(1.1)
Table 1.1 Pyrolysis processes for different biomass.
| Biomass | Catalyst | Temperature (°C) | Pressure (atm) | Biochar yield (%) | Biofuel yield (%) | Reference |
|---|---|---|---|---|---|---|
| Mesue ferra seed oil | Na2CO3 | 500-600 | 1 | 17.5 | 66 | [49] |
| Canola oil | HZSM-5 | 500 | 1 | 16.4 | 49.6 | [50] |
| C. inophyllum | KOH | 50 | NA | NA | 66.5 | [51] |
| Rise husk | KOH | 700 | 1 | 21 | 39 | [52] |
| Waste clay oil | CaO | 450 | NA | 23 | 72 | [53] |
| Rice straw & sugarcane | HZMN-5 | 500 | NA | 29.7 | 26.1 | [54] |
| Microcrystalline cellulose | NaY | 30-950 | N2 atm | 19 | 48 | [55] |
| Grape seeds | CaO | 550 | NA | 42.5 | 38.5 | [56] |
| Ajjwa date seed | Ni3Fe | 500 | 10 bar H2 | 15.8 | 58.1 | [57] |
NA = not available
As a result of pyrolysis of lignocellulosic solid wastes, three main products are obtained. These are semi-coking solid product (char), oil and gas as seen in Figure 1.2. As a result of pyrolysis of cellulose, char and active cellulose are obtained, the products of active cellulose which are not used as a result of biofuel production which are separated. Wastes can be converted into solid, liquid and gaseous products with the pyrolysis process. The composition and proportions of these products largely depend on the type of raw material and reaction conditions. During pyrolysis of solid wastes, a wide variety and quite complex reactions occur in series and parallel. These complex reactions occur because the breakdown of cellulose, hemicellulose and lignin in the biomass content forms different products [48].
In the pyrolysis process, the most important process variables can be considered as heating speed, temperature and duration of stay. Those variables have the effect of reactor type and particle size on the heating speed. By using any type of reactor, if the particle size is increased, thermal conductivity decreases in the plant structure, heating rate decreases in the particle and changes in product yields occur. For each selected particle size, the reactor type acts on the mechanism of heat transfer. In other words, radiation, convection and conduction mechanisms vary depending on the type of the reactor [46].
Heating of biomass at low temperatures, long gas and solid retention time are called slow or conventional pyrolysis. Depending on the system, the heating rate is in the range of 0.1–2 °C per second and commonly the pyrolysis temperature is about 500 °C. The retention time of the gas can vary from 5 seconds to minutes or days, depending on the biomass. With conventional pyrolysis, approximately equal amounts of gas, liquid and solid products are obtained [47].
Figure 1.2 Multistep collective mechanism in the degradation of cellulose [47].
Fast pyrolysis is a high-temperature process in which biomass is rapidly heated in an oxygen-free environment. Heating rate is in the range of 200 and 105 °C/s and pyrolysis temperature is generally higher than 550 °C. The liquid obtained as a result of fast pyrolysis constitutes approximately 70-80% of the biomass. Fast pyrolysis is the most preferred pyrolysis method today for the production of liquid products with features such as low cost, easier transportation and storage [48].
1.4.2 Types of Reactors Used in Pyrolysis
Reactor type is effective on the composition and yield of the products obtained, and constitutes approximately 10-15% of the total cost of an integrated pyrolysis system. Pyrolysis reactors include reactor bed types such as fixed bed, fluid and suspended bed, movable bed, free fall feed reactor, inclined rotating oven, etc. [58]. Reactors such as circulating fluidized bed, ablative, bubbling fluidized bed, rotating cone, wire mesh braided and drag flow reactor are some of the types of pyrolysis reactors used today [59, 60]. In Table 1.2, the yield of biofuels obtained under different conditions from different types of reactors is indicated. As can be seen, lignocellulosic material and the process temperature have high effects on biofuel yield corresponding to the reactor type.
1.4.2.1 Bubble Fluidized Bed Reactor
Compared to other reactors, the design and construction of fluidized bed reactors is simpler. It has many advantages, such as good gas solid contact, excellent heat transfer properties, better temperature control and large heat storage capacity. Fluid bed pyrolysis offers good and consistent performance from wood, usually with a high liquid product efficiency of 60-75% by weight [58].
1.4.2.2 Circulating Fluidized Bed and Transport Bed Reactor
Circulating fluidized bed pyrolysis reactors have similar properties as bubble pyrolysis reactors. Compared to the bubbling fluidized bed pyrolysis reactor, steam and