Alkali pretreatment uses bases (sodium, potassium, calcium or ammonia hydroxide) as catalysts. Lime [Ca(OH)2] treatment is most beneficial from an economic and health and safety point of view and lime can be easily recovered for re-use (Yang and Wyman 2008). Alkali treatment induces swelling of celluloses and selective removal of hemicelluloses coupled with lignin solubilisation, which renders the lignocellulose more accessible to enzymes and bacteria. Although the amount of inhibitors generated is lower than in acid pretreatments, hemicelluloses can undergo peeling reactions and be degraded into furans that, together with the solubilised lignin, could negatively impact the functioning of microorganisms. This technology can increase the methane yield of residual materials such as newsprint (Fox et al. 2003) but is less attractive for pretreatment of woody materials given the negative effects of high lignin content on the process.

Acid hydrolysis pretreatment is mostly performed using sulphuric acid catalysts, but other mineral acids such as hydrochloric, nitric and trifluoroacetic acids have also been applied. Processes utilising acid catalysts can either be carried out by concentrated-acid/low temperature or dilute acid/high temperature hydrolysis. Concentrated acid allows the hydrolysis of both cellulose and hemicellulose under moderate temperatures, but requires high acid concentrations (72 % H2SO4, 41 % HCl or 100 % TFA), which makes recovery costly and can lead to equipment corrosion (Gfrio et al. 2010). Lower concentrations (30 %) of acid have been reported to retrieve 41 % of the total theoretical glucose from pine sawdust (Miller and Hester 2007). Dilute acid selectively hydrolyses the hemicellulose fraction, while the cellulose remains in a solid fraction that can be hydrolysed by enzymes or by a second dilute acid step. Dilute acid has been applied to species of Eucalyptus (Mclnstoch et al. 2012; Silva et al. 2011; Wei et al. 2012), Acacia (Ferreira et al. 2011) and Pinus (Huang and Ragauskas 2012). Although dilute acid treatment implies lower acid consumption by the substrate when compared to concentrated acid treatments, the higher temperatures of operation can lead to greater equipment corrosion and higher levels of hemicellulose degradation. Both acid schemes entail a neutralization step prior to biochemical transformation.

Ozonolysis is achieved by the treatment of lignocellulose with oxidizing agents such as ozone, which mainly reduces lignin content, slightly affects hemicellulose and increases the sugar yield of enzymatic hydrolysis. It can be conducted at normal pressure and room temperature, so that it does not generate compounds that are toxic to further hydrolysis and fermentation. Ozonolysis could be effective for the pretreatment of lignocellulose-rich residues such as sawdust (Ncibi 2010), but the amounts of ozone required makes this process costly.

The organosolv process employs organic or aqueous organic solvent mixtures with ethanol, methanol, acetone, ethylene glycol and tetrahydrofurfuryl alcohol as potential components, which can be supplemented with an acid catalyst (HCl, H2SO4, oxalic or salicylic acid) to disrupt the link between hemicellulose and lignin. This improves the susceptibility of cellulose to enzymatic hydrolysis by increasing enzyme accessibility to cellulose in both hardwoods (Romani et al. 2011) and softwoods (Park et al. 2010). An additional advantage of this process is the recovery of relatively pure lignin.

The novel SPORL (Sulphite Pretreatment to Overcome Recalcitrance of Lig — nocellulose) approach to pretreatment is based on the pulping of biomass in the presence of sulphites and was developed to enhance the biochemical conversion of softwoods with large particle sizes (Zhu et al. 2009). This technology consists of sulphite/bisulphite treatment of wood chips under acidic conditions and, contrary to conventional pretreatment technologies, is followed by a reduction of particle size by means of disk milling. The removal of hemicelluloses (pulping spent liquor) and sulfonation of lignin is considered to be critical for enhanced cellulose conversion. Moreover, this technology reduces the energy consumption required for size-reduction to values equivalent to agricultural biomass (Zhu et al. 2010).

