Demineralization of Brazilian Coals for Use in Gasification and Oxy

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DEMINERALIZATION OF BRAZILIAN COALS FOR USE IN GASIFICATION AND OXY-FUEL COMBUSTION PROCESSES, AIMING TO REDUCE CO2 EMISSIONS Keila Guerra Pacheco Nunes, Kirstin Milbradt Engel, Eduardo Osorio, and Nilson Romeu Marcilio Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01025 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

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DEMINERALIZATION OF BRAZILIAN COALS FOR USE IN GASIFICATION AND OXY-FUEL COMBUSTION PROCESSES, AIMING TO REDUCE CO2 EMISSIONS Corresponding author: Keila Guerra Pacheco Nunes Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS) R. Eng. Luiz Englert, s/n. Campus Central. CEP: 90040-040 - Porto Alegre - RS Brazil E-mail: [email protected] Kirstin Milbradt Engel Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS) R. Eng. Luiz Englert, s/n. Campus Central. CEP: 90040-040 - Porto Alegre - RS Brazil [email protected] Eduardo Osório Iron and Steelmaking laboratory (LASID), University of Rio Grande do Sul Av Bento Gonçalves, 9500. Porto Alegre – RS Brazil [email protected] Nilson Romeu Marcílio Department of Chemical Engineering, Federal University of Rio Grande do Sul R. Eng. Luiz Englert, s/n. Campus Central. CEP: 90040-040 - Porto Alegre - RS Brazil [email protected]

ABSTRACT Mineral matter in coal has an environmentally negative effect, as it is responsible for the generation of soot and slag. Technically, the mineral matter causes corrosion of equipment and reduces the combustion rate of coal, as well as influencing the sizing of 1 ACS Paragon Plus Environment

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equipment. Therefore, removing that unwanted portion of the carbon matrix would reduce the need for imports of high-quality coals and minimize transportation costs. This work aims to study the effects of the removal of mineral matter on the structure of southern Brazilian coal at the temperature of gasification and considering the kinetic parameters of the oxy-fuel combustion reaction. The demineralization process was conducted using solutions of HF, HCl and HNO3 with concentrations of 20 % (v). The samples were characterized by elemental analysis, proximate analysis, SEM, FT-IR, petrographic analysis, XRF and XRD. The removal of the mineral portion introduced major changes in the structure of coals, reflecting on the increase of the Boudouard reaction start temperature to 160 K. The oxy-fuel reaction was performed at temperatures of 873, 973, 1073 and 1173 K with concentrations of O2/CO2 ranging from 10 to 30 % (v/v). Before the demineralization for the oxy-fuel reaction the activation energy was 10.8 kJ.mol-¹. The results obtained with demineralized char was 56.46 kJ.mol-¹. Keywords: coal, oxy-fuel combustion, demineralization, kinetic. 1. INTRODUCTION Approximately 50 % of the electricity produced worldwide comes from nonrenewable resources. The use of this fuel type generates a large amount of environmental pollution and cause considerable damage to human health. The burning of coal alone is responsible for 35 % of the emissions of greenhouse gases 1. Environmental issues associated with the accumulation of CO2 in the atmosphere constitute the main reason to return attention to renewable energy sources and cleaner fuels.

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Because of the global energy situation, coupled with the fact that high-quality coal reserves are declining, the production process of an ultraclean coal has gained more prominence. Ultraclean coal is a by-product of coal whose main feature is low ash, approximately 0.1 % 2. Thus, this new raw material has the potential to be used directly in a gasification combined cycle, gas turbines, and advanced technologies such as pressurized fluidized beds, where the efficiency can reach 48 %. In this way, highash coals/high sulfur, unsuitable for combustion, gasification, or liquefaction, become environmentally acceptable after the demineralization process, since the CO2 emissions are reduced by 25 – 35 % 3. The mineral matter in coal is composed of clays, sulphites, carbonates, sulphates, pyrites, halogenides, chlorates, silicates, oxides, hydroxides and phosphate. These represent all the elements present in coal except for those in organic components, such as C, H, N, S and O. There are some macroconstituents whose concentrations usually exceed 1 % of the weight of the ash. The principal elements are Al, Ca, Fe, S and Si, followed by K, Mg, Na, P, and Ti 4. The mineral matter has negative effects on the environment and equipment because it generates slag and soot as by-products, causes corrosion in equipment and reduces the overall rate of combustion. On the other hand, the catalytic action of alkali metals, alkaline earth metals and transition metals can have a positive effect. These elements have an influence not only on the combustion process, gasification and liquefaction but also on equipment design. The goal of treatment for the removal of mineral matter is to produce a high-quality coal with a high calorific value, reduce the amount of fly ash and air pollutant precursors emitted, reduce the cost of operation and maintenance of burners in thermal generation plants, minimize transportation and storage costs, and reduce the need for imports of high-quality coals. In addition, the 3 ACS Paragon Plus Environment

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reagents used can be recycled, thus increasing the economics of the gasification and pyrolysis process 5. It is known that during the process of thermal decomposition of coal, its properties are altered, and the formation of free radicals provides information about the chemical structure of the product generated. Although the mechanism of the reactions is still unknown due to the complex structure of the macromolecules, studies show catalytic effects of some metals present in the carbon network. The metals K, Na, Mg, and Ca qualitatively and quantitatively influence the products of pyrolysis and gasification. According Samaras et al. 6, impurities, such as Fe, K, Mg and Ca, catalyse the gasification reaction. The heating of coal during gasification produces carboxylate salts in the ash and generates CO2, leaving other inorganic species highly dispersed 7. Samaras et al 6. investigated the effect of treatment with different acid solutions on the reactivity and development of lignite pores. Using different acids (HF, HCl, HNO3) one could observe the effect of each on the coal. Nitric acid promotes a greater removal of pyrite and sulfur. The hydrofluoric acid removes silicate, and hydrochloric acid removes sulfides and carbonates. The catalytic behaviour of the ash can also be observed. After 45 minutes of heating at 1173 K, a sample containing 22 % ash reached 100 % conversion. At the same temperature, for a sample containing only 0.14 % of mineral matter, 7 hours were required to reach 98 % conversion. With the aid of surface area and porosity analysis by the BET method and thermogravimetric tests, it can be concluded that demineralization contributes to increased porosity and reduced reactivity.


