Biomedical Engineering Reference
In-Depth Information
(2) Thermochemical conversion
Thermochemical conversion decomposes natural lignocelluloses into liquid and
gaseous fuels at high temperature. Thermochemical conversion can improve fuel
quality and energy efficiency and enhance energy density, making them easy to
store and use. In addition, the natural cellulose have a relatively high content of
hydrogen and carbon, almost no sulfur, and low ash, so they are easier to handle
than coal. Thermal decomposition is the center of thermochemical conversion. The
natural cellulose materials are converted into a variety of products by thermal
decomposition in the absence of air or a small amount of air or steam. The
pyrolysis process of thermal decomposition is carried out without oxidants or under
low-oxidant conditions, which is generally an endothermic reaction. The polymer
compound is decomposed into a low molecular product, such as hydrogen, methane,
carbon monoxide, carbon dioxide, and other gases. The liquid portion of polymer
compound includes methanol, acetone, acetic acid, and other organic matter, such
as tar, solvents, oil, and aqueous solution. The solid part is mainly carbon black.
Burning is an exothermic reaction whose main products are carbon dioxide and
water.
The composition and ratio of pyrolysis product vary with thermal decomposition
conditions.
①
Gasification
The gas composition of thermal decomposition varies with materials, gasification
gas medium (gasification agent), devices, and heating conditions. There could be
20-40 % fuel gas received from coal in the process of gasification, while from
natural cellulose materials, it is 70-80 % fuel gas. Using air as the gasification agent,
because of nitrogen mixed with the generated gas, the gas has low calorific value
(700-1,800 kcal/m
3
). With oxygen as the gasification agent, the calorific value of
gas is 2,500-4,500 kcal/m
3
. The difference of gasification and carbonization is that,
in the gasification process, the pyrolysis material and carbonized residue continue to
react with air, steam, oxygen, carbon dioxide, or hydrogen; that is, some materials
are burned in controlling conditions, so that the rest is converted into a gas with
low calorific value. But, the heat generated in the exothermic process can be used
to maintain the reaction temperature required in the gasification system. However,
gasification is different from direct combustion because of its extremely rare smoke.
A gas furnace is the equipment to achieve the gasification process.
②
Liquefaction
Gases from high-speed thermal decomposition are converted to diesel, oil, or
gasoline, or CO
2
and H
2
are converted to a mixture of ethanol-containing methanol
in the presence of catalysts and pressure conditions. A liquefaction reaction can run
in the presence of hydrogen or carbon monoxide.
③
Dry distillation
Dry distillation can produce a mixture of solid, liquid, and gaseous fuels. The
solid product is wood vinegar. The wood vinegar and flammable wood gas can be
separated from the gas mixture through condensation in the dry distillation kettle.
