Introduction

Coal and char gasification has been studied extensively over the past decades and is an alternative technology to produce liquid hydrocarbons (gasification with Fischer–Tropsch synthesis). Catalytic coal gasification has also been studied in order to gain more insight and understanding into the reactivity of coal–char. The use of minerals during coal–char gasification has been shown to lower gasification temperatures and also to overcome the slow reaction of carbon with CO2 [1]. The addition of some minerals produces more porous coal–chars, thus enhancing char’s reactivity with CO2 [2]. Kinetic models consisting of elementary steps of the overall gasification reaction of carbon with CO2 have been proposed by many researchers [3–6]. However, these studies have focused predominantly on Northern Hemisphere vitrinite-rich coals and the inertinite-rich Southern African coals need to be investigated.

South African coals have high mineral matter contents, sometimes in excess of 20 % [7, 8]. The most abundant minerals are clays, carbonates, sulphides and quartz. Mineral matter in coal has a significant effect on coal utilization processes such as combustion, coking and gasification [9]. Mineral matter comprises of all inorganic material and elements present in coal as discrete (crystalline and non-crystalline) mineral phases [10] and may occur in coal as minerals, mineraloids and as organically associated inorganic elements [9, 10]. During heat treatment, these minerals may react and are transformed into new mineral phases [9–11]. Some disadvantages of the mineral phases formed during coal processing include: fouling, slagging, corrosion of equipment and the reduction in the overall rate of coal conversion processes. However, some of the mineral matter may exhibit a catalytic effect on the coal thermal processes [8, 9].

Leaching of the raw coal with hydrochloric acid (HCl) and hydrofluoric (HF) acid has been shown to alleviate the problem of mineral matter associated with coal. Acid leaching removes most of the inorganic constituents (mineral matter) without causing significant changes to the carbonaceous part of coal [12]. Acid leaching of coal to remove mineral matter from the coal sample is used to prepare coal samples for investigations regarding the effect of added minerals or inorganic compounds on the further thermal processing of the carbon part of the coal.

Potassium-containing minerals and salts can act as gasification catalysts for vitrinite-rich coal [10, 13]. However, potassium compounds are also known to react with mineral matter in coal, resulting in catalytically inactive silicates [13, 14].

Although South Africa is the fifth largest coal producer worldwide, there is comparatively little scientific knowledge of its inertinite-rich coals [7, 12, 15, 16]. No literature of thermogravimetric-mass spectrometry (TG-MS) studies on the effect of potassium salt additions on the hydrogen gas composition of gasification products from South African inertinite-rich coal could be found. This paper reports on a study investigating the influence of added potassium carbonate and potassium chloride on the hydrogen gas evolution during the CO2 reaction with the char derived from an inertinite-rich South African bituminous coal from a Highveld coalfield. To remove the influence of minerals and inorganic compounds present in the coal sample, the coal sample was acid treated to remove these before the potassium salts were added to the coal to prepare the char sample. The thermal behaviour of the acid-treated coal sample and the acid-treated coal samples with added potassium compounds was compared to give a relative indication of the influences of the potassium compounds on the gasification reactivity of the coal.

Experimental

Coal sample and demineralization

A typical Permian-aged South African Highveld inertinite-rich medium rank C bituminous coal was chosen for this study (particle size <75 µm). In order to obtain a coal sample with low mineral matter content, the coal was demineralized using an acid treatment method of HCl and HF [7, 12]. The coal sample (500 g) was leached with 4 dm3 of a 5 mol dm−3 HCl solution for 24 h, and the solution filtered in a polypropylene Buchner funnel under vacuum. The residue was leached with 4 dm−3 of a 5 mol dm−3 HF solution for 24 h, and the solution was again filtered under vacuum using a Buchner funnel. The resulting residue was again leached with 4 dm3 of 5 mol dm−3 HCl for 24 h, after which the product was filtered under vacuum and washed with large quantities of distilled water until a constant pH was observed. The resulting product was then dried overnight in an oven at 80 °C. All the leaching steps were carried out at room temperature, and the resulting leached coal samples were stored in a desiccator under nitrogen in a dark place until use.

