Desulfurization of thar lignite by oxidative alkali leaching under pressure

ABSTRACT In this research work, thar coal was treated with NaOH solution in dissolved oxygen with different reaction parameters in an isothermal reactor. All three types of sulfur were removed from coal through oxidative alkali leaching under pressure. About 90% pyretic sulfur, 78% organic sulfur and 50% sulfate sulfur were removed, and more than 82% of total sulfur removal was achieved. Effects of various reaction parameters were observed, it was investigated that reaction time, agitation speed and partial pressure of oxygen have positive effects on the rate of desulfurization. Whereas desulfurization was increased with increase in reaction temperature and alkali concentration till optimum value as 120°C for temperature and 2.5 Molarity of NaOH solution for alkali concentration . While desulfurization was decreased with further increase in values of temperature and alkali concentration. The rate of desulfurization was observed maximum with minimum particle size of 40 µm. An Optimized reaction was performed reaction time 120°C oxygen partial pressure 200 psi, particle size 40 µm, NaOH concetration was 0.25 Molarity, reaction time was 3 hours agitation speed was 2000 rpm, after getting the optimum values of all reaction parameters. Processed and raw thar coal was characterized with Thermogravimetric Analysis (TGA) and Testo smoke number to investigate the combustion behavior of processed coal. It was observed that the combustion properties of processed coal were improved than that of raw coal and black smoke in processed coal was reduced.


Introduction
Coal is a highly abundant fossil fuel on the earth's surface and has been used since long, it is about 70% of the total known fossil fuel resources in the world (Franco and Diaz 2008). Even it is reported that the coal reserves are more than other fossil fuels like oil and gas, which are more than enough to face the present energy requirement (Rehman et al. 2018). Still now, coal is fulfilling the requirements of energy generation. About more than 40% of electric power plants are utilizing coal fuel (Meshram et al. 2015). Pakistan is the seventh largest country in coal reserves, about 186 billion tons of coal are present in Pakistan and mostly 97% of coal is lignite (Malkani 2012). Hence Pakistan becomes the top lignite coalbearing country in the world. Thar is the largest coal mine in the country, about 175,506 million tons of coal are to be estimated. Thar coalfield spreads over 9000 km 2 and has 20 maximum coal seams. The ultimate and proximate composition of the coal varies from block to block and with different depths (Malkani and Malik 2017). Before the discovery of Thar coalfield, Pakistan did not appear in the list of coal-rich countries (Munir et al. 2017). Thar lignite coal contains a variable amount of sulfur concerning block and depth. The presence of sulfur in coal may enhance the calorific value of coal, but it causes serious destruction to the ecological system as production of SOx and hydrogen sulfide during combustion. Therefore it is needed to remove or reduce the Sulfur amount from coal (Mukherjee and Borthakur 2001).
Sulfur in coal is present in different forms, like inorganic Sulfur (mostly in the form of pyrite), organic Sulfur (mostly present in trace amounts). Organic Sulfur in coal is present as thiol, sulfide, thiophenol, thiophenes and disulfide in different ranges (Kozłowski, Wachowska, and Yperman 2003). Coal desulfurization before combustion is important to avoid air pollution (Prasassarakich and Thaweesri 1996). Sulfur is covalently bonded with carbon (C-S bond) in these compounds, this bond is difficult to break down by any physical process. Hence it requires chemical processing for cleaving off the bond for Sulfur removal. Chemical leaching has the ability to remove the minerals that are strongly bound to the coal, and that cannot be removed easily with the physical method (Mukherjee and Borthakur 2003).
