An assessment of chemical properties and hardgrove grindability index of Punjab coal.
Coal has prime importance among fuels in meeting the energy requirements of the whole world. It is by far the largest source of energy available to world economies today (Sanders et al., 2002). Its utilisation through several technologies like carbonisation, combustion, coal water slurry, fluidised combustion beds, gasification, liquefaction and beneficiation requires grinding of coal to finer size (Sengupta, 2002). Grindability of coal is a central characteristic of coal, which is closely related to pulverizer performance and mill capacity (ACARP, 1998) and is defined as ease of grinding of coal which is, in turn, linked to coal tenacity, fracture and hardness (Rubiera et al., 1999).
Grindability of coal is generally estimated by two indices: (a) work index in which energy consumption for obtaining product of constant fineness is measured; and (b) grindability index in which fineness of final product is measured for same grinding work of the standard samples. Determination of work index is time and labour consuming therefore, grindability index, most commonly known as hardgrove grindability index (HGI) is preferred for predicting the grinding behaviour of coal and energy consumption requirements of mill (Tichanek, 2008). HGI of coal is influenced by various factors including coal rank, composition and petrography, type and distribution of mineral matter particles and experimental conditions like temperature and pressure etc. Various physical and chemical properties of coal influence the value of HGI, including moisture contents, ash, volatile matter and fixed carbon, presence and proportion of various macerals and microlithotypes and composition and distribution of mineral matter (Jorjani et al., 2008; Hills, 2007; Sengupta, 2002). Blending of coal is also an important factor which affects the grindability of a given coal. Generally, blending of coal does not show weighted average or additive relation among the HGI values of individual coal component (Vuthaluru et al., 2003).
Rank of coal is an important factor which influences the grindability of coal. Generally, ease of coal grindability increases with the increase of rank but higher rank coals anthracite and lower rank coals lignite show more resistant to grinding than bituminous and sub-bituminous coals (Ozbayoglu et al., 2008). Sengupta (2002) performed HGI tests on more than 300 coal samples collected from all coalfields of India and investigated the effects of ash, moisture, volatile matter and fixed carbon on HGI values. Positive relation of HGI with moisture content and ash content of coal was observed, while the relation with volatile matter and fixed carbon was negative.
Earlier researchers have investigated the effects of maceral and microlithotypes on grindability of coal (Jorjani et al" 2008; Trimble and Hower, 2003; Hower, 1998; Hower and Calder, 1997; Gray and Patalsky, 1990; Hower and Wild, 1988). Generally, softness of coal increases with increasing vitrinite content and decreasing micronite and liptinite macerals. Ural and Akyildiz (2004) studied the effect of mineral matter on grindability of low-rank coals. They found a positive relation of grindability with mineral matter content especially with quartz.
This study deals with the assessment of hardgrove grindability index of Punjab coal reserves in relation to their chemical properties. Coal deposits of Punjab are located in two coalfields; salt range coalfield, and Makerwal coal field (Fig. 1).
The total coal reserves of Punjab are found to be 235 million tonnes of which 213 millions are deposited in salt range and 22 millions in Makerwal (Warwick and Husain, 1990). At present most of the mined coal from Punjab coalfields is utilised in brick making industry while part of it is being used in cement sector which needs comminution of these coals before their usage. Normally imported coal is blended with the local coal to acquire permissible limits of coal properties for their effective utilisation in cement industry.
Materials and Methods
Materials. A total of eight representative coal samples were collected from different coalfields of Punjab. Among these, three samples were taken from three different areas (Ara, Mahinwal-Basharat and Dalwal) of the eastern salt range and three from different coal mines situated in the Bhadrar area of the central salt range (Fig. 2), while the remaining two were collected from different sections of Makerwal coalfield.
Methodology. Proximate, analysis was performed on each sample of coal in accordance with ASTM standards (ASTM, 2004 D3173, 3174, 3175). All tests were conducted on air-dried basis. Total sulphur was determined using Eschka method (ASTM, 2004 E775-87). ASTM, 2004 (D409-02) was followed for determining hardgrove grindability index of coal samples. Each sample was crushed and screened to obtain a fraction of sample having size less than 4.75 mm, which was subsequently ground and screened to get an analytical sample of grain size of 0.6 to 1.18 mm. A charge of 50 g [+ or -] 0.01 g of coal was taken from analytical sample for performing grindability test in Hardgrove grindability machine (Fig. 3). After 60 revolutions of HGI grinder, the ground coal was screened for 10 min on 75 micron sieve. The weight of undersize was measured and HGI was calculated by following relation:
HGI = 13 + 6.93 (mass of -75[ micro]m fraction).
Results and Discussion
The results of proximate analysis and hardgrove grindability tests are shown in Table 1. The proximate analysis performed on eight coal samples collected from three different coal fields of Punjab reveal that most of the Punjab coal belongs to sub-bituminous category except the Tunnel C section coal of Makerwal and Iqbal Mineral coal of Dhalwal, which are high volatile B bituminous and lignite A category, respectively.
