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a sustainable energy future

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Discusses the main issues with producing sustainable energy.
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    veryone needs energy from fossil fuels in one form or another. They are what keeps us mobile through combustion engines and through electricity generation, power the gadgets and appliances that have improved our quality of life greatly from something as simple as a light bulb to a supercomputer. By far the greatest  primary source of energy used in the world is fossil fuel (coal, oil and natural gas).   Oil remains the world leading energy source accounting for 33% of global energy consumption in 2012 followed by coal at 30% and natural gas at 24%. The rest is split up  between hydro, nuclear and other renewables (e.g.  biofuel) as shown in Figure 1 [1] . The bulk of the primary sources of energy are predominantly used in electricity generation. This means in the pursuit to solve any issues with primary energy use and sustainability it is essential to scrutinize electricity generation. 67.4% of electricity generation was done using fossil fuels in 2010, mainly coal and gas [2] . Irrespective of the economic benefits of fossil fuels, the environmental impact, especially that coming from carbon dioxide emissions, is impossible to overlook. The issues Fossil fuels are carbon based fuel sources which release  predominantly carbon dioxide when they go through combustion. Carbon dioxide is a greenhouse gas which means it absorbs and emits radiation within the thermal infrared range in the atmosphere [4] . This process is what keeps earth at a relatively cool temperature. Carbon dioxide makes up almost 80% of anthropogenic (manmade) greenhouse gas emissions  [7] . Over the last century the amount of CO 2  in the atmosphere has risen, largely driven by fossil fuel use but also because of land use change and deforestation. Atmospheric concentrations of CO 2 have increased by 35% since the industrial revolution  [4] . It has been confirmed by various well respected research groups and the Intergovernmental Panel on Climate Change that global warming is occurring due to human an activity  [21] . Evidence of global warming can also be seen in the extreme weather events which have been occurring all over the world in the past decade [8] . The other issue with fossil fuels is their non-renewable nature. From current estimates oil, coal and gas will run out in approximately 52.9, 109 and 55.7 years respectively if energy consumption follows the current trend [1] . Even with these issues, a sustainable energy future will require using all the resources available to us over the short, medium and long term. At the same time, it means  producing and utilising all these energy sources in a way that minimises adverse impacts on the environment and maximises economic and social benefit. To make this  possible new oil, coal and gas extraction techniques are  being developed alongside a rapidly developing renewable energy industry. Various technologies are also  being developed to tackle the release of the other by- products of fossil fuel combustion which are also  pollutants. By-products such as oxides of sulphur (SOx) and nitrogen (NOx)  –   and particulate and trace elements, such as mercury [14] . The technologies mainly discussed, will be the ones which tackle the issues of fossil fuels  because they still have a high energy density and they will still play a big part in future electricity generation. Coal Coal has long dominated as the main fuel for electricity. It has been the fastest growing energy source in the last decade and this is largely driven by the growth of developing economies, mainly China [1] . In 2012 China alone accounted for 50.2% of global coal consumption. It is so popular because Coal is the most prevalent and E   N EW AND E MERGING E NERGY S UPPLY T ECHNOLOGIES F OR A G REENER F UTURE .    Figure 1: Global energy consumption 2012 [1]   widely distributed fossil fuel, accounting for 64% of globally economically recoverable fossil resources compared to 19% oil and 17% natural gas [14] . Coal is recoverable across every continent and every region which provides energy security across broad  political arenas [9] . Coals abundance and distribution, coupled with its relatively low and stable price pattern makes it a reliable supply of energy. This makes it a very attractive base load fuel. This means electricity generated from coal is one of the first sources to be dispatched throughout the electric grid. Super critical coal fire power plants are one of the most affordable methods of power generation in China, costing USD 33/MWh compared to USD 50 for hydro, USD 53 for nuclear and USD 71 for wind [17] . The problems of using coal Even though the market price of coal is low, the true cost is reflected in its impact on the environment and humans. To assess the true cost of coal you have to look at the supply chain, i.e. the mining of coal, combustion and waste disposal. The mining  causes widespread deforestation due to the excavation, soil erosion and pollution. Miners also suffer from Coal Workers’ Pneumoconiosis, colloquially known as black lung disease. Working in a coal mine overtime will lead to the accumulation of dust in the lungs which will cause many health problems [15] . The combustion of coal releases more of the anthropogenic CO 2 released into the atmosphere than any other fossil fuel. These include sulphur and nitrogen oxides and  particulate trace elements such as mercury. These  pollutants when released lead to the formation of  photochemical smog and acid rain [15] . The waste from coal combustion  known collectively as Coal Combustion Wastes (CCW). These are mostly ash which is toxic and often laced with lead, arsenic and cadmium. When placed in a landfill, these can seep into the ground and contaminate ground water [15] . The solution The answer to solving these issues will come through the development of efficient technologies which reduce the waste produced at each stage and neutralise the adverse environmental impact. Legislation has led to the development of technologies which remove the  particulates from coal combustion and this has proven effective [12] . A considerable amount of the waste from coal power plants is also recycled. Fly ash and bottom ash are used in concrete and blended cement and boiler slag is used in grit/roofing granules [22] . More efficient coal use By making electricity generation more efficient the amount of coal per megawatt will be reduced which in turn would reduce the effect of coal on the environment. Current pulverised coal fired plants (PCF) are mostly sub-critical power plants [17] . Significant efficiency improvements and reduction in CO 2 can be achieved if these are replaced by higher efficiency supercritical (SC) and ultra-supercritical plants (USC) which operate at higher pressure and temperatures. Another alternative would be Integrated Gasification Combined cycle plants (IGCC) which also offer improved efficiency. IGCC  plants use a gasifier to convert coal to syngas, which drives a combined cycle turbine. The average PCF plant efficiency is currently 33% compared to 45% for the more efficient SC, USC and IGCC plants [9] . Highly efficient modern power plants also emit up to 40% less CO 2 than the previous coal plants [9] . As a result improving the efficiency of the oldest and most inefficient plants would reduce CO 2  emissions from coal use by 27% representing nearly a 7% reduction in global CO 2 emissions [9] . With greater efficiency, comes greater running cost but the benefits to the environment outweigh this.  