Characteristics of Hybrid Coal from Co-Pyrolysis of Lignite and Corn Cob

Abstract


INTRODUCTION
Coal is one of Indonesia's energy sources.Potential coal reserves in Indonesia are mostly found on the islands of Kalimantan and Sumatra.Indonesia's coal reserves are 88.21 billion tons, Kalimantan holds most of the total reserves at 25.84 billion tons or 62.1% (Meyliawati et al., 2022).Coal is classified into different grades according to the quality of its combustion.As energy demand continues to increase, the focus is shifting to lowrank coals despite constraints of low calorific value, high moisture content, and high CO2 emissions.In addition, in some countries, including Indonesia and China, low-rank coals (lignite and sub-bituminous) are in ample supply compared to highgrade coals (Tahmasebi et al., 2016).
For combustion in pulverized coal burners, low-rank coals require additional processing to deal with excess moisture content.In some applications, high-temperature tail gas is used for thermal drying to remove moisture from low-rank coals.These extra treatments lowers the overall efficiency of the power plant (Xu et al., 2015).
Low rank coal can be used as fuel with a mixture of biomass .Among renewable energy sources, biomass-based energy sources are considered carbon-neutral because the amount of carbon dioxide released into the atmosphere during combustion is equal to the amount of carbon dioxide absorbed by plants through photosynthesis during their life cycle (Xiu & Shahbazi, 2012).Biomass energy in the form of corn cob waste is widely used due to the high corn production in Indonesia, especially at Balangan Regency in South Kalimantan.In 2020, Balangan Regency's corn planting area is 2896 hectares and yields 1856 tonnes (Herlinawati et al., 2022).The resulting corn cob waste accounts for 30% of production.Corn cob contained 37.2% cellulose, 41.9% hemicellulose, and 15.3% lignin (Singh et al., 2021).
Mixing low-rank coal (lignite) with biomass can be used as a feedstock for co-pyrolysis, converting it into solid (biochar), gas, and liquid products (Li et al., 2017).Biochar products can be used as solid fuels for power plants, raw materials for activated carbon, and even as catalyst supports and supercapacitor electrodes (Kan et al., 2016;Nanda et al., 2016;Qian et al., 2015), as well as effective adsorbents (Mukherjee et al., 2014).
Several studies related to pyrolysis of low rank coal and biomass have been carried out (Akyürek, 2019;Hossain et al., 2017;Pinto et al., 2015;Uzoejinwa et al., 2019).Co-pyrolysis of lignite and biomass (rice husk) produces hybrid coal products (Sasongko et al., 2017) .Some researchers (Soncini et al., 2013;Wei et al., 2011;Zhang et al., 2007) also conducts coal and biomass pyrolysis with a research focus on product distribution.Soncini et al. (2013) pyrolyzes sub-bituminous, lignite coal, and southern yellow pine as biomass and the results showed that gas and water increased with increasing the weight percentage of biomass, while the amount of remaining char decreased.Wei et al. (2011) used legume straw and pine sawdust as biomass, and Dayan lignite and Tiefa bituminous coal as pyrolysis raw materials.The results show that co-pyrolysis of legume straw/Tiefa bituminous can obtain higher liquid production rate than pine sawdust/Tiefa bituminous co-pyrolysis.The rank of coal has a significant impact on the liquid yield of co-pyrolysis.And used legume straw and Dayan lignite as raw materials at a pyrolysis temperature of 500 -700 o C, the results showed that as the pyrolysis temperature and the mixing ratio of legume straw/Dayan lignite increased, the char yield decreased and, correspondingly, the liquid yield increased.
This study used lignite and corn cob biomass as raw materials.The raw material mixture was pyrolyzed at a pyrolysis temperature of 350 -550 o C and a corn cob/lignite mixing ratio of 3:1, 1:1, and 1:3.Taking the charcoal product (hybrid coal) obtained from the pyrolysis process as the main product, its calorific value, and proximate parameters were analyzed, and the effects of pyrolysis temperature and mixing ratio on the characteristics of the hybrid coal product were studied.

Materials
The corn cob comes from the Balangan district of South Kalimantan, Indonesia.The coal was sourced from Asam-Asam in the Tanah Bumbu district of South Kalimantan, Indonesia.The coal used is lignite, which is a low-rank coal.

Pre-treatment of Raw Material
The coal was crushed and sieved to 0.3-1 mm.Corn cobs cleaned, cut into small pieces and sieved into a size of 0.4-2 mm.The material was then dried in an oven at 105°C for 24 hours.This process was to remove moisture, thereby preventing decay.The lignocellulose content, proximate analysis, and calorific value of the raw materials were analyzed.

