**2.5 Catalysts**

The reactions in gasification can proceed with higher yields and less energy input if appropriate catalysts are employed. Catalysts can facilitate the process by reducing slagging problems, by which in severe cases, gasifiers need to be shut down for maintenance. Together with slagging of low-melting-point inorganic compounds, tar and soot formation also interrupts the operation because matters can be vaporized at high temperature, then condense at cooler zones and clog the systems. Catalysis helps limit the formation of such undesired side-products or decompose them to workable substances by cracking reactions. The mechanism of tar catalytic cracking can be assumed as follows [28]:

feedstock to show input plasma energy was lowest while syngas formation yield was highest [39]. Experimental results showed that steam or catalysts added to plasma

Gasification is a complicated process, which is influenced by many factors, among which equipment design plays a very important role. Popular types of

There are three ways of arrangement for biomass and gasifying agents entering

• Updraft gasifiers (**Figure 3a**): in this type of reactor, biomass is fed downward from the top and gasifying agents is fed upward from the bottom in a counter flow arrangement. Ash is collected at the bottom of the equipment with airlock design. The biggest weakness of updraft gasifiers is the accumulation of tar, moisture, and soot on the top of the reactors, which becomes hard clogging blocks inside the equipment. **Figure 4** is the actual photo of a very thick and hard layer of tar and soot attached to the inner wall on the top of an updraft biomass gasification reactor (the photos were taken at the Laboratory of Biofuel and Biomass Research, Ho Chi Minh City University of Technology, HCMUT). This counter flow process also makes syngas from updraft gasifiers carries much contamination. In contrast, the operation of updraft gasifiers is the easiest among the three types of fix-bed gasifiers above. Its design is also

• Downdraft gasifiers (**Figure 3b**): a narrow throat at the combustion zone is the typical design of this type of equipment. Since syngas is obtained at the bottom of the reactor, biomass and gasifying agents move in a co-current direction and get in contact for combustion at the device throttle. The flow rate of the

to react with each other in the reactors: updraft, downdraft, and cross draft as

gasification can significantly reduce the formation of tars [40].

gasifiers are listed and briefly discussed as bellows.

simple and available for multi-feed stock purpose.

*Fix-bed gasifier types. (a) Updraft gasifier. (b) Downdraft gasifier. (c) Crossdraft gasifier.*

**3. Gasifiers**

*Gasification of Biomass*

*DOI: http://dx.doi.org/10.5772/intechopen.93954*

**Figure 3.**

**553**

**3.1 Fixed-bed gasifiers**

illustrated in **Figure 2a**–**c**.


In contrast, catalytic gasification has some disadvantages, such as material costs and fading catalyst performance over reaction time. Theoretically, catalysts can be recovered after the process. But in fact, they are easily poisoned and contaminated by variable products, which are formed from the complex interactions in gasification.

Alkali metal salts seem to be the earliest catalysts to be examined for gasification [29]. Alkali elements were studied to catalyze gasification of char and biomass, and they were proved to reduce the formation of tar and soot [30, 31]. The employment of catalysts is preferred for entrained-flow gasifiers, which will be discussed later [32].

Natural minerals, precious metal and synthetic catalysts are also studied for their application in biomass gasification, as well as coal and syngas conversion [33–35].

#### **2.6 Plasma gasification**

Plasma, which can be produced by an electric arc discharged to a gas, is a very hot and highly ionized gaseous mixture. The initial gas interacts with the electric arc to become dissociated into electrons and ions at temperatures often exceeding thousands of Celsius degree. When biomass and a non-oxidizing gasifying agent are fed into a plasma reactor, the gasification can proceed at high temperatures without combustion to generate heat as in conventional process. Therefore, plasma gasification can convert organic substances to syngas that preserve all its chemical and heat energy, while converts inorganic mineral ash to inert vitrified glass or slag. As a result, contamination and dilution of syngas are minimized and the process control is easy to yield expected syngas composition [36, 37].

Microwave was also used to generate plasma in plasma gasification [38]. However, microwave plasma system is not easy to scale up for industrial purposes like electric arc type.

With the principle of supplying intensive heat for endothermic reactions, plasma gasification was used to produce hydrogen with steam injection as discussed in Section 2.3 [20]. Carbon dioxide gasification was studied with a various biomass

feedstock to show input plasma energy was lowest while syngas formation yield was highest [39]. Experimental results showed that steam or catalysts added to plasma gasification can significantly reduce the formation of tars [40].
