[footnoteRef:1] [1: The date on which we submitted the paper for review is on 2nd Feb 2013. This work was supported in part by the Department of Petroleum Technology, Sri Aditya Engineering College, Surampalem.T.Awinash is with Sri Aditya Engineering College, Surampalem 5334337(e-mail:tadiawinash@gmail.com). I.Rohit was with Sri Aditya Engineering College, Surampalem 5334337(e-mail: rohitimandi@gmail.com).]

Coal to Liquid FuelsT.Avinash, Petroleum Technology, and I.Rohit, Petroleum Technology Jr., Members, SPEAbstractcontinued trust on oil is unsustainable and this has resulted in attention of alternative fuels. Coal to Liquids (CTL) can produce liquid fuels and have been successfully used in several cases as alternative for oil. This paper reviews CTL theory, process, and technology. Converting coal into a liquid fuel (coal liquefaction) is a tactile solution to the pressures felt by countries dependent on oil imports, adversely affected by high oil prices and having large reserves of coal.Conversion ratios for CTL are generally in the range of 1-2 barrels/ton coal. This results in strict limitation on future CTL capacity imposed by future coal production volumes. About 10% of the world coal production can be diverted to CTL. But the liquid fuel supply will be limited to only a few Mb/d. which directly affects the CTL from becoming a viable mitigation plan for liquid fuel shortage on a global scale. Even though CTL producing limited amount of liquid fuels, it is possible for individual countries having large reserves of coal for the sufficient fuel supply to CTL. But it is not realistic to assume that CTL provides a feasible to liquid fuels. For the major part, it can only be claimed as a minor contributor.

Terms: Coal-to-Liquids, synthetic fuels, conversion ratio, coal liquefactionINTRODUCTIONThe price of oil has increased in a brisk pace over the last few years. The oil price has reached about $100 per barrel in January 2008. This was happened before the price was rocketed to $147 per barrel in July 2008. After the huge collapse of the oil price in late 2008, the price of the oil has now recovered to over $60 per barrel. There were some signs of return to $20 per barrel which was the typical price in 1990s. The alternative fuels can be obtained from various other feed stocks just like converting the solid materials to the other forms. The alternative for these conventional fuels can be obtained from various other feed stocks like coal also. This technology is known as the (CTL) Coal to liquids. This (CTL) technology is mainly based on the liquefaction, which is done by three basic approaches they are: (1). Direct coal liquefaction (DCL), (2).Indirect coal liquefaction (ICL), (3).Pyrolysis. Based on the type of feedstock the, generally the synthetic fuel properties can be made almost identical to the conventional fuels which are obtained from crude oil.This technology (CTL) is one of those technologies which are the most dependable approach for the two world wars technically and economically established. So we can expect a significant future use of thisCTL as way to lessen the impact of the declining conventional fuels which are produced from petroleum.

A. Abbreviations and Acronyms CTL-carbon to liquid DCL-direct coal liquefaction ICL-indirect coal liquefaction FT-Fischer-Tropsch DME-di methyl etherHistorical overviewThis CTL technology is one of the technologies, developed at the beginning of the 20th century, and recently attracted the attention once more. Historically it has helped to fuel the German military during the two world wars. This CTL has provided about 92% of the Germanys air fuel and 50% of the petroleum supply in 1940s. This CTL is developed in the 1950s by South Africa; now plays a vital role in the South Africas national economy, providing 30% of their fuel demand.The best known of this CTL process is the FISCHER-TROPSCH synthesis. The inventors of this process are Franz Fischer and Hans Tropsch from the Kaiser William institute in the 1920s. The FT-synthesis is the basis for ICL technology. The Indirect coal liquefaction (ICL) was invented by a German Chemist Friedrich Bergius as a way to convert ignites into synthetic fuel in the year 1913. As a way to produce the smokeless fuel Karrick invented a low temperature carbonization process in the USA around the 1930s.

Fig 1: Coal to Liquid fuel technology is a well-established technology, performing since 100 years. Coal-derived fuels are ultra-clean, sulphur-free, low in particulates, and with low levels of oxides of nitrogen

The CTL technology has steadily improved since the Second World War. However, only a few number of commercial enterprises based on generating liquid fuels from coal have been undertaken, and most of them are based on the ICL technology. The most successful was the South African company SASOL, originally created a way to protect the countrys balance, of against the increasing demand on the oil. Even china implemented this technology; a new DCL plant has become operational in china as a beginning of the new era.

