Coal Based Energy in Botswana: Coal-to-Liquids, Coal-Bed Methane, and Underground Coal Gasification

Dumelang.* There is more to coal utilization than just burning it to make steam to generate electricity. In my previous post, I carried out a general overview of coal projects in Botswana, but it was heavily weighted towards coal mining and coal-fired electricity projects. In this post, I take a close look non-coal-fired electricity projects, such as coal-to-liquids, coal-bed methane, and underground coal gasification, to get a better understanding of the benefits and challenges associated with these other ways of drawing out the energy stored in coal. Even though some appear to offer advantages over traditional coal mining and coal-fired electricity, it is always important to understand the shortcomings associated with these alternatives. Various proposals for implementing these technologies in Botswana have been made, there has been a lot of information about these projects in the Botswana press over the past few years and some exploratory work has been carried out but, as yet, no commercial operations have been implemented. 

Coal to Liquids (CTL)

Let’s start with coal-to-liquids technology. A large CTL operation producing 20 000 barrels per day of petroleum liquids has been proposed for Botswana. The proposal incorporates a 300 MW coal-fired power plant, a fertilizer plant that will produce 300 600 tonnes/year of ammonium nitrate, and 15 200 tons of sulfur as a byproduct. The budget for this project is $4.6 billion and construction is supposed to start in 2016. When I read about this project, its size and scope surprised me because it is beyond the price and scale of anything that has ever been built in Botswana.

Let’s look first at the technology. The CTL process, also known as coal liquefaction, involves taking the carbon in coal and converting it into hydrocarbon molecules that can be used for a variety of purposes, including liquid fuels, such as diesel or petrol.

This is carried out in three ways:

• Coal is heated in an oxygen-free atmosphere to distill off coal tars and oils (also known as pyrolysis).
• Coal is treated at high temperature and high pressure in the presence of solvents, hydrogen gas, and catalysts to hydrogenate the coal and form hydrocarbons. 
• Coal is treated in a three-stage process, which first involves its high-pressure reaction with steam to form syngas – a mixture of carbon monoxide (CO) and hydrogen (H₂). Syngas is treated at high temperatures in the presence of a catalyst to form hydrocarbons, which then need to be refined and separated to produce individual hydrocarbon products.

This series of steps will usually convert one tonne of coal into one to two barrels of hydrocarbon fuel. If we consider that the heat content of one tonne of coal is ~24 000 megajoules (MJ) and that of one barrel of hydrocarbon fuel is 6120 MJ, you can see that the process is not very energy efficient: only ~26% of the energy in coal is converted and distilled into one barrel of fuel. That being said, the convenience of liquid fuels cannot be overstated – our transportation system is now dependent on liquid fuels like petrol and diesel and not, as at the start of the industrial age, on solid fuels like coal.

All these CTL options are technically complicated, capital-intensive, highly polluting operations with high operating costs. They involve sophisticated and large-scale high-pressure, high-temperature chemical processing equipment. Depending on their size, location, and coal supply, these operations only tend to be profitable in a world where crude oil costs more than $50 - $110 a barrel. The price of crude oil is presently around $40/barrel. As a result, there are a limited number of these operations in the world. The largest is the Sasol complex in Secunda, South Africa. There are some operations in China and several smaller operations scattered around the world. To give some sense of the polluting nature of these operations, Sasol’s Secunda operation, which produces ~150 000 barrels per day of synfuels, is the largest point source for CO₂ emissions on the planet!

Implementing the technology successfully involves requires large numbers of skilled, well-trained, and experienced personnel, and—in the case of Sasol—years and years of government subsidies. In a country like Botswana, there are insufficient trained personnel for such an operation and there will be a heavy reliance on expatriate personnel at the start. The water demand for these operations is also substantial; in Botswana, reliable, high-volume water supply is presently a difficult resource to come by. Moreover, these operations are very energy-intensive and require a reliable high-voltage electric grid. Reliable power supply can be difficult to achieve in Botswana at this time, so the incorporation of a 300 MW coal-fired power plant into the operation appears to be a wise, albeit expensive, way to guarantee reliable power.

Although the promises of local infrastructure, industrial development and employment opportunities are alluring, the scale of this project, as well as the demand for energy, water, and personnel resources required for a project of this size, suggests that caution and extensive scrutiny is warranted.

