Characterization of Some Nigerian Coals for Power Generation
Chapter One
Aim and Objectives
The aim of this research is to characterise coalsfrom five deposits in Nigeria which include Odagbo (Kogi State), Owukpa (Benue State), Ezimo (Enugu State), Amansiodo (Enugu State) and Inyi (Enugu State), thereby enriching the repository of existing data on properties of Nigerian coals and make a strong case for the utilisation of these coals in power generation.
The specific objectives of the research are:
- To carry out proximate, ultimate and ash composition analyses of the coal samples.
- The determination of calorific value of the coal
- To determine ash fusion temperature (in oxidising and reducing atmospheres)of the coal
- To carry out petrographic and thermogravimetric analyses of the coal
- To compare results of the above analyses with reference values of coal properties for power generation in order to determine the suitability of the coal samples for power generation.
CHAPTER TWO
LITERATURE REVIEW
Previous Research on Nigerian Coals
Olaleye(2003)investigatedsome Nigerian coalsand showed that the relatively low ash and moisture contents, low sulphur content and medium to high calorific value, make Nigerian coals compare favourably with other sub-bituminous coals in coal-producing countries of the world, thereby making them a good export commodity. They are superior as regards ease of ignition, combustion characteristics and freedom from clinker. The coals burn with a long flame and require large combustion space. They are acceptable boiler fuels if used in suitable appliances such as chain grate stokes or as pulverized fuel. Nigerian coals are suitable as energy fuel for electricity generation for the abandoned Oji Power Station in Enugu State and other proposed power stations at Kogi, Benue, Anambra and Delta States. With the current deregulation of power generation in the country, necessitated by the lack of sufficient power generation by the Power Holding Company of Nigeria (PHCN), Independent Power Producers stand better opportunities.
Kibiya (2012) collected and analysed five (5) coal samples from Lafia-Obi (Nasarawa State), Okaba (Kogi State), Lamja (Adamawa State), Okpara and Onyeama (Enugu State) to determine their properties and classify them by rank using proximate analysis. Auto-ignition temperatures, reactivity and activation energies of the various coal samples were also determined. Results obtained showed that Lafia-Obi coal could be ranked the highest as bituminous (low volatile) coal. Okpara and Onyeama coalsboth were both bituminous A coals, while Okaba coal was found to be sub-bituminous B coal. Lamja coal was found to be a lignite A coal. Findings also revealed a dependence of coal density, reactivity and activation energies on coal type and particle size. The study concluded that Lafia-Obi, Okpara and Onyeama coals may be used for power generation. Lamja and Okaba coals were not recommended for power generation because of their low calorific values.
Laudan (2008) analysed coal samples from Enugu State (Okpara West Area, Okpara West Bank and Onyeama) and Gombe State (Doho and Gamawa) to determine their physical properties and to classify them by rank using proximate analysis. An electric furnace fitted with a 3kW heating element capable of generating heat to a temperature of 900oC was constructed and used to determine the auto-ignition temperatures, reactivity and activation energies of the various coal samples. Findings revealed that the reactivity and activation energies of the various coal samples depend on the coal type, source and particle size. While for some coals, the reactivity and activation energies increase with decrease in particle size (Okpara West Bank coal), in others, reactivity and activation energies decrease with decrease in particle size (Onyeama, Okpara West Area and Doho coals). However, in all cases, reactivity increases with increase in temperature.
Jauro and Chukwu (2011) investigated three Nigerian coals (Onyeama, Lafia-Obi and Gairin Maiganga) to determine their suitability in developing formed coke for use as blast furnace coke. Parameters that were usedincluded the shatter index, expressed as percentage stability and friability and mecum index. The highest cumulative percentage stability and the lowest cumulative percentage friability were observed in Lafia-Obi with values of 67.54% and 32.46%, followed by Onyeama with 66.92% and 33.08% and then Garin Maiganga with 55.04% and 44.96% respectively. Medium and low temperature carbonisation of Onyeama and Lafia-Obi coal samples gave an improved and satisfactory percentage stability and friability for semi-cokes.
Suleiman (1992) analysed the moisture content and volatile matter of some Nigerian coals. The results obtained showed that the three coal samples (Enugu, Lafia and Okaba) collected have low moisture contents, and only the Lafia coal has low volatile matter, thus satisfying the metallurgical requirement for the production of coke.
Isah (1991) carried out proximate and ultimate analysis on Enugu, Lafia and Okaba coals to determine the percentage of ash and total sulphur. The Eschka method was used to determine the total sulphur present. The analysis showed that Lafia coals have high ash and sulphur contents, while Enugu coals have low ash and sulphur contents. The Okaba coals have a little bit higher ash and sulphur contents than the Enugu coals, but much lower values when compared to Lafia coals.
CHAPTER THREE
MATERIALS AND METHODS
Coal Samples
Coal samples, each weighing 5kg,were obtained through the Nigerian Coal Corporation, Enugu, for five different deposits in Nigeria,which include Odagbo (Kogi State), Owukpa (Benue State), Ezimo (Enugu State), Amansiodo (Enugu State) and Inyi (Enugu State). The samples were stored in appropriately labeled air-tight containers to retain their as-received conditions.
600g of representative sample ofcoal from each site wasused at the laboratory of Advanced Coal Technology (now Bureau Veritas Testing and Inspections South Africa, BV-TISA), Icon Industrial Park, Sunderland Ridge, Pretoria, South Africa for proximate analyses, ultimate analyses, determination of gross calorific values, ash composition, ash fusion temperatures (in oxidizing and reducing atmospheres) and petrographic analysis. Thermogravimetric analysis was carried out at the Institute of Applied Materials of the University of Pretoria.