Sample preparation

Potassium carbonate (K2CO3) and potassium chloride (KCl) were chosen as the inorganic salt dopants. The salts were dried in an oven overnight at 120 °C prior to all experiments. Catalytic salts were added to the demineralized coal prior to charring. The coals and salts were mixed together using a mortar and pestle and an electric mixer until uniform mixing. Samples with loadings of 0.5, 1, 3 and 5 potassium ion mass percentages for each of the salts were prepared.

Thermogravimetric (TG) analysis

Thermogravimetric (TG) studies of the coal samples were carried out using a TA Instruments’ SDT-Q600 thermogravimetric analyser. Approximately 10 mg of sample was used per experiment. The TG chamber was purged with nitrogen (purity >99.9 %) for 10 min to ensure an inert atmosphere, and the temperature was allowed to equilibrate (at room temperature). Char samples were prepared by heating the coal samples to 900 °C at a rate of 10 °C min−1 using a nitrogen flow rate of 75 mL min−1 followed by holding the temperature for 15 min at 900 °C. The samples were then allowed to cool down (in situ) to room temperature under the nitrogen atmosphere, to prevent the sample from oxidizing. Once the sample had cooled down to room temperature, the nitrogen flow was switched to CO2 (purity >99.9 %). The temperature and the masses of the resulting samples were allowed to stabilize, and after stabilizing, the sample was heated under CO2 flow to 1200 °C at a rate of 10 °C min−1 and a CO2 flow rate of 20 mL min−1.

The mass loss as a function of temperature was recorded. All samples were subjected to the same experimental conditions, and multiple runs were performed to ensure repeatability. The averaged thermogravimetric curves were used as relative measures to compare the effect of addition of the potassium salts on the thermal behaviour of the coal samples. This method of relative comparison of coal’s thermal behaviour is often used in coal science [4, 5, 16].

Mass spectrometry (MS)

A Cirrus mass spectrometer with a dual Faraday and multiplier detector was used. The mass spectrometer was attached to the outgas port of the TG instrument, and the evolved gases were simultaneously detected as the thermogravimetric curves were obtained. The evolved gaseous species from the sample in the TG were fed through the outgas port, via a heated capillary to the mass spectrometer. The hydrogen evolved was detected and recorded from the beginning of the reaction under the CO2 atmosphere up to a temperature of 1200 °C.

Results and discussion

Coal characterization

The original and acid-leached coal samples were submitted for proximate and ultimate analyses. These analyses were used to determine the effect of acid leaching on the mineral matter content in this coal sample. Proximate and ultimate analyses results are summarized in Table 1 and are consistent with previous findings of an inertinite-rich South African coal [7, 8].

Table 1 Normalized proximate and ultimate analysis results of raw coal and acid-treated coal
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A substantial reduction in the ash content (from 22.6 % to <3 %, dry basis) was observed from the proximate results, indicating that a large proportion of the inorganic matter was removed by sequential leaching of the coal sample.

The leaching method has been reported not to affect the coal molecular structure, except for a slight increase in –COOH content [12]. This increase in –COOH functional groups on the coal surface may be attributed to some oxidation during the acid leaching process.

Effect of catalyst loadings on the coal–char derived from demineralized coal

Thermogravimetric (TG) curves

The TG curves for the K2CO3- and KCl-loaded demineralized coal–char samples are presented in Figs. 1 and 2, respectively. No significant mass loss was observed below 600 °C for all the samples. These results are consistent with the fact that carbon gasification (C–CO2 reactions) only occurs at 700 °C and above [3–5]. Note that all the mass loss curves are reported on a potassium salt-free basis.

