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Protocol.
Introduction.
The title of this paper titled “Shifting the Paradigm: the Prospective Utilization of Coal Liquefaction for Liquid Fuel Production in Nigeria” is multi-disciplinary in nature. The word ‘prospective’ in the title implies ‘the paper intends to evaluate the potential use’ of coal deposits to produce clean combustible liquid fuels to drive the nation’s economy. The technology is designed to spur the migration from petroleum crude to coal, in our spirited effort to produce liquid transportation fuel to meet our domestic consumption, and the export market.
Mr. Chairman, I am truly delighted and honored to be called upon to do justice to this title of the paper, for the purpose of sparking -off a national discuss that offers a new promise for the rapid economic development of our country. Hence, the paper intends to discuss the new options of tackling the challenges of our historic overdependence on light sweet crude oil for transportation fuels in the coming decades as our finite crude oil resources reserve are fast depleting! It is heartwarming to note that new fuel sources with different characteristics are emerging to displace crude oil, and a series of new engine technologies are also being developed with the promise of improved efficiency and cleaner combustion. I hope that at the end of my delivery, the issues raised will not be an exercise in futility!
The coal liquefaction technology is a paradigm shift encouraged to confront the challenges of; heightened armed agitation for resource control in the Niger-delta region, environmental consequences of crude oil refining, delivery and usage; the fast depleting reserves; finding an alternative to the crude oil–centered economy; fuel scarcity and increasing production to meet the demands of the local consumption; and creating more jobs and wealth for a depressed economy such as ours.
Nigeria Energy Needs:
Energy needs and consumption patterns are barometer to measure the economic development of a nation. The statistics of global total energy consumption presents; China (3.034 Mtoe), United States (2,224 Mtoe), India (872 Mtoe), Russia (751 Mtoe), Japan (437 Mtoe), Germany (307 Mtoe), Brazil (306 Mtoe), South Korea (277 Mtoe), Canada (251 Mtoe) and France (243 Mtoe) as the top ten highest energy consumer nations, and coincidentally also doubles as the most industrialized1. Hence, a striking correlation exists between energy consumption patterns and economic development! It remains incontrovertible that Nigeria cannot even be found in the list of the top twenty energy consuming nation, reveals the magnitude of energy insecurity, and paints a ‘bleaker’ picture of our economic woes. This therefore calls for more deliberate policies that will promote the diversification of energy sourcing and technology for speedy economic growth. Presently, Nigeria generates less than 4000MW of electricity from its hydropower and coal and gas thermal power stations2.
This megawatts of electricity generated is hardly enough to provide lightings nationwide, let alone driving the cogs of our local industries! Hence, the consequences of higher energy cost for manufacturing of goods, unemployment, poverty and an unproductive economy. The price volatilities of crude oil products, such as; diesel and gasoline occasioned by the high exchange rate of the dollar and fluctuations of the cost of the product in the international market, and local refineries running far below capacity, makes the petroleum product either scarce or the cost of running local industries and manufactured products very prohibitive2. These aforementioned factors further underscore the need for an aggressive energy diversification policy.
Sourcing of Vehicular Fuel from Crude oil.
World over, transportation fuels are distilled in petroleum refineries as numerous products of crude oil in a fractionating column based on the varying boiling point of the products3 refer to figure 1. Nigeria has vast oil and gas history and potential. It is the 12th largest crude oil producer in the world, contributing about three percent of the global crude oil production, which make it the largest crude oil producer in Africa, with the second largest reserve. It is also the ninth country, in terms of gas reserves in the world, making it have the largest gas reserve in Africa and also contribute about eight percent of the global liquefied natural gas supply2.
In technical terms, one barrel of Nigerian crude oil has a volume yield of 6.6% automotive gas oil, 20.7% gasoline, 9.5% kerosene and jet fuel, 30.6% diesel, and 32.6% fuel oil and residues 4. They are all hydrocarbon products. Hydrocarbons separate due to the heat because they boil at a different temperature 5.
Figure 1. Fractional distillation of crude oil.
Despite its crude oil reserves profile and its installed refining capacity of 445,000 barrels per day, yet 70% of its domestic petroleum products demand is met through importation. This cost the country about $62 billion in 2014, an amount sufficient to meet the initial investment requirement for building four refinery plants, each with the operating capacity of the combined capacities of the existing dilapidating refineries in the country. This amount is also higher than what the country received (averagely around $56 billion) from crude oil exports in the same year2.
