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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 guarantee for the technical performance of their gasification technology. Though they are sold commercially, they are not delivered with the same kind of operational guarantee as e.g., a gas turbine. This shows the limited operational experience and lack of confidence in the technology, but in comparison with alternative routes to utilize cellulosic biomass gasification is well proven and one of the possible technologies to be introduced commercially as a major part of the energy route to biofuel.
The production of high-quality syngas from biomass, which is later used as a feedstock for biomass-to-liquids production, requires particular attention. This is due to the fact that the production of synthesis gas from biomass is indeed the novel component in the gas-to-liquids concept – obtaining syngas from fossil raw materials (natural gas and coal) is a relatively mature technology.
Various gasification concepts have been developed over the years, mainly for the purposes of power generation. However, efficient biomass-to-liquids production imposes completely different requirements for the composition of the gas because for power generation, the gas is used as a fuel, while in biomass-to-liquids processing, it is used as a chemical feedstock to obtain other products. This difference has implications with respect to the purity and composition of the gas.
The calorific value of the gas is the prime factor for power generation – the higher the value, the better. Hence, the availability in the gas of any compounds that increase calorific value is generally welcomed – product gas, which contains carbon monoxide (CO), hydrogen (H2) and various hydrocarbon derivatives [methane (CH4), ethylene (CH2=CH2), ethane (C2H6),