The Effect of Pretreated Palm Kernel Shell and Mukah Balingian Coal Co-gasification on Product Yield and Gaseous Composition

In this study, co-gasification of palm kernel shell (PKS) and low-rank Malaysian coal (MB) was carried out in a fixed bed reactor. For the pretreated samples, PKS was torrefied at 270°C (PKS_To) and MB was preheated at 250°C (MB_Pr) for 1 h, respectively, prior to co-gasification at 767°C, with a biomass blending ratio of 52% and a steam flow rate of 55 mL/min. The effect of different blending combinations was investigated towards product yields, namely gas, tar, char and gases composition. The co-gasification on both pretreated (PKS_To/MB_Pr) and catalyst-pretreated (Cat-PKS_To/MB_Pr) produced a greater gas yield, with lesser tar and char yield than both untreated PKS and MB (PKS_Un/MB_Un) and pretreated PKS and untreated MB (PKS_To/MB_Un). The PKS_To/MB_Pr was found to enhance the H₂ production by 63.9% and 41% than PKS_Un/MB_Un and PKS_To/MB_Un, respectively, at 45 min of reaction time. Thus, the pretreatment on both samples had a significant impact on the distribution and composition of product yields during co-gasification. As a conclusion, the pretreated sample, which has been upgraded on characteristics such as higher carbon and lower oxygen content than the untreated sample was revealed to enhance gas yield and H₂ production during co-gasification.

Biomass; Gasification; Low rank coal; Palm kernel shell; Pretreatment

Currently, the application of the world energy, which releases carbon dioxide, sulfur oxide and nitrogen oxide, has become an issue (Taba et al., 2012). The other problems are associated with the usage of fossil fuels and production of greenhouse gas. Thus, gasification, which is established as an energy-efficient technology, has an acknowledged important consideration (Sulaiman et al., 2012; Heidenreich and Foscolo, 2015). Presently, coal is the main feedstock in gasification and is expected to be applied as the energy resource for many decades ahead. However, this direction is difficult to achieve due to the increase in energy demand, which has caused the shortage of supply and the reduction of high-rank coal (Mohr et al., 2015). Consequently, one of the approaches is to utilize the abundant low-rank coal and biomass in gasification.

        The low-rank coal is almost partial towards the world's entire coal deposits compared to the high-rank coal.  The usage of low-rank coal in thermal conversion is economical due to its low pricing. However, low-rank coal as a substitute for high-rank coal has several limitations, such as low calorific value and high moisture and oxygen content (Rao et al., 2015). These drawbacks can be minimized by using the pretreated or upgraded low-rank coal in gasification (Xia et al., 2015). Similarly, the utilization of biomass, which is a renewable and environmentally friendly resource during gasification, created several problems. Untreated biomass has relatively low energy, high moisture and oxygenated compound, hygroscopic behavior and poor grindability (Chen et al., 2015). Accordingly, the pretreated biomass improved in energy density; hygroscopic characteristics and grindability overcome the disadvantage of untreated biomass and are suitable for further thermochemical conversion (Nhuchhen et al., 2014; Yuliansyah et al., 2019).

      Biomass commonly has higher hydrogen content than coal and it is appropriate to mix both together. Further, the alkali and alkaline earth metals (AAEM) in biomass catalyze the gasification of char resulting from coal pyrolysis. Equally, the high silica (SiO₂) content in coal acts as an effective catalyst for tar cracking to light hydrocarbon in thermal conversion (Mallick et al., 2017). However, the gasification of biomass indicated more drawback than coal gasification, where biomass has high oxygenated compound and moisture content and low energy density (Ahmad et al., 2014; Kasim et al., 2019; Ahmad et al., 2019). Thus, co-gasification of biomass and coal can be substituted for individual gasification, as it may improve their disadvantages on both feedstocks (Brar et al., 2012).

Co-gasification has been studied by some researchers. It improved the overall gas and hydrogen composition more than individual gasification (Howaniec and Smoli?ski, 2013) and showed the synergistic influence in terms of high gas yield, low tar and char yield at 1:1 biomass–coal ratio (Krerkkaiwan et al., 2013). There were synergistic effects in the decrease of char yield and increase of gas yield in the co-conversion of coal-biomass blending (Yuan et al., 2012). Consequently, the synergy between biomass and coal co-gasification increases the gas yield, gasification efficiency and reactivity of char and reduces the tar yield (Winaya et al., 2015). Upgraded biomass, such as torrefied pellets, was suitable to obtain low tar yield (Dudy?ski et al., 2015), and torrefied bamboo was also established to produce high syngas yields (Kuo et al., 2014). Moreover, the blending of pretreated biomass and sub-bituminous coal in co-gasification was found to minimize the formation of agglomerates in fluidized bed reactors (Strege et al., 2011). Definitely, torrefaction creates the gasification behavior of the biomass in its approach to coal where the H₂ composition in the syngas of torrefied biomass is comparable with coal.

Furthermore, one of the most efficient techniques of producing higher gas qualities is steam gasification. It offers the highest composition of hydrogen (Parthasarathy and Narayanan, 2014). Numerous studies have reported enhanced syngas yield and carbon conversion efficiency when steam was utilized as a gasifying agent (Howaniec et al., 2011; Moghadam et al., 2014; Naqvi et., 2016).

        Co-gasification of PKS and MB coal was done in a fixed bed reactor. The pretreatment of a blend of both samples produced a higher gas yield with lower tar and char yield than the untreated blend of both samples. The PKS_To/MB_Pr produced a higher H₂ composition of 31.3%, which was more than PKS_Un/MB_Un sample of 19.1% at 45 min reaction time. The Cat-PKS_To/MB_Pr showed a minor increase on H₂ composition of 32.6%, which is more than PKS_To/MB_Pr of 31.3% at 45 min reaction time. The lowest CO₂ composition at 12.5% was produced by PKS_To/MB_Pr compared with PKS_Un/MB_Un at 20.3% in a reaction time of 60 min. Thus, the PKS_To/MB_Pr, which had been enriched in their properties, improved the co-gasification performance in terms of product yield and gas composition.

This research project is funded by the Ministry of Higher Education, Malaysia, under the Fundamental Research Grant Scheme (FRGS), FRGS/1/2017/TK10/UITM/02/11. 

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