Short and Long Term Sustainability of Technologies for Liquid Biofuels

Two main roads to biomass conversion to transport fuels exist; the thermochemical and the biochemical conversion. Several fuels for road transport are in full scale production, mainly from the biochemical route in the form of first generation bio-ethanol and biodiesel. A second generation biodiesel production based on rendered animal fat was established in full scale in Denmark this year, and so was a large scale/semi full scale second generation bio-ethanol from straw.

The thermochemical conversion route has promising technological perspectives, and the prospect is that transport fuels produced from ligno-cellulosic biomass via the thermochemical route will be available on the market within a 5 year period. The key technologies of gasification and liquefaction are well known using other precursors than biomass. The technological perspectives are one thing, however, the feasibility and sustainability issues another. To assess the sustainability of biofuel technologies, one has to consider some proportions.

Proportions in demand and supply of biomass

Earth land coverage is around 15 Gha of which 5 Gha is in use for agricultural production either as cultivated land for crop production (around 1.5 Gha) or pastures for animal production (around 3.5 Gha). The remaining 10 Gha are either biophysically unsuited for cultivation or cultivable, but standing as uncultivated nature, i.e. primary forest, wetlands, and other biome categories.Till now, the cultivated part of the land has largely been cultivated for food and animal feed production. But many new customers to biomass resources enter the scene: electricity, heat, road transportation, aviation, polymers, organic bulk chemicals. A quick look at the proportions and the magnitude of these newcomers, compared to our agricultural sector as we know it, illustrates how big they are: With the average daily diet of around 2700 kcal per person, the total calorific food intake is around 25 EJ per year by the world's population. The global fossil fuel consumption today is around 400 EJ per year, i.e. 16 times larger than the energy content of the world's food intake.

The energy content of the crops deriving from agricultural land is, of course, bigger than the 25 EJ/year, due to the losses in the supply chain for food, and based on data from FAO Statistics Division in 2007, the gross energy production in agricultural crops today, thus, amounts to an estimated 100 - 150 EJ/year. Acknowledging that also biofuel/bioenergy production imply losses in the supply chain, the proportion is, thus, that using biomass for all energy demands, today satisfied by fossil fuels, would require four to five times the present energy output, on top of the existing agricultural production.

Moreover, consumption increases in both energy and food tend to exceed yield increases, for the food part not least due to a growing demand for meat. With present trends, therefore, yield increases do not seem to close the gap, on the contrary. Looking at possibilities for extension of agricultural land, leading geographers have found that earth can provide a maximum of 120% new cultivable land, most of which is found in tropical South America and Africa. The figure is theoretical, and actually cultivating this land would imply deforestation including violation of nature preservation, and the realistic magnitude of new land cultivation is much lower. A recent study commissioned by the Danish Ministry of Food and Agriculture finds the sustainable potential to be in the order of 30-40% new cultivable land only. Moreover, forest and other nature types have in many cases sequestered much more carbon than the agricultural land following the cultivation, and the release of carbon due to cultivation may be very high compared to the subsequent carbon offsetting by the crops substituting fossil fuels, and ?carbon pay-back periods' as high as 400 years are reported. The conclusion of this rough look at proportions is that while we need five times more cropland in order to fully replace fossil fuels by biomass, we can at maximum double our cropland, and we can do far from that without jeopardising nature conservation laws and without carbon releases exceeding carbon savings.

This quick scan of proportions matches well the fact that most scientific studies of biomass potentials estimate the maximum potential to be in the area of 20% of energy consumption around 2030, some of the most optimistic around 50%, and the ones looking not only on maximum potential but also at economic feasibility around 10%. Looking at the biomass residue potential only, being the biomass that can be extracted without competition with food production, the total population of studies estimate a potential in the interval of 15 ? 96 EJ/year, or around 2 ? 13% of projected fossil energy consumption in 2030.

The short term perspective

These proportions in mind, the issue of competition for the biomass and the need for prioritizing it become evident. For several decades ahead, we still depend heavily on fossil fuels, and we can only replace these to the extent and at the speed that alternatives become available. Any large-scale prioritisation of biomass for one purpose will imply a loss of alternative uses of the same biomass. The use of biomass for transport fuels shall, thus, be seen in the light of competition for and prioritisation of the constrained land and biomass resources, not only in relation to food and feed demands for biomass and land, but in relation to all new potential uses of biomass and arable land.

The studies using this proper holistic comparative perspective unambiguously show that more is lost than gained when prioritising biomass for transport biofuels at the expense of heat and power. Looking at both CO2 reduction and fossil fuel savings, the conversion of biomass to heat and power or its use in chemistry can imply from fifty up to several hundred percent higher savings per ton of biomass and/or per hectare than biofuels, depending on the type of biofuel.

In order to compete, the success criteria for any use of biomass for energy are: high crop yield, high conversion efficiency, and high cost efficiency. Acknowledging this, it is easy to explain the bad environmental performance of the liquid biofuels: Bio-diesel from vegetable oil implies a very low crop yield (temperate regions), up to 3 times lower yield of dry matter per hectare compared to other energy crops. Bioethanol, both 1st and 2nd generation, has a low energy-conversion efficiency being around 50-70% including the by-products. The reason is that bioethanol suffers from several significant conversion losses: pre-treatment (2nd generation), low metabolic conversion to ethanol, subsequent need to dry residual unconverted matter, and distillation to separate ethanol from water. The same will hold true for biobutanol, and also the thermochemial conversion routes imply significant losses. When using biomass to substitute fossil fuels in heat and power production on the contrary, a substitution efficiency close to 100% can be achieved.

On the short term, therefore, when biomass can be exchanged for oil or natural gas in the heat and power sectors, this is by all judgements a better option, then using the saved oil and gas for transport purposes. Oil and gas can in this way be saved for road transport and oil can be saved for aviation by further converting road transport to natural gas. Assuming properly designed incentives for the use of biomass for energy, conventional market forces would pick this strategy as the winner ? like would the pure environmental concerns.

The long term perspective

When oil and gas are no longer used in stationary applications, and biomass thus no longer can be exchanged to oil or gas in the heat or power sectors, alternative fuels for road transport are justified. At that time, the electric car for road transport is believed to be available being superior in terms of both energy efficiency and environmental impacts. When oil is no longer used in stationary appliances or in road transport, and oil, thus, no longer can be saved for aviation there, alternative aviation fuels are justified. At that time, BTL jet-fuels will have to compete with GTL and CTL as well as other routes to fuels, e.g. wind and solar based electro-hydrolysis and methanol production from H2 and CO2. In case BTL proves competitive, aviation is judged to be a priority customer for the available biomass. A trend based projection of aviation fuel consumption to 2030 estimates an aviation fuel consumption of around 25 EJ/year equivalent to 40-50 EJ/year of biomass, if bio-based. Projected chemical feedstock is likewise estimated around 50-60 EJ/year in 2030. These two sectors together can, thus, take more than the estimated maximum non-food biomass potential in 2030, and they are believed to have higher priority for carbon based fuel and feedstock than road transport. So has a percentage of biomass as a storable fuel to buffer electricity systems.

It is, thus, difficult to see the justification of the liquid biofuel technologies, as we know them, on the short term as well as the long term.