review on coal-to-liquid fuels and its coal consumption

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2010; 34:848–864Published online 30 July 2009 in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1596

A review on coal-to-liquid fuels and its coal consumption

Mikael Hook�,y and Kjell Aleklett

Department of Physics and Astronomy, Uppsala University, Global Energy Systems, Box 535, SE-751 21,Lagerhyddsvagen 1, Sweden

SUMMARY

Continued reliance on oil is unsustainable and this has resulted in interest in alternative fuels. Coal-to-liquids (CTL) cansupply liquid fuels and have been successfully used in several cases, particularly in South Africa. This article reviews CTLtheory and technology. Understanding the fundamental aspects of coal liquefaction technologies is vital for planning andpolicy-making, as future CTL systems will be integrated in a much larger global energy and fuel utilization system.Conversion ratios for CTL are generally estimated to be between 1 and 2 barrels/ton coal. This puts a strict limitation

on future CTL capacity imposed by future coal production volumes, regardless of other factors such as economics,emissions or environmental concerns. Assuming that 10% of world coal production can be diverted to CTL, thecontribution to liquid fuel supply will be limited to only a few mega barrels per day. This prevents CTL from becominga viable mitigation plan for liquid fuel shortage on a global scale. However, it is still possible for individual nations toderive significant shares of their fuel supply from CTL, but those nations must also have access to equally significantcoal production capacities. It is unrealistic to claim that CTL provides a feasible solution to liquid fuels shortagescreated by peak oil. For the most part, it can only be a minor contributor and must be combined with other strategies.Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: coal-to-liquids; synthetic fuels; conversion ratio; peak oil mitigation

1. INTRODUCTION

The oil price has risen dramatically over the last

few years. The price of oil reached $100 per barrel

in January 2008, before rocketing to $147 barrel in

July 2008. After the dramatic price collapse in

late 2008, oil prices have now recovered to over

$60/barrel, still an historic high. In summary, there

are few signs of a return to the $20 per barrel that

was a typical price in the 1990s and continued oil

dependence is environmentally, economically and

socially unsustainable [1]. Peak oil concerns,resulting in imminent oil production limitations,have been voiced by various studies [2–5]. This hasresulted in renewed interest in alternative fuels forthe future.

Alternative liquid hydrocarbon fuels can beobtained from various feedstocks, ranging fromsolids to gases. Coal-to-liquids (CTL) is a technologybased on the liquefaction of coal using three basicapproaches: pyrolysis, direct coal liquefaction(DCL) and indirect coal liquefaction (ICL) [6].

*Correspondence to: Mikael Hook, Department of Physics and Astronomy, Uppsala University, Global Energy Systems, Box 535,SE-751 21, Lagerhyddsvagen 1, Sweden.yE-mail: [email protected]

Received 12 January 2009

Revised 22 June 2009

Accepted 23 June 2009Copyright r 2009 John Wiley & Sons, Ltd.

Gas-to-liquids (GTL) and biomass-to-liquids(BTL) are related options, based on feedstockother than coal. Generally, synthetic fuel propertiescan be made almost identical to conventionalpetroleum fuels.

CTL is one of the more reasonable approachesfor alternative liquid fuels, having already beentechnically and commercially established. TheU.S. Department of Energy report Peaking ofWorld Oil Production: Impacts, Mitigation, andRisk Management [7], the National PetroleumCouncils report Facing Hard Truths about Energy[8] and other studies, expect significant futureuse of CTL as a way to lessen the impact fromdeclining conventional oil supply.

1.1. Historical overview

CTL is an old technique, developed at the begin-ning of the 20th century and has recently attractedattention once more. Historically, it helped to fuelthe German military during two world wars. CTLprovided 92% of Germany’s air fuel and over 50%of their petroleum supply in the 1940s [9]. SouthAfrica developed CTL technology in the 1950sduring an oil blockade and CTL now plays a vitalpart in South Africa’s national economy, providingover 30% of their fuel demand [10].

The best known CTL process is Fischer–

Tropsch (FT) synthesis, named after the inventors

Franz Fischer and Hans Tropsch from the Kaiser

Wilhelm Institute in the 1920s [11]. The FT

synthesis is the basis for ICL technology. Friedrich

Bergius, also a German chemist, invented DCL

as a way to convert lignite into synthetic oil in

1913 [11]. Karrick [12] invented a low-temperature

carbonization process in the U.S. around the

1930s, as a way to produce smokeless fuel and

liquids from oil shale.CTL technologies have steadily improved since

the Second World War. Technical developmenthas resulted in a variety of systems capable ofhandling a wide array of coal types. However, onlya very small number of commercial enterprisesbased on generating liquid fuels from coal havebeen undertaken, most of them based on ICLtechnology. The most successful is the SouthAfrican company Sasol, originally created as a

way to protect the country’s balance of paymentagainst the increasing dependence on foreign oil[10,13]. A new DCL plant has recently becomeoperational in China, possibly marking thebeginning of a new era.

1.2. Aim of this study

The theory and technology behind CTL arereviewed. Understanding the fundamental aspectsof coal liquefaction technologies is vital forplanning and policy-making, as future CTL systemswill be integrated in a much larger global energyand fuel utilization system. As a result, future CTLdevelopment will be affected by existing infrastruc-ture and systems. In our overview, we attempt topoint out some likely paths for CTL developmentand indicate possibilities and limitations. Based onthe various assumptions about the future, we willproject a number of outlooks regarding dominatingCTL technology in the future.

This study also compiles various assessments ofCTL conversion ratios. Furthermore, an empiricalcoal consumption estimate is made using Sasol, theworld leading commercial CTL enterprise, as areference case. This is compared with otherestimates as a simplistic sense check, aimed toinvestigate how well future outlooks agree withpractical experience. The derived conversion ratiosare also used to estimate future coal consumptionas a function of CTL capacity.

The annual decline in existing crude oilproduction has been determined as 4–7megabarrels per day (Mb/d) [14]. Similar productionvolumes would be challenging to offset, eitherpartially or in full, by new CTL projects. However,we overview some CTL forecasts and discuss theirpotential for counteracting a decline in oil pro-duction from a global perspective. Most especially,the required coal tonnage will be discussed todetermine the feasibility of a large-scale CTLindustry as a peak oil mitigation strategy.

