**4. Pyrolyzer**

Pyrolyzer is a reactor where pyrolysis of biomass is done [22]. The design of pyrolyzer depends on desired pyrolysis product(s). Therefore, it is another important factor that determines products obtainable from pyrolysis [10]. Some of designed pyrolyzers are discussed in sub-sections below.

### **4.1 Bed pyrolyzer**

This pyrolyzer can be made of firebrick, steel or concrete and is predesigned for bio-oil production. Its design contains feeding unit, non-condensing vapor collector and vapor condenser, others are filter and ash exit unit [29]. It is operated in such a way that biomass is fed downward in reactor through vertical shaft and counter current gas stream simultaneously fed-in in an upward direction. The biomass is heated by bed of hot solids (such as sand heated by external source or partial combustion of biomass) at high temperature, high heating rate and low vapor retention time in an inert environment to decompose it into non condensable gas, condensable vapor and char. The vapor is rapidly cooled to form high quality bio-oil with yield of about 70% mass of pyrolysis products [30]. Therefore, it is a simple and well designed fast pyrolyzer that gives precedence to production of bio-oil. Merits of this reactor are exemplary heat transfer system, very high relative velocity between the solid and liquid phase, good control of vapor holding time and pyrolysis reaction as a whole. Some of the existing designs are fixed bed, circulating fluidized and bubbling fluidized bed reactor [10].

### **4.2 Vacuum pyrolyzer**

This is a slow pyrolyzer with a moderate biomass degradation temperature, very low heat transfer rate and high vapor retention time. The biomass is feed into the pyrolyzer with the aid of conveyor belt made of metal containing stirrer that agitates the biomass mechanically. The biomass is heated with attached burner for decomposition into gas, liquid and solid. It produces bio-char of yield greater than 50% and bio-oil of less than 30% yield. Therefore, it is designed for bio-char production [31].

### **4.3 Microwave pyrolyzer**

Design of microwave pyrolyzer is done in such a way that biomass is heated to decompose into gaseous, liquid and solid products through heat generated from electrical microwave oven chamber. Inert gas is used to fashion out non oxygen environment. In this pyrolyzer, undesirable output (by-product) is not formed, there is effective heat transfer and heat process can be easily monitored. Among its merits are a very good chemical recovery, efficient control of heat transfer and heating system. This pyrolyzer is exclusively designed for syn-gas production with greater than 30% yield from feedstock biomass [32].

### **4.4 Solar pyrolyzer**

This makes use of clean energy from sun light. It is made of quartz tube containing opaque external wall. The opaque external wall when exposed to high intensity solar radiation is capable of generating temperature higher than 700°C for decomposition of biomass into syn-gas, bio-oil and bio-char [33, 34]. In its design, there is provision for solar energy storage inform of chemical energy. Therefore, at night and during poor sun shine weather, it can still work using already stored energy for

generating heat for pyrolysis process. It is a very fast pyrolyzer with bio-oil yield greater than 50%. Although, the yield of gaseous fraction is less than 30%, but is recommended for production of syn-gas because of poor quality of bio-oil formed according to Cornelissen et al. [28].