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Bolat et al 4. performed a chemical demineralization of Turkish coal with a high ash content using acid solutions with concentrations of 10, 20 and 30 % and acidic solutions combined with 0.5 N NaOH. The authors were able to observe that the degree of demineralization decreases with an increase in the concentration of the acid. This is due to a possible formation of stable compounds. It can also be noted that 10 % HCl removes more mineral matter that 10 % H2SO4 and 10 % HNO3, suggesting that smaller molecules more easily diffuse within the particles. In addition, calcareous minerals are abundant in coal and the reaction of CaO/CaCO3 with sulphuric acid creates insoluble gypsum. Haykiri-Açma et al. 8, using hydrofluoric acid and hydrochloric acid, studied the effect of demineralization on the reactivity of lignites. The authors observed an increase in the activation energy as the consequence of a reduced reactivity to combustion. There was an increase in porosity in the particle structure, but this did not affect the pore structure. Wu et al.

9

studied the effect of demineralization and the addition of a catalyst

on the formation of nitrogen compounds during coal pyrolysis and gasification of char. Coal has been demineralized with acid washing, and a catalyst has been added. A large effect on N2 emissions was observed during pyrolysis of these coals, reducing the emissions of that gas. The back-addition of 0.5 % Fe promoted the formation of N2, although the catalytic effect of the metal varied with the rank of the coal. According to the results for gasification, demineralization reduces the reactivity of coals of both high and low rank. Chemical processes for removing the ash are the most commonly used and include washing with acids, bases or other reagents. Table 1 shows chronologically the


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reagents used for the removal of mineral matter, according to each author and raw material.

Table 1 According to the information contained in Table 1, several authors performed the removal of the mineral matter contained in the coal using strong acids such as HCl and HNO3 for the removal of alkaline, earth alkaline and iron-containing compounds. For the removal of silica, HF is generally used and for the demineralization of biomass weaker acids such as organic acids have been used. In this case, the use of strong acids could affect the structure of carbonaceous materials such as cellulose, hemicellulose and lignin. The study of the demineralization of coal has already covered various areas, such as the formation of N2 during pyrolysis of demineralized coals

13

, the change in

chemical structure of coals after demineralization 11, and the effect on the formation of free radicals and phenols

43, 40

. Recent research has focused on the effect of

demineralization on the biomass structure

36, 38, 39, 42

, but little is known about the

influence of ash under oxy-fuel combustion conditions. The influence of metals and alkali metals in the coal gasification reaction is well known in the steel industry. Steelmaking initiates the production of pig iron in a blast furnace, where the CO generated by the coke gasification reaction (pyrolysis product of coal) is responsible for the reduction of Fe2O3 to FeO. To reduce the reagent consumption and promote the reduction of CO2 emissions during the production of steel, researchers have studied the impregnation of metals in coal samples and/or coke. Nevertheless, the influence of metals in the gasification, combustion or oxy-fuel combustion reactions of southern Brazilian coal for energy purposes is unknown. 6 ACS Paragon Plus Environment

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Southern Brazilian coal is mainly used in a powdered form for power generation in thermal power plants. Due to the high mineral matter content in these coal formations, the grinding process becomes very costly. Therefore, demineralization can give this low-rank coal a high quality for energy purposes, thus increasing the efficiency of power generation plants. This coal represents the largest energy reserves in Brazil, and thus, there is a constant search for processes that are more efficient from technical and economic points of view in order to improve the use of this energy resource. This work aimed to study the effects of the removal of mineral matter on the structure of southern Brazilian coal and its effect on the gasification temperature and to perform a kinetic analysis of the atmosphere during oxy-fuel combustion. The influences of the main metals present in the carbonaceous matrix in CO2 reactivity were assessed separately. 2. MATERIALS E METHODS 2.1. PREPARATION OF THE SAMPLE The coal used in this work is from the Candiota mine located in Rio Grande do Sul – Brazil. The Candiota mine has an estimated reserve of 1 billion tons of coal amenable to open-pit mining and is the largest coal mine in Brazil. The demineralization process was adapted from the procedures of Calahorro et al.

10

, Samaras et al. 6, Bolat et al.

4

and Asadieraghi and Daud

40

, where the authors

suggest the use of HF, HCl and HNO3 for the removal of mineral matter. During the process of removing the mineral matter of coal, the effect of the concentration of the acid solution was evaluated along with the particle size of coal and the reaction temperature. Acid solutions with concentration of 10 %, 20 % and 30 % by 7 ACS Paragon Plus Environment

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volume were prepared. Two granulometry fractions were selected in order to assess the effect of particle size during the removal of ash: the fraction A (0.212 – 0.250 mm) and fraction B (0.425 – 0.850 mm). The temperatures used were 293 K and 333 K. The tests were conducted as shown in Table 2.

Table 2 A coal sample of 50 g was weighed into a beaker to which was added 200 ml of HF solution. This solution was in contact with the coal for one hour under agitation. After the time determined, the reaction solution was filtered and washed with hot distilled water. The coal sample was returned to the beaker, and HCl was added to the solution. The coal was filtered and washed with hot distilled water after reaction for one hour with stirring. Finally, if HNO3 was added to the solution, the same experimental procedure as above was repeated. Finally, the coal was washed with hot distilled water and remained in the oven for 24 hours at 373 K for drying. After acid treatment, the sample was characterized according to proximate analysis, elemental analysis, SEM, petrographic analysis, FT-IR, XDR and XRF. 2.2.