2. BASIC CHEMISTRY OF CTL

The basis for all types of CTL syntheses is acarbon source combined with a hydrogen source,such as steam. Chemical reactions between carbon

A REVIEW ON COAL-TO-LIQUID FUELS AND ITS COAL CONSUMPTION 849

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. 2010; 34:848–864

DOI: 10.1002/er

and other compounds will eventually fabricatehydrocarbon molecules of the desired length. Theoriginal FT process was described by Formula 1.

ð2nþ 1ÞH2 þ nCO! nH2Oþ CnH2nþ2 ð1Þ

Carbon monoxide can be produced by gasifi-cation of coal or another carbon-rich compound.The necessary reaction energy is applied by addingoxygen or steam (Formula 2).

Cþ 12O2 ! CO ð2Þ

The resulting mixture of carbon monoxide andhydrogen is usually called synthesis gas (syngas). Itis used to construct hydrocarbon chains of differ-ent lengths using condensation and a suitablecatalyst. Generally, the FT process yields twotypes of products, described by two different re-actions (Formula 3).

nCOþ 2nH2 ! nH2Oþ CnH2n ðolefinsÞ

nCOþ ð2nþ 1ÞH2 ! nH2Oþ CnH2nþ2 ðparaffinsÞ

ð3Þ

The type of resulting products depends on thecatalysts used and the reactor-operating condi-tions. Olefin-rich products with n in the range 5–10(naphtha) can be used for making synthetic gaso-line and chemicals in high-temperature FT pro-cesses. Paraffin-rich products with n in the range of12–19 are suitable for making synthetic diesel andwaxes in low-temperature FT processes.

The Bergius process is the basis of DCL.Splitting coal into shorter hydrocarbons, resem-bling ordinary crude oil is done by adding hy-drogen under high pressure and temperature, thuseliminating the need for a gaseous middle stage(Formula 4).

nCþ ðnþ 1ÞH2 ! CnH2nþ2 ð4Þ

2.1. Catalysts

Both Bergius and FT processes use differentcatalysts to aid the chemical reactions. Commoncatalysts are transition metals such as iron,ruthenium or cobalt. Transition metal sulphides,amorphous zeolite and similar compounds havealso been utilized. In general, catalysts have a largeimpact on process efficiency as well as influence

over the resulting products. Many catalysts arenotoriously sensitive to sulphur compounds orother substances, which require special treatmentand separation techniques to avoid catalystpoisoning.

Much research has been made on differentcatalysts for CTL processes and in many wayssome of the greatest chemical challenges can befound in the right choice of catalysts and the op-timization of their performance. Closer discussionson catalysts used in CTL have been done by others[15–18].

2.2. Other process problems

Liquefaction can also be affected by the differentproperties of the coal feedstock. Different types ofcoal have different properties, requiring compat-ibility between CTL-reactor design and coal feed-stock.

Hydrodynamic problems can be caused bythermal fragmentation of coal, when the coalparticles are shattered into smaller grains capableof clogging gas outlets and causing other un-wanted disturbances. Drying coal and reducingmoisture will lower the probability of thermalfragmentation.

Caking occurs due to the plasticity of coal andcauses coal particles to meld into larger cakes,leading to pressure drops and channel burning,severely reducing the performance of the CTL re-actor. Mixing high-caking coals with low-cakingcoals creates a more manageable overall cakingprobability.

High ash coals will call for a gasification designcapable of removing large ash concentrationswithout loss of performance. Practical CTL designcan be found for all forms of coal [19]. Conse-quently, suitable designs are essential for obtaininggood performance.

3. CTL TECHNOLOGY OPTIONS

Experience from many different types of CTL fuelconversion techniques exists, while only some havebeen commercialized and proven feasible byindustry. CTL technology is an old concept thathas to be fused with modern processes and

M. HOOK AND K. ALEKLETT850


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machinery to be able to fulfil the economic andenvironmental constraints of today.

Existing technological infrastructure from con-ventional crude oil processing and gas liquefactioncan provide synergistic effects reducing costs andnecessary research. Research on synthetic fuelsfrom non-coal feedstock exists in several places,where coal is too expensive or impractical toacquire for example. In theory, FT synthesis can beused to create liquid fuels from very unconven-tional feedstock as long as hydrogen and carbonare available. For instance, carbon dioxide, carbonmonoxide and other combustion exhaust havebeen used as feedstock in patent applications [20].

3.1. Pyrolysis

The oldest method for obtaining liquids from coalis high-temperature pyrolysis. Typically, coal isheated to around 9501C in a closed container. Theheat causes decomposition and the volatile matteris driven away, increasing carbon content. This issimilar to the coke-making process and accom-panying tar-like liquid is mostly a side product.

The process results in very low liquid yields andupgrading costs are relatively high. Coal tar is nottraditionally used as a fuel in the transportationsector. However, it is used worldwide for manu-facturing roofing, waterproofing and insulationproducts and as a raw material for various dyes,drugs and paints.

Mild temperature pyrolysis uses temperaturesof 450–6501C. Much of the volatile matter is dri-ven off and other compounds are formed throughthermal decomposition. Liquid yields are higherthan for high-temperature pyrolysis, but reach amaximum at 20% [21]. The main product is char,semi-coke and coke (all smokeless solid fuels). Thistechnique has mostly been used to upgradelow-rank coals, by increasing calorific value andreducing sulphur content.

The Karrick process is a low-temperature car-bonization process that also yields liquids [12].The main product is, however, semi-coke. The tarliquids produced require further refining beforethey can be used as a transportation fuel.

In summary, pyrolysis provides low liquid yieldsand has inherently low efficiency. Furthermore, the

resulting liquids require further treatment beforethey can be used in existing vehicles. A demon-stration plant for coal upgrading was built inthe U.S. and was operational between 1992 and1997 [21]. However, there is little possibilitythat this process will yield economically viablevolumes of liquid fuel. Consequently, furtherinvestigation and analysis of coal pyrolysis are notundertaken.

