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ACS Omega
ACS Omega
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ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c03475
Article
Effect of Low-Rank and High-Rank Coal Blend Characteristics on the Gasification Performance in Fixed Bed Reactor
Sharma Sanjeev Kumar
Anand Amrit *
https://orcid.org/0009-0001-9399-8046
Gautam Shalini *
Chattopadhyay Milan
Department of Fuel, Minerals and Metallurgical Engineering, Indian Institute of Technology (ISM) Dhanbad, Dhanbad, Jharkhand 826004, India
* Email: amrit.vim@gmail.com.
* Email: shalinigautam@iitism.ac.in.
22 08 2024
03 09 2024
9 35 3703537043
10 04 2024
01 08 2024
31 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Coal gasification is the most demanding technology, increasing day by day for synthesis gas and chemical production in a clean environment. Coal is a primary source of energy or fuel. India has a high preservation of high-ash coal. Employment of this coal for gasification is tough due to its abrasive nature. Coal blending is an effective way to utilize such coal and to control the gasification performance. The present study focuses on using high-ash, low-rank Indian coal with high-rank imported coal in a suitable blend. The blending effect on raw gas yield (kg/kg of coal) and heating value (kcal/Nm3) was studied for coal blends—CH1 (20:80), CH2 (30:70), CH3 (40:60), CH4 (50:50), and CH5 (60:40) of RC (raw Indian coal) with RH (raw high-rank imported coal). Also, the gasification characteristics of the WC (washed Indian coal) were studied similarly. It may be seen that the raw gas yield with blends of raw coal and the rank of imported coal is 1.11 to 1.46 (ton/ton of coal), whereas in washed Indian coal, it is 0.95 (ton/ton of coal). A slight change was observed in the heating value of raw gas, and its average value in the blended CH1 to CH5 coal is 2860 kcal/Nm3, whereas in WC, it is 2956 kcal/Nm3. The maximum utilization of raw coal in the blend for gasification can be 60%, which is economical and 0.55 times more effective than WC gasification.

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The abstract graphic was corrected on August 23, 2024.
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pmc1 Introduction

It is great to hear that gasification technology garners interest for its versatility and environmental advantages. Gasification is a process that converts carbonaceous materials, such as coal, biomass, or waste, into a mixture of gases known as syngas (synthesis gas), which primarily consists of carbon monoxide, hydrogen, and sometimes methane.1 These syngas can be used for various applications, such as power generation, fuel production, or chemical synthesis.2−4 Syngas is an attractive alternative to traditional resources, such as petroleum crude and natural gas, in various applications. Syngas is a mixture of hydrogen (H2) and carbon monoxide (CO), and it can be produced from various feedstocks, including coal, natural gas, biomass, and even waste materials. It is a versatile intermediate that can produce a wide range of valuable products.5,6

There are three main gasifiers, i.e., fixed, fluidised, and entrained bed, in which fixed or moving bed gasifier can convert a more significant portion of the energy contained in the coal into useful work with high thermal efficiency.7,8 Fixed-bed gasifiers are indeed considered robust technologies, as coals from different locations differ in their properties, mainly ash, can be used widely. Typically, the ash in coal is up to 35%, which can be used in the fixed-bed gasifier with some modification on their operational parameters.9 In a fixed-bed gasifier, the coal or solid feedstock is loaded into a bed that remains stationary during gasification. The gasifying agent is introduced from the bottom of the bed, and as it flows upward through the bed, it reacts with the solid feedstock at high temperatures.10 Heat exchange takes place between the solid stage (coal or feedstock) and the gas phase (gasifying agent and resulting syngas).11 The specific choice of feedstock is crucial in fixed-bed gasifiers, as feedstocks that agglomerate or form clumps when exposed to high temperatures can lead to operational issues.12 Noncaking coal with a high ash fusion temperature (AFT) is preferred for fixed-bed gasifiers.13