INFLUENCE

OF

MINERAL

MATTER

ON

GASIFICATION

TEMPERATURE The demineralization process affects the petrographic composition of coal, a parameter that directly influences the reactivity of a coal sample 5. With the removal of mineral matter, the carbon content increases. In carbonaceous materials such as char from coal, the reactivity decreases with an increasing carbon content 45. To investigate the catalytic effect of ash on the Boudouard reaction (CO2 + C → CO2), the char gasification tests were conducted with CO2 in a thermobalance. During 8 ACS Paragon Plus Environment

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the experiments, an alumina crucible-type plate with a diameter of 17 mm was used. The reagent CO2 gas enters the top of the balance, and exhaust gases exit from the bottom. This arrangement seeks to minimize the diffusive effects that may occur during the char gasification reaction. The heating rate used was 20 K.min-¹ from room temperature to 1273 K with a CO2 flow of 100 ml.min-¹. 2.3. OXY-FUEL REACTIONS For the kinetic study of demineralized coals, a sample of char was first produced. The char (pyrolysed coal) was prepared in an atmosphere of N2 with a heating rate of 20 K.min-¹ from room temperature up to 1173 K. The sample remained at this final temperature for 1 hour. During the experiments, kinetic studies under the conditions of the oxy-fuel combustion reaction were performed on a thermobalance model NETZSCH 409 using an alumina crucible type plate. The reactant gases enter the top of the thermobalance, having direct contact with the sample of char. The influent flow of inert gas (N2) in the thermobalance was 100 ml.min-¹ during the pyrolysis step, and the influent flow of reactant gases (O2/CO2) was 100 ml.min-¹. The O2 concentrations used were 10 %, 20 % and 30 % v/v. The temperatures used in the oxy-fuel reaction combustion were 873, 973, 1073 and 1173 K. 3. RESULTS AND DISCUSSION 3.1. CHARACTERIZATION OF SAMPLES The characterization of samples before and after the demineralization process is of great importance to the understanding of the physical and chemical changes that occur in the structure of coal after acid treatment.


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The coal samples produced in tests 1 to 5 were characterized using proximate and elemental analyses; the results are in Table 3. TABLE 3 ¹ dry base ² obtained Nunes and Marcílio 46. Due to the high mineral matter content in their respective coal formations, the grinding process becomes very costly, as seen in Table 3. Calorific value analysis provides an indication that the process of removing the mineral matter contributes to an increase in the fuel coal power, as suggested by Spheigth 47. However, an economic analysis must be conducted in order to compare the costs of milling and the costs of coal demineralization. From the results presented in Table 3, it is possible to note a greater removal of mineral matter in test 4, with a reduction in ash content of 90 % after the acid wash. The fact that the degree of demineralization was not complete in either of the coals can be explained by the formation of stable and insoluble compounds, such as carbonates and sulfides, that remained in the sample, as mentioned by Bolat et al.

4

As the results of

tests 4 and 5 were similar, it was decided to treat the coal with less concentrated solutions. The reduction in the amount of ash generates a significant increase in the content of volatile matter and fixed carbon. This effect was also observed by Kizgut et al. 48 and Jiang et al.

36

, when they removed the mineral matter in coal and biomass samples. In

addition to the increase in volatile matter content, Kizgut et al.

48

observed an increase

in nitrogen and oxygen, which is the result of oxidation and nitration of the carbon


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matrix. As expected, there was an increase in the carbon content after the removal of mineral matter 2, 36. The increase in the nitrogen content can also be observed in this study. The authors Wijaya and Zhang 2 observed an increase in N and O contents in the chemical composition of the coal after treatment with HNO3. HNO3 is used in treatment after use of HF to remove possible insoluble compounds that have remained in the sample, mostly pyrite. The possible reactions that occur during treatment involving pyrite and HNO3 are: FeS2 + 2 HNO3 → Fe(NO3)2 + H2S + S

(1)

2 FeS2 + 6 HNO3 → 2 Fe(NO3)3 + 3 H2S + S

(2)

6 FeS2 + 30 HNO3 → 3 Fe2(SO4)3 + 3 H2SO4 + 30 NO + 12 H2O

(3)

Thus, the H2SO4 formed during reaction (3) reacts with HNO3 producing NO2+, causing the oxidation of carbon and nitrogen species forming a new structure, as shown in reaction (4). HNO3 + 2 H2SO4 ↔ NO2+ + H3O+ + 2 HSO4-1

(4)

The reduction in the sulfur content, present in the form of pyrite, may be confirmed by tests with XRD. The coal samples produced were also submitted to morphological analysis using a scanning electron microscope (SEM), as shown in Figure 1. With this test, the surface structure and particle porosity were evaluated. FIGURE 1


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By analysing the SEM images shown in Figure 1, it is noted that the acid washing carried out as described in test 5 (30 % concentration of acid solutions) left a less porous structure in the carbon sample. In test 4 (20 % concentration of acid solutions), the attack on the carbon surface was less vigorous, giving particles with a good porosity and a greater removal of mineral content. To characterize the composition of the ash remaining in the samples of tests 4, analyses with XRD and XRF were conducted. Figure 2 presents the X-ray diffractogram of the samples from the Candiota coal mine before and after the process of demineralization. From the incidence of peaks after acid washing, a significant reduction in the levels of Si (in the form of quartz) can be observed. After the demineralization step, the peaks relating to caolinita and pyrite are no longer observed, confirming the result obtained in the elemental analysis, where we observed a reduction in the level of S2.

Figure 2 In this work, it can be seen that the use of HNO3 to remove compounds such as pyrite was efficient for sample of the mineral coals used. The removal of the compounds content Al, Mg, K, Ca and Al showed good results with the use of HCl, as observed in the XRD analysis. According Mukherjee and Borthakur18, the HCl is an effective acid in demineralization due to the small molecules readily diffusing into the carbonaceous matrix. The results obtained with the XRD analysis can be confirmed by determining the chemical composition of the mineral matter remaining in the samples via XRF. The results are shown in Table 4.


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Table 4 The XRF analysis confirms the results observed in the XRD diffractogram in Figure 2. Analysing the results in Table 4, a reduction in the levels of all compounds is observed, as well as the emergence of Cl in the demineralized samples due to use of HCl in the process of removing mineral matter. Several authors have observed a large mass loss as a result of the use of acidic solutions for removal of mineral matter

2, 4, 6, 10, 36

. The authors reported a reduction in

organic matter content using HF in the demineralization process. According to Bolat et al. 4, in addition to this attack on silicon, the largest acid constituent of the mineral matter, it also promotes the dissolution of some organic species in coal. After the process of demineralization, the mass loss observed in the total sample was approximately 50 %. About 1 gram was lost during filtration and washing with distilled water. In the demineralized mass obtained, only 55 % remained of its initial size (granulometry B). Table 5 shows the maceral composition of the coal samples before and after the process of removal of their mineral matter.

Table 5 According to the results shown in Table 5, a large reduction in the vitrinite content was observed, indicating an increase in the inertinite fraction. According to Bengtsson et al.

