Umberto Desideri & Francesco Fantozzi

5.1 FEEDSTOCK PROPERTIES

5.1.1 Biomass and biofuels definition and classification

According to a general definition biomass may be considered as animal and plant resources and the wastes deriving from their treatment, which could be used, directly or after a pretreatment as a source of energy. It is therefore a resource directly or indirectly resulting from the photosynthesis process, represented by the following equation (Klass, 1998):

Living plant + CO2 + H2O + SunlightСЛl—^yU (CHmOn) + O2 — 480kJ/mol

For every mole of CO2 absorbed 1 mole of oxygen is released. Zhu etal. (2008) have shown that the maximum conversion efficiency of solar energy to biomass is 4.6% for C3 photosynthesis at 30°C and today’s 380 ppm atmospheric concentration of CO2, while C4 plants have an efficiency of about 6%. Losses are distributed thus: loss by reflectance of photo-synthetically active light (4.9% for example); loss in rapid relaxation of higher excited states of chlorophyll (6.6% for example); loss between the reaction center and carbohydrate synthesis (24.6% for C3 plants and 28.7% for C4 plants, for example); loss due to photorespiration (around 6.1% for C3 plants and 0% for C4 plants); loss due to respiration (1.9% for C3 plants and 2.5% for C4 plants). Figure 5.1 shows the minimum energy losses calculated for 1000 kJ of incident solar radiation.

When considering its use as a fuel the interest is focused on combustible materials result­ing directly from silviculture, agriculture, aquaculture, farming, and the related transformation

Biomass 46 kJ Biomass 60 kJ

Minimum energy losses calculated for 1000 kJ of incident solar radiation (Zhu et al, 2008)

industries (e. g. wood and food industries), or indirectly through their preprocessing to obtain better performing fuels (biofuels) with respect to the initial state (Williams et al., 2012; Tillman, 1991).

Different classifications of biomass are possible according to their origin, characteristics or use; however from an energy point of view its importance is linked to its potential to yield a competitive biofuel that may replace a fossil fuel, therefore a useful classification should consider the environmental and economic effectiveness of its energy conversion. From this point view, since the primal transformation is photosynthesis, biomass energy content is somehow deriving from low density solar energy and, most important, biomass is a geographically distributed resource scattered on a wide area, as it is solar radiation.

This turns into a low energy content per volume product which needs to be produced, collected and transported hence its economic and environmental competitiveness is strongly dependent on the overall balance resulting from the different phases (supply chain).

According to this view, a possible general classification of biomasses (and/or biofuels) could consider different categories as a function of how many phases (and their resulting economic and environmental burden) are necessary to obtain available biomass as a fuel or as a feedstock to obtain a biofuel. Three categories therefore can be identified as follows.

(a) Energy crops: They are dedicated crops specifically cultivated for energy purposes. This is the worst performing category since the economic-environmental burden of the production phase and of the gathering-transportation phase is totally allocated to the final product. However these biomasses may provide a useful (sometimes unique) solution for agriculture revival in depressed or contaminated areas.

They can be divided into no-food crops and food crops depending on their possible competition as a raw material for the food industry.

Among the no-food crops short rotation crops such as woody (poplar, black locust, eucalyptus, etc.) and herbaceous crops (miscanthus, giant reed, kenaf, sorghum, etc.) are utilized for direct combustion or for second-generation bioethanol production as a source of cellulose. There is also an increasing interest in algae for biomass and oil production.

Among the food crops the main interest is in high sugar or starch content crops, for bioethanol production (corn, sugarcane, sugar beet, etc.) or oily fruits for oil extraction to use directly as a fuel or for the production of biodiesel (sunflower, rapeseed, palm and soybean).

(b) Residual biomasses: They are residues of agricultural crops and forestry maintenance. These biomasses do not comprehend the economic-environmental burden of the production phase, since its cost is allocated on the primary product (vegetables or wood) while still comprehending the burden of the gathering-transportation phase. The gathering in particular may still be an issue if the harvesting of the primary product does not consider a proper handling of the byproduct.

Residual biomasses comprise:

• Agricultural residues (pruning, straw, corn/sunflower/tobacco stovers, etc.);

• Forestry residues (pruning, branches, tops, sawdust etc.);

• Urban green residues (pruning, branches, sawdust, etc.).

(c) Agro industrial and farming residues: They represent the byproducts of the food, wood, pulp and paper, and animal farming industry. This is the best performing category since the economic- environmental burden of the production phase and of the gathering-transportation phase is totally allocated to the final product, leaving a cost free biomass available in a single site. Moreover these residues are often to be disposed of therefore their energy conversion could also represent an avoided cost.

They can be divided into: [5]

• urban and industrial residues (organic fraction of MSW (Municipal Solid Waste), sewage sludge, pallets and packaging residues, paper and cardboard, etc.).

With some notable exceptions (waste frying oils, cereal husks, paper and cardboard, etc.) most of agro-industrial residues have a very high humidity content which is not suitable for direct combustion application, while biological treatment such as anaerobic digestion is preferable.

A general classification of biomass can be found in UNI-EN 14961-1 “Solid biofuels: Fuel specifications and classes — Part 1: General requirements (EN 14961-1, 2010)”.