3.2. DCL

This process is built around the Bergius process(Formula 4), where the basic process dissolves coalat high temperature and pressure. Addition ofhydrogen and a catalyst causes hydro-cracking,rupturing long carbon chains into shorter, liquidparts. The added hydrogen also improves the H/Cratio of the product.

Liquid yields can be in excess of 70% of the dryweight coal, with overall thermal efficiencies of60–70% [22,23]. The resulting liquids are of muchhigher quality, compared with pyrolysis, andcan be used unblended in power generation orother chemical processes as a synthetic crude oil(syncrude). However, further treatment is neededbefore they are usable as a transport fuel and re-fining stages are needed in the full process chain.Refining can be done directly at the CTL facility orby sending the synthetic crude oil to a conven-tional refinery. A mix of many gasoline-like anddiesel-like products, as well as propane, butaneand other products can be recovered from the re-fined syncrude.

Some smaller pilot-plants and testing facilitieshave provided positive results. In 2002, theShenhua Group Corporation, the largest state-owned mining company in China, was taskedwith designing and constructing the world’s firstDCL commercial plant in Inner MongoliaAutonomous Region [24], which recently becameoperational.

3.3. ICL

This approach involves a complete breakdown ofcoal into other compounds by gasification. Result-ing syngas is modified to obtain the requiredbalance of hydrogen and carbon monoxide. Later,



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the syngas is cleaned, removing sulphur and otherimpurities capable of disturbing further reactions.Finally, the syngas is reacted over a catalyst toprovide the desired product using FT reactions(Formula 1).

Alteration of catalysts and reaction conditionscan create a wide array of different products(Figure 1). For instance, methanol is one possibleproduct that can be produced directly or furtherconverted into high-quality gasoline via the Mobilprocess in additional stages [25]. In general, thereare two types of FT synthesis, a high-temperatureversion primarily yielding a gasoline-like fueland a low-temperature version, mainly providinga diesel-like fuel [26]. More details on FT synthesisvia ICL technology have been discussed byothers [6,26].

Sasol in South Africa owns the only commer-cial-scale ICL plants currently in operation withwell established and proven technology andtogether with a lot of operational experience [10].In total, Sasol has over 50 years of experienceof ICL and has produced over 1.5 billion barrels ofsynthetic oil during its existence [21]. A number ofdifferent ICL technologies have been developed bySasol; the oldest ones date from the 1950s andwas used to late 1980s. Today, advanced technol-ogies from the 1990s are utilized, including theSasol Advanced Synthol High-Temperature FT

synthesis and the Sasol Slurry Phase DistillateLow-Temperature FT-synthesis [13].

3.4. Comparison of DCL and ICL

The main candidates for future CTL technologyare DCL and ICL. In essence, DCL strives tomake coal liquefaction and refining as similar toordinary crude oil processing as possible bycreating a synthetic crude oil. By sidestepping thecomplete breakdown of coal, some efficiency canbe gained and the required amount of liquefactionequipment is reduced.

Coal includes a large number of different sub-stances in various amounts, several unwanted oreven toxic. Some substances can poison catalystsor be passed on to the resulting synthetic crude oil.Ever-changing environmental regulations mayforce adjustment in the DCL process, requiring itto meet new regulatory mandates, just as crudeoil processing has to be overhauled when newenvironmental protocols are introduced.

In comparison, ICL uses a designer fuelstrategy. A set of criteria for the desired fuel are setup and pursued, using products that can be madein FT synthesis. Many of the various processes willyield hydrocarbon fuels superior to conventionaloil-derived products. Eliminating inherent noxiousmaterials in coals is not just an option; it is a must

Figure 1. Possible uses of synthesis gas from ICL. Based on [8].



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to protect the synthesis reactor catalysts. Far fromall ICL-derived products are better than theirpetroleum-derived counterparts when it comes toenergy content or other characteristics. However,all ICL fuels are inherently clean and virtually freefrom nitrogen, sulphur and aromatics, generallygiving lower emissions when combusted [27,28].

Comprehensive comparison between DCL andICL 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 ICLusually results in final products. ICL has a longhistory of commercial performance, while DCL hasnot. Consequently, the economic behaviour of aDCL facility has only been estimated while ICLanalyses can rely on actual experience.

3.5. System efficiency

It is widely believed that DCL is more energy-efficient for making liquid fuels than ICL, justified bythe simplicity of DCL’s partial breakdown comparedwith the complete coal reconstruction used in ICL.Several other features, such as environmental impact,flexibility and reliability of process, should also betaken into account for a more complete systematicview of the technology options.

The estimated overall efficiency of the DCLprocess is 73% [31]. Other groups have estimatedthe thermal efficiency between 60 and 70% [21,30].SHELL estimated the theoretical maximumthermal efficiency of ICL to 60% [32,33]. Theoverall efficiencies of ICL (making methanol ordi-methyl-ether) is 58.3 and 55.1% [30]. Tijmensenet al. [34] give an overall energy efficiency of ICLof about 33–50% using various biomass-blends.Typical overall efficiencies for ICL are around50%. Detailed well-to-wheel analysis of energyflows for ICL diesel has been done by van Vlietet al. [35].

Caution must be exercised in making efficiencycomparisons, because DCL efficiencies are usuallyfor making unrefined syncrude, which requiresmore refining before utilization, and ICL effi-ciencies are often for making final products. Ifthe refining of DCL products is taken intoaccount, some ICL-derived fuels can be producedwith higher final end-use efficiency than their

DCL-counterparts [30]. It is also sometimes un-clear, whether the extra energy needed for processheat, hydrogen production, and process power isincluded in the analyses, making efficiency com-parisons even more delicate.

3.6. Process requirements

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 a 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 40 000–50 000m3 day�1 [36]. Therefore, water

availability is an essential factor to be considered

during placement of CTL facilities. Grinding of

coal and mixing it with water are another process

steps that will consume energy and water.The DCL system requires hydrogen to crack the

coal into syncrude. This hydrogen is the mostcostly part of the DCL system. High-efficiencydesigns often acquire hydrogen from steam re-forming of natural gas, but DCL systems can alsobe modified to produce hydrogen from coal by so-called water—gas shift reactions. Necessary heatprocess for obtaining syncrude is usually providedby coal.

