Pyrolysis, slow liquid yields

Pyrolysis of biomass is divided into slow pyrolysis, which is well known to produce charcoal, for example, fast pyrolysis, which produces a high yield of liquid biofuels and other chemicals (Bridgwater, 2000) and flash pyrolysis. Slow pyrolysis (or carbonisation) requires low temperatures and very long residence time. In the carbonisation process the amount of char is maximised. 

In pyrolysis the wood material is heated rapidly to about 500 °C at which temperature the wood decomposes to a maximum amount of liquid product. At lower temperatures more char is formed and less liquid and gas, and at higher tenperatures the energy requirements are higher without producing noticeably more liquid. The pyrolysis process is carried out in a fluidised bed where milled material is fed into the bed and the product stream is condensed at temperatures between 30 and 60 °C. The char is usually separated before the condenser and used as fuel - along with the gas -to provide heat to the fluidised bed. The fluidised bed may be bubbling or circulating. In both cases a fast pyrolysis is obtained in contrast to slow pyrolysis which usually yields lower amounts of liquids. 

Liquid yields of 35-50% on dry feed are typically obtained with higher char yields than fast pyrolysis systems. Conversely, the liquid yields are higher than in slow pyrolysis technologies because of fast removal of vapours from the reaction zone,... 

In this study, an attempt was given to produce bio-oil through slow pyrolysis of rice husk (RH) at different heating rates in order to determine the optimum reaction condition that will give maximum liquid yield. The characteristics of bio-oil produced at different heating rates are then analyzed. Apart from that, the properties of bio-oil that was produced at optimum operating condition are compared with those in literature. 

During the slow pyrolysis of polyethylene, for a temperature increase from 400 to 700° C, the yield in liquid phase remains higher than 80% with a very small increase in the yield of gas phase (less than 20%). On the other hand, in flash pyrolysis of polyethylene, an increase of temperature from 550 to 700°C leads to a decrease of the yield in the liquid phase to less than 40% with an increase in the yield of the gas phase up to 60%. 

The main product yield (after slow or flash pyrolysis) is the liquid phase and Sawagushi found that this is mainly composed styrene monomers, dimers and trimers. For a residence time of 60 min, increasing the temperature from 310 to 350°C increases the monomer fraction up to 78%. Table 10.16 shows the proportions of these three components in the liquid phase as a function of the temperature. 

Pyrolysis entails thermal decomposition of biomass molecules in the absenee of oxygen, usually at the temperatures up to 650-800 K. To produee liquid oils, the heating process should be short [i.e., short residenee time), thus this process is usually referred to as fast pyrolysis. At high temperature, the biomass is vaporized and then condensed upon cooling to produce a liquid oil mixture which may be comprised of more than 300 compounds such as alkanes, aromatic aliphatic, sugars, alcohols, ketones, aldehydes, acids and esters. If the residence time is longer (slow pyrolysis), the product mixture is likely to produce more solid coke than liquid fuels. An advantage of fast pyrolysis is that it is economical for use on a small scale ie., 50-100 tons biomass per day). Yields of bio-oil production in excess of 70% have been... 

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