It is evident from financial reports, that oil generates around 70 percent of the country’s revenue. Hence, the following factor has been identified as the principal causes of the huge under-capacity in the Nigerian oil industry: price volatilities of crude oil products due high exchange rate and fluctuations of the cost of the product in the international market; plummeting crude oil prices in the international market ; obsolescence of local refineries; overbearing government involvement, and legislative uncertainty, corruption; large-scale oil theft, and youth restiveness and militancy in the Niger Delta region has produced telling a consequence of higher energy cost for manufacturing of goods, unemployment, poverty and an unproductive economy2. Hence, this all the more has necessitated the need to diversify the energy sector by the exploration and utilization of other energy resources- that includes, the nation’s vast coal reserve – to improve the energy supply mix and power our economy.
Coal Power Revisited.
There are an estimated 892 billion tonnes of proven coal reserves worldwide. This means that there is enough coal to last us around 110 years at current rates of production. In contrast, proven oil and gas reserves are equivalent to around 52 and 54 years at current production levels6.
Coal is the most abundant of fossil fuels. It is available from a wide variety of mines distributed globally. It is used for electricity generation, but also in steel and aluminium production as well as in cement manufacture and as a liquid fuel. Global consumption of coal is growing and is expected to increase even more as developing countries expand their energy needs. Nonetheless, the introduction of various carbon management schemes, particularly carbon capture and storage (CCS), is vital to mitigate the impact of future coal use on the environment.
China, as the most densely populated country in the world relies on coal for most of her thermal energy generation. Shanxi coalfield7, in the northern part of China, houses 200 billion tons of coal, i.e. one third of China’s total coal reserves. In Shanxi province, Yangcheng International Power Company is now operating its enormous coal- fired power plant to generate 2,100 megawatts of electricity since the year 20027.
Availability of Coal in Nigeria.
According to the National Energy Policy document of the Energy Commission of Nigeria, coal of sub-bituminous grade occurs in about 22 coal fields in over 13 states of the federation8. The proven coal reserves so far in the country are about 639 million tones, while the inferred reserves are about 2.75 billion tones, consisting approximately of 49% sub-bituminous, 39% bituminous and 12% lignitic coals.
These resources are found largely in Enugu, Benue, Delta, Nassarawa and Plateau States of Nigeria. However, very recent geological investigation indicates that Gombe, Adamawa and Yobe States are likely to have some deposit [9, 10, and 11]. Notwithstanding, the challenges of greenhouse gas and particulate emission, it has become imperative, that the available of huge coal deposits, calls for its exploitation and utilization for the rapid economic development of the nation.
Coal Power: The Overarching Advantages.
The following details may greatly help in weighing the gains and challenges of coal power [12].
- Coal has an abundant supply, which is concentrated in several industrialized countries including the US, China, India and Russia.
- The utilization of coal technology is relatively inexpensive.
- It provides high load factor.
- It comes with continuous power for good application.
- It is a mature industry with substantial existing infrastructure.
- Carbon can be made low, and can also be cleaned using carbon, capture and storage (CCS), and different scrubbers.
- Coal may be converted into a gas or liquid form that burns cleaner.
- Coal power requires relatively low capital investment, compared with nuclear and gas energy.
Clean Coal Technology – An Overview!
Modern clean coal technology (CCT) is a product of several generations of technological advances that have led to more efficient combustion of coal with reduced emissions of sulfur dioxide and nitrogen oxide.
The most disturbing problem is the significant amount of CO2 (Carbon dioxide) emitted. According to statistics, coal contributes 31% of the total CO2, which is the biggest contributor of any source. However with CCT applications, power plants built today emit 90 percent less pollutants (SO2, NOx, particulates and mercury) than the plants they replace from the 1970s, according the National Energy Technology Laboratory (NETL). CCT program has resulted in more than 20 new lower costs, more efficient and environmentally compatible technologies for electric utilities, steel mills, cement plants and other industries13-14. The ensuing paragraph will be dealing with why overriding consideration is given to CCT applications14.
Clean Coal Technology in Modern Economy.