ICL utilizes huge amounts of steam to breakdown coal into syngas, requiring substantialenergy input. Treatment and purification of thesyngas is necessary for protecting the catalysts.This usually involves gas cooling and differentseparation stages, all necessitating additionalenergy. However, some of this energy can beproduced from sulphur and other compoundsseparated out from the syngas in the recyclingprocesses. Some ICL configurations actuallygenerate more electricity than they consume byconverting excess heat into electricity [30].

Finally, DCL or ICL refining and productupgrading requires additional heat, energy andhydrogen. This extra energy requirement is up to



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10% of the energy content of the syncrude and canalso be provided by coal. Additional energy mustbe also provided to reduce GHG and other emis-sions, if environmental concerns are to be taken into account.

3.7. System costs

The capital cost of a facility is usually the largestcost, with operation/management costs comingsecond. The coal costs are usually around 10–20%,varying due to local supply, quality, etc.

ICL plants in the U.S. provide break-even crudeoil-prices in the range of 25–40 U.S.$ per barrel,dependent on environmental measures (such asCO2 capture) undertaken [30]. Older and moremodest studies claim break-even crude oil pricesaround U.S.$35 per barrel [37]. Liquid fuel costsfor a Chinese DCL facility have been estimated ataround U.S.$24/barrel [38]. The development ofcoal prices and the economic situation in recentyears has influenced break-even prices. The mostrecent study of CTL costs available, suggestsa break-even price of 48–75 U.S.$/barrel [39].Expected costs for ICL and DCL do not seem todiffer much and can be assumed as virtuallyidentical. Table I lists some estimated costsfor construction in the U.S. of three different ca-pacities.

The DCL facility in Inner Mongolia in Chinahas an overall cost in the order of U.S.$4billion [41]. Sasol and China were planning twoadditional 80 000b/d ICL plants in Shaanxi(650 km west from Beijing) and Ningxia (1000 kmwest from Beijing), with U.S.$5 billion as estimatedcapital cost per plant [42]. Currently, only twoprojects are approved and the Chinese NationalDevelopment and Reform Commission suspendedall other CTL projects in September 2008 [41].

3.8. Emission properties

The low sulphur content of CTL products

compared with petroleum-derived fuels is a com-

mon trait for both DCL and ICL, which comes

from the necessity to protect catalysts from

poisoning. Aside from this similarity, emissions

and combustion characteristics of DCL and ICL

fuels differ. Comprehensive analysis of emission

characteristics of synthetic and conventional fuels

has been compared by others [43,44].DCL products are typically rich in polycyclic

aromatics and heteroatoms [45–48], while ICL haslower aromatics content. High-temperature FTsynthesis yields branched products and containsaromatics, whereas these are virtually absent inlow-temperature FT synthesis [49]. Recently, en-vironmental regulation trends have moved to-wards limiting the aromatic content intransportation fuels [30], giving the advantage toICL fuels.

Toxic trace metals and inorganic compounds,such as cadmium, selenium, arsenic, lead andmercury, can be passed on to the final fuel productin both DCL and ICL processes. In ICL systems,removal of mercury and other metals is generallytrivial and inexpensive [30]. For DCL, however, itwill be more complicated and more costly, but notimpossible.

Cetane and octane numbers also differ, result-

ing from the chemical properties of the various

products. ICL gives diesel of a high quality, which

is mostly due to the dominance of straight-chain

products. However, low densities are a problem

for ICL products, but this can be mitigated by

blending [48]. General differences of final products

are summarized in Table II. Typical properties for

specific ICL distillates can be found in Leckel [48].Both DCL and ICL fuels emit large amounts of

carbon dioxide compared with ordinary petro-leum-derived fuels. However, there are methodsfor reducing or even neutralizing emissions with-out raising production costs drastically. Sub-stantial differences exist between DCL and ICLtechnologies with regard to the potential and costof greenhouse gas emission mitigation [30].

Vallentin [39] concludes that DCL generatesabout 90% more CO2 than conventional fuel on a

Table I. Estimated costs of CTL-industries in the U.S.

One 20 000b/d plant

One 80 000b/d plant

One Mb/dIndustry

Capitalinvestment

$1.5–$4billions

$6–$24billions

$60–$160billions

�The costs for emission reductions are not included, nor aregovernment grants and funding. Source: [40].



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well-to-wheel basis. This is in agreement withother studies, but if reduction measures areimplemented, the emissions could be reduced to nomore than 30% extra compared with conventionalpetroleum fuels [30].

ICL technology generates approximately80–110% more CO2 emissions compared withconventional fuels, if the CO2 is vented [30,39].However, there are ICL system configurationswhere H2S1CO2 co-capture/co-storage can reduceemissions [30]. Well-to-wheel analysis has shownthat even with CCS, CTL production chain emis-sions are higher than for petroleum-derived fuels,mostly due to emissions from mining [35].

In summary, CTL fuels can improve emissioncharacteristics and reduce transportation emis-sions of sulphur, aromatics, NOx and particlescompared with conventional fuels [43]. However,there does not seem to be much potential for CO2

emission reductions if the full supply chain isanalysed for either DCL or ICL.

3.9. Infrastructure

Industrialized countries have generally discardedDCL technologies for making synthetic fuelsand focused more on ICL or GTL [30]. Conse-quently, DCL systems generally require newtechnology, with the important exception of theH2-production [30]. Scaling up current DCL plantsto a globally significant fuel industry is a daunting

task, but not impossible if proper investments anddevelopments are pursued.

In comparison, commercialization of ICL tech-nology began in 1934 [48], and have been con-tinuously redesigned and developed ever since. ICLsystems only require a relatively small amount of newtechnology as most components are already estab-lished. Similarities exist with GTL technologies andthe chemical industries based on syngas derived fromgasification [48], making future ICL developmentseasier. This common technical ground and experienceprovides a solid base for expansion into new ICLprojects. Realizing those and scaling up existingcapacity could still prove to be a significant challenge.