Repetition of chemical leaching process causes a reduction in the organic sulfur content due to the breakage of C-S bonds, which tends to decrease or even eliminate sulfur from the coal (Kozłowski, Wachowska, and Yperman 2003). Samite (Mukherjee and Borthakur 2003) found that alkali leaching is highly effective for pyritic sulfur removal, he leached the Indian coal with KOH 16% solution followed by acid treatment at 150°C, about 90% inorganic sulfur and 11% organic sulfur could be removed. He stated that demineralization of Indian coal was not found as high as with NaOH. Sharma achieved the 75% demineralization of Indian coal by leching with NaOH (Sharma and Gihar 1991) .70% pyritic Sulfur removal by NaOH leaching was reported by Li and Cho, at the reaction conditions of 0.4 Molarity of NaOH solution and at 90°C temperature (Xia and Xie 2017). Mursito performed leaching under pressure (hydrothermal treatment) of Banten Indonesian high sulfur coal. The total sulfur was reduced up to 90% at 0.025 mol/g of NaOH (Mursito, Hirajima, and Sasaki 2010). B. R. Utz (Liu et al. 2008) found the chemistry of molten alkali hydroxide for organic species in the coal. And found that organic sulfur present in the coal is very attractive to oxidation. (Saikia, Khound, and Baruah 2014). Oxidative desulfurization is promising technology for removing sulfur from coal in economical way (Taylor 2014), and can remove most of the pyritic sulfur as well as organic sulfur (Chiri and Schlegel 2017). Mketo (Joshi et al. 1983)  performed the leaching of South Africa coal with sodium hydroxide and hydrogen peroxide (as oxidative agent) and could achieved 55% sulfur reduction. M. Kozloski (Kozłowski, Wachowska, and Yperman 2003) has leached the American coal with potassium liquid ammonia solution and observed that organic sulfur (thiols) are to be easily converted into soluble sulfides. Lui (Benassi, Fiandri, and Taddei 1997) achieved the removal of 73% organic sulfur, and 83% pyritic sulfur by oxidative alkali leaching of Yangzhou (Eastern China) coal in dissolved oxygen and then further treated with acid reflux. He stated that loss of organic sulfur may be occurred due to removal of thiols compounds present in the coal that are very attractive to oxidation. They are easily converted to the disulfides under basic condition. He observed that mostly organic sulfur compounds on oxidation produces sulfoxides, sulfones, sulfonic acids, sulfonic acid, and sulfenic acid depending upon the nature of organic compound and oxidizing agent and conditions, although S = O group is common in the oxidized products. J. B. Joshi (Ahmad et al. 2019) has performed the selective oxidative reaction of pyrite in aqueous slurry and could remove most of the pyritic sulfur at 3600 second reaction time and at 0.68 MPa of dissolved oxygen. The desulfurization of coal with oxidative method highly reduces its corrosive effects (Utz, Friedman, and Soboczenski 1986). A detail mechanism of oxidation of organic disulfides and thiols of coal was investigated by the Rois Benassi (Wheelock 1981) he stated that oxidation of organic disulfides and thiols are to be converted into sulfonic acid as final product. The reaction of pyritic desulfurization is given by the equation 1 and 2. In first step pyrite reacts with dissolved oxygen and forms the precipitate layer of hematite and sulfuric acid, then sulfuric acid immediately neutralizes with alkali (NaOH) and produces sodium sulfate and water. The sodium sulfate is soluble in water and hematite forms precipitate in the solution.
Pyrite is naturally effective to react with water and oxygen (Andersen, Halpern, and Samis 1953). Chuang (Chiri and Schlegel 2017) reported that rate controlling step is the diffusion of oxygen through the product hematite (Fe 2 O 3 ) layer at temperature from 120°C to 180°C and partial pressure of oxygen from 0.41 MPa to 1.04 MPa. It was also observed by J.E Andersen (Chuang, Markuszewski, andWheelock.1983 1983) while he was leaching the galena with sodium hydroxide he concluded the kinetics of chemical reaction that, reaction of metallic sulfide (PbS) with Oxygen is in first step, while neutralization with alkali sodium has secondary effect on kinetics of reaction arising from its influence on the solubility and transport of oxygen with in the solution, and also hydrate (water) is the product of the reaction due to the neutralization. He calculated the activation energy for the reaction to be 6300 calory per mole.
