[ Some of the points made include:
- As a rule-of-thumb, a 50-60% thermal efficiency can be used for hydrocarbon liquefaction in general assessments. This implies that only half of the coal energy invested in liquefaction will come out as energy available as transportation fuel. CTL production is further limited to water-rich regions, since 6 to 15 tons of water are required per ton of CTL fuel.
- More than two thirds of current crude oil production may need replacement by 2030 simply to meet current demand. This is likely to prove extremely challenging, since conventional oil peaked in 2005. This could severely impact the world economy if alternative sources of energy and liquid fuels are unable to “fill the gap” between climbing demand and falling production on the timescale required. Coal-to-liquids (CTL) is often proposed as a possible mitigation strategy and has been an important component in several peak oil mitigation outlooks.
- At present, world production of conventional oil stands at around 85 million barrels/day (Mb/d) and has been roughly constant since mid-2004 . Current world CTL and GTL capacity is around 400,000 b/d. Existing estimates place the global decline in existing oil production rates at between 3 and 8% annually, or in other words, new capacity of 3–7 Mb/d is required every year.
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer]
Höök, M., et al. 2014. Hydrocarbon liquefaction: viability as a peak oil mitigation strategy. Philosophical Transactions. Series A: Mathematical, physical, and engineering science, 372. 34 pages
Current world capacity of hydrocarbon liquefaction is around 400,000 barrels per day (kb/d), providing a marginal share of the global liquid fuel supply. This study performs a broad review of technical, economic, environmental, and supply chains issues related to coal-to-liquids (CTL) and gas-to-liquids (GTL). We find 3 issues predominate.
- Significant amounts of coal and gas would be required to obtain anything more than a marginal production of liquids.
- The economics of CTL plants are clearly prohibitive, but are better for GTL. Nevertheless, large scale GTL plants still require very high upfront costs, and for three real world GTL plants out of four, the final cost has been so far approximately 3 times that initially budgeted. Small scale GTL holds potential for associated gas.
- CTL and GTL both incur significant environmental impacts, ranging from increased greenhouse gas emissions (in the case of CTL) to water contamination. Environmental concerns may significantly affect growth of these projects until adequate solutions are found.
Oil is the largest contributor to mankind’s energy needs and provides over 90% of all transportation energy (IPCC, 2007). Each year, new production must be brought on-stream to offset declining output from current production. More than two thirds of current crude oil production may need replacement by 2030 simply to meet current demand. This is likely to prove extremely challenging, and there is a significant risk of a peak of conventional oil production before 2020 (UKERC, 2009). Peaking global oil production would imply a peak in oil-sourced liquid fuels. This could potentially severely impact the world economy (Fantazzini et al., 2011), especially if alternative sources of energy and liquid fuels are unable to “fill the gap” between climbing demand and falling production on the timescale required. Coal-to-liquids (CTL) is often proposed as a possible mitigation strategy and has been an important component in several peak oil mitigation outlooks (SRES, 2000; Hirsch et al., 2005; Hirsch, 2008; IEA, 2011).
Proponents of CTL claim that it will be capable of full or partial mitigation of the expected shortfall of conventional oil due to a global oil peak. Similarly, liquefaction of natural methane gas (gas-to-liquids, or GTL) has emerged as a promising option to monetize stranded gas assets (Fleisch et al., 2002; Wood et al., 2012).
At present, world production of conventional oil stands at around 85 million barrels/day (Mb/d) and has been roughly constant since mid-2004 (Fantazzini et al., 2011). Current world CTL and GTL capacity is around 400,000 b/d (less than half a percent of oil production). Existing estimates place the global decline in existing oil production rates at between 3 and 8% annually, or in other words, new capacity of 3–7 Mb/d is required every year (Höök et al., 2009). Various observers advocate hydrocarbon liquefaction to provide everything from a minor role to production levels of several Mb/d. What expectations are reasonable? To assess this question, this paper reviews the technology, economics, environmental impact and supply chain of CTL and GTL.
Coal is a complex compound consisting of carbon, hydrogen, oxygen, sulfur, and minor proportions of other elements. It is an aggregate of microscopically distinguishable, physically distinct and chemically different subparts baked together. CTL works by breaking up the solid hydrocarbon structures found in coal. This may be accomplished by partial breakdown directly to liquid hydrocarbons (direct coal liquefaction or DCL) or by full breakdown into hydrogen and carbon that can be reassembled into H-C-chains of a desired length (indirect coal liquefaction or ICL). The chemical reactions involved in reality are significantly more complex than the simple overview presented here.
CTL processes are influenced significantly by the properties of the coal feedstock (ash content, grindability, sulfur content, plasticity, caking properties, etc.). GTL displays fewer issues in this respect since natural gas is a more homogenous feedstock. Process efficiency and yield are further influenced by the choice of catalyst. Only four group VIII-metals (Fe, Co, Ni, and Ru) have sufficiently high activities for hydrogenation of CO to merit their use as effective FT catalysts (Tavakoli et al., 2008).