In conclusion, the existing support infra-structure for ICL seems stronger than for DCL.Worldwide gasification capacity indicates thatsupporting gasification solutions are common forproducing chemicals, but FT fuel is increasing itsshare [50]. However, DCL can provide importantnew feedstock for refineries, as conventional crudeoil begins to become scarce in the wake of peak oil.

3.10. Transition properties

CTL systems must also be able to cope with an

uncertain future. Stricter environmental regula-

tions, increased demand for liquid fuels, energy

and electricity together with energy security

questions are important factors that can greatly

impact any technology. Williams and Larson [30]

have attributed to DCL poor prospects for making

high-quality diesel, which makes it an unsuitable

candidate for energy-efficient end-use technolo-

gies. Furthermore, fuel flexibility, such as being

able to blend the coal feedstock with biomass, tar

sand or waste material, is also an advantage worth

considering. FT synthesis also generates naphtha

as a by-product, which can be a valuable feedstock

for the chemical industries and allowing for

further business opportunities [35].One possible future outcome is the so-called

hydrogen society, where hydrocarbon fuels arephased out and replaced with hydrogen fuelcells. If the hydrogen scenario is taken intoaccount, major differences can be seen. DCLconsumes hydrogen during cracking stage and it isimpossible to obtain pure hydrogen in the DCL

Table II. Typical properties of DCL and ICL finalproducts.

DCL ICL

Distillable product mix 65% diesel,35% naphtha

80% diesel,20% naphtha

Diesel cetane number 42–47 70–75Diesel sulphur content o5 ppm o1 ppmDiesel aromatics 4.8% o4%Diesel specific gravity 0.865 0.780Naphtha octanenumber (RON)

4100 45–75

Naphtha sulphurcontent

o0.5 ppm Nil

Naphtha aromatics 5% 2%Naphtha specificgravity

0.764 0.673

�Modified from: [49].



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process. In comparison, ICL can produce hydro-gen and the modifications needed to switch pro-duction from hydrocarbons to hydrogen arerelatively minor [35].

Methanol, ethanol or other similar fuels areother alternative fuel possibilities with acceptableefficiencies [51], resulting in other transitions.Methanol, DME and many other fuels can moreeasily be produced by ICL than DCL. However,DCLs similarity to conventional petroleum pro-cessing should not be overlooked as it can providea means to mitigate shortages in conventionalcrude oil without having to construct entirely newsystems for new transportation fuels. Syncrudefrom DCL does offer the possibility of prolongingthe life and usability of current infrastructure asconventional petroleum supply decreases.

Another possible future is based on electrifica-

tion and the corresponding transition will be from

hydrocarbon fuels to electricity. Anthropogenic

climate change mitigation strategies request re-

ductions or eliminations of CO2 emissions, which

favour CCS technology. The CCS-capable power

plant design known as Integrated Gasification

Combine Cycle (IGCC) has many similarities to

ICL technology and is one of the flagships of

future clean coal technology [52]. In comparison,

DCL lacks any strong connections to future

coal-fired power generation. The IGCC concept

combined with ICL can generate various fuels,

chemical products, heat and electricity at the same

time and this would make it possible for coal to

play a major role in a climate and otherwise

environmentally constrained world [30].

4. SUMMARY AND FUTURE OUTLOOK

The two candidates for CTL are DCL and ICLtechnologies; the fraction of liquids obtained frompyrolysis is simply too small and of too lowquality. Energy-efficiency differences exist, but ifend-use is taken into account these differences willdecrease and become relatively minor. Fundamen-tal characteristics do not favour any of theapproaches, and feasibility is largely dependenton the future society and energy system that CTLshould be integrated into.

ICL fuels, especially ICL diesel, are generallycleaner and can outperform many DCL fuels andconventional fuels in terms of emissions. It is alsoeasier to implement CCS and GHG emission re-duction in ICL plants compared with DCL. Inessence, ICL technology will generally put the coalenergy system on a track more dedicated to en-vironmental concern, while DCL does not offerthis possibility to the same extent.

ICL offers more variable systems, capable ofproducing many more products than DCL sys-tems, especially in polygeneration designs [30]. Theexisting infrastructure supports further ICL de-velopments to a much larger extent than DCL.Furthermore, ICL is capable of dealing with morefuture outcomes and has generally better transi-tion properties. In comparison, DCL is a lessflexible transition technology with stronger ties tothe present conventional fuel system layout.However, this can also be an advantage asDCL can be used to maintain existing infra-structure as conventional crude oil becomesincreasingly scarce.

There is some interest in CTL technologyaround the world, especially in China. However, allbut two CTL projects were recently suspended [41].The objective was initially to produce 10Mtonannually of crude oil equivalents by 2010 fromdomestic coal and CTL technology and total out-put was expected to rise to 30 million tons of crudeoil equivalents by 2020, approximately 16% ofChina’s present crude oil production [53]. Cur-rently, China is reconsidering this plan and thefuture path remains uncertain.

The Alliance for Synthetic Fuels in Europe(ASFE) is a collaboration of DaimlerChrysler,Renault, Royal Dutch Shell, Sasol-Chevron,Volkswagen and Toyota, trying to promote GTL,BTL and CTL technology development in Europe.ASFE [54] sees synthetic fuels as a vital part ofdeveloping a less polluting vehicle park togetherwith creating energy security that enables trans-portation in to the future. However, the actualnumber of CTL projects in Europe is low. Mostlysmall-scale or laboratory-scale projects in lique-faction chemistry.

Recent interest in CTL technology has grown asa reaction to increased American dependence on



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imported oil. Several major American coal com-panies have expressed interest in CTL technologyor created blueprints and studies for CTL plants[55,56]. The U.S. military has expressed concernover the dependence on foreign oil and studiedalternatives derived from CTL or possiblyBTL [57,58]. Feasibility studies and evaluationprograms are being pursued, but no full-scaleprogram has yet been implemented. Uncertaintyabout the cost and performance of CTL plants,uncertainty about the future course of the oil priceand CO2 emission properties are three majorobstacles for future development [48].