K. C. (Hsu et al. 1977) Chuang leached the American coal alkaline NaOH solution in dissoved oxygen and air. He observed that rate of reaction (oxidation reaction) was proportional to the oxygen partial pressure, hence he decided that in first step pyrite reacts with oxygen forms hematite and sulfuric acid and then immediately this acid is to be neutralized with alkali, the reaction was first order reaction. The desulfurization of coal with oxidative method highly reduces its corrosive effects (Utz, Friedman, and Soboczenski 1986)

Materials and Method
Coal was collected from the site of Thar coal, lumps were crushed in jaw crusher, the crushed coal was dried in an oven at 110°C and then ground in ball mill to get pulverized coal. The pulverized coal was screened through standard sieving process to get different particle size from 30-40 µm to 200-400 µm. A basic solution of NaOH was prepared in distilled water with different concentrations from 0.1Molarity to 1Molarity for different runs of the experiment.
Raw coal was characterized by ultimate and proximate analysis and heating values to study the coal as presented in Table 1. The sulfur was determined by D2492, according to this method coal sample was dissolved in the HCL and HNO 3, Sulfate sulfur is soluble in HCL while Pyritic sulfur is soluble in Nitric acid, and organic sulfur is not soluble in acids. The organic sulfur is determined the difference. Proximate analysis by D7582, Ultimate analysis by D3176, and gross calorific value of coal by D5868, that was 3624.55 (Kcal/Kg). At the end raw coal and products were characterized with thermogravimetric analysis, and Testo smoke number to study the combustion behavior of coal.
An isothermal hot jacketed tank reactor was used to perform this reaction, manufactured by Edibon International. The total capacity of the reactor was about 3 lit. The reactor tank was made up of glass and jacketed with hot paraffin oil circulation to achieve and maintain the required temperature. The hot paraffin oil was circulated by centrifugal pump fixed at the base unit and was heated by immersed electrical heater in hot oil tank. A three-stage agitator was immersed into the reactor tank to agitate the solution, powered by a motor with speed regulator. Reactor tank has different valves for oxygen, reactant inlet, product outlet and for sampling. In this experiment, the solution and raw coal were fed manually. The temperature of circulating oil and reacting solution were taken with thermocouple and their reading was displayed on attached computer. The flow rate of oxygen was measured by rotameter installed in equipment.
Pulverized coal and prepared basic solution were poured into the reactor manually from the top of the reactor tank as shown in Figure 1. The heater was kept on heating circulating oil until the reaction temperature was achieved and oxygen flow was started. The reaction time for the process was different for each run. As soon as the reaction was completed, circulation of hot oil, gas flow and agitator all were turned off and solution was cooled at room temperature and product was withdrawn from the reactor. The solution was filtered with grade 1 filter paper to separate filtrate liquid and coal cake. Coal was washed with distilled water and air-dried for 24 hours and then oven dried at 110°C and then characterized in laboratory.

Effect of Alkali Concentration
A series of experiments were conducted to observe the effect of alkali concentration on desulfurization of Thar coal. The pulverized coal with particle size of 40 µm was treated with different concentrations of NaOH solution taken from 0.1 Molarity to 0.4 Molarity. Temperature was kept constant at 120°C while reaction time was maintained for 1.5 hour, and dissolved oxygen partial pressure was 50 psi and 100 psi respectively. It was observed that pyritic sulfur reduction was 34%, organic sulfur reduction was 8% and total sulfur reduction was 17% while sulfate sulfur could not be reduced while treating only with distilled. In the presence of NaOH solution and dissolved oxygen pyrite sulfur removal was achieved 67% at 0.25 M and 50 psi partial pressure of oxygen. Furthermore, at 100 psi of oxygen pyritic removal was about 70.3% and total sulfur removal was improved from 51% to 55.5%. Organic sulfur reduction was 32% at 50 psi and at 0.25 M and increased up to 35% sulfur reduction