**7. Research priority areas and solutions to the identified technical issues**

The FBGs such as the downdraft gasifier is characterized by four distinct reaction zones including the drying, pyrolysis, combustion, and reduction zones respectively; the specific functions of each of these zones are described in [32]. Of these distinct reaction zones, the combustion zone, also known as the oxidation zone, is considered the most important zone because heat is generated in this zone. However, the presence of cold spots (a factor linked to uneven heat distribution in and around the combustion zone of the downdraft gasifier), is the main reason why these types of gasifiers are limited to small-scale applications [13]. There are basically two methods that can provide a solution to the problem of uneven heat distribution in fixed bed systems: one method is to decrease the cross-sectional area of the gasifier at a certain height. This means altering the design characteristics of the throat angle and throat diameter of the gasifier by way of size-reduction. The other method is to centralize the air inlet and its velocity using nozzles that are positioned in a way that allows the throat circumference of the gasifier to be captured.

In the case of the FBGs, although a well-established technology (in terms of design concept) for heat and power generation, bed defluidization, as indicated in a previous section, is considered the main technical issue, which as previously described, occurs due to agglomeration and pressure drops, particularly when gasifying feedstocks with high amounts of ash such as agricultural residues and wastes. Alkali silicates such as calcium, potassium, and sodium silicates present in ash can form low-melting eutectics with silica, which is often used as the bed material in FBGs [22]. A quick and easy solution to the defluidization problems in FBGs is to replace the commonly used bed material (silica) with more advanced artificial materials such as aluminum oxide or magnesium carbonate. However, the cost associated with the use of these materials may constitute a major drawback. Therefore, the hydrodynamics of the FBG needs to be further investigated and the hydrodynamic study must incorporate devolatilization kinetics, char gasification, and gas species in relation to particle agglomeration and sintering.

For the high-pressure EFG, the production of molten ash (which mostly originates from the ash constituents of the feedstock and forms deposits on the walls of the gasifier) is a commonly encountered technical problem. Depending on the operating conditions of the gasification process, the molten ash deposits often solidify, causing plugging and the blockage of critical parts of the gasifier thereby hindering process efficiency. Therefore, just like the FBG, a solution to the problem of molten ash formation in the EFG is to further investigate the feedstock conversion mechanism and gasifier hydrodynamics, particularly when more complex low-grade feedstocks such as agricultural residues and biomass-based chars are used in the gasification process under high-pressure conditions.

Studies [33, 34] have shown that modeling work has accelerated the research progress made in the field of biomass gasification since gasifier design and operating conditions can be optimized at minimal time and costs. However, modeling and simulation cannot replace good experimental investigations. In fact, studies [35] have determined that mathematical modeling and simulation of high temperature and

pressure reaction systems involving gaseous, liquid, and solid phases is a major scientific challenge. Therefore, addressing the technical issues of the gasification technologies described in this chapter will not only require the development of a robust and sophisticated model that can be applied to a wider range of operating parameters of the gasifiers but also able to replicate actual operations of the gasification technologies with an acceptable level of anomaly.