5. COAL CONSUMPTION OF CTL

The consumed amount of coal in CTL is oftenoverlooked or just briefly discussed in manystudies. Conceptually, coal is a finite resourceand this puts limitations to the amount of fuelsthat can be produced by liquefying coal. Practicaldetails regarding coal supply, such as accessibility,transportation and production will impact CTLfeasibility.

Many estimates of coal consumption by CTL

have been performed in the literature. Couch [22]

and Malhotra [59] state yields of approximately

three barrels of unrefined syncrude per ton of

bituminous coal for DCL, with less efficiency for

low-rank coal. The Monash Energy CTL project

aims to produce liquid fuels, using 1.2 ton lignite

per barrel [60]. Milici [61] gives conversion ratios

of 1.3–1.8 barrels per ton bituminous coal, also

mentioning lower yields for lower coal ranks.

National Petroleum Council [8] has compiled

other American studies and gives conversion rates

ranging from 1 to 2 barrels/ton of coal. However,

liquid yield comparisons are tricky, as yield is

dependent on the chosen technical system, the coal

type used, system borders and many other factors.

Despite differences in methodologies, all estimates

of CTL coal consumption end up at approximately

the same figures.Sasol can be used to establish an empirical

estimate of the coal consumption of CTL, sincethey are the world’s leading CTL producer. TheSecunda site consists of two CTL plants with a

combined capacity of 150 000 b/d and more than40 million tons of coal per year is consumed [10]. In2003, the South African synthetic fuel industryconsumed 24% of all coal produced in SouthAfrica [62], since Sasol’s CTL facilities are the onlyproducer of synthetic fuels in South Africa, thismust also reflect their coal consumption(Figure 2). South African coal production was238Mton that year [63], and consequently, the coalconsumption of the CTL sector was 57Mton. AllSouth African coal is classified as bituminous [63].

Using 40Mton as a lower limit and 57Mton asan upper limit for Sasol coal consumption, one cancompute that one barrel of synthetic fuel consumes0.73–1.04 tons of bituminous coal, i.e. a conver-sion ratio of 1–1.4 barrels/ton coal. This agreeswith the estimates of other studies, but tends to bein the lower range. Differences between technicaland Sasol-derived estimates reflect disparitiesbetween theory and practice. Suboptimal condi-tions, losses, leaks and similar are unavoidableparts of reality, especially when performed ona large industrial scale. Including coal qualityissues, refining and further treatment, also makesit reasonable to expect lower yields. Hence, the

Figure 2. South African coal consumption in 2003divided into parts.



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empirical Sasol conversion ratios are deemed rea-sonable. Similar conversion efficiencies are alsorealistic for future large-scale CTL industries,especially since ICL is the more likely future CTLtechnology development path.

5.1. Coal consumption in various CTL forecasts

Any CTL production forecast must be related tocoal consumption. Some CTL forecasts do notmention corresponding coal consumption, whileothers present estimated consumption volumes.We will use the Sasol-analogy and comparewith other studies as a simple sense check, toinvestigate how well estimates agree with practicalexperience.

Outlooks that present CTL as a mitigation oreven a solution to the problem of declining con-ventional oil supply will be closely inspected. Forinstance, the National Petroleum Council [8] pre-sents a number of production forecasts, where themain message is that peak oil can be partiallysolved by substantial CTL development in theU.S. We intend to quantify what required coalvolumes are needed to offset decline in existingcrude oil production. This sheds some new light onthe discussion of future CTL potentials andrequirements. Furthermore, it is also usefulinformation for policy makers when planning forthe future, as the achievability of replacing oil withderivatives of another finite resource on a largescale can be disputed if sustainable development isthe ambition.

Hirsch et al. [7] assumed annual future con-struction of five CTL plants, each with a capacityof 100 000 b/d. No coal consumption figures orconversion ratios are given. Using Sasol experi-ence, corresponding increase of annual coal con-sumption is 133–190Mton. This is equivalentto �2.5% the world production of coal for2007 [63]. This is a significant increase, but prob-ably doable if proper investments are forthcoming.

The National Coal Council [64], also mentionedin [8], foresees a production of 2.7Mb/d by 2025and presents 430Mton as the corresponding coalconsumption, which equals a conversion ratio of2.3 barrels/ton coal. Using Sasol experience, coalrequirement would be 700–1000Mton, almost

twice as much as the National Coal Council as-sumes. In conclusion, the National Coal Council’sestimate is optimistic when compared with actualexperience, and will probably require a dramaticincrease in process efficiency and improved tech-nology or use of high-quality coals with excellentliquefaction properties.

The National Petroleum Council [8] also pre-sent a CTL forecast of 5.5Mb/d by 2030 withcorresponding coal consumption of 1439Mton,originally performed by the Southern StatesEnergy Board [65]. The conversion ratio is 1.4barrels/ton, in agreement with Sasol experience,but it should be noted that the consumption figurefrom Southern States Energy Board [65] is leaningtowards the optimistic side. Using the Sasolmodel, estimated coal consumption becomes1466–2100Mton, which is more than the entirecurrent coal production of the U.S. [63]. In sum-mary, we can conclude that this CTL forecast isentirely unrealistic, since it is not feasible to divertall coal to new CTL facilities, or to double the U.S.coal output in 20 years [66,67].

The Annual Energy Outlook 2007 (AEO2007)Reference Scenario features a CTL production of2.4Mb/d globally and 0.8Mb/d in the U.S. [68].No coal consumption figures are provided forglobal CTL production, but the U.S. CTL in-dustry is estimated to consume 112Mton, whichequals conversion ratio of 2.6 barrels/ton coal. Itshould also be noted that coal consumption forCTL has decreased 50% in AEO2007 comparedwith AEO2006. Applying the Sasol model, esti-mated annual coal consumption would be213–304Mton, which is twice as much as the EIAassumes. It should be remembered that a sig-nificant share of American coal is sub-bituminouscoal, i.e. more low ranking than the South Africancoals that Sasol utilize. In essence, the EIA mustbe assuming that future American CTL industrywill be twice as efficient as Sasol. Given the factthat Sasol is a world leading CTL enterprise, theEIA assumption seems very optimistic and onlyvaguely justifiable.