at 100 psi and at 0.25 M. The maximum sulfate sulfur reduction was 35% at 100 psi and 0.25 Molarity as shown in Fig. 1 and Fig. 2. Maximum sulfur reduction was occurred at 0.25 molarity of NaOH and further increase in molarity of NaOH showed decrease in sulfur reduction. Hence the 0.25 Molarity of NaOH was found as optimum value for alkali concentration for this reaction. It was observed that rate of coal desulfurization was increased with increase in concentration of alkali solution till optimum value of 2.5 Molarity of NaOH solution. And at higher concentration of alkali, the sulfur content in coal starts to increase. It means that desulfurization tends to reduce as shown in Fig. 2 and Fig. 3. Similar behavior was observed by Mukherjee (Mukherjee and Borthakur 2001), while leaching the Makum (Assam India) coal with different concentrations of NaOH, and found that demineralization of coal was increased with increase in alkali concertation, till optimum value, and then it tends to reduce due to the formation of [Na a (AlO 2 ) b (SiO 2 ) c NaOH.H 2 O] sodium aluminosilicate precipitates. E. Andersen reported the decrease in desulfurization degree due to an increase in alkali concertation after optimum values owing to less solubility of oxygen at higher concertation of alkali (Chuang, Markuszewski, andWheelock.1983 1983). TD Wheelock performed leaching of US coal in dissolved oxygen, and observed that desulfurization was  increased with increase in alkali concentration till optimum value (0.35 Molarity of sodium carbonate), and then at a higher value of alkali concentration it tends to reduce rapidly (Mukherjee and Borthakur 2004). Mukherjee had reacted the Boragolai and Ledo (Indian) coal with sodium hydroxide and observed that the desulfurization of coal increased with increase in alkali concentration from 2% to 8% NaOH solution, while at 16% it startd to decrease in Boragolai coal. He also noted that NaOH was more effective for pyritic sulfur removal as compared to organic and sulfate sulfur removal (Kara and Ceylan 1988).

Effect of Reaction Time
Eight experiments were conducted with a difference of 30 minutes to study the behavior of reaction time on desulfurization. During reaction (leaching), coal particle size was 40 µm, the molarity of NaOH solution was 0.25 M and the temperature was 120°C. While oxygen pressure was fixed at 50 psi and 100 psi for each run, respectively. It was observed that the influence of time on sulfur removal was uniform, i.e. with the increase in reaction time, desulfurization was also increased. Although the rate of desulfurization in pyrite was higher than that of organic and sulfate sulfur reduction as shown in Fig. 3 and Fig. 4. At maximum time of 4 hours, around 72.6% pyrite was reduced at 50psi, and 74% pyrite was removed at 100 psi. The maximum organic sulfur reduction was 48% and sulfate sulfur reduction was 50%. And total sulfur reduction was achieved up to 62.8% as shown in Fig. 4 and Fig. 5. The behavior of higher sulfur reduction due to higher reaction time was also investigated by Mukherjee and Borthakur (Kara and Ceylan 1988). He noted that the rate of desulfurization was increased with an increase in leaching time. Rehman and Waqar (Rehman et al. 2018) observed a similar effect in leaching the Chola Saidan Shah coal, they used the Quadratic module response (ANOVA) to analyze their experimental data. And investigated that while increasing the reaction time, also increased the degree of desulphurization, he concluded his results that when coal particle remained within the hot alkali solution for a longer time, more sulfur removal occurred. Husein Kara (Anwar et al. 2020) treated four Turkey coals (Beysehir, Dadagi, Ermenek and Ilgin) with NaOH, and discerned that Dadagi coal and Beysehir coal showed an increase in desulfurization of coal with increase in treatment time. Effect of reaction time on desulfurization, at partial pressure of oxygen 100 psi, particle size 40µ, NaOH concentration 0.25 Molarity, temperature 120°C and at 500 grams in 50ml solution of NaOH.
M. Guru (Utz, Friedman, and Soboczenski 1986) leached the Turkey Askale coal with nitric acid as an oxidant agent and he found that desulfurization was increased with increase in leaching time. The concentration of nitric acid was 65% the optimum leaching time was 16 minutes and he removed about 40% sulfur from Askale coal.