Results are reported as the average of duplicate tests as long as the results are within the repeatability limits set out in each ISO method.
General Sample Preparation
The principle of mixing and then dividing, using either mechanical or manual methods as described in ISO 13909-4:2001 was used to generate a representative pulverised sample of each coal, and then preparing each sample to the desired analytical specifications using an
array of sample preparation equipment. Crushing of the samples from a top-size of 150mm to a suitable size distribution range of 12.5 – 50.0mmwas carried out using an Osborn 10×6 inch jaw crusher shown in PlateVI. A double roller crusher, shown in PlateVII, was used to crush samples from a maximum top-size of 25mm to a size distribution of range of 1 – 6mm. A Retsch ZM200 ultra centrifugal mill, shown in PlateVIII, was then used to prepare samples from a top size of 6mm down to a suitable size distribution range of 150 – 212µm.
An automatic sieve shaker, shown in PlateIX, was used for sieving the samples to the desired size distribution required for each test while a hand rifler, shown in PlateX, was used to split the samples into representative sub-samples.
CHAPTER FOUR
RESULTS AND DISCUSSION
Proximate Analysis
The results obtained from proximate analyses ofthe five coal samples are graphically depicted in Figure 4.1(raw data are shown in Appendix A) on an air-dried basis while the calculated fuel ratio of each sample is shown in Figure 4.2.Fixed Carbon was calculated by difference using equation (3.1).
CHAPTER FIVE
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
Summary
Five coal samples from Odagbo (Benue State), Owukpa (Benue State), Ezimo (Enugu), Amansiodo (Enugu State) and Inyi (Enugu State) have been characterised to determine their suitability for power generation. Specifically, tests carried out on the coal samples include proximate, ultimate, ash composition, petrographic and thermogravimetric analyses. Calorific value and ash fusion temperatures were also determined. Results obtained were compared with requirements published by pulverised coal power plant operators around the world. Amansiodo coal sample had the highest gross calorific value of 27.48MJ/kg, followed by Owukpa, Odagbo, Ezimo and Inyi coal samples with 26.51MJ/kg,22.60MJ/kg, 20.96MJ/kg and19.39MJ/kg, respectively. Results of proximate analysis show that Owukpa coal had the highest volatile matter content of 39.1% while Inyi coal sample had the lowest moisture content of 3.8%.The highest fixed carbon content of 47.9% was recorded in Amansiodo coal. Elemental carbon was highest in Odagbo coal (67.82%).
Petrographic analysis results show that Owukpa coal had the highest huminite content of 75.8%, followed by Odagbo and Ezimo coals with 65.3% and 45.0%, respectively. Ezimo coal had the highest liptinite content (7.2%). Thermogravimetric analysis results show that Owukpa and Ezimo jointly recorded the lowest ignition temperature (283.63oC) while the lowest peak temperature at maximum weight loss rate was recorded in Odagbo coal (405.12oC). Odagbo coal also had the lowest burnout temperature of 504.60oC. The lowest ash fusion temperature recorded from analysis of all the samples was in the oxidising atmosphere with Odagbo coal (1461oC).
Conclusions
Based on analysis of tests carried out, the coals have been classified in termsof rank, grade and type. Amansiodo coal is a bituminous, low sulphur and medium ash coal; while Owukpa coal is a sub-bituminous A, low sulphur, low ash coal rich in huminites. In addition, Odagbo coal is a sub-bituminous B, medium sulphur, low ash coal rich in huminites; Ezimo coal is a sub-bituminous C, low sulphur, high ash coal; while Inyi coal is asub-bituminous C, low sulphur, high ash coal.
The ease of combustion of the coal samples in decreasing order is Odagbo, Owukpa, Inyi, Ezimo and Amansiodo. While the ignition temperatures of the coals increase with decreasing volatile matter content, their calorific values are strongly correlated with the fixed carbon, elemental carbon, volatile matter and hydrogen contents in decreasing order.
Benchmarking the properties of the five (5) coal samples against requirements of existing PC plants from around the world shows that these coals meet most requirements. Pulverised Coal (PC) Combustion is the most commonly used technology in coal-fired power plants and is a well developed technology that accounts for well over 90% of coal-fired capacity worldwide. Deficiencies observed may be corrected through beneficiation and/or blending with other appropriate coals.
Finally, the five (5) coal samples analysed can be used for power generation using circulating fluidised bed combustion (CFBC) technology. CFBCtechnology was developed to burn low grade and/or difficult-to-burn fuelsand can therefore tolerate a widevariety of coals and particle sizes.
Recommendations
From the conclusion of the International Energy Agency that developing economies have a particularly strong dependency on coal for power production, electricity generation from coal is the most feasible solution to the power supply problem bedeviling Nigeria. In keeping with its published Roadmap for Power Sector Reform, it is needful for the government to aggressively exploit Nigeria‟s abundant coal reserves for this purpose. Both pulverised coal combustion and fluidized bed combustion technologies may be deployed to ensure that Nigerian coals are used for generation of much-needed electricity.
Petrographic analysis of Amansiodo and Inyi coals, hardgrove grindability index and drop tube furnace tests of the five coal samples are recommended to complete the range of tests required for the characterization of these coals. In addition, a study of the characteristics of different ratios of blends of these coals with a superior bituminous coal is also recommended.
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