volatile matter and consequent high yields of gases and liquids on pyrolysis. A relatively high water yield results from the high oxygen concentration in biomass, and which consumes considerable hydrogen. Consequently, the advantages of the high H/C ratio associated with biomass are not reflected in the products to the extent that might be expected. In fact, pyrolysis gases can be deficient in pure hydrogen and pyrolysis liquids are highly oxygenated, viscous tars.
An additional and significant source of water vapor in biomass gases is the high moisture content of the source materials. In countercurrent flow schemes such as the Lurgi moving bed gasifier, this water is evolved in the relatively low temperature drying and pyrolysis zones and does not partake in gas phase or carbon-steam gasification reactions.
On the other hand, in fluidized bed systems, the moisture is evolved in the high- temperature well-mixed reaction zone and therefore does participate in the reactions. If the system is directly heated and air-blown, the additional heat required to evaporate the water will result in more nitrogen being introduced, and more carbon dioxide being produced, so reducing the calorific value of the product gas. As the gas from air-blown processes is, in any case, a low-calorific value product, this factor is probably of little consequence other than with very wet feedstock. In oxygen-blown systems, however, the additional pure oxygen required and higher carbon dioxide content of the medium calorific value off-gas may be of sufficient impact to dictate some degree of drying as a pretreatment.
Apart from drying, additional beneficiation may be undertaken to yield a resource of higher energy density. These operations will normally be undertaken at the source, so transport and subsequent storage costs may be reduced as well. Beneficiation steps include size reduction and densification. Waste heat, if available, may be used for drying, while size reduction and compression to form pellets or briquettes is estimated to require less than 2% of the energy in the dry biomass. Nevertheless, these operations are time consuming, and can be either labor or capital intensive.
Some advantages of biomass over conventional fossil fuels are the low sulfur content and highly reactive char. In addition, biomass materials do not cake and can therefore be easily handled in both fluidized and moving bed reactors. Finally, catalyst poisons are not present in biomass in significant concentrations. This can be important for the initial thermal processing as well as for subsequent upgrading operations.
Biomass – Pyrolysis
Biomass can be converted into gas, liquid, and char via pyrolysis (Table B-22). The exact proportion of the end products is dependent on the pyrolysis process parameter, such as the feedstock, the temperature, the pressure, and the residence time in the reaction zone). Thus, in fact, pyrolysis of biomass is an important process option, either as a pretreatment for gasification or as an independent process treatment. Pyrolysis takes place actively at a temperature on the order of 500°C (930°F) and produces gases and a liquid product (bio-oil). In fact, the pyrolysis of biomass is quite similar, as a process treatment, to oil shale pyrolysis and coal pyrolysis.
Table B-22 A general flow scheme for biomass conversion by pyrolysis.
| Feedstock | Process | Products (primary) | Products (secondary) |
|---|---|---|---|
| Biomass → | 550oC/no oxygen | Char | Heat |
| Power generation | |||
| Condensation | Vapors | Hydrogen | |
| Chemicals | |||
| Liquids | Fuels | ||
| Chemicals |
Bio-oil production via biomass pyrolysis is typically carried out via flash pyrolysis. The produced oil can be mixed with char to produce a bio-slurry, which can be more easily fed to the gasifier for efficient conversion. The slurry is pumpable and alleviates technical difficulties involved in solid biomass handling.
Typical end products are pyrolysis oil, char, and gas. The oil and char are more economical to transport than the original biomass feedstock and have heating values on the order of 10,000 Btu/lb and 12,000 Btu/lb, respectively. The pyrolysis gas, which has a nominal heating value of 150 Btu/scf, is not considered an end product since it is directly used in the cogeneration system.
The conversion of biomass to crude oil can have an efficiency of up to 70% for flash pyrolysis processes. The biooil (biocrude) can be used in engines and turbines and has the potential to be used as a refinery feedstock, although issues need to be overcome. These include poor thermal stability and corrosivity of the oil. Upgrading by lowering the oxygen content and removing alkalis by means of hydrogenation and catalytic cracking of the oil may be required for certain applications.
See also: Biofuels, Biomass, Biomass – Liquefaction, Torrefaction.
Biomass – Synthesis Gas Production
Gasification is the process of gaseous fuel production by partial oxidation of a solid fuel. This means in common terms to burn with oxygen deficit. The gasification of coal is well known, and has a history back to year 1800. The oil-shortage of World War II imposed an introduction of almost a million gasifiers to fuel cars, trucks, and busses. One major advantage with gasification is the wide range of biomass resources available, ranging from agricultural crops, and dedicated energy crops to residues and organic wastes. The feedstock might have a highly various quality, but still the produced gas is quite standardized and produces a homogeneous product. This makes it possible to choose the feedstock that is the most available and economic at all times.
Gasification occurs in a number of sequential steps: (i) drying to evaporate moisture, (ii) pyrolysis to give gas, vaporized tars or oils and a solid char residue, and (iii) gasification or partial oxidation of the solid char, pyrolysis tars and pyrolysis gases.
Not all the liquids from the pyrolysis are converted to syngas, due to physical limitations of the reactor and chemical limitations of the reactions. These residues form contaminant tars in the product gas, and have to be removed prior to a Fischer-Tropsch reactor. Other impurities in the producer gas are the organic BTX [benzene, toluene and xylene (benzene components with one or two methyl groups attached)], and inorganic impurities as NH3, HCN, H2S, COS and HCl. There are also volatile metals, dust, and soot. The tars have to be cracked or removed first, to enable the use of conventional dry gas cleaning or advanced wet gas cleaning of the remaining impurities. There are mainly three ways of tar removing/cracking: thermal cracking, catalytic cracking, or scrubbing.
Despite the long experience with gasification of biomass, there are some problems with large-scale reliable operation. No manufacturer of gasifier is willing to give full
