## 1 Introduction  
The worldwide energy crisis, the rising costs of crude oil and natural gas, environmental pollution and climate change in recent years urgently require a transition to sustainable and cleaner energy forms. Among fossil fuels, coal covers about 29% of primary energy and 40% of electricity production globally [1]. Its great reserves and lower costs indicate that coal will continue to be the main resource of energy in the future [2] if clean technologies are used, which could concomitantly maintain the economic sustainability of power plants. One such clean technology is coal gasification, converting coal into valuable fuel gas (CO, H\({}_{2}\), CH\({}_{4}\)) through a reaction with non-stoichiometric air, steam or carbon dioxide, at high temperatures (above 700 \({}^{\circ}\)C). This process is not only environmentally superior to combustion, avoiding gaseous emissions that cause acid rain, but also offers greater flexibility for the generated gas to be used, besides as an energy source, for the production of biofuels and added-value chemicals [3]. When steam is used as the gasifying agent, the yield of high-quality hydrogen gas is enhanced [4, 5, 6], whereas catalysts [7, 8], absorbent materials, or a pre-pyrolysis process can increase the reactivity of the fuels and reduce the amount of tar and carbon dioxide in the product mixture [3], and thus, reduce consequent operational and environmental problems. On the other hand, the use of carbon dioxide from residual streams as the gasifying agent could provide a potential solution to the greenhouse gas effect [9]. The reaction of carbon with carbon dioxide is also important in large-scale fixed-bed gasifiers, where the feedstock is subjected to different reaction zones such as drying, pyrolysis, air oxidation and carbon dioxide gasification [10]. For coal gasification, however, in order to achieve high efficiency under less severe operating conditions and generate a gas free of by-products, the cost is drastically higher [6]. Therefore, a sustainable solution to this problem could be achieved through the partial substitution of coal with alternative renewable energy sources. As such, biomass presenting great similarities to coal, in terms of chemical composition and methods of utilization [11; 12], seems the most attractive candidate. Biomass, accounting for about 15% of the world's energy supply [12], offers not only low-cost availability, but also carbon neutrality [3; 12], in line with global policies for de-carbonized energy and mitigation of the carbon footprint on the environment through the promotion of renewables and circular economy [13]. Additionally, the co-gasification of coal and biomass overcomes the drawbacks of large investments in new biomass plants, which suffer due to the seasonal availability of materials, their low energy density, or their slagging/fouling tendency [14; 15]. As agricultural wastes are generated in large quantities in most countries around the world, at low or no cost, with energy availability ranging between 5 EJ/y and 27 EJ/y [11], the majority of past investigations have studied their co-gasification with high-rank coals, in order to promote their penetration of energy markets and fulfill the Agricultural and Renewables Policy [11; 16], as well as promoting circular economy [17; 18; 19]. Sorghum, fruit waste, spirit-based distillers, empty fruit bunches, sugar cane bagasse, rice straw and walnut shells [20; 21; 22; 23] were among the residues tested. The reaction temperature, flow rate of the gasifying agent, blending ratio, chemical and structural characteristics of feedstocks and composition of ashes were addressed as the key factors affecting the performance of the co-gasification process. Synergetic effects were mostly reported between coal and biomass depending on these factors, increasing or decreasing the reactivity of the fuels and the yield of product gases [17; 18; 19; 20; 21].