Earlier work has been carried out on the laboratory, pilot, and commercial scale of the FBDB gasifier.14 They show interest in the upstream and downstream parameters like pressure, particle size distribution, temperature, residence time, AFT, and thermal fragment on gasification performance.15−18 India has a very high amount of high-ash coal, having an average ash of >45%, which cannot be directly used in the gasifiers. So, to utilize such indigenous coals, it is washed in washeries to minimize the ash to <35% and blended with high-rank coal (RH) to maintain the ash below 30%. However, due to the drift origin of coal, the presence of a higher percentage of near-gravity material in raw coal can pose challenges during the washing process and result in lower yield.19−23 This makes the overall process economically less viable and less preferable due to excess water usage.14 Hence, the present study focuses on using low-rank indigenous coal without washing and blending it with RH, which may have a better scope for its utilization. So, blended coal gasification performance is assessed and compared with that of washed coal (WC) gasification studies. On the other hand, the effect of coal properties and operating parameters on gasification performance has not been conventional for indigenous coal, and the relation between coal properties and gasification performance has not been well established.24 So, it is ultimately important to understand the coal properties vis-à-vis the expected behavior of the targeted coal feedstock in a gasifier. On this behalf, RH blends with run of mines (ROMs) to maintain the ash percentage, preferably suitable for gasification. Therefore, the ash % of blended coal samples is set aside in the 24.29–32.66% range.

Hence, the current study will analyze the effect of coal properties mainly fixed carbon (FC) content on gasification performance, mainly gas yield, and the gas’s heating value and also focus on the maximum utilization of raw coal in blended coal samples for gasification in the lab-scale reactor. In this regard, eight samples like raw Indian coal (RC), RH, WC, and blends of RC and RH in the ratios of 20:80 (CH1), 30:70 (CH2), 40:60 (CH3), 50:50 (CH4), and 60:40 (CH5) were cast off as gasifier feedstock, and their gasification performance were evaluated.

2 Materials and Methods

2.1 Sample Collection and Preparation

The ROM coal from the MCL coal of Talcher coalfield, WC, and RH was collected from Jindal Steel and Power Angul, Odisha. The ROM coal was crushed 5 to 50 mm in size in the coal handling plant. Then, it was transferred to the coal beneficiation plant, where ROM coal was washed to get the clean coal (ash <35%). A total head of 50 kg of each sample, i.e., ROM coal, WC, and RH, was collected following the standard ASTM D 346 from their stacks.19 Furthermore, 100 g of each sample from the head sample of the blends of RH with ROM coal (RH, RC) and WC was prepared in 212 μ sizes for characterization study as per the standard ASTM D 2013.23 For gasification, 2 kg of each blended and individual sample, i.e., RC, RH, CH, and WC with a sample size of +5 to −50 mm, was used. The ash percentage of the blend in the fixed bed was kept at <35%, and the optimized blends CH1, CH2, CH3, CH4, and CH5 were prepared from RC and RH in the ratio of 20:80, 30:70, 40:60, 50:50, and 60:40, respectively; and 100% WC and 100% RH are also considered as feed samples. Further sampling of raw gas, obtained from blends CH1, CH2, CH3, CH4, CH5, and WC gasification, was carried out by collecting the gas in the bladder at room temperature.