The advantage of DCL is its very high liquid yield – potentially >70% of the dry weight coal (Benito et al., 1994; Couch, 2008). DCL liquids are typically of higher quality (i.e. less nitrogen, sulphur, phenols, aromatics, etc.), due to hydrogen addition, than liquids obtained from pyrolysis. The DCL liquids are effectively a synthetic crude oil (syncrude) and are directly usable in power generation or in petrochemical processes. However, they require further refining before they can be used as a transport fuel. Refining can be done directly at the CTL facility or by sending the synthetic crude oil to a conventional refinery, where it can be made into gasoline- and diesel-like fuels as well as propane, butane and many other products.
GTL technology options
There are many ways to liquefy natural gas, and several pilot plants, trial projects and research initiatives exist. However, only two companies – Sasol and Shell – have built large-scale commercial plants (>5,000 b/d capacity). The GTL industry is essentially immature and many important patents are held by relatively few companies (Wood et al., 2012). Established GTL approaches have much in common with ICL technologies, as they both work with FT synthesis and gaseous chemistry. Both high temperature and low temperature FT synthesis can be used to provide liquid fuels. There are commonly three main stages in a GTL facility: synthesis gas generation, FT reaction and product upgrading (Panahi et al., 2012). Auto-thermal reforming is the preferred technology to generate syngas since it offers better H2/CO ratio compared to all alternatives (Iandoli and Kjelstrup, 2007). The syngas generation stage is often the most capital intensive part of a GTL plant. A schematic process chain of a GTL complex can be seen in Figure 1.
Commercial GTL developments
Sasol developed the Sasol slurry phase distillate GTL process in the 1980s from their CTL technologies. Hot syngas is bubbled through a slurry of catalyst particles and liquid reaction products. Initially iron was used as a catalyst, but recent developments have used cobalt-based catalysts providing greater conversion rates. Developments by Sasol resulted in the construction of a stand-alone GTL plant with a capacity of 22,000 b/d in Mossel Bay in 1992. The project became known as Mossgas and is considered the first commercial GTL plant. A subsequent collaboration between Sasol and Qatar Petroleum used auto-thermal syngas production, slurry phase FT reactors and an isocracking product upgrading technology to develop a 32,400 b/d facility known as the Oryx plant in Qatar. The project agreement was signed in 2001 and completed in 2008 after delays and budget overruns. Sasol later engaged in a GTL feasibility study for a Nigerian plant together with Texaco in 1998. The Escravos project was agreed in 2002 and involved Sasol, Chevron Corporation and Nigerian National Petroleum Company, using the same design as the Oryx project. Delays and cost overruns caused Sasol to withdraw in 2009, although their technologies are still used under license. The capacity of the Escravos GTL plant will be 32,400 b/d when completed in 2013 – similar to Oryx in design and size. In 1993, a joint-venture consisting of Shell, Mitsubishi, Petronas and Sarawak State completed a GTL plant in Bintulu, with a final capacity of 14,700 b/d. Experience from Bintulu was used by Shell in the design of the world’s largest GTL project, the Pearl GTL plant in Ras Laffan Industrial City, Qatar. Construction started in 2007 as a joint venture between Shell and Qatar Petroleum, and production began in 2011, reaching full capacity of 140,000 b/d in 2012.
Thermal or energy efficiency is the percentage of the energy in the feed-stock that is converted into energy output as products.
Low thermal efficiencies, often in the range of 45– 55%, have been a major argument against hydrocarbon liquefaction (Liu et al., 2010). DCL is commonly seen as more efficient for producing liquid fuels than ICL because only partial breakdown of the coal is required. However, such claims can be misleading because published DCL efficiencies usually refer to the formation of an unrefined syncrude requiring additional processing into useable liquid fuels. In contrast, ICL efficiencies often refer to the final products. Caution should always be exercised when dealing with efficiencies.
The estimated overall efficiency of the DCL-process is 73% (Comolli, 1999). Other groups have estimated a thermal efficiency of 50-70%. However, Sovacool et al. (2011) criticized these estimates as misleading, because industry tends to compare the heating value of the resulting liquids with that of the inputs. Hydrogen production, product refining and other steps necessary to complete the entire product supply chain are not always included in the efficiency calculations; one needs to pay attention to how those assessments have been made. Representative efficiency for FT-synthesis used in ICL and GTL is around 50%, while the theoretical maximum has been estimated at 60–65%. Tijmensen et al., (2002) give overall energy efficiencies ranging from 33–50% for ICL co-using various biomass blends. Detailed
In essence, there is no significant efficiency advantage for either DCL or ICL, while GTL is somewhat more efficient. As a rule-of-thumb, a 50-60% thermal efficiency can be used for hydrocarbon liquefaction in general assessments. This implies that only half of the coal energy invested in liquefaction will come out as energy available as transportation fuel.
