Notes from 23 page: Höök, M. & Aleklett, K. 2010. A review on coal-to-liquid fuels & its coal consumption. International journal of energy research Vol. 34 10:848-864
Annual decline in existing crude oil production is around 4-8%, equivalent to an annual production decrease of 3-7 Million barrels per day (Mb/d) .
If 10% of world coal production were diverted to CTL, only a few Mb/d could be produced.
This prevents CTL from becoming a viable mitigation plan for liquid fuel shortages on a global scale, and therefore unrealistic to claim CTL provides a feasible solution to liquid fossil fuel shortages created by peak oil when it can only be a minor contributor.
Sasol in South Africa gets one barrel of CTL synthetic fuel per 0.73- 1.04 tons of bituminous coal, i.e. a conversion ratio of 1-1.4 barrels/ton coal. This puts a strict limitation on future CTL capacity imposed by future coal production volumes.
Water will also limit volumes of CTL produced: Water is a vital part of the process, either as hot steam or as a feedstock for hydrogen production. Water for cooling and the boiler must also be provided, and for a larger plant the amount of water consumed can be very large. The water consumption for a 50,000 b/d facility with American coal would be in the region of 40,000 to 50,000 cubic meters per day . In addition, the grinding of coal and mixing it with water will consume both water and energy.
Capital will also limit CTL plants, which are very expensive 
- 20,000 b/d $ 1.5-$ 4 billion
- 80,000 b/d $ 6- $24 billion
- 1,000,000 b/d $60-$160 billion
Liquid hydrocarbon fuels can be obtained from various feedstocks, ranging from solids to gases. Coal-to-Liquids (CTL) is a technology based on the liquefaction of coal using three basic approaches; pyrolysis, direct coal liquefaction (DCL) and indirect coal liquefaction (ICL) . Gas-to-Liquids (GTL) and Biomass- toLiquids (BTL) are related options, based on feedstock other than coal. Generally, synthetic fuel properties can be made almost identical to conventional petroleum fuels.
South Africa developed CTL-technology in the 1950s during an oil blockade and CTL now plays a vital part in South Africa’s national economy, providing over 30% of their fuel demand .
The annual decline in existing crude oil production has been determined as 3-7 Mb/d . Similar production volumes would be challenging to offset, either partially or in full, by new CTL-projects.
The oldest method for obtaining liquids from coal is high temperature pyrolysis. Typically, coal is heated to around 950° C in a closed container. The heat causes decomposition and the volatile matter is driven away, increasing carbon content. This is similar to the coke-making process and accompanying tar-like liquid is mostly a side product. The process results in very low liquid yields and upgrading costs are relatively high. Coal tar is not traditionally used as a fuel in the transportation sector. However, it is used worldwide for manufacturing roofing, waterproofing and insulation products and as a raw material for various dyes, drugs and paints. Mild temperature pyrolysis uses temperatures of 450-650 °C. Much of the volatile matter is driven off and other compounds are formed through thermal decomposition. Liquid yields are higher than for high temperature pyrolysis, but reach a maximum at 20% . The main product is char, semi-coke and coke (all smokeless solid fuels). This technique has mostly been used to upgrade low-rank coals, by increasing calorific value and reducing Sulphur content.
Pyrolysis provides low liquid yields and has inherently low efficiency. Furthermore, the resulting liquids require further treatment before they can be used in existing vehicles. A demonstration plant for coal upgrading was built in the USA and was operational between 1992 and 1997 . However, there is little possibility that this process will yield economically viable volumes of liquid fuel. Consequently, further investigation and analysis of coal pyrolysis is not undertaken.
Direct coal liquefaction (DCL)
This process is built around the Bergius-process (Formula 4), where the basic process dissolves coal at high temperature and pressure. Addition of hydrogen and a catalyst causes “hydro-cracking”, rupturing long carbon chains into shorter, liquid parts. The added hydrogen also improves the H/C-ratio of the product. Liquid yields can be in excess of 70% of the dry weight coal, with overall thermal efficiencies of 60-70% [22, 23]. The resulting liquids are of much higher quality, compared to pyrolysis, and can be used unblended in power generation or other chemical processes as a synthetic crude oil (syncrude). However, further treatment is needed before they are usable as a transport fuel and refining stages are needed in the full process chain. Refining can be done directly at the CTL-facility or by sending the synthetic crude oil to a conventional refinery. A mix of many gasoline-like and diesel- like products, as well as propane, butane and other products can be recovered from the refined syncrude.