2.2 Methodology

The proximate analysis of coal was carried out as per standard ASTM D 514219,25 using a thermogravimetric analyzer (TGA-701) (make: LECO), where inherent moisture (IM), volatile matter (VM), ash, and FC content were determined, and ultimate analysis was performed by a CHNS-689 (Ultimate) analyzer (make: LECO) as per standard D 5373, where the elemental composition as total carbon, hydrogen, nitrogen, sulfur, and oxygen (by difference) were determined.25,26 The gross calorific value (GCV) of coal was determined by a bomb calorimeter (Parr-6100, USA) using ASTM D 5865.4 The ash sample of coal was prepared in a muffle furnace at 800 °C for 1 h. Ash composition was determined by wavelength dispersive X-ray fluorescence (WDXRF, Rigaku, Primus IV) as per ASTM standard D 4326-21.27 Ash was mixed with boric acid, and the pellet was made in an aluminum cup by applying 200 MPa pressure. The instrument was calibrated using a certified reference material. The error of the instrument was 0.0005–0.0010%. AFT was determined by the ash fusibility instrument (make: Sylab) as per standard ASTM D 1857. For AFT analysis, around 1 g of coal was slightly dissolved in isopropyl alcohol, and the paste was placed on a cone mold, then the molded sample was placed on refractory support inside the furnace. Images were carefully observed, and the fluidity was estimated. The AFT analysis was carried out under oxidizing conditions using air as an oxidizing agent. The instrument was calibrated using gold as a reference standard. The standard error of the instrument was ±2 °C. The CO2 reactivity of coal was determined using the Mettler Toledo CO2 reactivity analyzer.28 In post-gasification, the raw gas volumetric composition and heating value were obtained by gas chromatography as per ASTM 3588.14,29

2.3 Experimental Setup and Procedure

The CH1, CH2, CH3, CH4, and CH5 samples are prepared using the head sample, having a ratio of 20:30, 30:40, 40:60, 50:50, and 60:40 of RC and RH. A total of 2 kg of sample of both RC and RH was used in the laboratory-scale batch fixed-bed reactor, made up of stainless steel (SS 304), as depicted in Figure 1. The indirect heating of the reactor was performed by furnace with a heating rate of 20 °C/min and reached up to 1200 °C. A perforated plate is placed at the bottom of the reactor for uniform distribution of gasifying agents, i.e., oxygen and steam, inside the reactor. Initially, the furnace was heated up to 500 °C, and then the reactor with charged sample was kept inside the furnace. Nitrogen gas was purged to remove the IM up to 200 °C.

Figure 1 Small-scale fixed-bed reactor.

Further steam (@62 g/min) and oxygen (@6–8 L/min) after premixing were introduced across the reactor for superheating and discharged at the bottom of the reactor, and the furnace temperature was maintained at 800 °C. The produced raw gas was passed through a condenser, liquid and gaseous products were separated in the liquid gas separator, and the clean raw gas was collected in a bladder for compositional analysis. In each experiment, the produced raw gas sample is collected after 15 min intervals, considering the total reaction time of 50 min. Three raw gas samples were tested in each experiment, and an average of these three results is considered for overall gas yield along with heat value evaluation, and their average value is predicted in Table 5.

3 Results and Discussion

3.1 Proximate, Ultimate, and GCV of Coal

The proximate and ultimate analysis of eight coal samples is shown in Table 1. The experimental data in Table 1 show that the FC % of RH is 52.93, RC is 28.21, WC is 32.68, and blended coal is between 38.12 and 47.88%. VM % is 23.69 in RH, 27.91 in RC, 28.55 in WC, and in other blended coal, it is between 24.61 and 26.37. The ash % of RH is 20.05, RC is 40.99, WC is 30.17, and other blended coal is 24.29 to 32.66%. From the ultimate analysis, it is found that in WC, the C, H, N, S, and O are 55.99, 4.23, 1.19, 0.81, and 6.20; in RH, they are 67.85, 4.03, 1.25, 0.50, and 4.13; in RC, they are 43.62, 4.23, 1.01, 0.5, and 10.33; while in CH1, CH2, CH3, CH4, and CH5 blend coals, they are 53.25 to 65.09, 3.84 to 4.61, 1.16 to 1.58, 0.45 to 0.81, and 5.68 to 10.87, respectively.