Ionic liquids (ILs), also named “green solvents”, are organic salts composed mainly of organic cations with small amounts of either organic or inorganic anions, with the ability to dissolve a wide range of organic and inorganic compounds. IL solvents have several valuable properties including a low melting point, chemical and thermal stability, negligible vapour pressure and relatively low toxicity (Liu et al. 2012). Additionally, its solvent properties can be adapted for a particle substrate by adjusting the ratio of cations to anions. The synergy of ILs with other compounds such as acids (Diedericks at al. 2012) or solid super acids (Br0nsted superacids or Lewis acids) have also been investigated (Gfrio et al. 2010). The most common ILs can be classified according to their cations in four groups: quaternary ammonium ILs, N-alkylpyridinium ILs, N-alkyl-isoquinolinium ILs, and 1-alkyl-3-methylimidazolium ILs. Most of these solvents remove lignin and alter cellulose structure, which increases the accessibility of cellulolytic enzymes. Imidazolium-based ionic liquids have been used to dissolve hardwoods and soft­woods (Mora-Pale et al. 2011). The ionic liquid 1-ethyl-3-methyl imidazolium acetate ([C2mim][OAc]) has been shown to increase cellulose digestibility of species of Eucalyptus (Qetinkol et al. 2010) and Pinus (Torr et al. 2012).

Although the technology based on ILs would require less equipment and energy input compared to conventional pretreatments, efficient methods for both recovery of the different fractions and the recycling of ILs should be developed for large scale application. The development of such processes would circumvent the negative impact that some ILs have on enzyme activity and effective microorganism functioning (Wang et al. 2011).

Compared to agricultural biomass (i. e. wheat straw), woody biomass has a higher density, higher cellulose and lignin content and lower hemicelluloses content, which means it is more resistant to biological conversion. Enzymatic hydrolysis and digestibility of lignocellulose are the key steps in biological conversion and they depend strongly on the feedstock composition and structure, pre-processing of the material and dosage and efficiency of the enzymes used for hydrolysis.

Biological conversion of lignocellulose is enhanced by mechanical comminution that reduces the particle size as well as the degree of polymerization of the cellulose. For biochemical conversion the biomass should have a loose structure that can easily be penetrated by enzymes. Therefore additional pre-treatment is needed to improve accessibility of the cellulose and increase the digestibility to above 50 % for enzymatic hydrolysis (Vidal et al. 2011). High crystallinity of the cellulose and a high degree of polymerization limit enzymatic hydrolysis, mainly the initial hydrolysis rate.

For biochemical conversion feedstock with a high moisture content is preferred and drying reduces the accessibility of biomass to chemicals, steam and enzymes (Stephen et al. 2010; Liu et al. 2002). Special care must therefore be taken to avoid drying and the associated “hornification” phenomenon. Freshly harvested wood chips are the optimal woody feedstock for bioconversion of lignocellulose.

The disruption of the lignocellulose structure by pre-treatments can significantly reduce the recalcitrance of lignocellulose to biological degradation. Most of the pretreatments alter not only the chemical composition but also the physical structure of the biomass by increasing the accessible surface area and pore volume, thereby enhancing cellulase attack.

An increase in lignin content reduces the digestibility of biomass, thereby decreasing the rate and extent of enzymatic hydrolysis. Lignin with a high syringyl content (typical for hardwoods) can be easier degraded by pre-treatments (diluted acid, alkali, hydrogen peroxide, etc.). Apart from being a physical barrier to hydrolysis, lignin can adsorb irreversibly to the enzymes, thereby reducing the yield and increasing time required for effective conversion. The adsorption capacity of lignin depends on the type of lignin and pretreatment applied. Unlike in the pulping process, where the objective is to maintain the fiber integrity, these pretreat­ments aim for maximum digestibility by removing lignin and loosening the fibre structure.

The type of hemicelluloses also affects the choice of pre-treatment and the enzymes used for the fermentation process. For example, the acetyl content in hemicelluloses from hardwoods is involved in autohydrolysis reactions during thermochemical pretreatments, enhancing the cellulose accessibility. On the other hand, residual acetyl groups in pre-treated material constitute a steric hindrance for the enzymatic hydrolysis.

However, the pretreatment of lignocellulose to improve digestibility may also result in the production of sugar and lignin degradation compounds, which may lead to hydrolysis/fermentation inhibition.

Biomass with a low ash content is preferred for biological conversion, not only because it maximizes the availability of carbohydrates and lignin for the conversion process, but also because the buffering capacity of ash may increase the chemicals requirement in acid-catalysed pretreatments for biological conversion.

After the goal, the product(s) and the system have been decided on, the functional unit needs to be defined. The functional unit corresponds with a reference flow to which all other modelled flows of the system are related (Cherubini et al. 2009). This is why the functional unit needs to be quantitative. The functional unit provides a reference to which the input and output process data are normalised, and the basis on which the final results are presented. Generally four types of functional units can be found in the bioenergy-LCA literature (Cherubini and Str0mman 2011):

1 Input related — the functional unit is the unit of input biomass, measured in either mass or energy. With this type of functional unit, results are independent of conversion processes and types of end-products. This unit can be selected by referring to studies that aim at comparing the best uses for a given biomass feedstock.