The Annual Energy Outlook 2009 (AEO2009)has reduced U.S. CTL production in the ReferenceScenario to only 0.26Mb/d by 2030 [69]. The coalconsumption presented is only 24.6Mton, which



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would equal a conversion ratio of 2.9 barrels/ton.Corresponding coal usage would be 68–95Mton,using the Sasol model. Although the expected CTLcapacity has been reduced, the conversion ratiohas increased compared with earlier estimates andis even further away from the real numbers. Wecan only conclude that the conversion ratios usedby EIA seem extremely high and lack any realcounterpart. The EIA seems to be using purelytheoretical values, rather than sound numbersderived from practical experience.

AEO2007 [68] foresees a global CTL produc-tion of 2.4Mb/d in the reference case, and thiswould annually consume 640–912Mton of coal.This is equivalent to around 12% of the currentworld production of coal. AEO2009 [69] has low-ered the global CTL/GTL production to only1.6Mb/d, without showing individual contribu-tions to this figure. The reduction is justified byconcern for CO2 emissions. The global CTL pro-duction in AEO2009 would require something inthe range of 400–500Mton coal annually, usingthe Sasol model.

Annual decline in existing crude oil productionis around 4–8%, equivalent to an annual produc-tion decrease of 3–7Mb/d [14]. Such massive vo-lumes are theoretically possible to produce, butwould require astronomical investments regardlessof the chosen technology. Related coal usagewould be 782–2555Mton, using the Sasol model.Such vast volumes of coal cannot be realisticallyliquefied just to offset a single years decline inexisting world oil production. Consequently, itmust be asked whether the investment and the coalitself can be used more efficiently in ways otherthan CTL and if other mitigation strategies shouldbe preferred.

These findings also have repercussions for fu-ture climate policies, as several of the Inter-governmental Panel on Climate Change (IPCC)emission scenarios [70], used for projections oftemperature increases and anthropogenic emis-sions, depict significant contribution from CTL inthe future. In the dynamic technology scenariogroup (A1T), liquid fuels from coal are assumed tobe readily available at less than U.S.$30/barrelwith prices falling even further. The en-vironmentally B2 scenario family sees CTL

production costs decline from U.S.$43/barrelto U.S.$16/barrel. Details on conversion ratiosare not given, nor related coal consumptionvolumes.

As an example, the B2 Message scenario givesa global CTL production of 32Mb/d (71.8 EJ)in 2100, which is more than the 23.2Mb/d(52EJ) derived from oil production in the sameyear. Equivalent coal consumption would be8342–11 680Mton, using Sasol conversion ratios,and still very extensive even if better efficiencieswere reached in the future. The world coal pro-duction is given as 300EJ in 2100, meaning that24% goes to CTL. Can so much coal be reallyproduced and diverted to CTL in a realistic case orshould some emission scenarios be revised? Eitherway, more details should be shown regardingassumed conversion rations, technologies andother factors.

In summary, we find that many forecasts orscenarios do not discuss CTL coal consumption orconversion ratios in any detail. In some cases, ac-tual numbers are given but they are often veryoptimistic compared with practical experience orpeer-viewed literature. Our sense check seems toindicate that several CTL outlooks have pooragreement with practical experience and empiricaldata. Scenarios and guidelines for future planningshould not be use such vaguely justified numbersor assumed conversion ratios.

5.2. Coal consumption summary

Using the empirical Sasol estimate and estimatesfound in the literature, it is possible to establishfour different conversion ratios. We have chosento set the low conversion ratio at 1 barrel/ton, themean value at 1.5 barrels/ton and the high estimateat 2 barrels/ton. This is in agreement withboth Sasol empirical data and other studies. Thetechnical CTL conversion ratio is assumed at3 barrels/ton coal, based on Couch [22] andMalhotra [59]. The results are summarized inTables III and IV. The approximate coal con-sumption for an arbitrary CTL capacity is shownin Figure 3.

Conversion ratios are of significant importanceas current world coal production can give



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everything from 17–53Mb/d in equivalentCTL capacity (Figure 3). However, empiricalexperience from Sasol indicates that realisticconversion ratios are in the order of 1–1.5 barrels/ton coal. This puts a strict limitation onfuture CTL capacity imposed by future coal pro-duction volumes, regardless of other factorssuch as economics, emissions or environmentalconcerns.

Future CTL production will ultimately be lim-ited by coal production and the share that can bediverted to liquefaction. How large this share canbecome is dependent on alternatives and priorities,but assuming that 10% of world coal productioncan be diverted to CTL, the contribution to liquid

fuels supply will be limited to only a few Mb/d(Figure 3). This prevents CTL from becoming aviable mitigation plan for liquid fuel shortages ona global scale.

However, it is still possible for individualnations to derive significant shares of their fuelsupply from CTL, but those nations mustalso have access to equally significant coal pro-duction capacities. The worlds coal reserves areunevenly distributed with the vast majority locatedin a small number of countries [63]. Combinedwith the fact that most coal is consumed in thesame country as it was produced due to un-favourable long distance transportation proper-ties, the number of countries that can sustain

Table III. Summary of forecasted CTL capacities and their estimated coal consumption.

CapacityAnnual consumption (Mton)

Scenario (Mb/d) Low Mean High Tech.

Sasol synfuel 0.15 54.8 36.5 27.4 18.3Hirsch 0.50 182.5 121.7 91.3 60.8NCC 2025 2.60 949.0 632.7 474.5 316.3SSEB 2025 5.50 2007.5 1338.3 1003.8 669.2EIA U.S. Ref 2007 0.80 292.0 194.7 146.0 97.3EIA U.S. High 2007 1.70 620.5 413.7 310.3 206.8EIA Global Ref 2007 2.40 876.0 584.0 438.0 292.0EIA U.S. Ref 2009 0.26 94.9 63.3 47.5 31.64% Decline 3.00 1095.0 730.0 547.5 365.08% Decline 7.00 2555.0 1703.3 1277.5 851.7

Table IV. Summary of forecasted CTL capacities and their estimated coal consumption in four cases as shares of worldcoal production in 2007.

