Development Potential Bio-Oil
The major drawback of biomass is that the energy density is an order of magnitude lower than that of crude oil (Table 4-17). When it is further considered that biomass for fuel is a difficult to handle solid (grain is an exception in that it almost flows like water), this implies that fuel biomass can never be shipped economically over long intercontinental distances.
|
Table 4-17 Energy Densities of Various Fuels |
|||
|
Fuel |
Particle Density |
Bulk Density |
Energy Density |
|
GJ/m3 bulk |
|||
|
kg/m3 |
kg/m3 |
product |
|
|
Crude oil |
855 |
35.8 |
|
|
Coal |
1350 |
700 |
21 |
|
Natural gas (80 bar) |
57 |
2.9 |
|
|
Biomass |
450 |
230 |
3.7 |
|
Bio-oil |
1200 |
20 |
|
|
Gasoline |
760 |
35 |
|
|
Methanol |
784 |
19 |
Therefore, when contemplating really large biomass production, the first conversion that has to take place is to convert the biomass into a transportable product. The best way to accomplish this is by flash pyrolysis, which is a fast pyrolysis at a high temperature of 450^-75 °С. In this way the solid biomass is converted into a transportable liquid with an energy density of about 20GJ/m3 (see Table 4-17), which is similar to that of methanol. The pyrolysis plants can be located anywhere, because they can be economically built on a reasonably small scale. A stand-alone gasifier is only likely to be economic for world-scale plants, and hence they always depend on biomass from a variety of sources. The energy conversion of flash pyrolysis is now 75% but could well increase to 80% in the future. The bio-oil product is a transportable material that has as its only disadvantage that it is corrosive due to the presence of organic acids. A big advantage is that it can be gasified with oxygen in the same commercial gasifiers that can process heavy oil fractions. An added advantage of bio-oil is that it is easily homogenized. Holt and van der Burgt (1997) proposed such a concept of decentralized flash pyrolysis plants feeding a single large gasification installation. Research on details of actual implementation is continuing (Henrich, Dinjus, and Meier 2002).
Table 4-18 lists some of the most important properties of bio-oils. Additional data, in particular of some important organic compounds in bio-oil, is contained in Henrich, Dijus, and Meier (2002). In terms of its suitability for gasification in a standard oil gasifier, the most important aspects are the pH (material selection of the feed train) and the alkaline ash content. This latter may favor the use of a radiation screen instead of simple refractory lining.