Effect of Particle Size
Four experiments were conducted to observe the effect of particle size on desulfurization. Different particle size for each experiment was selected, ranging from 40 µm to 400 µm and all other parameters were kept constant. The partial pressure was 100 psi, treatment time was 2 hours, NaOH concentration was 0.25 Molarity and temperature was 120°C. It was noted that desulfurization was decreased with increase in Thar coal particle size. Or in other words at smaller particle size, the rate of desulfurization was higher. The amount of sulfur content with respect to the particle size is shown in Fig. 6.
Joshi observed that the fine particle size requires less reaction time for the conversion of pyrite to ferric oxide as compared to coarse particle size (Ahmad et al. 2019). Faraz Anwar (Gürü, Sariöz, and Çakanyildirim 1008) leached the coal (from Jehlum district) with potassium hydroxide with different particle sizes from 60 to 220 meshes, he came to know that when particle size was decreased the rate of desulfurization was increased, he could remove as maximum as 75% of total sulfur. Pattarapan (Prasassarakich and Thaweesri 1996) treated the Mae Moh coal (from Lampang Thailand) with sodium Benz oxide, he also observed that amount of desulfurization increases with decrease in particle size, he used the particle size from 250 µm to 850 µm, temperature from 190 o C to 250°C, and was able to remove sulfate and pyritic sulfur up to 65% to 85% respectively. However organic sulfur removal was up to 30%. He concluded that the rate of reaction was inversely proportional to the coal particle size, and the mass transfer rate was inversely proportional to the square root of coal particle size, however, optimum particle size should be decided for desulfurization, because a sufficient amount of energy is required for grinding and weighing of coal during size reduction. Chuang (Tai, Graves, and Wheelock 1977) observed that the time for complete conversion of FeS 2 to hematite depends upon the square root of particle size. Guru (Karaca and Akyu 2003) performed the leaching of Turkey coal with H 2 O 2 as an oxidative agent and investigated that rate of desulfurization was decreased with increase in particle size.

Effect of Temperature
The effect of temperature on desulfurization of Thar coal by oxidative alkali treatment is shown in Fig. 7(a) and 7(b), different experiments were conducted to investigate the effect of temperature on desulfurization. It was observed that the degree of desulfurization was increased with increase in temperature till an optimum value, then it tends to reduce. The optimum value for temperature was 120°C and other parameters were; particle size 40 µm, time was 1.5 hour and 3 hours, partial pressure of oxygen was 100 psi and agitation speed was 700 rpm, the flow rate of oxygen was 1.2 liters per minute, the ratio of Thar coal to alkali solution was 50 grams of coal in 500 ml of alkali solution, it was observed that sulfur reduction was increased with increase in temperature from 90°C to 120°C, the maximum sulfur reduction was observed at a temperature of 120°C. And then desulfurization rate was decreased at a temperature higher than 120°C as shown in figure.
The decrease in desulfurization at higher temperature (more than 120°C) may be due to the lower solubility of oxygen at higher temperature as reported by T.D Wheelock (Mukherjee and Borthakur 2004). A similar effect was observed by Waqas Ahmed (Utz, Friedman, and Soboczenski 1986) that desulfurization of coal was increased till an optimum value and then it tends to reduce, he selected the Lakhra coal for his study and treated the Lakhra coal with three different alkali compounds sodium hydroxide, sodium carbonate, and potassium hydroxide at different concentrations, time and temperatures. He investigated that its optimum temperature was 40°C and about 50% sulfur was removed at that temperature, although at higher temperature desulfurization tends to reduce. This behavior of temperature was also observed by Semra Karaca (Li, Sun, and Jia 2010), he leached the Askale Turkish coal with nitric acid as an oxidative agent at different temperatures and observed that desulfurization was increased with increase in temperature and it was started Sulfur Removal (%) Figure 5. (a). Sulfur removal % at maximum reaction time 4-hour, partial pressure of oxygen 100 psi, particle size 40µ, NaOH concentration 0.25 Molarity, temperature 120°C (b) Sulfur removal % at maximum reaction time 4-hour, partial pressure of oxygen 50 psi, particle size 40 µm, NaOH concentration 0.25 Molarity, temperature 120°C.