Pyrolysis of Coal and Biomass
Lignite and corn cob were mixed in a mass ratio of 3:1, 1:1, 1:3, lignite: corn cob.400 g of the raw material mixture was placed in the pyrolysis reactor and heated at a temperature of 350°C, 400°C, 450°C, 500°C, and 550°C for 1 hour with a nitrogen flow rate of 1.5 L/min.After the reaction was complete, the reactor was cooled to room temperature and the char was collected.Then the proximate and calorific value analyses were carried out.As a comparison, lignite and corn cobs were pyrolyzed without mixing at 500 o C. The sample codes used in this study are shown in Table 1.Aduunchuluun coal (a)   This study Shariff et al (b) Biswass et al (c)

Characterization
The calorific value of hybrid coal samples was quantified using a bomb calorimeter (Gallenkamp Adiabatic Bomb Calorimeter CBA-305).Also, proximate analysis is determined according to ASTM D3172-07a.

RESULTS AND DISCUSSION
Corn cob were analyzed for lignocellulosic content.Corn cob consist of hemicellulose (36%), cellulose (36.58%) and lignin (15.88%).The results of the calorific value and the proximate analysis of raw material are shown in Table 2.The results from this study are also compared with the results from other literature.Then the results of hybrid coal analysis from co-pyrolysis of lignite and corn cob are shown in Table 3.

Effect of Pyrolysis on The Characteristics of Lignite and Corn Cob
Table 2 shows the calorific value and approximate properties of lignite and corn cob before pyrolysis and Table 3 shows these number after the pyrolysis process.The raw material analysis results in Table 2 show that the calorific values of lignite and corn cob before pyrolysis are 4,765.29 cal/g and 4,247.32cal/g, respectively.This indicates that the calorific value of lignite and corn cob does not meet the charcoal briquetting quality standard of SNI 13-4931-2010, which is 5,000-6,000 cal/g.The calorific value for pyrolysis of lignite is 5,770.56cal/g.The corn cob value after pyrolysis was 6,397.33 cal/g.This indicates that pyrolysis increases the calorific value of lignite about 21.0956% and corn cob about 50.6204%.Klaas et al (2020) carried out pyrolysis of corn cob at 300 o C and obtained an increase in the calorific value of 19.4805%, where the calorific value of the raw material was 3,678.227cal/g and increase to 4,394.765cal/g after pyrolysis (Klaas et al., 2020).In this study, a greater increase in heating value was obtained because it used a higher pyrolysis temperature, 500 o C.
It can be seen from Table 2 that corn cob has high volatile matter content.Due to the high content of volatiles in corn cob, corn cob was suggested to be a suitable feedstock for thermochemical transformation processes such as pyrolysis.The high volatile content also makes biomass a highly reactive fuel that burns faster.It also can be observed that the ash content of corncob is quite low compared to lignite.A low ash content in the biomass feedstock can reduce slagging and fouling in furnaces used in thermochemical conversion processes caused by the alkali content in the ash-rich biomass.The different analysis results of corn cob from the literature may be due to some factors, such as different types of corn, different weather conditions for growing corn and different soil types for growing corn.As well as the different characteristics of lignite coal due to different mining locations in different countries (Shariff et al., 2016).
The instant analysis results of lignite and corn cob before and after the pyrolysis process in Table 3 show that the pyrolysis process reduces the moisture content of the product and increases the ash content of the product.The decrease in water content is due to the evaporation of more water due to the high pyrolysis temperature.The increase in ash is due to the presence of combustion residue.The ash content indicates that the inorganic components in the raw materials exist in the form of metal oxides (Mohammed et al., 2017).In addition, there was an increase in lignite volatile matter but it decreased in corn cob.The level of volatile matter in the material is caused by the perfection of the carbonization process, and is also affected by the time and temperature of the decomposition process (Wang et al., 2020).

Yield Char
The co-pyrolysis product is influenced by several factors, namely the nature of the material, the type of reactor, the residence time, heating rate and temperature of the co-pyrolysis (Garcia-Nunez et al., 2017;Kan et al., 2016;Sasongko et al., 2017).The results of this study indicate that temperature differences affect the yield of solid products (hybrid coal).The effect of co-pyrolysis temperature of lignite and corn cob on hybrid coal yield is shown in Figure 1.
Figure 1 shows that the yield of hybrid coal (char) is inversely proportional to temperature.Hybrid coal yield decreases with increasing co-pyrolysis temperature.Yield obtained at temperatures of 350 o C, 400 o C, 450 o C, 500 o C and 550 o C respectively were 83.00%; 78.25%; 73.52%; 62.06% and 61.81%.This shows that the yield of hybrid coal has decreased from 83.00% at 350 o C to 61.81% at 550 o C.This study is in accordance with previous research conducted by Zullaikah (2015) where hybrid coal decreased from 37.16 grams (37.16%) at a temperature of 300 o C to 18.44 grams (18.44%) at a temperature of 700 o C. The decreasing yield of hybrid coal indicates that the raw material decomposes more with increasing temperature.
Figure 1.Yield char of co-pyrolysis of lignite and corn cob at various temperature.