One of the axioms of the energy business is: It takes energy to get energy. We have to expend energy drilling into oil deposits, pumping, refining, and transporting the oil before we can use it as petrol to power our cars; we have to expend energy mining, processing, and transporting coal before we can burn it; even in the renewable energy field, where we have free fuel sources, such as sunlight, we have to expend energy creating the components of solar panels, transporting, and installing them. Every energy source requires an investment of a certain amount of energy to recover energy that we can use somewhere else. The trick is to be sure that the energy invested is less than the energy produced. This concept called EROI – energy return on investment (also referred to as EROEI – energy recovered on energy invested). The formula for EROI is straightforward:

EROI = Energy recovered/Energy Invested = Energy output/Energy input

One always wants the EROI to be greater than 1, i.e., one wants to recover more output energy than input energy. There are many ways to calculate the inputs and outputs, but the most rigorous way is to undertake a life-cycle analysis and compare all the inputs and outputs over the whole life of the project. The range of EROI values in the energy field is wide: for hydropower, it is of the order of 80; for ethanol from biomass, it is of the order of 5. The figure below provides ranges of EROI for some important energy sources. It is interesting to observe the wide range. The reason I have raised this concept is that questions have been raised regarding the EROI of coal to liquids technology. As I noted above, this is an energy- (and water-) intensive technology and there are concerns that the input energy might be greater than the output energy. There may well be other compelling reasons to consider CTL processes, such as job creation, access to and the convenience of liquid fuels – it is difficult to fill up a car with coal – but, nevertheless, the EROI is important factor to consider, particularly in a country like Botswana that is already energy-challenged.

Source: Hall, C.A.S, et al

Coal-Bed Methane (CBM)

A number of CBM methane projects have been proposed for Botswana. Methane is the simplest hydrocarbon. It is a gas that contains one carbon and four hydrogen atoms and it is a very useful chemical. It can be used as a starting block to build longer chain hydrocarbons, such as the molecules that are found in fuels like petrol and diesel, and it can be burned directly to generate electricity. One of the great advantages of methane, also called natural gas, is that natural gas-fired power plants have higher conversion efficiencies than coal plants and are far less polluting. Coal-fired plants typically have conversion efficiencies of 30% to 35%, whereas modern natural gas-fired plants have efficiencies of 50% or higher. On a per kilowatt hour of electricity basis, coal plants tend to generate 40 to 50% more carbon dioxide emissions than natural gas operations. As a result, methane is a highly desirable fuel: the large volumes of natural gas that have been released from shale deposits in the USA via fracking technology have had a profound impact on US energy markets and a depressing effect on their coal mines and coal-generated electricity industries.

Methane is found in underground deposits, such as underground reservoirs and gas-rich shales. It is also often a byproduct of crude oil production as well. Methane is recovered by drilling down into these deposits and pumping out the gas. In shale deposits, horizontal drilling is utilized, followed by fracturing the deposit with high-pressure water in a process known as “fracking” to create a web of small channels that release the natural gas locked up in the shale.

Methane is also found associated with coal deposits, where it is a byproduct of the process that led to the formation of coal. In fact, methane is often the cause for the terrible explosions that occur in coal mines that can lead to the deaths of many miners.

Coal-bed methane technology involves drilling holes into coal deposits and then pumping out water to lower the pressure. This creates the conditions that lead to release of methane which can then be pumped out of the coal bed. However, the methane produced by this process often needs to cleaned up to remove water, carbon dioxide, and nitrogen, which reduce the heating value of natural gas. Once methane is treated, it then needs to be transported to its point of utilization. This requires a pipeline network with compressor stations every 80 to 100 km. Pipelines are expensive, they need to run parallel to an electric grid to power the compressor stations, they run across peoples’ property, they require maintenance to run, and are vulnerable to damage. That is why it might be best to build a generating plant close to the source of the coal-bed methane, provided the source of methane is large enough to support such an investment.

Source: http://www.ehp.qld.gov.au/management/ucg/

The figure above shows the basic operation of a CBM operation. One of its drawbacks is the water issue. Water has to be pumped out from the coal bed to create the low-pressure conditions that lead to the release of methane. The removal of large amounts of water from a coal deposit can result in the lowering of water tables in local underground reservoirs if there is a linkage. At first one might think that water is not necessary a problematic byproduct, especially in an arid country like Botswana, but the snag is that the water is often saline, which leads to disposal challenges.

CBM is an interesting technology and might have applicability in Botswana, however the transportation of gas across hundreds of kilometers of non-electrified areas and the water disposal issue need to be carefully considered.