to reduce at a higher temperature. He found that the optimum temperature was 103°C at 25% solution of nitric acid and investigated that at higher temperature the leaching agent starts to destroy. At the initial stage, an increment in desulfurization with increase in temperature was observed and then an equilibrate was observed by Zhiling Li (Ahmed et al. 2008). It was also observed that at 1.5 hour pyrite sulfur reduction was about 70.3%, organic sulfur reduction was 44%, sulfate sulfur was 35% while the total sulfur reduction was achieved about 57%. Although at 3-hour time and at 120°C temperature the pyritic sulfur reduction was improved up to 80.6%, organic sulfur reduction up to 66%, and sulfate sulfur up to 45%, while total sulfur was reduced up to 70% as shown in Fig. 8 (a) and 8 (b).

Effect of Partial Pressure of Oxygen
The partial pressure shows a great influence on the desulfurization, a sufficient reduction in all pyritic, sulfate and organic sulfur was observed at a higher value of partial pressure of oxygen. About 5 different experiments were conducted to study the behavior of oxygen partial pressure. The pressure of oxygen was from 25 psi to 200 psi, while other parameters were constant as; the temperature was 120°C, the Figure 6 concentration of NaOH was 0.25Molariy, particle size was 40 µm, reaction time was 3 hours, oxygen flow was from 1 liter per min to 2.5 liter per min (different for each value of oxygen pressure), agitation speed was 700 rpm, and Thar Figure 7 lignite coal to solution ratio was 50 grams of coal to 500 ml of NaOH solution.
It was investigated that the higher the value of oxygen partial pressure, lower the value of sulfur in processed coal (higher rate of desulfurization). Although the pyritic sulfur reduction was higher than that of organic sulfur and sulfate sulfur. The behavior of sulfur reduction with respect to the oxygen partial pressure was uniform as shown in Fig. 10. About 88% pyritic sulfur was removed at 200 psi partial pressure of oxygen. The organic sulfur reduction was achieved up to 74%, while sulfate sulfur removal was about 50% and around 79.8% of total sulfur was removed at that pressure as shown in Fig. 9. The related behavior was observed by Chaung (Chiri and Schlegel 2017), that when the oxygen partial pressure was raised the conversion rate of pyrite to hematite was also increased. He leached the iron pyrite (of Lova state) with sodium carbonate in dissolved oxygen, and found that more than 75% sulfur from pyrite was removed. And concluded that (a) Effect of temperature on desulfurization of Thar coal at partial pressure of oxygen 100 psi, NaOH concentration of 0.2 Molarity, agitation speed 700 rpm, flow rate of oxygen 1.2 liter per minute, particle size 4 µm, coal to solution ratio was 500 grams in 50 ml of solution of NaOH, reaction time was 1.5 hours. (b) Effect of temperature on desulfurization of Thar coal at partial pressure of oxygen 100 psi, NaOH concentration of 0.2 Molarity, agitation speed 700 rpm, particle size 4 µm, reaction time was 3 hours.
the rate of desulfurization of pyrite depends up on the overall rate of diffusion of dissolved oxygen. This study temperature effect at two different oxygen partial pressures indicated that more organic sulfur was removed at the higher oxygen partial pressure than that at the lower oxygen partial pressure. These results suggested that oxygen partial pressure has an important effect on the removal of both organic and inorganic sulfur. Sulfur Removal (%) Figure 8. (a) Sulfur removal % at an optimum reaction temperature of 120°C, partial pressure of oxygen 100 psi, particle size 40 µm, NaOH concentration 0.2 Molarity, and reaction time 1.5 hours. (b) Sulfur removal % at an optimum reaction temperature of 120o C, partial pressure of oxygen 100 psi, particle size 40 µm, NaOH concentration 0.2 Molarity, and reaction time 3 hours.