Table 1 Proximate (Air-Dried Basis), Ultimate (Dry Basis), and GCV Analysis of Coal

sample	IM (%)	VM (%)	FC (%)	ash (%)	C (%)	H (%)	N (%)	O (%)	S (%)	GCV (kcal/kg)	H/C	O/C	
RH	3.33	23.69	52.93	20.05	67.85	4.03	1.25	4.13	0.50	6098	0.71	0.05	
RC	2.89	27.91	28.21	40.99	43.62	4.23	1.01	10.33	0.55	4128	1.16	0.18	
CH1	3.22	24.61	47.88	24.29	64.44	4.38	1.54	6.18	0.81	5618	0.82	0.07	
CH2	3.09	24.98	45.05	26.38	65.09	4.61	1.58	5.68	0.80	5254	0.85	0.07	
CH3	3.19	25.27	43.12	28.42	59.13	4.27	1.16	7.91	0.48	5168	0.87	0.10	
CH4	3.02	25.87	40.59	30.52	56.71	4.16	1.26	9.77	0.52	4611	0.88	0.13	
CH5	2.85	26.37	38.12	32.66	53.25	4.03	1.21	10.87	0.45	4341	0.91	0.15	
WC	8.60	28.55	32.68	30.17	55.99	4.23	1.19	6.20	0.81	4637	0.91	0.08	

Experimental data of proximate analysis from Table 1 show that the FC content in blended and RH is high, whereas in RC, it is low because of the effect of RH in blends. Ultimate analysis data from Table 1 indicates that blended coals’ average C and O are higher than RC but lower than RH. This is true due to the presence of RC in different ratios.

In the combustion and gasification of coal, both the H/C and the O/C ratio are crucial. According to Table 1, the H/C and O/C ratios for blending coal in this study vary from 0.82 to 0.91 and 0.07 to 0.15, respectively, while for RC, they are, respectively, 1.16 and 0.18, and in RH, they are 0.71 and 0.05.

A high H/C ratio implies high VM and a higher heating value. The H/C ratio and O/C ratio also suggest a trend toward lessening aromaticity and an increase in the functional hydroxyl carboxylic ether group found in oxygen, which boosts coal’s reactivity.12,30

In the van Krevelen diagram (Figure 2), it is seen that the values of the O/C of CH are on the slightly left side compared to that of the WC, and the values of the H/C of CH are substantially lower than that of the WC, thereby evincing higher reactivity of CH than WC. Furthermore, the RC is on the right side and above HC and WC, the RH is nearest to the corner (origin), and WC is between and at the right side of WC and RC. So, in terms of reactivity, the coal samples follow the order (descending): lignite > RC > WC > CH > RH.14

Figure 2 Van Krevelen diagram of blends (RC, WC, RH, and blended coal).

3.2 XRF and AFT

Table 2 presents an XRF analysis of the ash compositions of RC, WC, RH, and blends. The mineral composition of the coal sample is SiO2 > Al2O3 > Fe2O3 > CaO > K2O > MgO > TiO2 > P2O5. Compared to RC and WC, RH has larger levels of Fe2O3, which lowers the ash fusibility temperature, as shown in Table 3. The B/A value for slagging worry is between 0.13 and 0.16, which is less than the low slagging index value of 0.18 to be exact.31,32 The FI value, below the low fouling value of 0.26,31,32 with regard to the fouling index, is further in the range of 0.23 to 0.28. Given the above information, the likelihood of slagging and fouling during gasification for the WC and RH samples is quite low. So, further analysis is not required because of the less significant effect of ash composition on operational issues; however, a slight softening of ash is always needed in practical applications to avoid any grate pitting and improve the followability performance of ash in the grate.

