Biomass yield, conversion factors

Two conclusions can be made at this stage of the discussion. Firstly, when surface area constraints are important, direct conversion of solar energy wins out over biomass. Estimates of an increase in biomass yields using bio-engineering approaches amount to not more than a factor of 2 [7]. 

Ethanol Feedstock Costs (Canadian I), for Upper and Lower Values of Jerusalem Artichoke Tuber and Tops Yield Ranges, for Different Land Prices in Western Canada and Quebec, Given a Conversion Factor of Biomass to Ethanol of 100 I t-1... 

However from the standpoint of green chemistry, the use of isolated enzymes (or dead whole cells) is highly preferred because it avoids the generation of copious amounts of biomass. It must be emphasized that the productivity of microbial conversions is usually low, since non-natural substrates are only tolerated at concentrations as low as 0.1-0.3% [106]. The large amount of biomass present in the reaction medium causes low overall yields and makes product recovery troublesome. Therefore the E-factors for whole cell processes can be extremely high. Moreover the use of wild-type cells often causes problems because an array of enzymes is present which can interfere in the reduction of a specific ketone (giving opposite selectivities). The use of recombinant techniques, however, which only express the desired enzyme can overcome this problem [108]. 

To any energy planter, the most meaningful measure of solar energy conversion to biomass is the number of kilograms of dry matter produced per hectare per year (or tons per acre per year). While photosynthetic processes per se remain an important factor, equally important are all other processes and constraints of plant growth and development which come into play as photosynthate is elaborated to harvestable biomass. Each of these factors finds expression in the energy planter s gross yield of biomass. Annual dry matter yields in the order of 22,500 kg/ha (10 tons/acre) are common for a few species but the majority of herbaceous land plants probably yield less than 4500 kg/ha (2 tons/acre). 

Therefore the demand for land and other agricultural resources required to support biobased industrial products is probably not a factor for chemicals and materials, but will be an issue for biobased fuels. The demand for land to supply liquid fuels depends on the yields of biomass from the land, the yield of fuel from the biomass and the miles traveled per unit of fuel. All three factors are important and must be considered in conjunction. Increasing the efficiency (yield) of each step will increase the overall system efficiency. In particular, biomass conversion to fuel ethanol must be highly efficient and low cost if the product is to compete with petroleum-derived fuels. 

Figure 9.8 shows the flowsheet of an algae-based biorefinery case study. The mass balance was determined using spreadsheet calculations, and the conversion and yield factors of the various process units are presented in Table 9.8. The production of lOOOkgh" of dry algae biomass is used as basis. The composition of algae biomass is shown in Table 9.9. 

---