The envisaged impact of CCT in the economy of nations could be evaluated in terms of its relevance in the electricity generation, and provision of transportation fuels for the citizenry. In this regards, CCT could be viewed to be positively impacting for the following reasons16:
- The nation’s abundant coal reserve could be exploited in a cost effective manner to help satisfy the increasing demand for electricity.
- A coal-fuelled electricity industry and its public sector partners can meet the environmental challenges we face, while continuing to provide reliable, low-cost electricity.
- Employment and economic opportunities could be provided by deploying advanced coal-based power plants with carbon capture and storage technologies.
- The prospect of using coal to provide almost half of the electricity we use to run our factories, heat our homes, and grow our economy is realizable.
Coal Liquefaction Technology: An overview.
The two common types of coal liquefaction technologies are discussed as follows17, 18:
- Direct Coal Liquefaction (DCL).
In this case, pulverized coal is treated at high temperature and pressure with solvent comprising recyclable oil slurry. The large coal molecules are broken down into smaller molecules. The H/C ratio is increased by adding gaseous H2 to the slurry of coal and coal-derived liquids. Catalysts can speed some of the required reactions, and the product is a liquid which is broadly comparable to a synthetic crude oil. Liquid yields in excess of 70% by weight of coal feed, have been demonstrated for some processes, albeit under favorable circumstances.
There are two main variants of DCL. They are as follows:
- Single-stage liquefaction process; provides distillates via one primary reactor, or in a train of reactors in series.
- Two-stage process provides distillates via two reactors or reactor trains, in series. The main function of the first stage is the coal dissolution, and it is operated either without a catalyst, or with only a low-activity disposable catalyst. The heavy coal liquids are hydro-treated in the second stage in the presence of a high-activity catalyst to produce distillable products.
Furthermore, a range of partially refined gasoline- and diesel-like products (as well as propane and butane) can be recovered from the synthetic crude by fractional distillation. These products tend to be highly aromatic, which makes them difficult to use as high quality transport fuels, although they can be rich in octane aromatics making a good gasoline substitute.
Co-processing is a variant on other direct liquefaction processes involving the simultaneous upgrading of coal and a non-coal-derived liquid hydrocarbon. The latter serves as the slurring and transport medium for the coal. It is usually a low-value high-boiling point material, such as bitumen, an ultra-heavy crude oil or a distillation residue or tar from crude oil processing. The overall aim of co-processing is to upgrade the petroleum-derived solvent at the same time as the coal is liquefied, thereby reducing capital and operating costs per unit of product.
- Indirect Coal Liquefaction
This is a high temperature, high pressure process, comprising of a gasification stage, syngas cleanup, and F-T and methanol synthesis used in the production of liquid fuels. In this case, oxygen blown gasification of the coal produces a syngas consisting mainly of CO and H2,
The syngas contains a number of impurities, including particulates, sulphur compounds, and nitrogen, which are removed in a series of clean-up stages, after which the CO2 is separated. The cleaned syngas molecules (CO+H2) are catalytically combined/rebuilt to make the distillable liquids, such as synthetic gasoline or diesel, and/or oxygenated fuels, together with a wide range of other possible products17,18.
Hydropyrolysis could be considered as another type of indirect coal liquefaction method. It is a simple method to convert coal into higher valuable fuels involving only a heating step to split coal into gas, tar and char. When pyrolysis is carried out in a hydrogen environment it is called hydropyrolysis.
Under pressure chemical reactions can occur between the hydrogen and the free radicals in the primary coal decomposition products. The dynamic equilibrium between cracking and polymerization, which compete with each other, determines product formation and quality, In principle, the process parameters, that influence the position of the equilibrium and thereby the product distribution, can be used to control the overall reaction and to increase the devolatilisation rate.
Chemical bonds in the macromolecular coal structure are relatively weak; the primary decomposition of the coal is a very fast reaction and occurs n a few minutes if coal is heated rapidly to the reaction temperature. Hydrogen can stabilize the primary carbonization products and further crack the higher molecular coal fragments. This leads to enhanced yields of volatiles, and a shift in the product distribution compared to carbonization in inert gas. Pyrolysis and hydropyrolysis can be carried out in fixed bed, fluidized bed and entrained phase reactors (i.e.gasifiers).
Figure 4. Schematic diagram of hydropyrolysis.
Advantages of Indirect Coal Liquefaction Technology.