Underground Coal Gasification (UCG)

Another way to exploit coal resources without direct mining involves a combination of the CBM and CTL approaches, known as underground coal gasification and there has been some interest in applying this technology in Botswana. UCG involves the direct controlled burning of coal in an underground deposit and then using the generated heat and injected air, oxygen, and steam to generate syngas (that mixture of carbon monoxide and hydrogen gas referred to in CTL technology). The syngas is then drawn to the surface where it is dried, cleaned, and then can be combusted to turn a gas turbine to generate electricity or it can be transported in pipelines elsewhere where it can be used to produce liquid hydrocarbon fuels.

An underground gasification operation usually involves the drilling of two wells into an underground coal deposit. One is the injection well where air, oxygen, and or steam are injected into the burning coal bed and the other is the production well where the produced syngas is drawn out from underground. The key is obtaining a linkage between the two wells so that heat, steam, and the syngas produced in-situ can move from one well to another. Once the coal bed is ignited, the extent of combustion is controlled by air or oxygen injection. In the process, the combustion zone grows and moves towards the production well.

Source: http://www.ehp.qld.gov.au/management/ucg/

One of the attractive features of the UG process is that the coal ash is left underground and this removes a disposal problem associated with aboveground coal gasification. The process also allows exploitation of low-value and difficult-to-mine coal deposits and, because no direct mining is involved, it is far more energy efficient than aboveground gasification. Compared with traditional coal mining and CTL technology, the capital and operating costs are lower and the water consumption is relatively low. Because no mining is undertaken, the surface disturbances are much less and there are no open shafts or depleted mine works to deal with once the coal has been mined. The emissions from UCG, such as carbon dioxide, metals, particulates, sulfur, etc., are reported to be less as well.

However, and this is a big however, it needs to be understood that UCG involves setting fire to underground coal deposits. There is a possibility of runaway or poorly controlled combustion that could result in disastrous consequences, such as uncontrolled emissions of carbon dioxide, carbon monoxide, methane, and other hydrocarbon gases, contamination of local water tables, and uncontrolled subsidence into the burnt-out chambers. The very high temperatures of combustion could also affect the local geology, with the formation of cracks and fissures that result in uncontrolled flow of water and emissions into areas where control is not possible. There are thousands of uncontrolled coal seam fires presently burning in countries across the planet: just across the border in South Africa in the coal mining area of eMalahleni, abandoned coal mines have been burning for years. In the US, the town of Centralia in Pennsylvania had to be abandoned in 1984 due to an uncontrolled underground coal fire that made living and safety conditions unbearable. Once they start, these fires are difficult to reach and almost impossible to extinguish.

Another disadvantage associated with UCG is that produced syngas has to be transported to its point of use so a surface pipeline network has to be built to transport it. Additional complications are introduced by the nature of the syngas produced from UCG: it contains a great deal of nitrogen from the air used in burning and therefore has lower heating value than syngas produced above ground. Depending on the final application of the syngas, i.e., direct burning or hydrocarbon production, the nitrogen might have to be removed.

UCG is an intriguing and attractive concept and numerous pilot-scale tests have been carried out around the planet with mixed success: despite the promise of UCG, it still has to take off. Problems of uncontrolled gas emissions, pollution of farmland, and acidification of water supplies have occurred. The longest running underground gasification plant is the Linc Energy Yerostigaz plant in Uzbekistan. This has been running since 1961 and produces 1 million cubic meters per day of syngas that is used to power a nearby power station. There have been recent reports of commercial-scale UCG operations in China.

This post has covered a lot of ground from coal-to-liquids technology, to coal-bed methane, to underground coal gasification. In all cases, these operations appear to provide some advantages over traditional coal mining, but their complexity, costs, and potential environmental impacts create challenges that could be difficult for a resource-constrained country like Botswana to deal with. Some of these approaches have promise, but, in every case, require caution and extensive scrutiny so that we clearly understand and appreciate the tradeoffs between energy production and energy input, water, expertise required, and long-term environmental impact. These tradeoffs can be difficult and, at times, unpopular to consider in the face of demands for natural resource utilization, industrial development, and job creation, but prudent, unbiased, and in-depth evaluation of these interesting approaches is warranted.

Until next time, remember to turn the lights out when you leave the room so that less coal has to be burnt.

Tsamayang Sentle**

Mike Mooiman

mooimanm@franklinpierce.edu

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