Despite differences in methodologies, all coal consumption estimates end up at approximately similar figures. As expected from the relatively low thermal efficiencies, a significant amount of coal is required to generate liquid fuels in any substantial amount. Significant CTL production is viable only in areas with abundant coal reserves. It has been estimated that large scale CTL production will be limited to about 6 countries with large coal reserves and the ability to divert significant fractions of that coal to liquefaction (Höök and Aleklett, 2010).
Obtaining reliable GTL gas consumption figures is harder. For example, the Pearl project is designed to consume 45.3 Mm3 per day to yield 120,000 b/d of condensate, propane, butane, and ethane and 140,000 b/d of GTL products (Wood et al., 2012). The National Petroleum Council (2007) gives an average conversion factor of 283 m3 natural gas per barrel GTL product. Others have estimated the carbon efficiency, i.e. the amount of carbon in the feed gas converted to saleable products, at 53-77% (Fleisch et al., 2002). Water is a vital part of the conversion processes and CTL is highly water intensive (Mielke et al., 2010). Zhang et al. (2009) state that each ton of synthetic oil output requires 8–9 tons of freshwater for DCL and 12–14 tons of fresh water in ICL. In contrast, the US Department of Energy found that water consumption is approximately equivalent for DCL and ICL at around 5–6 ton water/ton of oil (National Energy Technology Laboratory, 2006) and RAND calculated 6–12 ton water/ton of oil (RAND, 2008). 3.
Environmental impacts from CTL
When considering CTL, first we will examine impacts that apply equally to all industrial applications of coal, including landscape modification, particulate emissions and acid mine drainage. We will then examine water consumption, water contamination and greenhouse gas emissions specific to CTL.
Three types of surface mining are generally used to extract shallow coal, open-pit, strip mining and mountaintop removal. In all cases the overburden is removed to expose the coal.
- Open-pit mining creates a large crater-like depression.
- In strip-mining, as the overburden of a strip is excavated, it is placed in the excavation of the previous strip.
- Mountaintop removal is applicable to horizontal coal seams in mountainous country, notably the Appalachian Mountains in the United States. At current coal prices, mountaintop removal is often the only cost-effective way to mine coal in this area. Explosives are used to remove the entire mountain top overburden and vegetation, including forests, which is placed directly in stream valleys. According to the US Environmental Protection Agency, between 1992 and 2002 surface coal mining in Appalachia damaged or destroyed more than 1900 km of streams and deforested 150,000 hectares of land, while 34,000 hectares of valleys were filled (Environmental Protection Agency, 2005).
Particulate emissions and coal processing
Particulates are emitted both when coal is mined and via wind erosion until new vegetation covers reclaimed land. Hendryx et al. (2008) found that, after accounting for other variables, lung cancer mortality was higher in Appalachian counties with extensive coal mining. Coal dust contains carcinogenic compounds and metals including zinc, cadmium, nickel and arsenic. The mining and cleaning of coal at local processing sites creates large quantities of ambient particulate matter as well as contaminated water. Water contamination and water consumption Water is used extensively throughout the coal mining and liquefaction process. Surface mines use water for dust abatement and all coal must be washed to remove soil and other contaminants before further processing. Water requirements often cause local aquifers to be depleted near coal mines. As rainwater drains through the mine it reacts with and oxidizes pyrite (FeS2) in the coal, producing sulphuric acid that may leach into local aquifers in a process called acid mine drainage (AMD). AMD often continues after the mine is no longer operational. Other contaminants that may leach into the water supply from the entire mining process include cadmium, selenium, arsenic, copper, lead, mercury, ammonia, sulphur, sulphate, nitrates, nitric acid, tars, oils, fluorides, chlorides, sodium, iron and cyanide (Spath et al., 1999; Palmer et al., 2010).
The total amount of water required for liquefaction depends on factors like plant design, location, humidity, and coal properties. CTL is classified as a water intensive process (Mielke et al., 2010), and consumption estimates range from 6–15 tons of water per ton of fuel, as noted earlier. CTL may generate or amplify water shortages in certain regions. Cooling, boiler and process water in CTL plants needs to be of reasonable quality to prevent corrosion and/or deposit formation, and treatment is typically needed. Discharged water must be treated before it can be released to the environment without causing harm (Lei and Zhang, 2009).
Rong and Victor (2011) point to water availability and water quality issues as important factors behind the recent caution toward CTL in China.