Table 2 Ash Compositions of Coal and Blend Samplesa

ash compositions	
sample	SiO2	Fe2O3	Al2O3	CaO	MgO	Na2O	K2O	SO3	MnO	BaO	TiO2	P2O5	B/A	FI	SI	
RH	50.19	8.70	30.15	3.85	0.67	0.2	1.26	1.29	0.075	0.06	1.73	1.48	0.18	0.26	0.23	
RC	58.55	6.56	25.92	2.36	0.63	0.21	1.71	0.26	0.06	0.09	1.97	0.9	0.13	0.25	0.03	
CH1	54.11	6.98	28.28	3.78	0.65	0.2	1.44	1.22	0.071	0.06	1.51	1.31	0.16	0.26	0.19	
CH2	54.98	6.99	27.41	3.71	0.65	0.22	1.58	1.21	0.066	0.07	1.55	1.23	0.16	0.28	0.19	
CH3	55.84	6.68	27.51	3.39	0.58	0.19	1.55	0.98	0.061	0.065	1.68	1.15	0.15	0.25	0.14	
CH4	57.29	6.01	26.84	3.21	0.61	0.2	1.51	0.91	0.059	0.06	1.71	1.05	0.13	0.23	0.12	
CH5	56.81	5.89	27.32	3.41	0.66	0.22	1.61	0.88	0.065	0.07	1.61	1.21	0.14	0.25	0.12	
WC	56.22	5.86	27.99	3.92	0.79	0.08	1.33	0.6	0.06	0.09	1.91	0.82	0.14	0.20	0.08	
a B/A = (Fe2O3 + CaO + MgO + K2O + Na2O)/(SiO2 + Al2O3 + TiO2); FI = (B/A) × (Na2O + K2O); SI = (B/A) × (S).

Table 3 AFTs of Coal and Blend Samples

ash fusion temperatures	
sample	IDT (°C)	ST (°C)	HST (°C)	FT (°C)	
RH	1298	1420	1484	>1500	
RC	1340	>1500	>1500	>1500	
CH1	1290	1490	>1500	>1500	
CH2	1297	1444	1491	>1500	
CH3	1310	1467	1498	>1500	
CH4	1310	1471	>1500	>1500	
CH5	1320	1485	>1500	>1500	
WC	1340	1498	>1500	>1500	

3.3 CO2 Reactivity

CO2 reactivity is a 50% mass loss of C per unit of time under the CO2 atmosphere.33 It displays the gasification reaction’s speed. CO2 reactivity (1/h) ranges from 0.90 to 1.51, depending on the source. Table 4 includes information about the reactivity of coal and coal mixes. According to the results, RH has the lowest reactivity, or 0.90 (1/h), while RC has the highest reactivity among the samples, 1.51 (1/h).

Table 4 CO2 Reactivity of Coal and Blend Samples

CO2 reactivity (1/h)	
WC	1.31	
RC	1.51	
RH	0.90	
CH1	1.11	
CH2	1.15	
CH3	1.18	
CH4	1.23	
CH5	1.29	

Additionally, RC has a higher level of reactivity than WC since it contains more RC in the blend. Because RC is more reactive, it is anticipated that the gasification reaction and the dominant combustion will occur more quickly. The oxygen input was decreased (0.22 kg/kg of coal) in blends of CH1, CH2, CH3, CH4, and CH5 coal (Table 5) to maintain a favorable condition, which may lead to higher heat generation for gasification.34

Table 5 Gasifying Agent, Raw Gas Yield, and Heat Value for Coal and Blend Samples

sample	raw gas yield (kg/kg of coal)	heat value (kcal/Nm3)	oxygen (kg/kg of coal)	steam (kg/kg of coal)	
CH1	1.46	2964	0.29	1.40	
CH2	1.39	2904	0.25	1.25	
CH3	1.28	2879	0.23	1.22	
CH4	1.21	2799	0.22	1.19	
CH5	1.11	2754	0.21	1.19	
WC	0.95	2956	0.28	1.21	