BASIC CHEMISTRY OF CTLSource combined with a hydrogen source, generally steam. Chemical reactions between carbon and other compounds the fundamental for all types of CTL-syntheses process is a carbon will eventually forge the hydrocarbon molecules of the craving length. The original Fischer-Tropsch (FT) process was depicted by the below Formula.

Carbon monoxide can be produced by gasification of coal or other carbon rich compound. The necessary energy is applied by adding oxygen or steam.

The net mixture of carbon monoxide (CO) and hydrogen () is usually called synthesis gas (syngas). It is used to build the hydrocarbon chains of different sizes using condensation process and a politic catalyst. Generally, the FT-process produces two types of products, described by two reactions.

The type of emerging products returned, depends upon the type of CATALYST and the reactor operating conditions. Olefin rich products can be used for making Synthetic gasoline and Chemicals at high temperatures and Paraffin rich products can be used for making Synthetic diesel and Waxes at low temperatures. The required range of n for olefin and paraffin rich products are 5-10 (naphtha) and 12-19 respectively at FT-process operating temperatures. Some of the commonly used catalysts are Transition metals like iron, cobalt or ruthenium. And the transition metal sulphides, amorphous zeolite and similar compounds are also utilized.

CTL TECHNOLOGY OPTIONSGenerally, there are different types of technologies for the conversion of coal to liquid fuels. Whileonly some have been proved as commercially feasible for the industry, Coal to liquid (CTL) technology is an old concept that has to be federated with modish processes and machinery to be able to fulfill the economic and environmental constraints of today. Surviving technological framework from conventional crude oil processing and gas liquefaction can render collusive effects decreasing costs and necessary research. Ground work on synthetic fuels from non-coal feedstock exists in several places, where coal is too expensive or abstract to access. For example in theory, FT-synthesis can be used to produce liquid fuels from unconventional feedstock as long as required amount of hydrogen and carbon are available. For exponent, carbon dioxide, carbon monoxide and other combustion poop out have been used as feedstock in crystal clear applications.

A .Pyrolysis The primeval method for annexing liquids from coal is high temperature pyrolysis. Usually, coal is heated to around 950 C in a closed container. The heat causes decomposition and the volatile matter is compelled away, which increases the carbon content. This is parallel to the coke-making process and supplementing tar-like liquid where it is mostly a side product. The operation results invery low liquid yields and elevating costs are relatively high. Coal tar is not consistently used as a fuel in the transportation stratum. But, it is used globally for manufacturing roofing, waterproofing and insulation products and also used as a raw material for various dyes, drugs and paints. Mild temperature pyrolysis operates at a temperature of 450-650 C. Mostof the volatile matter is ride off and the remaining compounds are formed through thermal decomposition. Liquid yields are higher when compared to high temperature pyrolysis, but reach a maximum at 20%. The main products are char, semi-coke and coke (all smokeless solid fuels). This technique has mostly been used to improve low-rank coals, by increasing calorific value and reducing sulphur content.

Fig 2: Pyrolysis is an older technique, for the conversion of coal to liquids. In which coal is heated at high temperatures which results in producing of char, coke, semi coke, bio-oil, and all smokeless solid fuels.

The Karrick process is a low temperature carbonization process that also yields liquids. Generally the main product is, semi-coke. And the tar liquids produced here should require further refining before they can be used as a transportation fuel. In arbitrary, pyrolysis produce low liquid yields and has innately low efficiency. Moreover, the resulting liquids require further treatment before they can be used in existing vehicles. A manifestation plant for coal upgrading was built in the USA and was operational between the years 1992 and 1997. However, there is less possibility for this process to yield economically feasible volumes of liquid fuel. Consequently, further investigation and analysis of coal pyrolysis is not undertaken.

B. Direct Coal Liquefaction The basic process involved in this direct coal liquefaction technique is dissolving the coal at high temperatures and pressures. Adding of hydrogen and a worthy catalyst results in hydro-cracking, splitting long carbon chains into shorter, liquid parts. The added hydrogen also advances the H/C-ratio of the product. Liquid yields can be 70% excess of the dry weight coal, with net thermal efficiencies of 60-70%. The deriving liquids are of higher quality, when compared to pyrolysis, and can be used directly in power generation or any other chemical processes as a synthetic crude oil (syncrude).