Figure 2: Energy prices  Carbon capture and storage  Figure 3: Carbon capture and storage [16]   These plants are also easier to fit with carbon capture and storage technology (CCS) which can reduce CO 2 emissions to the atmosphere by 80-90%. This involves capturing CO 2  that would otherwise be released to the atmosphere and injecting it to be stored in deep geographical formations. There are several ways to carry out carbon capture [16] . Capture    Post combustion    –   directly separating CO 2 from regular flue gas after the combustion process. The flue gas is cooled and treated to remove  particulate matter. Once cooled, the gas enters an absorber which consists of a liquid solvent (usually chilled ammonia) which absorbs the CO 2 . The CO 2-  rich solution is then sent to a stripper where it is separated into two gases: the  pure amine gas, which is recycled into the stripper and a CO 2 stream which is then dehydrated, compressed and sent to storage [16] .    Pre- combustion    –   carbon and hydrogen are separated prior to combustion. In this process, air is purified to extract pure oxygen (O 2 ). The O 2  is then sent to the gasifier where it reacts with the fuel source (coal/natural gas) to create a synthesis gas, or syngas. The syngas is purified which leaves a mixture of hydrogen (H 2 ) and carbon monoxide (CO). Steam is then added to a shift reactor which converts the CO to CO 2  and H 2 . The H 2 gas stream can be burned cleanly to  produce steam to run the turbines for electricity generation. The pure CO 2  stream is then dehydrated, compressed and sent to storage [16] .    Oxyfuel Combustion    –   also involves purifying the air to extract pure O 2 . The coal is then combusted with pure O 2  which is used to create steam to run the turbines for electricity generation. The flue gas produced will have a very high concentration of CO 2 due to the absence of nitrogen (N 2 ). This can then be captured as in post combustion capture, then stored [16] . Post combustion and oxyfuel capture can be retrofitted to existing power stations and new power plants constructed over the next 10-20 years [21] . Pre combustion on the other hand requires utilising IGCC cannot be retrofitted  but it makes coal potentially more flexible because the hydrogen produced can also be used in hydrogen fuel cell transportation [21] . Storage After the capture of the CO 2 the storage becomes the next issue to deal with. The geographical features being considered for CO 2  storage fall into three categories; deep saline formations, depleted oil and gas fields and unmineable coal seams. The Intergovernmental Panel on Climate Change (IPCC) special report on carbon capture and storage found that the risk of leakage was very likely to be less than 1 % over 100 years [21] . Deep saline formations    –   Very large,  porous rock formations typically atleast 800m below the surface and containing water that is unusable because of its high mineral content and/or salt. This saltwater is around 10 times saltier than ocean water and it’s trapped by impermeable rock called “cap rock” for millions of years. CO 2  injected into the  Figure 4: carbon storage potential [21]    formation is contained beneath the cap rock and over time it will dissolve into the saline water. Saline formations have the largest storage potential but are the least well explored [21] . Depleted Oil and Gasfields  –   well explored and have  proven ability to store hydrocarbons over a long period of time. CO 2 is already widely used in Enhanced Oil Recovery (EOR) from mature oil fields [18] . Injecting CO 2 into an existing oil field reduces the oils viscosity making it flow easier, and also acts as a pressurising agent  pushing more oil out of the rock. In EOR, the CO 2 can therefore have a positive commercial value [21] . Unmineable coal seams  –   when the CO 2 is injected into the seam it is adsorbed by the coal in preference to other gases e.g. methane which are displaced. This is feasible when used in conjunction with Enhanced Coal Bed Methane (ECBM) because the methane released can be captured and used as a very clean fuel source [18] . CCS has great potential in creating a sustainable energy future but it is still a relatively new field which still requires a lot of research and development. The first commercial example was setup as recent as 2000 in Weyburn oil field in Midale, Canada. Figure 5 shows the different stages each aspect of CCS is on in developmental terms. Carbon capture can also be used in any fossil fuel power plant such as a natural gas plant [21] .  Natural Gas  Natural gas has been widely discussed as a less carbon- intensive alternative to coal. It is the second most used fuel source having a 22.2% share in electricity generation [1] . Coal production in 2012 increased by only 2.5% (lower than the usual trend) which is mostly due to a large increase of natural gas consumption of 4.1% in the US [11] . Combustion of natural gas releases around 50% less CO 2  than coal per unit of electricity output and significantly less SO 2  which means before the wider implementation of CCS, natural gas is a better option for electricity generation [12] . In 2008, in the US only 21% of natural gas fired electricity was generated by steam turbine plants and simple cycle turbine plants which are relatively inefficient [11] . 79% was produced by combined turbine plants, which use waste heat to run steam turbines  boasting efficiencies on average 49% which is much higher than coal plants. It is widely considered as a “potential bridge fuel” for addressing climate change and transitioning into a future powered by low-carbon renewables [20] . An issue could be the  possibility of natural gas running out  but new gas extraction techniques are  being developed to allow access to this cleaner source of fuel.  Figure 5: Stages of develoment of CCS
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