50

, inertinite is the maceral responsible for a higher combustion

temperature, followed by vitrinite and liptinite. The influence of the reduction in the content of this maceral may be observed in the CO2 reactivity tests.


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As part of the organic matter was removed along with the mineral matter, it is believed that some chemical bonds have been broken and/or rearranged. To evaluate the formation of chemical bonds in the organic matter of the coals studied, Fourier transform infrared (FT-IR) tests were performed on the coals of Candiota before and after the process of demineralization. FT-IR diagrams are presented in Figure 3.

Figure 3 Analysing the FT-IR diagram for Candiota coal shown in Figure 3, it is observed that after the demineralization process, functional groups containing oxygen disappear, suggesting the formation of a polycyclic structure

11

. This phenomenon can be

confirmed by an increase in the intensity of the bands at approximately 1400 cm -¹ and 1650 cm-¹, characteristic of an aromatic C = C bond

39

. The band observed between

3000 and 3600 cm-¹ may be attributed to the stretching O - H of phenolic, alcoholic or carboxylic groups or possibly humidity present in the sample. Intense bands in the same region were also observed by the authors Asadieraghi and Daud

39

, after the

demineralization process of biomasses. 3.2

INFLUENCE

OF

MINERAL

MATTER

ON

GASIFICATION

TEMPERATURE Figure 4 shows the mass loss derived from the mass loss graphics with time for the demineralized coal.

Figure 4 It can be observed for both the coals that the pyrolysis step occurs between the temperatures of 373 and 1173 K. It is noted that this temperature is high enough for the 14 ACS Paragon Plus Environment

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complete removal of the mineral matter. The authors Seo et al. 50, Wu et al. 13, Salatino et al. 51 used pyrolysis temperatures in the range 1173 – 1223 K in studies on pyrolysis, gasification and reactivity with CO2. Radovic et al.

52

observed that in pyrolysis with a

fast heating rate, the amount of volatile matter released was independent of temperature. The temperatures used in the study were 973 – 1473 K. In Figure 4, the mass loss curve in a CO2 atmosphere for Candiota demineralized coal starts after 1173 K. Nunes et al.

53

studying Candiota ROM (Run of Mine) coal,

found that coal gasification is observed approximately 1013 K. An increase in gasification start temperatures of approximately 160 K is therefore observed for demineralized Candiota coal. This increase in temperature may indicate that the mineral matter would be acting as a Boudouard reaction catalyst, as suggested by Alonso et al

54

Wigley et al.

55

; Backreedy et al.

56

; Liu

57

; Su et al.

58

;

Vamvuka et al. 22. 4.6 Production of char from demineralized coal samples Table 6 present the characterization of char produced from the demineralized Candiota coal (test 4).

Table 6 ¹dry basis Considering the data presented in Table 6 in light of the results published by Nunes et al.

53

a reduction in surface area and an increased average pore diameter are

observed compared with the Candiota char.


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With the results obtained via the BET and BJH methods, the effect of the acid solution on the structure of the carbonaceous matrix can be observed, enlarging and decreasing the surface area. This effect was also observed by Jiang et al. 36. Calahorro et al

10

attributed the changes in the surface area of the char to the removal of inorganic

matter from the pores and the displacement of insoluble inorganic matter in the carbonaceous matrix to the pores. The HCl favours the increase in the micro- and mesoporous structure. The HNO3 and HF used in this work already contribute to a reduction in the surface area 10. In addition, the release of the volatile matter out of the coal during heating in N2 might result in widening of the pores decreasing specific surface area.

Figure 5 The images produced by the SEM shown in Figure 5 reinforce the results obtained through BET. Comparing the surface of samples of ROM and demineralized char, there is a reduction in the exposed surface of the demineralized samples as a result of the acid treatment process employed for the removal of mineral matter. With this result, during the oxy-fuel combustion reaction a change in reaction control to diffusion kinetics could be expected. Once the surface is not as available for the reaction and there is a widening in the pore diameter, one would expect a diffusion-reaction-control gaseous layer. 4.7 Thermogravimetric analysis of demineralized samples 4.7.1. Effect of temperature The temperatures used for the oxy-fuel combustion reaction were 873, 973, 1073 and 1173 K. The temperatures previously were selected based on thermogravimetric testing with the ROM samples, according to Nunes et al.

46

and Nunes and Marcilio 53. 16

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So that we could compare the results obtained with demineralized and ROM samples using the same temperatures and O2 concentrations in CO2. Figure 6 below shows the curves of the conversion of Candiota chars with time at temperatures of 873, 973, 1073 and 1173 K and concentrations of 10 % O2/CO2, 20 % O2/CO2 and 30 % O2/CO2.

Figure 6 Analysing Figure 6, it is observed that increasing the temperature also increases the speed of consumption of char, i.e., increases the reaction rate for the concentrations of 10 and 20 % O2 in CO2. This behaviour shows that the kinetic regime controls the reaction process. For the concentration of 30 % O2 in CO2, the effect of temperature becomes less evident and can be observed by overlapping the curves of the conversion rate. 4.7.2. Effect of O2 content in the gas mixture Figure 7 shows the curves of Candiota char conversion at different concentrations of O2 at the temperatures 873, 973, 1073 and 1173 K.

Figure 7 For the four temperatures studied, it was possible to observe that by increasing the concentration of O2 in the reagent gas an increase in the oxy-fuel combustion reaction rate occurs in the demineralized char samples. No overlap of the curves is observed with increasing temperature, as seen in the ROM samples and as stated by Nunes et al. 53. Despite the increase in O2 concentration, similar behaviour of the curves indicates the strong resistance to mass transfer. 4.7.3. Kinetic model 17 ACS Paragon Plus Environment

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The experimental data obtained for the demineralized samples were modelled according to the unreacted core model. According to Szekely and Evans,

(59)

the core

unreacted model has some deficiencies as to the limits of reaction control and does not consider solid characteristics, such as porosity and pore distribution, in the modelling. Since structural parameters are incorporated into the calculations, working with the poro model or grain model is suggested. Figure 8 presents the adjustments of the unreacted core model to experimental data, at the temperatures 873, 973, 1073 and 1173 K for the char from Candiota, in three situations: (a) when the diffusion in the outer gaseous layer of the particle controls the process, (b) when the diffusion of the reagent in the ash layer controls the process and (c) when the chemical reaction controls the process.