Fig 3: Direct Coal Liquefaction process is highly efficient, but the liquids products require further refining to achieve high grade fuel characteristics

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 mixture of many gasoline-like and diesel-like products, as well as other chemical compounds like propane, butane and other products can be recovered from the refined syncrude. Some smaller pilot-plants and testing facilities have provided positive results. In 2002, the Shenhua Group Corporation, the largest state-owned mining company in China, was tasked with designing and constructing the worlds first DCL commercial plant in Inner Mongolia Autonomous Region, which recently became operational.

C. Indirect Coal LiquefactionThis process involves in a complete disintegration of coal into other compounds by the gasification process. Resulting syngas is mutated to acquire the demanded balance of hydrogen and carbon monoxide. Later, the syngas is cleaned, by removing the sulphur and other impurities efficient of disturbing further reactions. Finally, the syngas is reacted with a catalyst to provide the required product using FT-reactions.Mending of catalysts and reaction conditions can evolve a wide array of different products. For instance, methanol is one possible product that can be produced directly or further converted into high quality gasoline via the Mobil process in additional stages. 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.Sasol in South Africa owns the only commercial-scale ICL plants currently in operation with well-established and proven technology and together with a lot of operational experience. In total, Sasol has over 50 years of experience of Indirect Coal Liquid (ICL) and has produced over 1.5 billion barrels of synthetic oil during its presence. A number of different ICL-technologies have been developed by Sasol, the oldest ones date from the 1950s and was used to late 1980s. Today, advanced technologies from the 1990s are utilized, including the Sasol Advanced Synthol High Temperature FT-synthesis and the Sasol Slurry Phase Distillate Low Temperature FT-synthesis

.Fig 4: Indirect Coal Liquefaction first gasifies the coal with steam to form syngas, which on further condensation over a catalyst gives high quality, ultra clean products such as petrol, diesel, synthetic waxes lubricants etc.

RESOURCE AVAILABILITYGenerally this CTL fuel technology is suitable for the countries which are heavily depended on the oil imports and have large of coal reserves. We know thatconverting coal into liquid fuels is more costly than current energy technologies; both in phases of production costs and the amount of greenhouse gases are emitted. Production of coal-to-liquid fuel has a large carbon stepping, releasing more than twice that of lifecycle greenhouse gases of conventional petroleum fuels. So this conversionof coal to fuels is only accessible for the countries which are highly dependent on oil and natural gas imports and also having large amounts of coal like USA, china, India.

Fig 5: The graph represents worlds main coal reserves accordingly. And the Asian countries are Russia, China, India, Kazakhstan etc.

The countries which are having large amounts of coal and highly dependent on oil imports are generally India, China, South Africa, Germany etc. nearly from 100 years Germany is notably converting these coal to liquids successfully and the production peak is in the year 1940, and then South Africa from the year 1950 implemented this technique and today around 30% of the gasoline and diesel needs are produced from this indigenous coal. And in the year 2008 China had started the first CTL facility plant in the world, which can produce 1 million tons of coal based liquid fuels per year.

BENEFITS OF CTL FUELSThe products which are produced from the conversion of coal are Ultra-clean petrol, Ultra-clean diesel, synthetic waxes, lubricants, chemical feed stocks, and alternative liquid fuels such as methanol and dimethyl ether (DME).The fuels produced from these conversions can be directly used as transportation fuels without changing any design of the engine. These indigenous coal fuels can bring cleaner fuel for transportation, reduce greenhouse emission through carbon capture storage systems and it will helps in the Energy basket of a country.

Fig 6: Coal to liquids are also used directly in the petrol engines, and also uses in the stationary power plants as a fuel.

1 .Coal is affordable and available worldwide enabling countries to access domestic coal reserves 2. A well-supplied international market 3. Decrease reliance on oil imports, improving energy security.4. Coal liquids can be used for transport, cooking, stationary power generation, and in the chemicals industry.5. Coal-derived fuels are sulphur-free, low in particulates, and low in nitrogen oxides.6. Liquid fuels from coal provide ultra-clean cooking fuels, alleviating health risks from indoor air pollution