Calorific Value
The higher the co-pyrolysis temperature, the higher the calorific value of the hybrid coal (Qiram et al., 2015).The effect of co-pyrolysis temperature of lignite and corn cob on hybrid coal caloric value is shown in Figure 2. Figure 2 shows the calorific value of hybrid coal increases with the increase in co-pyrolysis temperature so that the highest heating value is obtained at a temperature of 550 o C, which is 5,749.35cal/g.Meanwhile, the lowest calorific value was 4,793.43 cal/g in the sample with a temperature variation of 350 o C. The calorific value at 450 o C, 500 o C and 550 o C corresponds to the quality standard of coal briquettes, which is 5,000-6,000 cal/g.But at 350 o C and 400 o C, the calorific value does not meet the quality standard of coal briquettes.At temperatures 300-400°C, carbonization begins, releasing carbon monoxide, carbon dioxide and other short hydrocarbons.Carbonization above 400°C can significantly increase the calorific value of the hybrid coal (Sasongko et al., 2017).The calorific value has a major influence on the quality of the briquettes produced.The calorific value of briquettes describes the value of the heat of combustion that the briquettes can produce (Abdelsayed et al., 2019).

Proximate Analysis
From the data in Table 3, it can be seen that the higher the co-pyrolysis temperature, the lower the moisture content and volatile matter and increase the ash content and fixed carbon of hybrid coal.This is due to thermal decomposition releasing low molecular weight components (water and volatiles) leaving solids that fix carbon and ash (Jeong et al., 2014).The reduction in water content is due to the high pyrolysis temperature allowing more water to evaporate.The water contained in the solid product affects the quality of the briquettes produced.The moisture content of the briquettes should be as low as possible to generate a high calorific value and to burn easily (Saeed et al., 2020).

Yield Char
In biomass pyrolysis, cellulose content increases tar production, lignin is the main component used to make char products, and hemicellulose contributes to tar and gas production (Naqvi et al., 2019) .However, the presence of lignite as a feedstock for pyrolysis together with biomass will greatly affect the char product, especially when the lignite used has a larger composition.The yield charproducts at various mixing ratios at constant temperature (500°C) is shown in Figure 3. Figure 3 shows that the more corn cob (in a mixture of lignite and corn cob) produces less char because corn cob contain higher cellulose, the decomposition of cellulose produces more tar.The same product distribution also occurs in the pyrolysis of lignite and rice husk (Li et al., 2019).As a result, increasing the addition of corn cob resulted in a decrease in lignite as the main source of char, and consequently in char yields from lignite and corn cob pyrolysis.The resulting char yields ranged from 56.4-81.3%indicating that most of the material was converted to char.

Calorific Value
The calorific value of hybrid coal can be influenced by the composition of the raw materials (mixing ratio) shown in Figure 4.
Figure 4 shows that the calorific value of lignite typically increases to 5,865.74 after pyrolysis at a 1:3 lignite: corn cob mixing ratio.Increasing the mixing ratio or adding corn cob in the pyrolysis of lignite can increase the calorific value of the hybrid coal.However, when the mixing ratio is lower than 1:3, the calorific value decreases.Because the calorific value of corn cob is higher than that of lignite, the calorific value of the more lignite is lower because the calorific value of the produced briquette is affected by the calorific value of the raw material (Tumuluru & Fillerup, 2020).Thus, the addition of a maximum of 75% corn cob is the optimum mixing ratio to increase the calorific value of hybrid coal.

Proximate Analysis
The more mixing of corn cob with lignite will reduce the moisture content and volatile matter and increase the ash content and fixed carbon of hybrid coal produced (Table 2).The moisture content of the mixture of lignite and corn cob ranged from 4.98-5.27%,ash content 5.78-6.13%,volatile matter 32.41-35.08%,and fixed carbon 53.87-56.82%.The results of the best proximate analysis that meet the standards of SNI 13-4931-2010 were produced at a mixing ratio of 1:3, lignite:corn cob.In general, the volatile matter hybrid coal produced in this study did not meet the coal briquette standard (max 22%).The volatiles are caused by the improvement of the carbonization process and are also affected by the time and temperature of the decomposition process.The higher the temperature and burn time, the more volatiles are wasted, resulting in low volatility of the sample (Tomczyk, Sokołowska, & Boguta, 2020).

CONCLUSION
Hybrid coal from lignite and biomass is quite promising as an alternative to renewable solid fuels.Its manufacture through a co-pyrolysis process is quite simple, and efficient, and does not harm the environment.Based on the variation used in the study, it was found that the optimum temperature for obtaining large quantities of hybrid coal was 450 o C, with a mixing ratio of lignite: corn cob, 1:3.Under these conditions, it also fulfills the requirements of a material to be used as fuel based on its calorific value, which is more than 5,000 cal/g.

Figure 2 .
Figure 2. Calorific value of hybrid coal at various co-pyrolysis temperatures.

Figure 3 .
Figure 3. Yield char of co-pyrolysis of lignite and corn cob at various mixing ratio.

Figure 4 .
Figure 4. Calorific value of hybrid coal at various mixing ratio.

Table 3 .
Yield char, calorific value and proximate analysis of hybrid coal.