Figure 8 As seen in Figure 8(a), there is not a good fit of the model with the experimental data because the models do not adequately reproduce the experimental measures. In Figures 8(b) and 8(c), the fit of the model to the experimental data was similar, mainly at higher temperatures. In this case, there is a change from chemical reaction control to diffusion kinetics. The limiting step of the chemical reaction will be used to calculate the kinetic parameters of the reaction. Figure 9 shows a sample of char before the oxy-fuel combustion reaction and a sample of Candiota after the oxy-fuel combustion reaction in the thermobalance. As these samples had much of the mineral matter removed, there is a small reduction in the size of the particles.

Figure 9


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Energy & Fuels

Using the unreacted core model and considering that the reaction time (τ) for the complete conversion of the particle is obtained from equation (5), according to Levenspiel 60: =

5 12 −

where ρ is the particle specific mass of char determined via measurement by pycnometer (ρ = 2,50 g.cm-³ to Candiota char), R is the radius of the particle (R = 0,03375 cm), Cf is the fraction of fixed carbon (Cf = 0,84 for Candiota char), (-rA) is the reaction rate (g.cm-².min-¹) and τ is an experimental value obtained from the char conversion versus time curves, as shown in Figure 7 for Candiota coal. Thus, the reaction rate can be calculated using equation (6): − =

6 12

Assuming that the reaction rate (-rA) obeys equation (7) for the reaction C + O2 → CO2: − = 7

With equation (7), the value of the specific reaction rate k and the reaction order n can be determined, where PO2 is the partial pressure of O2. As the Boudouard equation occurs only at a temperature of 1173 K and above as noted in Figure 4 and this was the maximum temperature used, it will not be considered in the calculations of the occurrence of gasification. Using the linearized Arrhenius equation, the pre-exponential factor k0 and activation energy Ea can then be calculated for the oxidation reaction (8). 19 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

=

Page 20 of 43

!

"

#$ 8

Figure 10 shows the graph of the linearized Arrhenius equation. The results show that there is a satisfactory adjustment to the model. This indicates that the temperatures at which the tests were performed worked within the chemical scheme.

Figure 10 Analysing Figure 10, it is observed from the linear regression coefficient that a good fit exists between the Arrhenius equation and the data obtained for the reaction C + O2 → CO2 for the Candiota char. The results of the kinetic parameters of the oxy-fuel combustion reaction are shown in Table 7.

Table 7 Thus, the reaction C + O2 → CO2 is written as shown in equation (9): 56,46 − = 5,37.10* # 9 " The values for the order of the reaction, n, showed little variation with an average value of 0.8. No data were found in the literature for the kinetic parameters of demineralized coal. Table 8 presents the results obtained for the activation energy, Ea, and the preexponential factor, k0, for the oxy-fuel combustion reaction of the demineralized chars and ROM.

Table 8 1

Nunes et al.53

2

Nunes e Marcilio 46 20 ACS Paragon Plus Environment

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By analysing Table 8, an increase in the values of the activation energy and preexponential factor is observed after the removal of the mineral matter. With this result, it can be inferred that the ash previously present in the mined coal would be acting as an oxidation-reaction catalyst. 4.8. Candiota coal impregnating demineralized with metals Table 9 shows the impregnated metal content over the original samples (ROM) and demineralized samples.

Table 9 The levels of metals in the demineralized and impregnated samples were measured by atomic absorption. After the impregnation step, the samples were subjected to gasification tests to evaluate the effect of each metal on the CO2 reactivity. For comparative purposes, the same tests for ROM and demineralized samples were performed. Figure 11 shows the conversion rate curves for the ROM, demineralized and samples impregnated with the metals Al, Fe and Ca at a temperature of 1273 K.

Figure 11 Analysing the results shown in Figure 11, it is observed that the demineralization process reduces the coal conversion rate with respect to the ROM coal, thus decreasing its reactivity. This effect was also observed in the comparative curves in Figure 5. With respect to the impregnated samples, it was observed that calcium is a metal that promotes a faster coal conversion rate in a CO2 atmosphere, followed by the samples impregnated with iron and aluminium. In the ROM sample, there are possibly some inhibiting elements and other gasification reaction catalysts. This combination may 21 ACS Paragon Plus Environment

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Page 22 of 43

explain why the reactivity of the ROM sample is less than that of the impregnated sample with only one element, in this case calcium. 4. CONCLUSIONS Based on the results presented for the demineralization process of the southern Brazilian coal, it is concluded that the procedure adopted for the removal of mineral matter was satisfactory. A 90 % removal of ash content was obtained using a 20 % solution by volume of HF, HCl and HNO3. These removals were confirmed by XRD patterns obtained from XRD and XRF. Along with removal of the mineral portion contained in the sample, a loss of chemical groups was observed in the FT-IR diagram, as well as an increase in carbon content according to the proximate and elemental analyses. This increase in the carbon portion is responsible for the reduction in CO2 reactivity, demonstrated by an increase in temperature for the Boudouard reaction. An increase in the activation energy values was observed for the demineralized samples when subjected to the oxy-fuel combustion reaction under the same conditions as the ROM samples. Before the demineralization process the values for the activation energy and pre-exponential were 10.8 kJ.mol-1 and 13.817 gmol.min-1.cm-2, respectively. The new values for the activation energy and the pre-exponential factor, respectively, were 56.46 kJ.mol-1 and 5.37.10³ gmol.min-1.cm-2. Starting from demineralized Candiota coal, a sample was impregnated with an alkaline earth metal, Ca, a sample with a transition metal, Fe, and a sample with a semimetal, Al. This assessed the reactivity of the impregnated carbons in a CO2 atmosphere, where it was observed that calcium has a catalytic effect superior to the other metals used. The reaction rate of the ROM sample was higher than the reaction rate of the 22 ACS Paragon Plus Environment