Effect of Agitation Speed
The agitation speed has also a great influence on desulfurization. To study the effect of agitation speed Thar coal was leached with 0.25 Molarity of NaOH, leaching time was 3 hours and oxygen partial pressure was 100 psi, the oxygen flow rate was 1.2 liters per minute, leaching temperature was 120°C, ratio of raw Thar coal to NaOH was 50 grams in 500 ml of solution and particle size was 40 µm, the speed of agitation was different for each experiment ranged from 150 rpm to 2000 rpm. At 2000 rpm the pyritic sulfur reduction was achieved about 78.8% and sulfate sulfur reduction was about 60% while the organic sulfur reduction was observed as 58% and total sulfur reduction was achieved up to 70% as shown in Fig. 12. It was observed that with an increase in agitation speed from 150 rpm to 1500 rpm the rate of desulfurization was also increased and above 1500 rpm it became constant and did not change as shown in Fig. 11. Hence 1500rpm was found to be an optimum speed of agitation. Sulfur reduced slightly in organic as well as in sulfate sulfur, while in pyritic sulfur, the great sulfur reduction was observed. Ali Ahmed (Ken et al. 2018) leached the Lakhra coal with Hydrogen peroxide (H 2 O 2 ) and nitric acid was used as Oxidative agents for desulfurization of coal, he investigated that desulfurization of coal was increased with increase in rate of agitation. K. Chuang (Tai, Graves, and Wheelock 1977) Figure 11. Effect of agitation speed on desulfurization of Thar coal at NaOH concentration 0.2 Molarity, particle size 4 µm, coal to solution ratio 50 grams in 500 ml solution of NaOH, reaction time 3 hours, leaching temperature 120°C, oxygen partial pressure 100 psi. American coal with dilute sodium carbonate solution in dissolved oxygen under pressure, he studied the effect of agitation on desulfurization rate, he observed that coal desulfurization was increased rapidly with increase in agitation.

Optimize Reaction
After getting the optimum value of each factor. An optimized reaction was performed to get the practical optimized value of sulfur removal from Thar coal. In this reaction the temperature was 120°C, partial pressure of oxygen was 200 psi, reaction time was 4 hours, particle size was 4 µm and agitation speed was 1500 rpm. After lab analysis of product coal, pyrite sulfur reduction was achieved up to 90.3% (0.17% pyrite was left in processed coal), and organic sulfur was removed up to 78% (0.11% organic sulfur left in processed coal) and sulfate sulfur removal was up to 50%, while total sulfur removal was achieved up to 82%, and the processed coal contained only 0.83% total sulfur. This data shows that oxidative alkali leaching is very effective for pyritic sulfur removal as well as for organic and sulfate sulfur reduction.

TGA of Raw Coal and Processed Coal
Desulfurization of coal effects on the combustion behavior of coal (Gundogar and Kok 2014), the thermogravimetric test is a well-known technique to identify the various important changes in the combustion of coal owing to coal desulfurization (Cong et al. 2018), TGA gives the mass of fuel changes during combustion as a function of time, to study this variation the raw coal and processed samples were characterized by TGA test as shown in Fig. 13. Raw coal and three processed samples were selected for this study. Sample 1 was coal that was treated at optimum temperature 120°C for three hours, having the composition as; pyrite sulfur 0.34%, sulfate sulfur 0.14%, organic sulfur 0.17%, and total sulfur 0.65%. Sample 2 was processed coal that was treated at an optimum pressure of 200psi, its composition was; pyrite sulfur 0.21%, sulfate sulfur 0.1%, organic sulfur 0.13% while total sulfur was 0.44%. And sample 3 was the processed coal that was treated under optimized conditions with composition as; pyrite 0.17%, sulfate sulfur 0.1%, organic sulfur 0.11% and total sulfur 0.38%. It was observed that in raw Thar lignite coal a little amount of mass loss was observed up to the temperature of 150°C, this mass loss was due to the decomposition of light volatile matters, as observed by Kulnil (Sikarwar et al. 2018) and then shows the static behavior (no slope in TGA graph) of coal till the temperature reached 400°C, that showed the thermal stability (Marinov et al. 2010) (Ozbas, Kök, and Hicyilmaz 2002) of Thar lignite till 380°C. After 380°C mass of coal was rapidly reduced, about 75% of coal mass lost as the temperature was further raised about 300°C (up to 680°C), this was owing to the rapid combustion of coal between 380°C to 680°C, this mass loss was due to release of volatile matter and burning of coal and this demonstrates coal combustion region or primary reaction region in TGA test as reported by Kok [41]. Hence the peak combustion temperature of Thar lignite was 380°C, after that temperature the Thar coal started to degrade. While in processed coal loss of mass with a rise in temperature was smooth as compared to raw coal. Kok and Hicyilmaz observed that coal leaching showed a significant effect on coal combustion, they studied the raw and cleaned coal by TGA characterization and found that the coal leaching reduced the peak combustion temperature and showed the smooth mass reduction with the temperature increase. The similar behavior was observed by Bhupendra (Gundogar and Kok 2014) while examining the TGA analysis of raw Indian coal and processed coal with KOH of various concentrations, he observed that coal mass was rapidly lost from temperature 330°C to 520°C, while coal samples that were processed with KOH for longer time showed some uniformity of mass loss per temperature rise.