Indirect coal liquefaction has a number of advantages over and above direct coal liquefaction. Some of which are as follows17,18:
- The principal product from the first stage is a gas which leaves behind most of the mineral matter of the coal in the gasifier, apart from any volatile components;
- Undesirable components such as; sulphur compounds are more readily cleaned out from the gas and removed;
- It is easier to control the build-up of the required products;
- There is good operational flexibility in that syngas made from any source (coal, natural gas, or biomass) can be used;
- The CO2 produced can, in principle be captured for subsequent storage;
- The products from ICL are ultraclean, with near-zero aromatics and no sulphur. With minimal further refining it is possible to produce ultraclean diesel or jet fuel.
Production of Synthetic Fuels.
Coal liquefaction technology (also known as “Fischer-Tropsch” technology) offers a range of important products that are produced by liquefying coal using the coal liquefaction method. These methods and technologies produce synthetic fuels and waxes that are environmentally friendly and extremely useful to various industries 19, 20..
During the 1920’s, German scientists working at the Kaiser Wilhelm Institute discovered a way to liquefy coal for the production of synthetic fuels. The process became known as “F-T Synthesis” after its creators, chemists Franz Fischer and Hans Tropsch. During World War II, F-T fuels were used to power planes and tanks for the German army.
In recent decades, the application of F-T technology has resurfaced with renewed vigor. Scientists, researchers and lawmakers are working together to bring coal liquefaction programs into the mainstream of national economies to provide substitutes for petroleum based fuels. The ensuing paragraph will briefly discuss the dual process employed in synthetic liquid fuel production.
Coal Gasification.
Coal gasification, is a process that converts carbon materials into carbon monoxide and hydrogen. Many materials are gasified for this purpose, including petroleum, petroleum coke, and biomass and of course, coal. Coal is fed into a reacting vessel called a gasifier. Within the gasifier, controlled amounts of heat, pressure and oxygen are added to break up the molecular structure of the coal. The gasifier only allows a portion of the coal to burn, resulting in the partial oxidation of the coal. This reaction produces carbon monoxide and hydrogen rich synthesis gas. By removing these emissions during the gasification process, the resulting fuel products are considered “clean-burning.” 19, 20. The three common types of above-ground gasifiers are briefly mentioned below.
Type of Gasifiers.
The three types of above-ground gasifiers are21, 22, 23,24:
- Moving bed reactor (Lurgi – dry ash and BGL – slagging); In this case, a counter-current flow of coal and oxidizing blast is configured. The blast is composed of air and hot syngas, so low oxygen consumption. It operates on reactive carbon sources, and good heat transfer heats-up the carbon source creating methane and tar. Post production cleaning and scrubbing requires greater energy use
- Fluidized-bed reactor (Winkler, HTW, CFB-dry ash; KRW, U-Gas – Agglomerating). In the fluidized bed reactor, air fluidizes a bed and carbon containing particles added. Proper mixing of fuel and oxidant provide good mass transfer and heat transfer. The fine particles escape with syngas and needs to be cleaned. The very good heat/mass transfer profile partially -reacted carbon may settle with ash. The use of slagging will reduce fluidization, so temp remains below softening point for ash.
- Entrained flow reactors (Shell, Texaco, E-gas,KT- Slagging). In entrained flow reactors, carbon source is made of very fine particles in a liquid or slurry for very good mass transfer. The residence time in the reactor is usually very short, for co-current flow with oxygen to be achieved at high temperatures. In this case, low heat transfer means hot exiting gas with no methane or tar, but more oxygen is required. It was observed that its high temperature profile and very small carbon sources make it an ideal process for coal gasification.
Conversion of synthesis gas (syngas) to liquid fuels.
This could be achieved either by F-T synthesis and Methanol synthesis respectively 19,20.
- F-T Synthesis.
The syngas, is then fed into an F-T reactor where it is condensed over a catalyst. A “catalyst” is a substance that accelerates a chemical reaction without being consumed. Catalysts used in the F-T process are typically iron or cobalt, but vary based on the desired product. The exposure to the catalyst converts the syngas into liquid and wax products that can be refined into synthetic fuels. The conversion of the syngas is known as “Fischer-Tropsch Synthesis.” The term “Coal-to-Liquids” has become synonymous with F-T products and technologies.
| a. Moving bed reactor | b. Fluidized-bed reactor |
| c. Entrained flow reactors |
Figure 8. Gasifier types.