Indirect coal liquefaction (ICL)
This approach involves a complete breakdown of coal into other compounds by gasification. Resulting syngas is modified to obtain the required balance of hydrogen and carbon monoxide. Later, the syngas is cleaned, removing sulfur and other impurities capable of disturbing further reactions. Finally, the syngas is reacted over a catalyst to provide the desired product using FT-reactions (Formula 1).
In general, there are two types of FT-synthesis, a high temperature version primarily yielding a gasoline-like fuel and a low temperature version, mainly providing a diesel-like fuel . More details on FT-synthesis via ICL- technology have been discussed by others [6, 26].
The main candidates for future CTL-technology are DCL and ICL. In essence, DCL strives to make coal liquefaction and refining as similar to ordinary crude oil processing as possible by creating a synthetic crude oil. By sidestepping the complete breakdown of coal, some efficiency can be gained and the required amount of liquefaction equipment is reduced. Coal includes a large number of different substances in various amounts, several unwanted or even toxic. Some substances can poison catalysts or be passed on to the resulting synthetic crude oil. Ever-changing environmental regulations may force adjustment in the DCL process, requiring it to meet new regulatory mandates, just as crude oil processing has to be overhauled when new environmental protocols are introduced.
In comparison, ICL uses a “designer fuel strategy”. A set of criteria for the desired fuel are set up and pursued, using products that can be made in FT synthesis. Many of the various processes will yield hydrocarbon fuels superior to conventional oil derived-products. Eliminating inherent noxious materials in coals is not just an option; it is a must to protect the synthesis reactor catalysts. Far from all ICL-derived products are better than their petroleum- derived counterparts when it comes to energy content or other characteristics. Comprehensive comparison between DCL and ICL has been performed by other studies [22, 29-30]. In general, it is not easy to compare them directly, as DCL yields unrefined syncrude while ICL usually results in final products.
ICL has a long history of commercial performance, while DCL has not. Consequently, the economic behavior of a DCL-facility has only been estimated while ICL-analyses can rely on actual experience.
System efficiency. It is widely believed that DCL is more energy-efficient for making liquid fuels than ICL, justified by the simplicity of DCL’s partial breakdown compared to the complete coal reconstruction used in ICL. Several other features, like environmental impact, flexibility and reliability of process, should also be taken into account for a more complete systematic view of the technology options. The estimated overall efficiency of the DCL-process is 73% . Other groups have estimated the thermal efficiency between 60-70% [21, 30].
SHELL estimated the theoretical maximum thermal efficiency of ICL to 60% [32, 33]. The overall efficiency of ICL (making methanol or di-methyl-ether) is 58.3% and 55.1% . Tijmensen et al.  give an overall energy efficiency of ICL of about 33-50% using various biomass blends. Typical overall efficiencies for ICL are around 50%. Detailed well-to-wheel analysis of energy flows for ICL diesel has been done by van Vliet et al.  Caution must be exercised in making efficiency comparisons, because DCL efficiencies are usually for making unrefined syncrude, which requires more refining before utilization, and ICL efficiencies are often for making final products. If the refining of DCL products is taken into account, some ICL-derived fuels can be produced with higher final end-use efficiency than their DCL-counterparts . It is also sometimes unclear, whether the extra energy needed for process heat, hydrogen production, and process power is included in the analyses, making efficiency comparisons even more delicate.
CTL requires more than coal to produce usable fuel. Heat, energy, catalysts and other chemicals are necessary to maintain functioning production. Water is a vital part of the process, either as hot steam or as a feedstock for hydrogen production. Water for cooling and the boiler must also be provided, and for a larger plant the amount of water consumed can be very large indeed. Water consumption is approximately equivalent for DCL and ICL. The water consumption for a 50,000 b/d facility with American coal would be in the region of 40,000 to 50,000 cubic meters per day . Therefore, water availability is an essential factor to be considered during placement of CTL-facilities. Grinding of coal and mixing it with water will consume energy and water.
DCL or ICL refining and product upgrading requires additional heat, energy and hydrogen. This extra energy requirement is up to 10% of the energy content of the syncrude and can also be provided by coal. Additional energy must be also provided to reduce GHG and other emissions, if environmental concerns are to be taken in to account.