3.4 Gasification Performance Parameters and Optimization

The present study uses WC, RC, RH, and blends of CH1 to CH5 coal in different ratios. For study samples, 2 kg each were created. The raw gas yield and heat value of six blend samples are 1.11 to 1.46 kg/kg of coal and 2797–2964 kcal/Nm3, respectively (Table 5). The steam and oxygen input for six blended coals are 1.19 to 1.40 and 0.21 to 0.29, respectively. Regarding WC, the equivalent numbers are 0.25, 1.21, and 2956 kcal/Nm3, respectively (Table 5). Due to the strong reactivity of RC, the oxygen input for blended coal is kept slightly lower than that for WC to maintain the reaction zone’s temperature and lower the CO2 concentration in the product raw gas for better fuel quality.35 Because of this, blended coal CH will have a greater relative yield of the CO2 component than WC, which could lead to a higher yield of raw gas in blends CH1 to CH5 than WC (Table 5). Therefore, blended coal CH1 to CH5 has less heat value, even if it produces higher raw gas than WC.

On the other hand, the steam supply is decreased to maintain the gasification zone temperature since RH has less reactivity (Table 4). Even though WC has the highest heat value, the gas output is the lowest.14 This is primarily due to less FC, a high VM, and a higher IM content than blended and RC (Table 1). Thus, it can be concluded that the gasification of blended CH1 to CH5 coal is generally superior to that of WC in terms of yield and heat value of the resulting raw gas. As a result, the gasification of RC and RH blends is highly preferred because the weak yield of raw gas from WC does not support gasification alone.

3.4.1 Raw Gas Composition

Post-gasification, raw gas was cooled and collected in a bladder, and the volumetric composition of the gas was analyzed by gas chromatography. Table 6 shows that in the blend sample CH1 to CH5, the composition (%) of H2, CO2, CO, CH4, and OHC is in the range of 41.97–49.01, 20.05–31.72, 20.64–25.07, 4.69–4.91, and 0.23–0.42, respectively.

Table 6 Volumetric Raw Gas Composition (%)

volumetric raw gas composition (%)	
sample	H2	CO2	O2	N2	CO	CH4	H2S	OHC	H2/CO	CO/CO2	
CH1	49.01	20.05	0.02	0.24	25.07	4.91	0.28	0.23	2.44	1.25	
CH2	47.82	24.28	0.04	0.23	22.37	4.81	0.21	0.24	1.97	0.92	
CH3	45.98	26.96	0.05	0.29	21.52	4.76	0.19	0.25	1.71	0.8	
CH4	43.95	29.6	0.05	0.33	20.91	4.72	0.2	0.27	1.48	0.71	
CH5	41.97	31.72	0.07	0.27	20.64	4.69	0.22	0.42	1.32	0.65	
WC	44.12	28.29	0.02	0.31	19.84	6.45	0.26	0.71	1.56	0.7	

The current study, however, is conducted at atmospheric pressure. However, according to the literature, the methanation process under pressured conditions in the pyrolysis zone accounts for 10–12% of the usual percentage of CH4 in fixed-bed gasification at about 25–30 bar.36

3.4.2 Gasification Performance Evaluation

This section includes the effect of FC, VM, and H/C ratio of blends and WC on the raw gas yield, composition, and heating value. The blend samples (CH1–CH5) and WC samples are colored blue and red, respectively.

3.4.2.1 Effect of FC and VM on Raw Gas Yield

According to van Dyk et al.,24 increasing the FC content in feed coal boosts raw gas output. From Figure 3, it can be seen that CH blends have a higher raw gas yield than WC, which may be due to the very low FC content in WC than CH (Table 1). It was previously explained in the literature23 that the feed’s FC is directly connected to the production of the total raw syngas, so raw gas increases with the feed coal’s FC. In the case of WC, the FC content is 32.68, less than the blended coal samples, with an FC (%) content of 38.12–47.88. The higher FC content requires higher oxygen and steam inputs, leading to lower CO and higher CO2 production in raw gas and higher gas yield. Also, the CO2 content in the raw gas of WC is less than that of different CH samples despite higher oxygen inputs. The higher CO2 in the raw gas of the CH (Table 6, Figure 2) results from a higher reactivity than WC, leading to more combustion.37,38 So, relatively less oxygen input for the CH blend is required to maintain the combustion zone temperature and drive the gasification reactions. Endothermic reactions use the heat produced during combustion because FC is essential in the combustion and gasification zones.39,40 The endothermic gasification reaction in the reduction zone and pyrolysis zone of the gasifier is driven by the exothermic reaction in the oxidation zone, which produces CO and CO2 as well as a significant quantity of heat.15,17,41