Greenhouse gas emissions
The CTL process produces significant amounts of carbon dioxide, the greenhouse gas primarily driving anthropogenic global warming. From a life-cycle perspective, it is also important to include the emission contributions from mining. Coalification, the natural process by which coal is made, traps significant amounts of methane as the coal rock is formed, called coal bed methane (CBM) that is released during the mining of coal. Methane represents approximately 14% of global GHG emissions (in CO2-equivalent) and CBM accounts for approximately 8% of total methane emissions (World Coal Association, 2012). Annual worldwide CBM release in 2000 was estimated to be 0.24 Gt CO Annual worldwide CBM release in 2000 was estimated to be 0.24 Gt CO2 equivalent. This compares with approximately 35 Gt of anthropogenic CO2 released annually (IPCC 2007). Brandt and Farrell (2007) find that even a partial transition to coal-to-liquids synfuels could raise upstream GHG emissions by several Gt of carbon per year by mid-century, approximately 7% of the current total carbon emissions, unless mitigation steps are taken. However, there are CTL plant configurations using CO2 recycling/capture/storage that may be capable of reducing emissions significantly (Williams and Larson, 2003). Mantripragada and Rubin (2009) explore some of those configurations, but also stress that handling CO2 responsibly dramatically raises CTL costs.
Our analysis highlights a strong risk for CTL plants to become financial black holes, and helps explain why China has strongly slowed down the development of its CTL program (Rong and Victor, 2011).
Unfortunately, cost escalation often occurs: apart from the Oryx plant, the final total cost has so far been approximately three times that initially budgeted. In the case of the World GTL plant in Trinidad and Tobago, the original CAPEX was expected to be 0.125 billion$, while the last estimate is roughly 0.400 billion $ (Ramdass, 2010). The unit cost is approximately $178,000 b/d, in between the Pearl and the Escravos plants — and more than 3 times the original estimate.
Supply chain issues
Supply chain risks, vulnerabilities and uncertainties (Simangunsong et al., 2012) are another important topic for energy strategies involving major hydrocarbon liquefaction undertakings. High oil prices or oil shortages that make CTL more attractive may also bring about problems for parts of the liquefaction supply chain. Business risks have been broadly reviewed by Oke and Gopalakrishnan (2009). For a CTL/GTL supply chain, we have identified three major risk categories. In a joint report, Lloyd’s of London and Chatham House have advised all businesses to begin scenario-planning exercises for the oil price spike they assert is coming in the medium term (Lloyd’s, 2010). It will prove imperative that business addresses this Schumpetarian shock in a timely fashion (Barney, 1991).
Material flow risks
Material flows involve physical movements within and between supply chain elements, such as coal transportation, movement of spare parts for CTL/GTL facilities and delivering CTL/GTL products to consumers. These concerns, and issues such as capacity change over time relate to typical supply chain design problems (Carle et al., 2012; Singh et al., 2012) complicated by many of the risks discussed above that are specific to the chemical industry (Floudas et al., 2012; Liu et al., 2011). Today, petroleum products supply 95% of all energy used in global transportation (IPCC, 2007). Oil price volatility or supply disruptions may have a major impact on transportation and this may completely change the competitiveness of CTL facilities located at a distance from coal mines. For the USA, coal accounts for 44% of the railroad tonnage (McCollum and Ogden, 2009), while the corresponding figure for China is more than 50% (Rong and Victor, 2011). Rail capacity issues and bottlenecks have been a persistent problem in several cases and future rail policies can have significant impact on CTL supply chains. The only exception is CTL facilities at mine-mouth locations. It should also be noted here that CTL/GTL may also impact existing hydrocarbon supply chains negatively and these concerns have led to the abandonment of certain projects (Rong and Victor, 2011).
Information flow risks. Supply chains are also influenced by information flows such as demand, inventory status, order fulfillments, design changes and capacity updates. Some observers even perceive information as a bonding agent between material and financial flows. Information system security and disruptions could arise from internally ill-managed systems or potentially by outside sources such as industrial espionage, hackers or similar (Faisal et al., 2007).
We also note that coal production requirement is a major factor in CTL feasibility. Significant CTL production requires equally significant coal production and resources that only a few countries or regions realistically can develop. CTL capacities in the Mb/d-range will effectively be limited to the largest coal producing countries in the world: China, USA, India, Russia, Australia and South Africa. Even if several Mb/d could be derived from CTL, this would account for only a minor share of global oil production and barely offset the decline in existing oil production (Höök and Aleklett, 2010).
We also note that almost all papers admit that financing CTL projects can be difficult unless public incentives and subsidies are provided.
GTL faces similar problems and risks. Those include high capital costs, technical efficiency and reliability issues, oil price volatility, uncertainty of petroleum product markets, project financing: in this regard, for three real GTL plants out of four, the final cost has been so far approximately three times initial budget. One additional factor is access to technology, as only a small number of companies hold many important patents (Wood et al., 2012). However, GTL faces a better situation compared to CTL, due to cheaper inputs and lower water requirements.
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