System costs. The capital cost of a facility is usually the largest cost, with operation/management costs coming second. The coal costs are usually around 10-20%, varying due to local supply, quality etc.
Using 40 Mt as a lower limit and 57 Mt as an upper limit for Sasol coal consumption, one can compute that one barrel of synthetic fuel consumes 0.73- 1.04 tons of bituminous coal, i.e. a conversion ratio of 1-1.4 barrels/ton coal. This agrees with the estimates of other studies, but tends to be in the lower range. Differences between technical and Sasol-derived estimates reflect disparities between theory and practice. Suboptimal conditions, losses, leaks and similar are unavoidable parts of reality, especially when performed on a large industrial scale. Including coal quality issues, refining and further treatment, also makes it reasonable to expect lower yields. Hence, the empirical Sasol conversion ratios are deemed reasonable. Similar conversion efficiencies are also realistic for future large scale CTL-industries, especially since ICL is the more likely future CTL-technology development path.
Outlooks that present CTL as a mitigation or even a solution to the problem of declining conventional oil supply will be closely inspected. For instance, the National Petroleum Council  presents a number of production forecasts, where the main message is that peak oil can be partially solved by substantial CTL- development in the USA. We intend to quantify what required coal volumes are needed to offset decline in existing crude oil production. This sheds some new light on the discussion of future CTL potentials and requirements. Furthermore, it is also useful information for policy makers when planning for the future, as the achievability of replacing oil with derivatives of another finite resource on a large scale can be disputed if sustainable development is the ambition.
Hirsch et al.  assumed annual future construction of 5 CTL-plants, each with a capacity of 100,000 b/d. No coal consumption figures or conversion ratios are given. Using Sasol experience, corresponding increase of annual coal consumption is 133-190 Mt. This is equivalent to ~2.5% the world production of coal for 2007 . This is a significant increase, but probably doable if proper investments are forthcoming. The National Coal Council , also mentioned in , foresees a production of 2.7 Mb/d by 2025 and presents 430 Mt as the corresponding coal consumption, which equals a conversion ratio of 2.3 barrels/ton coal. Using Sasol experience, coal requirement would be 700-1000 Mt, almost twice as much as the National Coal Council assumes.
In conclusion, the National Coal Council’s estimate is optimistic when compared to actual experience, and will probably require a dramatic increase in process efficiency and improved technology or use of high quality coals with excellent liquefaction properties. The National Petroleum Council  also present a CTL forecast of 5.5 Mb/d by 2030 with corresponding coal consumption of 1439 Mt, originally performed by the Southern States Energy Board . The conversion ratio is 1.4 barrels/ton, in agreement with Sasol experience, but it should be noted that the consumption figure from Southern States Energy Board  is leaning toward the optimistic side. Using the Sasol model, estimated coal consumption becomes 1466-2100 Mt, which is more than the entire current coal production of the US . This CTL forecast is entirely unrealistic, since it is not feasible to divert all coal to new CTL facilities, or to double the US coal output in 20 years [66, 67].
The Annual Energy Outlook 2007 (AEO2007) Reference Scenario features a CTL production of 2.4 Mb/d globally and 0.8 Mb/d in the USA . No coal consumption figures are provided for global CTL production, but the USA CTL industry is estimated to consume 112 Mt, which equals conversion ratio of 2.6 barrels/ton coal. It should also be noted that coal consumption for CTL has decreased 50% in AEO2007 compared to 304 Mt, which is twice as much as the EIA assumes. It should be remembered that a significant share of American coal is subbituminous coal, i.e. more low-ranking than the South African coals that Sasol utilize. In essence, the EIA must be assuming that future American CTL- industry will be twice as efficient as Sasol. Given the fact that Sasol is a world leading CTL-enterprise, the EIA assumption seems very optimistic. The Annual Energy Outlook 2009 (AEO2009) has reduced US CTL production in the Reference Scenario to only 0.26 Mb/d by 2030 . The coal consumption presented is only 24.6 Mt, which would equal a conversion ratio of 2.9 barrels/ton. Corresponding coal usage would be 68-95 Mt, using the Sasol model. Although the expected CTL capacity has been reduced, the conversion ratio has increased compared to earlier estimates and is even further away from the real numbers. We can only conclude that the conversion ratios used by EIA seem extremely high and lack any real counterpart. The EIA seems to be using purely theoretical values, rather than sound numbers derived from practical experience. AEO2007  foresees a global CTL-production of 2.4 Mb/d in the reference case, and this would annually consume 640-912 Mt of coal. This is equivalent to around 12% of the current world production of coal. AEO2009  has lowered the global CTL/GTL-production to only 1.6 Mb/d, without showing individual contributions to this figure. The reduction is justified by concern for CO2 emissions. The global CTL production in AEO2009 would require something in the range of 400-500 Mt coal annually, using the Sasol model.
