Fossil fuel feedstocks, efficient

To convert these feedstocks into useful chemicals, mainly fermentation, chemical modification or thermochemical methods were applied. However, these processes were later abandoned in favor of the more economic and efficient processes based on fossil resources, in particular oil. Easier transport and more stable chemical composition (biomass feedstocks are highly diverse, depending on the source) are two relevant additional factors in favor of fossil fuels. Therefore, although the concept of biorefinery is attractive, there are several barriers to economically feasible. 

The higher efficiency biomass case, again with biomass feedstock produced by the management-intensive system and no credit for ener co-products, is projected to be capable of dehvering about 3.4 times as much energy in the form of ethanol than the fossil-fuel inputs needed to operate the sj tem. The NEPRs ot each ol the thermochemical biomass cases would be higher still if low-grade feedstocks were used instead of those produced by the system described in Tables 14.8 and 14.9. 

The relative performance of biomass energy systems in reducing net greenhouse gas emissions depends on the sustainability of the sources of biomass feedstock, the energy requirements of the conversion systems, and the overall conversion efficiencies. Unlike fossil fuels, biomass production systems recapture emissions of carbon dioxide. It has been well documented that... 

The fact that menthol is produced from both renewable and fossil feedstocks allows for an interesting study in sustainability. In order to produce the same crop year after year, it is necessary to use fertilisers to replenish the nitrogen and minerals which the plant takes from the soil. Secondary metabolites such as menthol and essential oils occur at a level of, at most, only a few per cent of the dry weight of the herb. Therefore, in order to produce an economic return, it is necessary to use efficient, mechanical methods of cultivation and harvesting. A full life cycle analysis of menthol production reveals that production from cultivation of mint plants consumes more fossil fuel, produces more carbon dioxide effluent and has more environmental impact than either of the leading synthetic routes. 

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