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sample impregnated with iron and Al but lower than the reaction rate of the sample impregnated with Ca. It is possible that in the constitution of coal there are inhibitors and elements that act as catalysts for the Boudouard reaction. This combination could justify the difference in reactivity. REFERENCE (1) IEA – International Energy Agency. CO2 emission from fuel combustion, France, 2013. (2) Wijaya, N.; Choo, T.K.; Zhang, L. Generation of ultra-clean coal from Victorian brown coal: effect of hydrothermal treatment and particle size on coal demineralization and the extraction kinetic of individual metals. Energ Fuel. 2012, 26, 5028 – 5035. (3) Steel, K. M..; Besida, J.; O’Donnell, A.; Wood, D.G. Part I – Dissolution behavior of mineral matter in black coal toward hydrochloric and hydrofluoric acid, Fuel Process Technol, 2011, 70, 171 – 192. (4) Bolat, E.; Saglam, S.; Piskin, S. Chemical demineralization of a Turkish high ash bituminous coal, Fuel Process Technol, 1998, 57, 93 – 99. (5) Meshram, P.; Purohit, B.K.; Sinha, M.K.; Sahu, S.K.; Pandey, B.D. Demineralization of low grade coal – A review. Renew Sust Energ Rev. 2015, 41, 745 – 761. (6) Samaras, P.; Diamadopoulos, E.; Sakellaropoulos, G. P. Acid treatment of lignite and its effect on activation. Carbon. 1994, 32, 771 – 776. (7) Hengel, T.D.; Walker Jr., P.L. Catalysis of lignite char gasification by exchangeable calcium and magnesium, Fuel. 1984, 63, 1214 – 1220.


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Haykiri-Açma,

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Ersoy-Meriçboyu,

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(16) Wu, Z. The influence of mineral matter and catalyst on nitrogen release during slow pyrolysis of coal and related material: A comparative study, Energ Fuel. 2002, 16, 451 – 456. (17) Mukherjee, S. Demineralization and desulfurization of high-sulfur assam coal with alkali treatment, Energ Fuel. 2003, 17, 559 – 564. (18) Mukherjee, S.; Borthakur, P.C. Demineralization of subbituminous high sulphur coal using mineral acids, Fuel Process Technol. 2003, 85, 157 – 164. (19) Zhao, Z.; Qiu, J.; Li, W.; Chen, H.; Li, B. Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion, Fuel. 2003, 82, 949 – 957. (20) Das, P.; Ganesh, A.; Wangikar, P. Influence of pretreatment for deashing of sugarcane bagasse on pyrolysis products, Biomass Bioenerg 2004, 27, 445 – 457. (21) Yagmur, E.; Simsek, E.H.; Aktas, Z.; Togrul, T. Effect of demineralization process on the liquefaction of Turkish coals in tetralin with microwave energy: Determination of particle size distribution and surface area, Fuel. 2005, 84, 2316 – 2323. (22) Vamvuka, D.; Troulinos, S.; Kastanaki, E. The effect of mineral matter on the physical and chemical activation of low rank coal and biomass materials, Fuel. 2006, 85, 1763 – 1771. (23) Fierro, V.; Torné-Fernandes, V.; Celzard, A. Methodical study of the chemical activation of Kraft lignin with KOH and NaOH, Microporous and mesoporous materials, Micropor Mesopor Mat. 2007, 101, 419 – 431.


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(24) Wu, Z.; Steel, K.M. Demineralization of UK bituminous coal using HF and ferric ions. Fuel. 2007, 86, 2194 – 2200. (25) Keown, D.M.; Hayashi, J.; Li, C. Effects of volatile–char interactions on the volatilization of alkali and alkaline earth metallic species during the pyrolysis of biomass, Fuel. 2008, 87, 1187 – 1194. (26) Ahmed, I. I.; Nipattummakul, N.; Gupta, A. K. Characteristics of syngas from cogasification of polyethylene and woodchips. Appl Energ 2011, 88, 165 – 174. (27) Sun, S.; Zhang, J.; Hu, X.; Qiu, P.; Qian, J.; Qin, Y. Kinetic analysis of NO-char reaction, Korean J Chem Eng. 2009, 26, 554 – 559. (28) Eom, I.; Kim, K.; Kim, J.; Lee, S.; Yeo, H.; Choi, I. Characterization of primary thermal degradation features of lignocellulosic biomass after removal of inorganic metals by diverse solventes, Bioresource Technol 2011, 102, 3437 – 3444. (29) Liu, X.; Bi, X.T. Removal of inorganic constituents from pine barks and switchgrass. Fuel Process Technol. 2011, 92, 1273 – 1279. (30) Mourant, D.; Wang, Z.; He, M.; Wang, X.S.; Garcia-Perez, M.; Ling, K.; Li, C. Mallee wood fast pyrolysis: Effects of alkali and alkaline earth metallic species on the yield and composition of bio-oil, Fuel. 2011, 90, 2915 – 2922. (31) Suzuki, T.; Nakajima, H. Effect of mineral matters in biomass on the gasification rate of their chars, Biomass Conv. Bioref. 2011, 1, 17 – 28. (32) Majob, B. Chemical demineralization of high volatile Indian bituminous coal by carboxylic acid and characterization of the products by sem/eds, J Environ Res Develop. 2012, 6, 653 – 659.


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(33) Mandapati, R.N.; Daggupati, S.; Mahajani, S.M.; Aghalayam, P.; Sapru, R.K.; Sharma, R.K.; Ganesh, A. Experiments and kinetic modeling for CO2 gasification of Indian coal chars in the context of underground coal gasification, Ind Eng Chem Res. 2012, 51, 15041 – 15052. (34) Mayer, A.Z.; Apfelbacher, A.; Hornung, A. Effect of sample preparation on the thermal degradation of metal-added biomass, J Anal Appl Pyrol. 2012, 94, 170 – 176. (35) Wang, C.; Du, Y.; Che, D. Investigation on the NO reduction with coal char and high concentration CO during oxy-fuel combustion, Energ Fuel. 2012, 26, 7367 – 7377. (36) Jiang, L.; Hu, S.; Sun, L.; Su, S.; Xu, K.; He, L.; Xiang, J. Influence of different demineralization treatments on physicochemical structure and thermal degradation of biomass. Bioresource Technol. 2013, 146, 254 – 260. (37) Liu, Z.; Zhang, Y.; Zhong, L.; Orndroff, W.; Zhao, H.; Cao, Y.; Zhang, K.; Pan, W.P. Synergistic effects of mineral matter on the combustion of coal blended with biomass, J Therm Anal Calorim. 2013, 113, 489 – 496. (38) Reichel, D.; Klinger, M.; Krzack, S.; Meyer, B. Effect of ash components on devolatilization behavior of coal in comparison with biomass – Product yields, composition, and heating values, Fuel. 2013, 114, 64 – 70. (39) Asadieraghi, M.; Daud, W.M.A.W. Characterization of lignocellulosic biomass thermal degradation and physiochemical structure: Effects of demineralization by diverse acid solutions, Energ Convers Manage. 2014, 82, 71 – 82. (40) Liu, J.; Jiang, X.; Han, X.; Shen, J.; Zhang, H. Chemical properties of superfine pulverized coals. Part 2. Demineralization effects on free radical characteristics, Fuel. 2014, 115, 685 – 696. 27 ACS Paragon Plus Environment