Annual decline in existing crude oil production is around 4-8%, equivalent to an annual production decrease of 3-7 Mb/d .
Such massive volumes are theoretically possible to produce, but would require astronomical investments regardless of the chosen technology. Related coal usage would be 782-2555 Mt, using the Sasol model. Such vast volumes of coal cannot be realistically liquefied just to offset a single years decline in existing world oil production. Consequently, it must be asked whether the investment and the coal itself can be used more efficiently in ways other than CTL and if other mitigation strategies should be preferred.
These findings also have repercussions for future climate policies, as several of the Intergovernmental Panel on Climate Change (IPCC) emission scenarios , used for projections of temperature increases and anthropogenic emissions, depict significant contribution from CTL in the future. In the dynamic technology scenario group (A1T), liquid fuels from coal are assumed to be readily available at less than US$30/barrel with prices falling even further. The environmentally B2 scenario family sees CTL production costs decline from US$43/barrel to US$16/barrel. Details on conversion ratios are not given, nor related coal consumption volumes. As an example, the B2 Message scenario gives a global CTL production of 32 Mb/d (71.8 EJ) in 2100, which is more than the 23.2 Mb/d (52 EJ) derived from oil production in the same year. Equivalent coal consumption would be 8342-11680 Mt, using Sasol conversion ratios, and still very extensive even if better efficiencies were reached in the future. The world coal production is given as 300 EJ in 2100, meaning that 24% goes to CTL. Can so much coal be really produced and diverted to CTL in a realistic case or should some emission scenarios be revised? Either way, more details should be shown regarding assumed conversion rations, technologies and other factors. In summary, we find that many forecasts or scenarios do not discuss CTL coal consumption or conversion ratios in any detail.
The US has the world’s largest coal reserves and has been subjected to many CTL feasibility studies and projects. In 1980, Perry  pointed out that the construction of a synthetic fuels industry will be very costly and will provide only a small amount of increased energy independence. This situation has obviously not changed as Couch  states that replacing only 10% of the US transport fuel consumption with CTL would require over US$70 billion in capital investments and about a 250 Mt of annual coal production increase. Achieving required increases in coal production has been deemed questionable by other studies [66, 67]. Correspondingly, Milici  concluded that the US coal industry only could handle liquefaction of 54-64 Mt coal annually without premature depletion of the coal reserves, and states that attempts to replace all oil imports would deplete the national coal reserves by 2100. The resulting volumes of synthetic fuels are insignificant compared to the present and expected demand.
World oil production currently stands at more than 80 Mb/d . The total cost for replacing a significant amount of the world’s oil production by CTL would be astronomical, regardless of the chosen system approach. Necessary investments for a large CTL industry are evidently colossal, but the greatest issue lies perhaps in coal consumption. Coal will account for a large part of the costs, and with the required volumes being vast, accompanying changes in coal price and additional costs of increasing coal feedstock production will greatly affect the future economics of CTL. This is a topic that deserves more attention in future studies. In addition, the social and environmental impacts of large scale development of CTL must be considered. The political challenge of becoming very reliant on such a carbon dioxide-intensive fuel as coal is a major obstacle for many countries where greenhouse gas emissions are an important issue. Even if CCS and/or low emission CTL technologies are implemented, the vast required coal amounts will create serious environmental impact due to mining. Obtaining public acceptance, and later political acceptance, for CTL might become challenging because of its unavoidable environmental impact. 40% of the world coal production is required (Table 4). Clearly, this cannot be regarded as feasible in any realistic case. Even if technical efficiencies were achieved, significant shares of world coal would disappear into CTL-plants for a relatively modest contribution to world oil supply. If a 10% share of world coal production could be diverted, it would limit the CTL-production to only a few Mb/d at most. Consequently, it is unrealistic to claim that CTL provides a feasible solution to liquid fuels shortages created by peak oil. For the most part, it can only be a minor contributor and must be combined with other strategies.
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