2 Output unit related — here the functional unit is the unit of output, e. g., units of heat or power produced, or kilometres of transportation provided. This type of functional unit is usually selected by referring to studies aimed at comparing the provision of a given service using different feedstocks.

3 Unit of agricultural land — this functional unit refers, for instance, to hectare of land used to produce the biomass feedstock. This unit should be the first parameter to take into account when biomass is produced from dedicated bioenergy crops.

4 Time — results of the assessment are reported on a time basis. This type of functional unit is used in studies characterised by multiple final products, since it allows the avoidance of an allocation step.

Typical functional units in a bioenergy context are emissions/sequestrations per unit of energy produced, emissions/sequestrations per unit of land required.

The first production phase, primary production, is common to all the alternatives and takes all the activities and processes in the establishment and maintenance of the SRC plantations into account. It comprises, inter alia, mechanical and chemical land preparation, planting of fast-growing trees, weed control operations prior and after planting, and fertilising operations in order to enhance the growth rate of the trees, particularly during the first years after planting, until canopy closure is reached, after which competition from weeds is eliminated (Little et al. 1997).

The second production phase, harvesting and primary transport, comprises five harvesting system modules, including three different harvesting technologies and three types of primary transportation. The harvesting technologies modelled are motor-manual, mechanised forestry, and modified agricultural machinery. A for­warder fitted with a crane; a tractor-trailer combination loaded and unloaded, either manually or with a three-wheeler loader; and a tractor-container-trailer combination were assumed for the primary transportation.

The third production phase, pre-treatment of the biomass, entails three types of activities, namely comminution, drying and mobile fast pyrolysis. Depending on the harvesting system applied, two locations for comminution were proposed, i. e. mobile comminution at the roadside and stationary comminution at a landing of the central conversion plant. Similarly, both the location of the stored biomass and the form of the biomass (comminuted or un-comminuted) depend on the harvesting systems applied. In the case of four of the harvesting systems, uncomminuted biomass is stored in-field to air-dry for several weeks until the biomass has reached moisture content levels of about 40 % (dry basis). Once this level of moisture has been reached, the biomass is extracted to roadside for further processing. In the case of the remaining harvesting system, the trees are felled and comminuted in a single process, resulting in wood chips with moisture content levels of around 80 % (dry basis).

Irrespectively of the harvesting technology applied, additional drying of the biomass is required to reach the moisture content levels stipulated for the upgrading and conversion process. This is assumed to be achieved by using the exhaust heat from the respective conversion system. As no additional energy will be required, no additional costs and emissions arise from the active drying process.

As illustrated in Table 11.2, some of the alternatives use mobile fast pyrolysis. This is a process whereby the biomass is degraded in the absence of an oxidising agent, i. e. the volatile components of a solid carbonaceous feedstock are vaporised in primary reactions by heating, leaving a residue consisting of bio-char and ash. Pyrolysis always produces a gas vapour that can be collected as a liquid and as a solid char. Fast pyrolysis processes are designed and operated to maximise the liquid fraction by up to 75 wt% (dry basis). Thus, although fast pyrolysis can be understood as some form of pre-treatment of the biomass, it also represents one of the possible pathways for upgrading low-bulk-density biomass into densified, more homogeneous energy carriers (bio-oil and bio-char).

The fourth production phase encompasses the secondary transport of the bioen­ergy feedstock from the roadside to a central conversion plant. Un-comminuted biomass is assumed to be transported with a truck-trailer combination, comminuted biomass and bio-char with a truck-container-trailer combination, and bio-oil from a mobile fast pyrolysis system by a dedicated truck-tanker combination.

Five configurations of bioenergy conversion systems (BCS) were assumed for the fifth production phase. BCS I is an integrated steam-turbine system where biomass at a maximum 20 % moisture content (dry basis) is combusted to generate steam, which is then used in a steam turbine to generate electricity. The same moisture content is required for BCS II, an integrated gasifier-gas-turbine system, where the biomass is upgraded to bio-gas, which in turn, is fed into a gas turbine. BCS III consists of a stationary fast-pyrolysis plant converting biomass (10 % MC) into bio-oil and bio-char. The upgraded products are then fed into an integrated boiler — steam-turbine system to generate electricity. An integrated steam-turbine system is also assumed for BCS IV, also using bio-oil and bio-char that is produced in a mobile fast-pyrolysis system at the roadside, close to the primary biomass production sites. The last bioenergy conversion system (BCS V) also encompasses mobile fast — pyrolysis systems, but differs in the final conversion step, where only bio-oil is used to generate electricity, by directly injecting the liquid into a gas turbine. The bio­char by-product is assumed to be sold to the fertilising industry. To some extent, this effectively works as a way of capturing and storing carbon.