EMMISSIONSThe sulphur content of CTL products are low when compared to petroleum-derived fuels, and it is a common trait for both DCL and ICL, which comes from the mandatory reason to protect catalysts from poisoning. Apart from this equality, emissions and combustion characteristics of DCL and ICL type of fuels will differ. Comprehensive analysis of emission characteristics of synthetic and conventional fuels have been compared by others departments. DCL types of products are generally rich in polycyclic aromatics and heteroatoms, but ICL type of products has lower aromatics content. In high temperature conditions FT-synthesis yields branched products and contains aromatics type of compounds, but these compounds are virtually absent in low temperature FT-synthesis operations. Recently, environmental regulation trends have been moved towards reducing the aromatic content in transportation fuels, and giving the advantage to ICL-fuels. Toxic and trace metals and inorganic compounds, such as cadmium, selenium, arsenic, lead and mercury, can be passed over the final fuel products in both DCL and ICL type of-processes. In ICL-systems, eliminating of mercury and other metals is generally easierand inexpensive. But for DCL, it will be more complicated and more costly, but it is not impossible.

Fig 7: Vallentin concludes that DCL produce 90% more carbon dioxide and ICL produce 80-110% more carbon dioxide when compared to normal petroleum-derived fuels

Cetane and octane numbers also differ, resulting 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 lowered by blending.Both DCL and ICL fuels emit large amounts of carbon dioxide compared to ordinary petroleum-derived fuels. But, there are methods for mitigating or even neutralizing emissions without increasing production costs drastically. Substantial differences exist between DCL and ICL technologies with regard to the potential and cost of greenhouse gas emission mitigation. Vallentin concludes that DCL generates about 90% more than conventional fuel on a well-to-wheel basis. This is in agreement with other studies, but if reduction measures are implemented, the emissions could be reduced to no more than 30% extra compared to conventional petroleum fuels. ICL-technology generates approximately 80-110% more emissions compared to conventional fuels, if the is vented. However, there are ICL-system configurations where co-capture/co-storage can reduce emissions. Well-to-wheel analysis has shown that even with CTL production chain emissions are higher than for petroleum-derived fuels, mostly due to emissions from mining. Carbon capture and storage involves the capture of Carbon dioxide emissions from the source, followed by the transportation to, and storage in, geological formations. Carbon Capture Storage is particularly applicable to the CTL process as the Carbon dioxide stream produced from CTL is at a very high concentration, and a very little of the costly of Carbon dioxide separation is required before the transportation and the storage. If the Carbon dioxide has been captured then there are a number of storage options available. The CO2 can be stored in deep saline aquifers or it can be used to assist in enhanced oil recovery and subsequent Carbon dioxide storage systems. CO2 can also becaptured and sold to the industries like food and beverages, which displaces the CO2 currently sourced specifically for this purpose from naturally occurring CO2 deposits. This also provides an extra source of revenue for the project.

Fig 8: The percentage of nitrogen oxides, carbon monoxide, particulate matter, and carbon dioxide can be reduced by using co-storage systems etc.

In summary, CTL fuels can improve emission characteristics and reduce transportation emissions of sulphur, aromatics,and particles compared to conventional fuels. However, there does not seem to be much potential foremission reductions if the full supply chain is analysed for either DCL or ICL.

CONCLUSIONCoal-to-liquid technology is a well- established process that does not necessitate future research to successfully produce liquid transportation fuels. However, making the greenhouse gas emissions of synthetic fuels derived from coal comparable to those of oil requires further research into emissions from CTL and CBTL production and large-scale carbon sequestration. Although sequestering massive amounts of CO2is possible in theory, questions still remain about the length of time the sequestered CO2would remain in place, the long-term ecological impacts of sequestration, and the costs of constructing a CO2storage facility, the geologic formations that make the best storage sites, monitoring leaks, and general commercial viability. Based up on the heavy dependence on oil imports, and having coalas a basic energy source, these research steps will likely be taken regardless of whether CTL technology becomes widespread.REFERENCES[1]. World coal association, www.worldcoal.org.[2]. advancing science serving society policy brief: CTL technology, www.aaas.org

T.Avinash:was born in Rajamundry City, in 1993. He is pursuing undergraduate degree in Petroleum Technology from Sri Aditya Engineering College. His research interests include development of other unconventional sources of energy like Gas hydrates; and green (algae) crude oil.He is a junior member of Society of Petroleum Engineers since February 2012.

Rohit Imandi (M20) was born in Kakinada, Andhra pradesh, in 1993. He is pursuing undergraduate degree in Petroleum Technology from Sri Aditya Engineering College. His research interests include development of other unconventional sources of energy like Gas hydrates, liquefied petroleum from coal.He is a junior member of Society of Petroleum Engineers since February 2012.4