Testo Smoke Number
The raw coal and treated coal were characterized with testo smoke number to study the difference in combustion smoke of raw coal and two samples of treated coal. Sample 1 was treated at optimum pressure of 200psi with composition as; pyrite sulfur 0.21%, sulfate sulfur 0.1%, organic sulfur 0.13% and total sulfur 0.44%. And sample 2 was treated under optimum condition, its composition was; pyrite 0.17%, sulfate sulfur 0.1%, organic sulfur 0.11% and total sulfur 0.38%.
The Thar coal was characterized with testo smoke number. Initially, the smoke number of raw Thar coal was 2, but after 5 minutes it became 6, and after 10 minutes smoke of raw coal was totally black and was measured as 9 on testo smoke number, hence it reached the highest number of coal smoke. Similarly, the testo smoke number of processed coals was also observed, initially, the testo smoke number of the sample was 1 after five minutes the smoke number became 4 and after ten minutes the smoke number was reduced to 3 and the sample was very clean than that of raw Thar coal. Sample 2 also showed similar results initially but after five minutes the smoke Figure 14 number was 5 and then after ten minutes it became 3. So, the testo smoke number showed that smoke property of processed coal was enhanced and the coal become environmental friendly.

Conclusion
Not only pyritic sulfur could be removed by this alkali oxidative leaching, but also sulfate and organic sulfur were removed simultaneously. About 90.3% pyritic sulfur was removed and other sulfur contaminations types present in coal as organic sulfur was removed up to 78% and sulfate sulfur was removed 50% and total sulfur removal was up to 82.5%. Hence a sufficient amount of sulfur reduction was achieved.
As the reaction starts pyrite reacts with water and oxygen to form the hematite and sulfuric acid, this sulfuric acid immediately neutralizes with sodium hydroxide (NaOH) and forms sodium sulfate precipitate (noncombustible sulfur) and water molecules. Oxygen plays an important role in a chemical reaction and is responsible to convert the pyrite to hematite, owing to this maximum amount of pyritic sulfur and total sulfur was removed at a higher amount of oxygen partial pressure. While studying the effect of various factors individually, almost in every case pyritic sulfur was removed in greater amount than that of sulfate or organic sulfur.
Desulfurization was maximum at higher values of reaction time, partial pressure of oxygen, and agitation speed. The value for partial pressure for maximum desulfurization was found to be 200 psi, reaction time was 4 hours at an agitation speed of 1500rpm. Although desulfurization was observed to be greater with smaller particle size. The optimum value for NaOH concentration was 0.25 Molarity and that of temperature was 120°C.
Raw and processed coal samples were Characterized with thermogravimetric analysis to investigate the change in the ratio of mass reduction per time, it was observed that mass reduction per unit time in processed coal is uniform as compared to raw coal. And also raw coal and processed coal sample were characterized with Testo Smoke number to investigate the quality of smoke during combustion. It was noted that processed coal smoke quality was enhanced as compared to raw coal.