Duncanson et al. (2010) reported that space borne LiDAR data is useful for aboveground biomass (AGB) estimation over a wide range of biomass values and forest types, but the application of these data is limited, given their spatially discrete nature. The authors then used an integration of ICESat Geospatial Laser Altimeter System (GLAS) LiDAR and Landsat data to develop methods to estimate AGB in an area of south-central British Columbia, Canada. They compared estimates with a reliable AGB map of the area, derived from high-resolution airborne LiDAR data, to assess the accuracy of satellite-based AGB estimates. GLAS AGB models were shown to reliably estimate AGB (R2 = 0.77) over a range of biomass conditions. A partial least squares AGB model, using Landsat input data to estimate AGB (derived from GLAS), had an R2 of 0.60 and was found to underestimate AGB by an average of 26 Mg ha_1 per pixel when applied to areas outside of the GLAS transect. This study demonstrates that Landsat and GLAS data integration are most useful for forests with less than 120 Mg ha_1 of AGB, less than 60 years of age, and less than 60 % canopy cover. These techniques have high associated errors when applied to areas with greater than 200 Mg ha_1 of AGB. Airborne studies, however, have shown reasonable accuracies and precisions when it comes to forest biomass or volume estimation, e. g., Lefsky et al. (2002a, b), Popescu et al. (2002,2004), and van Aardt et al. (2006). In fact, van Aardt et al. (2008) have proven that wall-to-wall enumeration is possible at the taxanomic group level at high accuracies. It is likely obvious that (i) estimation quality improves as stands become more homogeneous, (ii) validation and calibration protocol for remote sensing assessments need to be put in place, and (iii) estimation outcomes are often time, species, and site dependent.

The system involves leaving behind few trees of different species over a harvested area. The regrowth has the same pole and timber qualities as that from complete clear cutting near the ground. The high value species are left until maturity whilst the other species are clear cut and the regeneration is managed to produce a range of small dimension wood products such as firewood and poles (Tuite and Gardiner 1990; Lowore and Abbot 1995; Shackleton and Clarke 2007). The trees that are left may be important for either timber or fruit or fodder. Additionally, trees that do not coppice may also be left behind to ensure regeneration perpetuation of such species. For example, in Zambia most of the trees that were left over a cut area were timber species. However, the system has been proposed for use by small-scale farmers in Malawi as a means of maximizing the production of firewood and poles whilst retaining high value species that produce non-wood products and other services, such as fruit trees with spiritual significance (Lowore and Abbot 1995). The system has an advantage of retaining a portion of tree cover and protecting the site from erosion and sun scorch. This silvicultural system requires that fires are excluded in the early stages so that the coppices are not affected by fire.

Secondary transport of biomass aims to move units of energy from source to conversion facility at the lowest cost. As a large part of transport cost is made up of fuel costs, this is synonymous with minimizing energy consumption in supply. Biomass ready for transport to the conversion plant exists in many forms with varying degrees of moisture content and differing assumptions on costs and efficiencies can give different suggestions on transport form (Tahvanainen and Anttila 2011). Biomass has a varying but generally low bulk density. For wood chips roughly 40 %, for trees and tree sections about 35 % and for harvesting residues, only 15-20 % of the load volume is solid matter.

Although there are significant differences in the capital investments for pyrolysis­upgrading, gasification-synthesis and biochemical conversion of lignocellulose to liquid biofuels for transportation, these technologies all require large-scale imple­mentation to make the high capital costs worthwhile (Anex et al. 2010; Kazi et al. 2010; Stephen et al. 2012; Kumar et al. 2012; Brown et al. 2013). Furthermore, it is also expected that the first commercial plants for these technologies will perform at less than design capacity and have capital costs that are higher than anticipated (Anex et al. 2010). Despite the need for bioenergy in transportation, the high capital costs and technology risks associated with newer technologies represent hurdles on the pathway to commercialisation (Stephen et al. 2012; Anex et al. 2010). The wide range of multiple, competing, commercially unproven technologies for the production of liquid transportation fuels from lignocellulose also indicates that standardisation of the conversion process, with associated economies-of-scale in plant component production to reduce capital costs, will not be achieved soon (Stephen et al. 2012).