Fig. 1
figure 1

Thermogravimetric mass loss curves of the char derived from the acid-treated coal with K2CO3 loadings during heat treatment in a CO2 atmosphere

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Fig. 2
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Thermogravimetric mass loss curves of the char derived from the acid-treated coal with KCl loadings during heat treatment in a CO2 atmosphere

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The average final remaining unreactive contents of the K2CO3 experiments were observed to range between 1 and 9 % (Fig. 1) and that of the KCl-loaded char samples range between 4 and 8 % (Fig. 2). Approximately 3 % of this remaining unreacted part of the coal–char samples is due to the ash yield (as measured using proximate analyses) and the rest is due to <6 % of the coal carbon structure that does not react under these conditions. The results are consistent with the literature and as expected for the heterogeneous nature of the coal molecular structure [7, 15, 16].

Figures 1 and 2 indicate catalytic behaviour by K2CO3 and KCl on the CO2 gasification reactions of the coal–char samples, as the reactions are observed to occur over a smaller temperature range using the same experimental conditions. An increase in rate of conversion (decrease in temperature range of the reaction) of the coal–char with increasing potassium salt loadings (up to 5 potassium ion mass percentage) was also observed.

Differential thermogravimetric (DTG) curves

The first derivatives of the TG curves, the DTG curves, for the K2CO3- and KCl-loaded coal–char samples were obtained to further evaluate the thermal behaviour of these samples. DTG curves of the K2CO3- and KCl-loaded demineralized coal–char samples are presented in Figs. 3 and 4, respectively.

Fig. 3
figure 3

DTG curves of char derived from the acid-treated coal with K2CO3 loadings during heat treatment in CO2 atmosphere

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Fig. 4
figure 4

DTG curves of char derived from the acid-treated coal with KCl loadings during heat treatment in CO2 atmosphere

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The DTG curves enable determination of the ‘characteristic’ temperatures of interest. These are the temperature at which maximum rate of mass loss (T max) took place (under the specific experimental conditions), as well as the temperature at the initial (T i ) and termination (T f) of mass loss. Figure 5 presents an illustration of how these temperatures were determined.

Fig. 5
figure 5

Illustration of characteristic temperatures on a DTG curve

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It was observed that the conversion of the sample with no potassium salt loading occurred over a broader temperature range as opposed to that with catalyst additions (i.e., char conversion of the potassium salt-loaded samples occurs over a narrower temperature range). These characteristic temperature values are listed in Table 2.

Table 2 Temperatures at initial (T i), termination (T f) and maximum (T max) rate of mass loss/°C
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The DTG curves showed that when the catalyst loading was increased, the temperature at maximum rate of mass loss was also lowered. The relative gasification reactivities (1/T max) were calculated from T max on the DTG curves for the different K2CO3- and KCl-loaded samples, and the variation of the relative gasification reactivity with potassium salt loading is presented in Fig. 6. This method of calculating and approximating the relative coal gasification reactivity is used in coal science as a relative measure of determining various additions’ and conditions’ influence of the gasification process. It was observed that the relative coal gasification reactivity increased with increasing loadings of K2CO3 and KCl, indicative of an increased in rate of the gasification reaction with increased potassium salt loading.

Fig. 6
figure 6

Relative gasification reactivity (1/T max) of the char derived from the acid-leached coal samples with K2CO3 and KCl loadings

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TG results indicated that K2CO3 and KCl (similar potassium loadings) are catalytic active during heat treatments of char derived from the acid-treated coal. The relative gasification reactivities of the coal–char sample with potassium ion mass loadings of 3 and 5 % were higher for the K2CO3-loaded samples than for the KCl-loaded samples, with the highest relative gasification reactivity measured for the 5 % K2CO3 addition. These results and observations indicate that the catalytic effect of K2CO3 is greater than that of KCl on the gasification reactions of the acid-treated inertinite-rich coal.

MS gas evolution profiles

H2 evolution profiles

The hydrogen (H2) evolution profiles of the coal–char samples derived from the acid-treated coal–char samples, with and without potassium salt loadings, are presented in Fig. 7. Qualitative determination of H2 evolution during the heat treatment of the chars in CO2 atmosphere for the samples with and without K2CO3 and KCl loadings were performed. As H2 is one of the main synthesis gases produced by the coal gasification process, following the evolution thereof during thermal processing of coal in a CO2 atmosphere (partially the gasification process) is used to better understand the influences of various added minerals or salts to the leached and neat coal samples. The H2 identification using the mass spectrometer was based on measuring the key fragment H +2 , corresponding to mass number (m/z) of 2.

