Biomass conversion technologies

Multiple conversion technologies can process a variety of biomass inputs. This section introduces these technologies and links them with compatible types of biomass. The chapter divides the technologies according to the state of the output product:

• Gaseous: anaerobic digestion, thermal gasification

• Liquid: hydrothermal liquefaction (HTL), pyrolysis, fermentation, hydroprocessing, and transesterification.

The gaseous products in the first category are biogas from anaerobic digestion and syngas from thermal gasification. Biogas is a mix of CH4 and CO2 plus impurities.

When used in stationary applications, biogas does not require CO2 removal or the cleaning of impurities, although the impurities will need to be removed in the case of further fuel synthesis.

Syngas is a mix of various components, including CO, CO2, N2, H2, H2O and other impurities. Like biogas, syngas can be combusted directly in stationary applications, but fuel synthesis requires cleaning and balancing the H2 and CO in specific stoichiometric ratios, depending on the subsequent fuel synthesis.

The liquid products in the second category are bio-oils, bio-ethanol and bio-diesel.

Bio-ethanol is the product of fermentation and can be used directly in combustion engines, on its own, or blended with other fuels. Bio-oils are a complex mix and generally require a series of treatment processes that involve hydrogenation to remove oxygen, nitrogen and sulphur to improves their physical qualities. The hydrogen addition is, however, modest, at 12% of the biomass input (on the lower heating value) for HTL and hydrotreated vegetable oils (HVO) [83] and should not be confused with the secondary conversion (used to increase yields) discussed in Section 5.3.

In the HTL process, biomass and water are heated at high pressure to produce a stable bio-oil with high energy content and low oxygen that may be processed in existing oil refineries due to its similarities with fossil oil [91] or used directly in shipping to replace marine gas oil (MGO). Several variations of pyrolysis exist, and known configurations include fast pyrolysis and catalytic hydropyrolysis [83]. Fast pyrolysis bio-oil has a high oxygen content with a relatively low energy content [92], and its deoxygenation potential makes it uncertain for commercial-scale applications [93].

5. COMPONENTS FOR RENEWABLE FUEL PATHWAYS

Catalytic hydropyrolysis deals with this issue and produces a more stable bio-oil with low oxygen content and higher energy content, making it more similar to the fossil equivalents.

Hydroprocessing produces HVO and hydroprocessed esters and fatty acids (HEFA) and, as with HTL and pyrolysis, involves a certain level of hydrogen to improve the physical qualities of the product. HVO is also known as renewable diesel, a fuel with superior qualities to fossil diesel, while HEFA is one of the few certified bio-jet fuels that can already be blended up to 50% with fossil jet fuel [94].

Transesterification uses similar feedstocks to those used in hydroprocessing, and its product is also known as bio-diesel; however, instead of hydrogen, it uses methanol or ethanol as reactants [83]. Ethanol is also the product of fermentation from sugar and starchy crops and is currently used in blends with gasoline or on its own.

The briefly described biomass conversion processes are compatible with certain biomass types, as illustrated in Table 1. Some biomass types, such as agricultural residues or energy crops, have more extensive compatibility with more biomass conversion technologies, but they are all dependent on the overall biomass availability.

Table 1: Biomass conversion technologies and the feedstocks they use.

Woody biomass

Solid agri.

residues Manure Organic waste Sludge Energy crops Veg.

oils, fats Anaerobic

digestion X X X X X

Thermal

gasification X X X

Hydrothermal

liquefaction X X X X X X

Pyrolysis X X X

Hydro-processing X X

Transesterification X X

Fermentation X X

Woody biomass is suitable for thermal gasification, HTL or pyrolysis at comparable efficiencies [83], producing either gaseous or liquid fuel outputs. Woody biomass can originate from forestry products, including tree plantation waste and wood residues.

Solid agricultural wastes are mainly residues from agricultural cultivation and are one of the two types of biomass compatible in all conversion processes except for those including oily and fat inputs. Straw is the most common resource in this category and is often referred to in connection with biogas plants, where it can increase yields.

However, straw may be used on its own for the production of syngas, oils or bio-ethanol.

Manure, organic waste and sludge are categorised as waste products, albeit with good potential for conversion to fuels in dedicated facilities, such as anaerobic digestion or HTL. These waste products are often associated with high GHG emissions and other hazardous effects, so waste treatment is essential.

Energy crops are grown explicitly for energy purposes and may be used in all mentioned biomass conversion processes, depending on the energy crop type. Sugar and starch crops are specific inputs in 1^st generation ethanol, while oily crops can be used in hydroprocessing or transesterification. Other crops, like willow, poplar or grassy crops, are suitable for the other conversion processes that can deal with lignocellulosic biomass.

The feedstock availability differs for each of the conversion processes, as illustrated previously in Figure 6, and it is one of the factors that can shape a technology’s large-scale deployment. A defining factor is the type of output, i.e. gaseous or liquid, which will influence the end-use applications. Regarding flexible operation, such equipment is typically kept in continuous operation as the potential for regulation is often limited [83] or unnecessary, depending on the end-use of the products.

Among the technologies described above, anaerobic digestion, hydroprocessing and transesterification are generally commercially available technologies, even though further optimisations are necessary for using new feedstocks, e.g. straw in biogas and bio-ethanol plants [83,95,96]. Thermal gasification has been demonstrated for a long time in Denmark and abroad [97,98], mainly connected with electricity and heat production, but the technology still needs to overcome technical and non-technical barriers before entering the market [83,99]. HTL and pyrolysis are both in the early development stages, with further research needed before any commercial units can be deployed [83,91].

Not all biomass conversion technologies are analysed throughout the three studies.

Anaerobic digestion takes a central role in Study 1, while thermal gasification is central to Study 2. Both technologies are analysed in Study 3, including HVO, but future analyses should include all technologies.

5. COMPONENTS FOR RENEWABLE FUEL PATHWAYS