**5. Technical challenges of the gasification technology**

Although the gasification technology has experienced development over several decades and has been commercialized in a number of countries like those previously mentioned [14]; its successful operation is not as simple as can be imagined because of the thermodynamics of the operation of the technology are not well understood. Further exploitation of the technology still needs to overcome a considerable number of technical issues. A description of the technological barriers that are associated with each type of gasification technology is presented thus:

#### **5.1 Limitations of the fixed bed gasifier**

The fixed bed gasifiers (updraft, downdraft, and crossdraft) are the simplest of all the types of gasifiers and are mainly suitable for small-scale applications (<10 MWth) [5]. Although they (fixed bed gasifiers) are very advantageous in terms of their simplicity and ease of operation, they generally suffer from poor mixing and poor heat transfer within the gasifier, which makes it difficult to achieve even distribution of fuel and temperature across gasifier geometry hence scale-up of this type of gasifier is

#### *The Technical Challenges of the Gasification Technologies Currently in Use and Ways… DOI: http://dx.doi.org/10.5772/intechopen.102593*

difficult. The fixed bed downdraft gasifier, which has not satisfactorily performed with feedstock capacity beyond 425 kg/h [18], is a typical example of the described technical issue. This is because air cannot travel up the center of the gasifier, which creates cold spots in and around the combustion zone of the gasifier during operation and results in reduced gasification efficiency. This limitation has been attributed to design characteristics in terms of gasifier geometry (throat angle and throat diameter) and air inlet velocity. In the case of the updraft gasifier however, its high tar production rate (5–20%) [19] remains a challenge to date and renders this type of gasifier unsuitable where a clean product gas (syngas) is desired. Due to its high tar production rate, the updraft gasifier is well-suited for the gasification of low-volatile feedstocks like charcoal [5].

#### **5.2 Limitations of the fluidized bed gasifier**

In terms of configuration, the fluidized bed gasifier (FBG) operates on the principle of fluidization where a gas stream is forced through a particle bed vessel that behaves like a fluid under certain conditions such as high particle flow velocity. The commonly used fluidization media include air, steam, or mixtures of steam/oxygen. The FBG is the most efficient of all types of gasifiers and its efficiency is mainly dependent upon the thermochemical and fluid behavior inside the gasifier; this type of gasifier is more appropriate for medium-scale units of about 5–100 MWth [5, 20, 21]. From a system performance and technical point of view, the operation of the FBG is quite complex because of the need to simultaneously control air supply, bed material, and feedstock during operation of the gasifier. As a result, the product gas obtained from the gasification process may be very high in particulates, which can circulate and cause equipment erosion. Although it may sway the gasification process, the FBG is operated at high-pressure conditions, which can result in low volumetric gas flow rates, condensation during compression, and other operational complications such as defluidization from particle agglomeration particularly when agricultural crops and wastes are used as feedstock in the gasification process. This is because agricultural crops and wastes contain an increased amount of ash/alkali and, the alkali content of ash (such as sodium and potassium alkali) can form low-melting eutectics with the silica in the sand, which is the regularly used bed material in FBG processes [22]. Under this condition, agglomeration and sintering will occur, triggering the formation of a thin sticky substance around the bed particles with an instant loss of bed fluidization (defluidization). Typical factors influencing agglomeration and the loss of fluidization in FBGs are presented in **Table 2**.

Even if more sophisticated bed materials such as alumina and magnesite are used in the FBG process of feedstocks with high ash/alkali content, process cost will become an issue of concern. These types of technical issues call to question the feedstock flexibility of the FBG systems.

#### **5.3 Limitations of the entrained flow gasifier**

The entrained flow gasifier (EFG) is an old alternative energy production technology used on a large-scale (>50 MWth) [5] in the petroleum industry for the gasification of petroleum residues. This type of gasifier offers greater rates of collision between solid particles and is considered excellent in terms of performance because of vigorous mixing of feedstock and oxidizing agent as well as better feed conversion efficiencies in comparison to other types of gasifiers [25]. However, even though the


#### **Table 2**

*The summary of the impact of operating parameters on agglomeration and loss of fluidization [23, 24].*

EFG has been in existence for centuries, it has not been exploited to its full potential partly because the fundamental principles underpinning its operation are still vague, particularly with regards to the type of material suitable as feedstock. The mechanisms involved in the feedstock conversion process are still under debate. In addition, the EFG is operated at very high temperatures (1,200 – 2,000°C) and pressures, under these operating conditions, fuel-oxygen mixtures are turned into a turbulent flame of dust that ensures the production of liquid ash, which are deposited on gasifier walls. This constitutes a technical issue of concern, particularly when analyzing the ash melting behavior of the material used as feedstock in the gasification process. Due to this high operating pressure, numerical modeling and experimental validation of the EFG tend to be onerous. Furthermore, due to its operating conditions, only specific types of materials are used as feedstock.

### **6. Current research status**

Extensive studies have been undertaken on gasification technology over the last decade. Despite the numerous studies, however, there are still pending researchrelated issues (such as those described in preceding sections) that require further improvements. For example, Kaushal et al. [26] developed a one-dimensional steadystate model specific to the bubbling fluidized bed gasifier (BFBG). Gómez-Barea et al. [27] also reviewed the performance optimization of a small-scale FBG plant with the aim of maximizing char conversion rate and minimizing secondary gas treatments. The process performance of the downdraft gasifier was evaluated by Biagini et al. [28] in which the performance parameters such as syngas production, syngas heating value, cold gas efficiency, and the net efficiency of the gasifier were monitored using corn cobs as feedstock. Furthermore, the performance of a pilot-scale pressurized entrained-flow (EFG) plant using stem wood made from pine and spruce as feedstocks was assessed by Weiland et al. [29]. A combined system involving gasification, hydrothermal carbonization (HTC), and solid oxide fuel cell (SOFC) technologies was developed by Papa et al. [30] using commercial process simulation software (ASPEN Plus), where the focus was to investigate the efficiency of the system under various

operating conditions. The challenges and opportunities of modeling the gasification technology using Aspen Plus were also detailed by Mutlu and Zeng who alluded to the issues of the gasification technology as hindering the widespread commercialization of the technology [31].