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(41) Li, X.F.; Xu, Q.; Fu, Y.; Guo, Q.X. Preparation and characterization of activated carbon from Kraft lignin via KOH activation, Environ Prog Sustain Energ. 2014, 33, 519 – 526. (42) Kong, Y.; Kim, J.; Chun, D.; Lee, S.; Rhim, Y.; Lim, J.; Choi, H.; Kim, S.; Yoo, J. Comparative studies on steam gasification of ash-free coals and their original raw coals, Int J of Hydrogen Energ. 2014, 39, 9212 – 9220. (43) Bai, Y.; Yan, L.; Li, G.; Zhao, R.; Li, F. Effects of demineralization on phenols distribution and formation during coal pyrolysis, Fuel, 2014, 134, 368 – 374. (44) Prationo, W.; Zhang, J.; Abbas, H.A.A.; Wu, X.; Chen, X.; Zhang, L. Influence of external clay and inherent minerals on lignite optical ignition and volatile flame propagation in air-firing and oxy-firing, Ind Eng Chem Res. 2014, 53, 2594 – 2604. (45) Sahu, S.G.; Chakraborty, N.; Sarkar, P. Coal–biomass co-combustion: An overview, Renew Sust Energ Rev. 2014, 39, 575 – 586. (46) Nunes, K. G. P.; Marcílio, N. R. Determination of the kinetics parameters of oxyfuel combustion of coal with a high ash content, Braz J Chem Eng. 2015, 32, 211 – 223. (47) Spheigth, J.G. Handbook of coal analysis, New Jersey, 2005. (48) Kizgut, S.; Baris, K.; Yilmaz, S. Effect of chemical demineralization on thermal behavior of bituminous coals, J Therm Anal Calorim. 2006, 86, 483 – 488. (49) Bengtsson, M. Combustion behavior for a coal containing a high proportion of pseudovitrinite, Fuel Process Technol. 1987, 15, 201 – 212.


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(50) Seo, D.K.; Park, S.S.; Kim, Y.T.; Hwang, J.; Yu, T. Study of coal pyrolysis by thermo-gravimetric analysis (TGA) and concentration measurements of the evolved species, J Anal Appl Pyrol. 2011, 92, 209 – 216. (51) Salatino, P.; Senneca, O.; Masi, S. Gasification of coal char by oxygen and carbon dioxide, Carbon. 1998, 36, 443 - 452. (52) Radovic, L.R.; Steczko, K.; Walker, P.L.; Jenkins, R.G. Combined effects of inorganic constituents and pyrolysis conditions on the gasification reactivity of coal chars, Fuel Process Technol. 1985, 10, 311 – 326. (53) Nunes, K.G.P.; Osório, E.; Marcílio, N.R. Kinetic of the oxy-fuel combustion of high-ash content coal from the Candiota – RS mine, Energ Fuel. 2016, 30, 1958 – 1964. (54) Alonso, M.J.G.; Borrego, A.G.; Alvarez, D.; Parra, J.B.; Menendez, R. Influence of pyrolysis temperature on char optical texture and reactivity, J Anal Appl Pyrol. 2001, 58 – 59, 887 – 909. (55) Wigley, F.; Williamson, J.; Gibb, W.H. The distribution of mineral matter in pulverized coal particles in relation to burnout behavior, Fuel. 1997, 76, 1283 – 1288. (56) Backreedy, R.I.; Jones, J.M.; Pourkashanian, M.; Williams, A. Burn-out of pulverized coal and biomass chars, Fuel. 2003, 82, 2097 – 2105. (57) Liu, X.; Zhang, M.; Lu, J.; Yang, H. Effect of furnace pressure drop on heat transfer in a 135 MW CFB boiler. Powder Technol. 2015, 284, 19 – 24. (58) Su, S.; Pohl, J.H.; Holcombe, D.; Hart, J.A. A proposed maceral index to predict combustion behavior of coal, Fuel. 2001, 80, 699 – 706.


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(59) Szekely, J., Evans, J. W. A structural model for gas — solid reactions with a moving boundary, Chem Eng Sci, 1970, 25, 1091 – 1107. (60) Levenspiel, O. Chemical Reaction Engineering, 3th ed., New York: John Wiley & Sons, 1976.

Table 2: Reagents used for the removal of mineral matter, according to each author, in chronological order. RAW AUTHORS