CapacityShare of world coal production 2007

Scenario (Mb/d) Low (%) Mean (%) High (%) Tech. (%)

Sasol Synfuel 0.15 0.86 0.57 0.43 0.29Hirsch 0.50 2.85 1.90 1.43 0.95NCC 2025 2.60 14.84 9.89 7.42 4.95SSEB 2025 5.50 31.39 20.93 15.69 10.46EIA U.S. Ref 2007 0.80 4.57 3.04 2.28 1.52EIA U.S. High 2007 1.70 9.70 6.47 4.85 3.23EIA Global Ref 2007 2.40 13.70 9.13 6.85 4.57EIA U.S. Ref 2009 0.20 1.48 0.99 0.74 0.494% Decline 3.00 17.12 11.41 8.56 5.718% Decline 7.00 39.95 26.63 19.97 13.32



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significant CTL projects in the future becomesquite limited.

6. CONCLUSIONS

ICL seems to be the more likely option for futureCTL projects, based on its higher flexibility, betterenvironmental capabilities and stronger support-ing experience and infrastructure. Furthermore,the fuel properties seem to benefit ICL comparedwith DCL, especially if end-use efficiencies areconsidered instead of just process efficiencies.Estimated costs between the two system typesseem similar and do not favour either approach.However, more meticulous economic studies arerequired for a comprehensive discussion but thelack of commercial DCL experience is proble-matic.

Estimates for coal consumption of coal lique-faction have been presented in many studies[8,22,59,61]. A pragmatic estimate can be derivedfrom the Sasol experience, and used as a com-plementary approximation in addition to morepurely technical assessments. This estimate is

further justified by the likelihood of ICL asthe primary candidate for future large-scale CTLindustries. The differences between coal consump-tion estimated from Sasol experience and otherassessments are small. Generally, CTL conversionratios are in the order of 1–2 barrels/ton coal.

Comparing empirical coal consumption esti-mates and conversion ratios with various CTLforecasts gives a reasonable agreement, althoughEIA [68] and SSEB [65] show significant optimismwithout more than vague justifications. In general,many future CTL scenarios assume conversionratios much higher than Sasol, thus resulting insignificantly lower coal consumption. This biasmight be a case of questionable optimism or per-haps even a result of wishful thinking. Anotherpossible explanation is that certain parts of theprocess, for instance heating process water andproducing process heat, have been omitted.

In our compilation and analysis, we find thatthe coal consumption is a major factor for CTLfeasibility. Significant CTL production requiresequally significant coal production and resources.We anticipate that only a few countries or regions

Figure 3. Estimated coal consumption as a function of CTL capacity for four different conversion rations ranging from1 to 3 barrels/ton. Current world coal production can be converted to 17–54Mb/d, depending on assumed conversion

ratio. However, practical experience indicates that the low or mean cases are the most realistic.



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can realistically develop a large-scale CTL in-dustry. Effectively, CTL will be limited to thedominating coal reserve holders that can divertshares of their production to liquefaction.

The U.S. has the world’s largest coal reservesand has been subjected to many CTL feasibilitystudies and projects. In 1980, Perry [71] pointedout that the construction of a synthetic fuelsindustry will be very costly and will provide only asmall amount of increased energy independence.This situation has obviously not changed asCouch [22] states that replacing only 10% of theU.S. transport fuel consumption with CTL wouldrequire over U.S.$70 billion in capital investmentsand about a 250Mton of annual coal productionincrease. Achieving required increases in coalproduction has been deemed questionable by otherstudies [66,67]. Correspondingly, Milici [61] con-cluded that the U.S. coal industry only couldhandle liquefaction of 54–64Mton coal annuallywithout premature depletion of the coal reserves,and states that attempts to replace all oil importswould deplete the national coal reserves by 2100.The resulting volumes of synthetic fuels are insig-nificant compared with the present and expecteddemand.

World oil production currently stands at morethan 80Mb/d [63]. The total cost for replacing asignificant amount of the world’s oil production byCTL would be astronomical, regardless of thechosen system approach. Necessary investmentsfor a large CTL industry are evidently colossal, butthe greatest issue lies perhaps in coal consumption.Coal will account for a large part of the costs, andwith the required volumes being vast, accom-panying changes in coal price and additionalcosts of increasing coal feedstock production willgreatly affect the future economics of CTL. This isa topic that deserves more attention in futurestudies.

In addition, the social and environmentalimpacts of large-scale development of CTL mustbe considered. The political challenge of becomingvery reliant on such a carbon dioxide-intensive fuelas coal is a major obstacle for many countrieswhere greenhouse gas emissions are an importantissue. Even if CCS and/or low-emission CTLtechnologies are implemented, the vast required

coal amounts will create serious environmentalimpact due to mining. Obtaining public accep-tance, and later political acceptance, for CTLmight become challenging because of its unavoid-able environmental impact.

In order to offset decline in existing oil pro-duction for just 1 year, around 10–40% of theworld coal production is required (Table IV).Clearly, this cannot be regarded as feasible in anyrealistic case. Even if technical efficiencies wereachieved, significant shares of world coal woulddisappear into CTL plants for a relatively modestcontribution to world oil supply. If a 10% share ofworld coal production could be diverted, it wouldlimit the CTL production to only a few Mb/d atmost. Consequently, it is unrealistic to claim thatCTL provides a feasible solution to liquid fuelsshortages created by peak oil. For the most part, itcan only be a minor contributor and must becombined with other strategies.

NOMENCLATURE

b/d 5 barrels per dayBTL 5 biomass-to-liquidsCTL 5 coal-to-liquidsDCL 5 direct coal liquefactionFT 5 Fischer–TropschGTL 5 gas-to-liquidsICL 5 indirect coal liquefaction

ACKNOWLEDGEMENTS

The authors thank Dr Herbert West for providinginspiration. Simon Snowden at the Liverpool Universityhas our sincerest gratitude for assisting with proof-reading and valuable comments. They also thank thereviewers for very helpful remarks that greatly improvedthis article.

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DOI: 10.1002/er

review on coal-to-liquid fuels and its coal consumption

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