The large production scales required for cost effective production of liquid transportation fuels from lignocellulose have to be balanced against the cost of feedstock supply. The transport cost of biomass supply generally increases with an increase in production capacity (Gnansounou and Dauriat 2010; Seabra et al. 2010; see Chap. 6). Regional conditions for the supply of biomass will determine optimal plant size, since the reduction of capital investment per unit energy produced as production scale increases is balanced by an increased requirement of feedstocks, the local cost and availability of which determines the maximum scale-dependant cost-benefit that can be achieved (Stephen et al. 2010). Various combinations of lignocellulose feedstock may therefore be used to increase biomass supply in order to reach the desired economies of scale (Seabra et al. 2010). In some scenarios the increase in cost effectiveness with an increase in production scale may outweigh the associated increase in feedstock cost (Amigun et al. 2010). It has been demonstrated that for torrefaction processes there is no benefit in terms of economies of scale for production scale beyond 40MWth (Uslu 2005; Uslu et al. 2008).

7.5 Conclusion

Combustion, pyrolysis and anaerobic digestion of lignocellulose are well — established technologies available for commercial application; while gasification, liquefaction, hydrolysis-fermentation and fractionation hold promise for future implementation.

Overall, it is expected that the production of bioenergy will increase in the Southern hemisphere, based on the overall sustainability of biomass production and cost comparisons with fossil-derived energy.

Rural producers can play a key role in growing biomass for biofuel production. It is, however, important to consider their exposure to risks brought on by aspects such as crop failures and delays between planting and harvesting of the crop. In the case where the small scale producer bears all the risk it can be seen as a direct threat to security of livelihoods. Jatropha (Fig. 9.3) is for instance a labour intensive crop that is harvested by hand and is regarded as being ideally suited to small scale producers. Farmers that sign up to grow Jatropha can, however, face years of investment before they can harvest seeds. Studies in Mozambique and Swaziland found that many subsistence farmers gave up growing Jatropha after the first year due to difficulty with growing the plant. Without a harvest to sell they have no income from the land and if food crops were replaced by Jatropha it can leave farmers without food and income (Friends of the Earth 2010). Risk reducing strategies based on a production portfolio of biomass and food crops on a larger scale, as discussed in Chap. 5 for commercial plantations, are often not a viable option for small scale producers, since their manpower and access to land is often limited. Therefore innovative options for risk reduction must be found.

Initiatives to spread the risk between biofuel companies and small scale farmers could include diversification of crops where biofuels are intercropped with food crops and the use of unproductive land for biofuel crops. In the cases where there is a time delay in biofuel production, farmers could be encouraged to grow short rotation food crops (Practical Action Consulting 2009).

Risks for small scale farmers also arise from market isolation and lack of awareness of market trends and prices. Initiatives that encourage cooperatives

and producer groups enable joint bargaining and the pooling of resources for mechanisms such as bridging loans could help to reduce the risk for each producer (Practical Action Consulting 2009). Linkages between small scale farmers and bioenergy companies can reduce risk by functioning as a market-based ecosystem that allows companies and farmers to act together and create wealth in a symbiotic relationship. These actors depend on each other, the system adapts and evolves and will often be both resilient and flexible (Prahalad 2006).

In the following example of a feasible approach to biomass sampling for above­ground components difficulties are outlined and possible solutions shown. The sampling is based on the selection of a number of representative trees from a population according to the principles of statistical sampling as described in Chap. 2. After this step further decisions about the sampling method that have to be made will include:

• the relevant biomass components;

• the sampling design of the stem biomass;

• the sampling design of the crown biomass; and

• the model to scale up from samples to the individual trees.

A plethora of different ways can be identified in the published literature to define the biomass components of a tree. A straightforward approach to calculate the aboveground biomass of trees, that can be used successfully in a good proportion of applications, is to divide the tree hierarchically. It is first divided in stem and crown, the stem is then further subdivided into bark and wood biomass and the crown into foliage and branches. Depending on the objective it might be useful to further differentiate fruits/cones, heart and sapwood, merchantable branches, dead branches or to add twigs as finer branch material to the components. High differentiation levels are usually warranted if nutrient budgets for sustainability assessments of biomass harvesting have to be established (see Chap. 10 and Seifert et al. 2006; Block et al. 2008; Dovey 2009; Ackerman et al. 2013b). In this example of a mid­rotation Pinus radiata tree (age 14 years) only the bark and wood of the stem, branches (bark and wood aggregated) and needles were used as components.