Figure 3 FC (%) vs raw gas yield (kg/kg of coal).

As demonstrated in Figure 4, the raw gas yield falls when the VM concentration in both WC and blends rises. It is important to note that the VM participates in the pyrolysis zone in fixed-bed gasification. During the pyrolysis, liquid hydrocarbons and lighter hydrocarbons are also formed, although their contributions to the gas output are not particularly significant, as shown here. So, the OHC concentration rises due to high VM in the CH blend and WC sample (Table 1).

Figure 4 VM (%) vs raw gas yield (kg/kg of coal).

3.4.2.2 Effect of FC on Heating Value and Volumetric Gas Composition

The heating value is the gross volumetric calorific value of the raw gas produced. According to11,42 Monazam and Shadle (1998), the amount of flammable components in raw gas increases with composition. It is observed (Figure 5) that the heating value of raw gas produced from CH1, CH2, CH3, CH4, and CH5 blended coal is on the lower side than WC due to the lower content of CH4 and OHC. Increasing the RC in the blend sample decreases the heat value due to the lower concentration (%) of H2 and CO in the raw gas. On the other hand, the hydrocarbon and methane are on the higher side in WC coal than in blended coal due to low FC and high VM as well as low reactivity. The H2/CO ratio is decreasing, which is desirable in the blend sample. As shown in Table 6, higher FC concentrations encourage the Boudouard and water gas reaction and increase the raw gas CO and H2 content. High reactivity increases CO2 concentration in blend samples.43 Hence, we can directly utilize the RC by blending it in a gasifier instead of washing the coal sample qualitatively by 60%.

Figure 5 FC (wt %) vs heat value of raw gas (kcal/Nm3).

3.4.2.3 Effect of H/C Ratio on Raw Gas Yield

It is observed from Figure 6 that as the H/C ratio increases, the gas yield decreases in WC and blended coal in the same manner. It may be due to the actuality that the only carbon content of the feed coal performs during the gasification, which is previously explained by van Dyk et al.24 The FC of the feed is directly connected to the production of raw gas. This indicates that the raw gas or syngas increases with the feed coal’s FC increases. Here, we see that the FC % in WC is 32.68, while in CH1–CH5 blend coal, it ranges from 38.12 to 47.88. That is why the gas yield increases in the blend coal. The H/C ratio in WC is 0.91; in blend coal CH1–CH5, it is 0.82 to 0.91. Additionally, it can be said that as the H/C ratio rises, the ignition temperature falls, indirectly improving gasification performance (Figure 6).

Figure 6 Effect of the H/C ratio on the raw gas yield.

4 Statistical Analysis

This section includes the correlation developed between the heating value and gaseous composition through regression and compares the variance, fittings, and significant parameters.

4.1 Effect of Raw Gas Composition on Heating Value of Gaseous Fuel

Regression (variable vs response): H2/CO, CH4, OHC vs heating value.

4.1.1 Regression Equation

In Section 4.1, Table 7, the regression coefficient value is greater than 91%, which is acceptable, and it is concluded that the response and variable depicted in the heating value regression equation are well-fitted.

Table 7 Model Summary

S	R-sq	R-sq(adjusted)	R-sq(predicted)	
35.6480	91.93%	91.00%	89.45%	

From Table 8, in the analysis of variance, the probability (P) value signifies that methane (CH4) and other hydrocarbons (OHC) are important variables, followed by the H2/CO ratio. However, from the variance inflation factor value, the H2/CO variable is best fitted, followed by those of the OHC and CH4.