Figure 9. Gasification based power system.
The F-T process for making synthetic hydrocarbons can be summarized, by the following two catalytic reactions that build up large hydrocarbon molecules from the small CO and H2 molecules produced by gasification, with the oxygen in the CO feed being rejected in steam:
n CO + 2n H2 ® n H2O + CnH2n (olefins)
n CO + (2n + 1) H2 ® n H2O + CnH2n+2 (paraffins)
The slate of products generated depends on the catalysts used and reactor operating conditions. Olefin-rich products with n in the range 5 to 10 (naphtha) can be used for making synthetic gasoline and chemicals in high-temperature F-T processes. Paraffin-rich products with n in the range 12 to 19 (distillates) are well suited for making synthetic diesel and/or waxes in low-temperature F-T processes.
Coal-to-liquid products are versatile. They can be used to run a variety of vehicles including cars, trucks, tanks and jets. In addition, F-T waxes may be stored indefinitely. Depending on the catalyst and conditions in the F-T reactor, coal-to-liquid products vary in density, composition and prospective use. Excess steam from the gasification process can be used to generate electricity.
In South Africa, SASOL has demonstrated that transport fuels produced from coal using ICL compares favorably with gasoline and diesel fuel. At the moment, the technology is being used to convert about 42 milliont (Mt) of coal into 6 billion liters (Gl) of synthetic fuels and 2 Gl of chemicals annually.
Figure 10. Process diagram of coal –to –liquid fuel conversion
.
MeOH is a well-established chemical commodity used throughout the world. It can potentially also be used indirectly or directly as a fuel. The primary reactions involved in making MeOH from syngas are as follows:
CO + H2O ® CO2 + H2 (water gas shift)
CO + 2 H2 ® CH3OH (methanol synthesis)
The MeOH produced can be further processed to make gasoline by the Mobil process (a commercial technology that can provide gasoline at attractive costs from low-cost stranded
natural gas. DME by MeOH dehydration (which involves de-watering methanol in the presence of a dehydration catalyst such as alumina) , or the MeOH can be used directly as fuel.
DME is a non-carcinogenic and virtually non-toxic chemical produced. It is also usable as a fuel. Currently DME is made by MeOH dehydration:
2 CH3OH ® CH3OCH3 + H2O.
DME can also be made at lower cost in a single step by combining mainly three reactions in a single reactor.
CO + H2O ® CO2 + H2 (water gas shift)
CO + 2 H2 ® CH3OH (methanol synthesis)
2 CH3OH ® CH3OCH3 + H2O (methanol dehydration).
In China, the Institute of Coal Chemistry (ICC) with the Shanxi New Style Fuel and Stove Company constructed a 500 t/year DME plant in Xi’an based on MeOH dehydration for use as a domestic cooking fuel as an alternative to LP.
Figure 11. Process diagram of synthetic fuel production.
Challenges of Coal Liquefaction.
CO2 emissions is a leading cause of global warming, it is usually released when coal is liquefied and again when the CTL fuel is burned. In order for CTL products to enter the mainstream as a viable source of transportation fuel, scientists and researchers must find a way to limit CO2 emissions.
CCS provides an avenue to limit CO2 emissions. This technology would capture and condense carbon dioxide during coal gasification. The CO2 could be stored safely and permanently in underground structures such as saline aquifers or captured carbon could also be sold commercially to oil companies and other fuel producing industries for enhanced oil yield/recovery15.
Concluding Note.
There is no doubt that Nigeria is blessed with abundant coal deposits, Hence, a robust clean coal technology policy implementation, will immeasurably boost the complementary role of clean coal technology applications in the nation’s energy-mix, to guarantee the conservation of our crude oil reserve, ameliorate our rising energy demands, and mitigate challenges of greenhouse and noxious gas emissions consequent on fossil fuel combustion, and create more jobs and wealth for the citizenry. All that is required is policy prioritization, and the political will for its timely implementation. This paper has situated itself within the energy policy framework of Nigeria; by promoting the effective utilization of the nation’s coal deposits to meet the energy needs of the country; by encouraging investment activities and indigenous participation in the coal industry, and minimizing environmental pollution largely associated with coal combustion.
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Nice write-up!