pneumatic feeding of pulverized material (a mandatory feeding option for finely pulverized fuels) into the reactor. The amount of inert gas depends on the bulk density of fuels – the lower the density, the larger the amount. The low bulk density of biomass implies large consumption of carbon dioxide, and alternative forms of biomass feedstock (via pre-treatment) have to be considered for entrained flow gasifiers. The most feasible biomass pre-treatment options are torrefaction, pyrolysis, and pre-gasification.
See also: Biomass – Gasification, Gasifiers, Synthesis Gas.
Biomass to Energy
The old way of converting biomass to energy, practiced for thousands of years, is simply to burn it to produce heat (one of final forms of energy). The problems with burning biomass are that much of the energy is wasted and that it can cause some pollution if it is not carefully controlled. A number of non-combustion methods are available for converting biomass to energy. These processes convert raw biomass into a variety of gaseous, liquid, or solid fuels that can then be used directly in a power plant for energy generation. The carbohydrates in biomass, which are comprised of oxygen, carbon, and hydrogen, can be broken down into a variety of chemicals, some of which are useful fuels. This conversion can be done in three ways: (i) thermochemical methods, (ii) biochemical methods, and (iii) physical methods.
When plant matter is thermally decomposed, it breaks down into various gases, liquids, and solids. These products can then be further processed and refined into useful fuels such as methane and alcohol. Biomass gasifiers capture methane released from the plants and burn it in a gas turbine to produce electricity. Another approach is to take these fuels and run them through fuel cells, converting the hydrogen-rich fuels into electricity and water, with few or no emissions.
Biochemical methods of biomass breakdown involve the use of bacteria, yeasts, and enzymes, which also break down carbohydrates. Fermentation, the process used to make wine, changes biomass liquids into alcohol, a combustible fuel. A similar process is used to turn corn into grain alcohol or ethanol, which is mixed with gasoline to make gasohol. Also, when bacteria break down biomass, methane and carbon dioxide are produced. This methane can be captured, in sewage treatment plants and landfills, for example, and burned for heat and power.
Biomass oils, like soybean and canola oil, can be chemically converted into a liquid fuel similar to diesel fuel, and into gasoline additives. Cooking oil from restaurants, for example, has been used as a source to make biodiesel for trucks.
See also: Biofuels, Biochemical Platform, Biomass Conversion, Thermochemical Platform.
Biomass to Liquids
The biomass-to-liquid (BTL) process concept is a process for converting various types of biomass to liquid fuels and uses a variety of processes (Table B-23).
Table B-23 Conversion of biomass to fuels.
| Starch/sugar crops | ||
|---|---|---|
| Hydrolysis | ||
| Sugar | ||
| Fermentation | ||
| Refining | ||
| Ethanol | ||
| Lignocellulose biomass | ||
| Pyrolysis/liquefaction | Gasification | Anaerobic digestion |
| Bio-oil | Synthesis Gas | Bio-oil |
| Hydrotreating | Fischer-Tropsch | Biogas |
| Hydrocarbon fuels | Hydrocarbon fuels | Gas treating |
| Methane | ||
| Oil-producing plants | ||
| Pretreatment | ||
| Vegetable oil | ||
| Esterification | ||
| Biodiesel | ||
There are two types of biomass feedstock that can be employed to produce BTL fuels – woody and herbaceous. Woody feedstock comprises wood chips, wood powder and sawdust, obtained from ordinary forestry (wood logs), short-rotation forestry, various wood residues, and wood waste. Herbaceous feedstock includes chaffed dedicated energy crops and straw.
Owing to biomass composition, woody feedstock is better suited for energy applications than herbaceous feedstock. Woody biomass also has a larger production potential for energy (including BTL) application; however, the production potential of herbaceous biomass is currently under-explored as the path to useful products is open, for example (Table B-24):
Table B-24 Routes to liquid fuels from biomass.
| Biomass | ||||||
| Pretreatment | ||||||
| Gasification | ||||||
| Synthesis gas | ||||||
| Fischer-Tropsch | ||||||
