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.