The outcome of HGI tests indicate that Makerwal and eastern salt range coals are softer coals while central salt range coals are relatively harder. It is interesting to note that Tunnel C section coal of Makerwal which has the highest heating value and volatile matter and the lowest total sulphur contents among the tested coal samples shows the highest value of HGI. It is due to the fact that bituminous coals are easier to grind (Ozbayoghu et al., 2008). PUNJMIN coal of Central Salt Range having the highest total sulphur content reveals the lowest value of HGI, indicating the hardest coal among the eight coal samples. This is probably due to the presence of most of the sulphur in the pyritic form in PUNJMIN coal.
Although, lignite coals show resistance to grinding (Ozbayoghu et al., 2008) but lignite coal of Dalwal presents higher value of HGI i.e., softer coal. This is due to the presence of higher ash content which increase the softness of Dalwal coal (Sengupta, 2002).
Figure 4 shows the various graphs drawn between HGI versus moisture content, volatile matter, ash contents, fixed carbon, total sulphur and gross calorific value. The general trends show positive relation with ash content, volatile matter and gross calorific value, while the relation is negative with moisture content, fixed carbon and total sulphur. The trend for volatile matter and ash content and fixed carbon is in accordance with that found by Sengupta (2002) but it is opposite for moisture content. The main reason for this difference is the range of moisture content in coal from 0.7 to 19%, while for Punjab coal it is from 1.0 to 4.5%, which indicates that effect of moisture on HGI vanishes at lower values of moisture and other parameters play prime role in determining HGI.
Chemical properties and hardgrove grindability index of various coal types of Punjab were determined. It was found that most of the Punjab coal belongs to sub-bituminous category and the HGI values vary from 57 to 92. HGI is frequently utilised in development design of grinding circuits for coal breakage. The results obtained from this study apparently indicate that grinding of Punjab coals is very smooth because its HGI values exceed 50 i.e., value at which most of the grinders are designed. Makerwal coal of Tunnel-C section is the most suitable coal for utilisation in cement sector and power industry as it has relatively high value of HGI with least total sulphur and ash contents. It was concluded that ash contents and volatile matter significantly, affect HGI of coal with moisture content less than 3% and fixed carbon less than 30%.
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Muhammad Shahzad *, Muhammad Mansoor Iqbal, Syed Alley Hassan, Shahab Saqib and Muhammad Waqas
Mining Engineering Department, University of Engineering & Technology, Lahore, Pakistan
(received April 15, 2013; revised October 2, 2013; accepted October 4, 2013)
* Author for correspondence; E-mail: email@example.com
Table 1. Proximate analysis and Hardgrove grindability index of Punjab coal Coal type Moisture Ash Volatile Fixed contents contents matter carbon (%) (%) (%) (%) Eastern Salt Range coal Modem Engineering Coal Mine, Ara 3.9 30.6 34.8 30.7 Hassan Kishor Coal Mine, Basharat 2.1 31.1 36.9 29.9 Iqbal Mineral Coal Mine, Dalwal 1.0 50.4 29.1 19.5 Central Salt Range coal (Bhadrar) PUNJMIN Coal Mine 3.0 31.9 33.8 31.3 Sangha Coal Mine 2.3 25.4 38.8 33.5 K. D. Well Coal Mine 2.8 18.5 43.2 38.3 Makerwal coalfield Aslam Karandi section 4.5 28.7 43.9 22.9 Tunnel-C section 2.9 13.0 48.7 35.4 Coal type Total Gross sulphur calorific HGI Coal class * (%) value (Kcal/kg) Eastern Salt Range coal Modem Engineering Coal Mine, Ara 3.82 5262 72 Sub-bituminous C Hassan Kishor Coal Mine, Basharat 3.93 5431 76 Sub-bituminous B Iqbal Mineral Coal Mine, Dalwal 5.66 3597 89 Lignite A Central Salt Range coal (Bhadrar) PUNJMIN Coal Mine 8.24 4750 57 Sub-bituminous C Sangha Coal Mine 6.24 5090 61 Sub-bituminous C K. D. Well Coal Mine 6.77 5350 63 Sub-bituminous B Makerwal coalfield Aslam Karandi section 4.51 4840 69 Sub-bituminous C Tunnel-C section 1.52 7466 92 High volatile B Bituminous * = based on ASTM classification (ASTM D388).
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|Author:||Shahzad, Muhammad; Iqbal, Muhammad Mansoor; Hassan, Syed Alley; Saqib, Shahab; Waqas, Muhammad|
|Publication:||Pakistan Journal of Scientific and Industrial Research Series A: Physical Sciences|
|Date:||Sep 1, 2014|
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