a one-off registration.
This article is republished with permission from the IEA Clean Coal Centre www.iea-coal.org