Biomass gasification is a promising approach for bioenergy conversion. Usually, biomass gasification is facing interruption in feedstock supply due to seasonal availability of biomass. In biomass gasification, formation of tar also affects the gasification efficiency. Therefore, in this study, catalytic air co-gasification of two palm wastes (coconut shells; CS, oil palm fronds; OPF) was investigated for syngas (H₂+CO) and methane production in downdraft gasifier using three mineral catalysts such as Portland cement, dolomite, and limestone to address the issues. The three main process variables were investigated within the specific range, the temperature of 700-900 °C, catalyst loading of 0-30 wt%, and the biomass blending ratio of 20-80 wt%. Response Surface Methodology, Box-Behnken Design was used for process optimization. The results showed that temperature was the most influencing parameter for syngas production, followed by catalyst loading and blending ratio. The maximum methane produced from Portland cement catalyst followed by limestone and dolomite. The syngas and methane yield was obtained 38.81 vol% and 19.96 vol% respectively at optimized conditions of catalyst loading of 20 wt%, temperature of 900 °C, and blending ratio of CS20:OPF80 using Portland cement as a catalyst. The higher syngas and methane yields from catalytic co-gasification as compared to non-catalyst co-gasification was due to the catalytic effect of Ca, Fe, Mg, K, P, and Al oxides present in catalysts and biomass materials.

Graphical Abstract

Response surface methodology (RSM) with a standard technique of Box-Behnken Design (BBD) was chosen to develop the matrix of experiments for catalytic co-gasification runs using Design Expert 11® software. RSM is an important optimization technique, which develops 3-D response surface between output response and operating parameters to analyze the effect of mutual interactions of operating parameters on the response. It also develops the empirical relationship between the operating parameters and response. The optimum variable's value obtained from response surface based on the desired output. The three influencing variables, temperature (A), catalyst loading (B), and blending ratio of CS/OPF (C) were used to observe the response on syngas (H₂+CO) and CH₄ (vol%) production using three three-level BBD. Table 5 illustrates the ranges of operating parameters which include 700-900 °C, 0-30 wt%, 20-80 wt% were selected for temperature, catalyst loading, and blending ratio of CS/OPF respectively. By default, the high levels of the factors are coded as +1, and the low levels are coded as -1. The higher and lower boundaries of the input variables were determined based on the preliminary results, literature, and considering the experimental setup facility. The experimental design matrix devolved by BBD and response results of syngas (H₂+CO) and CH₄ vol% production in the presence of Portland cement, dolomite, and limestone are presented in Table 6. The prominence feature of BBD, it avoids to perform experiments under extreme conditions and does not involve all combinations of factors which are all at once their highest or lowest levels.

Experimental facility used for catalytic co-gasification of CS/OPF blends

Optimization of Process Parameters: RSM allows for the optimization of various process parameters such as temperature, pressure, catalyst loading, and biomass-to-catalyst ratio. By systematically varying these parameters and analyzing their effects on the gasification process, RSM can identify the optimal conditions for maximum gasification efficiency and bioenergy production.

Improved Efficiency: Through RSM, researchers can identify the ideal combination of catalyst type, concentration, and operating conditions to enhance the gasification process. This leads to improved efficiency in terms of higher gas yields, increased bioenergy conversion rates, and reduced energy consumption.

Reduced Environmental Impact: Efficient gasification of palm wastes using mineral catalysts can contribute to reducing the environmental impact associated with waste disposal. By converting biomass into valuable bioenergy products such as syngas (a mixture of hydrogen, carbon monoxide, and methane), the process helps mitigate greenhouse gas emissions and dependence on fossil fuels.

Resource Utilization: Palm wastes, such as empty fruit bunches and palm kernel shells, are abundant agricultural residues. By utilizing these wastes as feedstock for gasification, RSM-guided processes enable the efficient utilization of renewable biomass resources for bioenergy production, thereby reducing reliance on non-renewable energy sources.

3-D response surface graphs of methane production the combined effects of operating variables (temperature, 700-900 °C, catalyst loading, 0-30 wt%, blending ratio, 20-80 wt%) for (a-c) Portland cement, (d-f) dolomite, and (g-i) limestone as a

Growing Demand for Renewable Energy: With increasing concerns about climate change and the depletion of fossil fuel reserves, there is a growing global demand for renewable energy sources. Bioenergy, derived from biomass such as palm wastes, holds promise as a sustainable alternative. RSM-guided optimization of catalytic co-gasification processes can enhance the efficiency and economics of bioenergy production, thereby tapping into this expanding market.

Focus on Waste Valorization: Palm wastes, including empty fruit bunches and palm kernel shells, are abundant agricultural residues often considered as waste. However, there is a growing trend towards waste valorization, where these residues are utilized for energy generation and other valuable products. The application of RSM to optimize catalytic co-gasification processes aligns with this trend, unlocking the market potential for converting waste biomass into valuable bioenergy products.

Government Incentives and Regulations: Many governments around the world are implementing policies and regulations to promote renewable energy development and reduce greenhouse gas emissions. Incentives such as feed-in tariffs, tax credits, and renewable energy mandates create a favorable market environment for bioenergy projects. RSM-guided optimization of catalytic co-gasification processes can help bioenergy producers meet regulatory requirements while maximizing their economic returns.

Technological Advancements and Research Funding: Ongoing advancements in catalysis, biomass gasification, and process optimization techniques contribute to the market potential for RSM applications in bioenergy conversion. Research funding from government agencies, private sector investors, and international organizations further drives innovation in this field, supporting the development and commercialization of RSM-guided catalytic co-gasification technologies.

3-D response surface graphs of methane production the combined effects of operating variables (temperature, 700-900 °C, catalyst loading, 0-30 wt%, blending ratio, 20-80 wt%) for (a-c) Portland cement, (d-f) dolomite, and (g-i) limestone