Table 8 Analysis of Variance

source	coefficient	P-value	VIF	
constant	1244	 	 	
H2/CO	316	0.176	1.60	
CH4	118.6	0.000	3.84	
OHC	166.8	0.000	2.99	

From Figure 7, the Pareto chart shows that CH4 is the most influential variable for the heating value of the gas produced. The probability chart is well-fitted and correlated following the straight-line path, except for three outlier points depicted in Figure 8.

Figure 7 Pareto chart of raw gas composition.

Figure 8 Residual plots for the heating value of raw gas.

5 Economic Analysis

This study utilized three different categories of coal RC, WC, and RH for gasification performance evaluation.14 The blend samples CH1 to CH5 consist of RC and RH, having a ratio (X1/X2) 20:80, 30:70, 40:60, 50:50, and 60:40, respectively.Cost of RC (CR) = 3.20 Rs/kg

Cost of WC (CW) = 4.40 kg

Cost of RH (CHR) = 5.50 Rs/kg

Cost for operation and management (COM) = 3.41 Rs/kg

Cost of byproduct (CP) = 1.51 Rs/kg

Net cost of operation and management (CN) = (COM – CP)

Cost of blend sample (CH1) = CR × X1 + CHR × X2

Total input cost of raw gas production (CI) =

Minimum selling price of syngas (Csyn) = 627.86 Rs/GJ ≈ 0.002627 Rs/kcal44,45

Selling price of raw gas (CRG) = HHV of gas × Csyn

Profit (%) = (CRG – CI) × 100/(CI)

Effectiveness w.r.t. WC (E) =

As observed from the above data in Table 9, we can say that all the blend samples are economical than WC, especially for CH1, which is 45.28%, which is very much significant. The sample CH5 is also 28.92% profitable. In terms of effectiveness, the CH1 is 2.44 times profitable than WC. In context of maximum utilization of RC in the blend, CH4 (50:50) is recommended for gasification as the ash % is also less than 30 (Table 1), which is desirable for fixed-bed gasifier.14,30 So, it can be inferred (Table 9) that blending RC with RH is feasible and economical.

Table 9 Economic Analysis

sample	blending ratio (X1/X2)	cost (Rs/kg)	CI (Rs/kg)	CR (Rs/kg)	profit (%)	E	
CH1	20:80	5.04	5.35	7.79	45.48	2.44	
CH2	30:70	4.81	5.36	7.63	42.32	2.27	
CH3	40:60	4.58	5.48	7.56	38.06	2.04	
CH4	50:50	4.35	5.50	7.35	33.81	1.81	
CH5	60:40	4.12	5.61	7.23	28.92	1.55	
WC	 	4.40	6.53	7.75	18.65	NA	

6 Conclusions

Gasification performance, specifically gas yield and heating value, is significantly influenced by coal qualities like FC and operational parameters (oxygen and steam). This study used WC and blended coal in different ratios (20:80, 30:70, 40:60, 50:50, and 60:40) in a fixed-bed reactor on a laboratory scale. The performance parameters, such as gas yield and heating value, were assessed, and the results were as follows.

The raw gas yield of WC is minimum, so it is not recommended for direct gasification.

Raw gas yield and its heat value improved with an increase in FC content of the coal blend.

The raw gas composition comprises H2, CO, OHC, and CH4, which are the main contributors to gas heating value. H2/CO and OHC are the most significant variables, followed by CH4.

Utilization of RC blends up to 60% is economical and 0.55 times more effective than WC.

50% of RC blended with RH is recommended for gasification.

The authors declare no competing financial interest.

Acknowledgments

The authors express their thankfulness to the Director, Indian Institute of Technology (ISM), Dhanbad, for his kind support in carrying out this work and permission to publish this paper.
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