Fig. 7
figure 7

Mass spectroscopic spectra of H2 (m/z = 2) of the acid-treated coal with and without potassium ion mass percentage loadings of a 0 %, b 0.5 %, c 1 %, d 3 % and e 5 % during heating in a CO2 atmosphere

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Hydrogen evolution during heat treatment of coal–char is associated with different functional groups present on the surface of the carbon matrix in the coal–char [17]. The H2 observed in the gaseous phase is attributed to the abstraction and removal of the hydrogen from the hydrocarbons functional groups due to temperature increase. The remaining parts of the functional groups on the coal carbon matrix’s surface then act as carbon active sites for further gasification reactions [18]. As a result, chemisorption of the CO2 gas molecule takes place at these active sites and these steps are described as the initial stages of coal gasification [19–21]. Removal of the edge hydrogen atoms is proposed to take place at 700–900 °C [18]. Evolution of hydrogen at lower temperatures or an increase in the rate of removal of hydrogen thus indicates that the gasification process also has shifted to lower temperatures.

Hydrogen evolution for all the samples (Fig. 7) began at approximately the same temperature of 800 °C; however, potassium salt addition caused the H2 evolution to take place over a narrower temperature range than that observed for the sample without any potassium salt loading. As can be seen from Fig. 7, addition of the potassium salts to the leached coal samples shifted the temperature at which the maximum amount of H2 is evolved (on the MS curves) to lower temperatures. In Table 3, the temperatures at maximum amounts of hydrogen evolution as measured on the mass spectroscopic curves for hydrogen are listed. These temperatures follow a similar trend than the temperatures (T max) at maximum rate of the gasification reaction as observed on the DTG curves (Figs. 4, 6). An increasing shift to lower temperatures at maximum reaction rates was observed with increasing potassium salt loadings (up to a maximum loading of 5 mass percentages) for both the K2CO3- and KCl-loaded samples. This implies that K2CO3 and KCl facilitate the evolution of H2 by speeding/enhancing the rates of the reaction steps and/or mechanisms where H2 is given off, resulting in faster H2 formation and release. It is proposed that additions of potassium carbonate and potassium chloride to coal enhance gasification and that these salts thus act as gasification catalysts.

Table 3 Temperatures at maximum rate of mass spectrometric determined H2 evolution (\(T_{{\text{max,H}}_{2}}\)) of the acid-treated coal–char samples with/without indicated potassium salt loadings/°C
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Conclusions

The proximate and ultimate analyses results of the neat coal sample and the acid-treated coal sample indicate that the leaching process successfully reduced the inorganic matter associated with coal, from 23 % to <3 % (dry basis). Thermogravimetric analyses of the prepared coal–char samples with and without the potassium salts indicated that the added salts influenced the CO2 gasification steps of the process. The relative coal gasification reactivity, measured as 1/T max, with T max the temperature at maximum rate of the reaction as measured on the DTG curves, increased with increasing loadings (up to a maximum of 5 potassium ion mass percentages) of K2CO3 and KCl, indicative of an increased in rate of reaction with increased potassium salt loading. The potassium carbonate and potassium chloride salts thus seem to act as catalysts for the coal gasification reactions.

Addition of K2CO3, at a potassium ion mass percentage loading of 5 %, has the greatest catalytic effect on the gasification of the inertinite-rich acid-leached coal. K2CO3 and KCl additions to the inertinite-rich acid-treated coal cause H2 evolution to take place over a narrower temperature range than that observed for the samples without potassium salt loading. Industrial coal gasification may thus be catalysed by the addition K2CO3 or KCl to the coal feed, and a K2CO3 loading of 5 % potassium ion mass percentage is indicated as an option to perform gasification at lower temperatures.