**3. Gasifiers**

**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

• Organic and hydrocarbon compounds are dissociated from the biomass and

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

Alkali metal salts seem to be the earliest catalysts to be examined for gasification

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].

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

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

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

[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

• Water is hydroxylated to OH radicals, which oxidize the hydrocarbon

by variable products, which are formed from the complex interactions in

• Syngas, CH4, and lighter hydrocarbons are formed then.

tar catalytic cracking can be assumed as follows [28]:

• Catalytic dehydrogenation reactions happen.

is easy to yield expected syngas composition [36, 37].

absorbed on the catalytic sites.

*Biotechnological Applications of Biomass*

fragments.

gasification.

discussed later [32].

**2.6 Plasma gasification**

electric arc type.

**552**

Gasification is a complicated process, which is influenced by many factors, among which equipment design plays a very important role. Popular types of gasifiers are listed and briefly discussed as bellows.