YEAR

REAGENTS MATERIAL

Hengel and Walker 7

1984

HF, HCl

Coal

Calahorro et al 10

1987

HF, HCl, HNO3

Coal

Kister et al 11

1987

HF, HCl

Coal

Kusakabe et al. 13

1989

KOH, NaOH, HCl

Coal

Samaras et al 6

1994

HF, HCl, HNO3, CH3COOH

Coal

Sentorun, et al. 12

1996

HCl, HF

Coal and biomass Wu and Ohtsuka 13

1997

HCl

Coal

Bolat et al 4

1998

HF, HCl, HNO3, H2SO4, NaOH

Coal

Haykiri-Açma et al 8

2000

HCl, HF

Coal

Linares-Solano et al. 14

2000

HF, HCl

Coal

Davidsson et al. 15

2002

CH3COOH

Biomass

Wu et al 16

2002

Ethanol, HCl

Coal


Page 31 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Wu et al. 9

2003

HCl

Coal

Mukherjee 17

2003

NaOH, KOH, HCl

Coal

Mukherjee; Borthakur 18

2003

HCl, HNO3, H2SO4

Coal

Zhao et al 19

2003

HF, HCl

Coal

Das et al. 20

2004

HF

Biomass

Yagmur et al. 21

2005

HF, HCl

Coal

HCl, CH3COOH

Biomass

Vamvuka et al 22

2006 HCl, HF

Coal

Fierro et al. 23

2007

H2SO4

Biomass

Wu; Steel 24

2007

HF

Coal

Keown et al. 25

2008

H2SO4

Biomass

Ahmad et al. 26

2009

HCl

Coal

Sun et al. 27

2009

HF, HCl

Coal

Eom et al. 28

2011

HCl, HF

Biomass

Liu and Bi 29

2011

CH3COOH

Biomass

Mourant et al. 30

2011

HNO3

Biomass

Suzuki and Nakajima 31

2011

HCl

Biomass

Majob 32

2012

C6H12O7, C6H8O7, EDTA, Coal CH3COOH, HO2CCO2H

Mandapati et al. 33

2012

HCl, HF

Coal

Mayer et al. 34

2012

HCl

Biomass

Wijaya et al 2

2012

acid pyroligneous, C6H8O7, Coal EDTA-Na

Wang et al. 35

2012

HCl, HF

Coal

Jiang et al 36

2013

CH3COOH, HCl, H2SO4, HNO3,

Biomass


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 43

H3PO4 Coal and Liu et al. 37

2013

HCl biomass Biomass and

Reichel et al.38

2013

HCl, HF Coal

Asadieraghi and Daud 39

2014

H2SO4, HClO4, HF, HNO3, HCl

Biomass

Liu et al 40

2014

HCl, HF

Coal

Li et al. 41

2014

H2SO4 e NaOH

Biomass

Kong et al. 42

2014

1-methylnaphthalene

Biomass

Bai et al. 43

2014

HF, HCl

Coal

Prationo et al. 44

2014

HF, HCl

Coal

Table 2: Experiments carried out for the removal of mineral matter in coal Candiota. Test

Concentration of the solution (%)

Granulometry

1

Heating (K) 298

B 2

10

3

323 A

298

4

20

B

298

5

30

B

298

A = 0.212 – 0.250 mm B = 0.425 – 0.850 mm


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Energy & Fuels

Table 3: Composition of coal produced after the acid wash. Levels (%) Analysis¹ ROM

Test 1

Test 2

Test 3

Test 4

Test 5

B

B

B

A

B

B

Volatile matter

21.1

34.8

38.3

42.4

41.4

45.7

Ash

52.9

17.8

9.3

8.2

5.7

5.1

Fixed carbon

26.1

47.4

52.4

49.4

52.9

49.2

C

35.3

66.3

65.2

57.6

65.5

64.0

N

0.7

3.3

4.2

3.8

4.2

4.8

S

1.6

1.5

1.3

0.9

1.1

0.9

H

3.9

4.6

4.7

4.0

4.6

4.3

O

58.5

24.3

24.6

33.7

24.6

26.0

Calorific value (cal.g-¹)

3186

-

-

-

5292

-

Granulometry Proximate

Elemental

¹ dry base ROM – Run of mine A = 0.212 – 0.250 mm B = 0.425 – 0.850 mm Table 4: Analysis of XRF for ROM and demineralized samples of Candiota. Levels (%) Elements ROM

Demineralized

SiO2

28.93

5.34

Al2O3

7.91

0.47

Fe2O3

2.91

0.07


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Page 34 of 43

SO3

3.36

1.91

TiO2

0.79

0.12

K2O

0.90

0.07

CaO

0.79

0.02

MgO

0.30

-

ZrO2

0.05

-

Y2O3

0.03

-

-

0.17

54.12

91.81

Cl CO2* * organic matter

Table 5: The maceral composition of coal samples before and after the process of removing mineral matter. Maceral group (%)

Coal ROM

Coal demineralized

Vitrinite

72

66

Liptinite

15

11

Inertinite

13

23

Table 6: Characterization of char produced from the coal demineralized of Candiota. Analysis

Levels (%)

Proximate¹ Volatile matter

3.1

Ash

9.4

Fixed carbon

87.5

Elemental¹


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C

86.2

H

0.8

N

2.3

S

0.6

Others

10.1

BET – surface area(m²g-¹)

6.05

BJH – average pore diameter (nm)

3.47

¹dry basis Table 7: The kinetic parameters for the reactions C + O2 → CO2 Candiota char demineralized. T (K)

n

873

0.7

973

0.8

1073

0.8

1173

0.9

Ea = 56.46 kJ.mol-1 k0 = 5.37.10³ gmol.min-1.cm-2

Table 8: Kinetics parameters of oxy-fuel combustion reaction of Candiota char ROM and demineralized samples. ROM 1

Demineralized

Ea (kJ.mol-¹)

10.8

56.46

k0 (gmol.min-1.cm-2.atm-n)

13.8

5.37.10³

Kinetic parameters

¹ Nunes et al.59


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Table 9: Levels of metals in the demineralized and impregnated samples. Levels (%) Metals Demineralized

Impregnated

Al

0.5

1.0

Fe

0.07

0.7

Ca

0.02

0.7

Figure 1: Samples of coal demineralized from mine Candiota.


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Figure 2: Result of the analysis of XRD for coal sample mine of Candiota before and after demineralization.

Figure 3: Diagram of FT-IR for ROM and demineralized Candiota coal.


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Figure 4: Curves of mass loss and derived from mass loss on time for the demineralized coals of Candiota.

Figure 5: Image obtained using SEM for Candiota char samples (a) ROM and (b) demineralized.


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Figure 6: Conversion rate for Candiota demineralized chars.


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Figure 7: Conversion rate curves versus time for the samples of char of Candiota demineralized in temperatures of 873, 973, 1073 and 1173 K, respectively.


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Figure 8: Unreacted core model for Candiota char demineralized.


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Figure 9: Demineralized char of Candiota before and after the oxy-fuel combustion reaction at temperature of 973 K.

Figure 10: Arrhenius equation fit to the data obtained by MNR under chemical reaction control for oxy-fuel combustion reaction.


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Figure 11: Conversion rate curves versus time for Candiota ROM coal, demineralized and impregnated in temperature of 1273 K.


Demineralization of Brazilian Coals for Use in Gasification and Oxy

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