I've spent many years studying biofuels and talking to some of the best and most far sighted researchers in the world. The idea is *generally* a non starter for many reasons. Mainly, because many different (and difficult) processes and technologies are required to extract, produce specific chemical products etc, and because generally anything you could do in a desert you also could do with food production and greenhouses with condensing systems (ie using night time cooling). The main exception is when using brine or sea water, in which case some synnergy (when using condensate especially) can be envisaged with both systems, or when using other unsuitable waste water that would contaminate food crops. In theory, another advantage (only with algea) is that commercially growing it in tanks or lakes requires less fresh water since surface area of a body of water is lower than for crops. But, we can design greenhouses to condense water evaporated so this needn't be a reason not to grow agriculturally for food production. The big problem for biofuels is that turning waste biomass (ie lignan rich material) into fuel is hard to do and both financially and energetically expensive (even with solar thermal inputs) and then needs shipping from remote locations. It also ignores the fact that breeding of crops and permaculture can greatly reduce waste biomass, and that the best way to use biomass waste is to use its structural qualities (as seen in the plant stalks) to produce a structural material like carboard for packaging, which we need anyway. This would be desirable as it can better protect foods and reduce food waste and deliver a product that can then be turned to energy by the end user. This would need much simpler processing with less energy input and biomass wastage (ie, with thermal repolymerisation, a proportion of biomass is usually lost as gasses, which can be burnt to power the process, but not easily fed into the grid due to distance and further processing requirements, or subsequently converted to liquid biofuels easily and efficiently, due in part to high temperatures needed and undesirable Hydrogen to Carbon ratio and other product losses). Many synfuel processes using biomass have been analysed but the problem is the same - biological material tends to produce a variety of products (ie on gassification it produces CO, H20, CO2, CH4, H2 and H2S as well as other gasses and a variety of condensable liquid hydrocarbons, and needs energy input to do it, or loses conversion efficiency ie from autothermal reforming which makes CO2 and H20 from a valuable portion of material and its energy content). For wastes to adequately supply civilisation, close to 100% of the material must be converted into fuel energy, and NONE must be generated in a way that competes with primary agricultural output (food). This can be achieved prospectively with added solar energy (to perform molecular re-polymerisation) but it is not easy or cheap, especially on the agricultural scale. It is better to use wastes as packaging and reduce the ammount of waste produced per unit of harvest, and to use land and greenhousing for food production rather than biofuels if they are the primary crop intended, under most circumstances (greenhousing means less land is needed per ton of food/year, and there are extended growing seasons, and can be placed as easily as algea farms in most places). The same land that is good for algea is usually good for greenhousing, unless it is in undevelop-able flood plains. Key biofuel concepts (ie microalgea) look good because of inflated claims as to productivity per unit of land use area and their photosynthetic efficiency. However in practice this is actually available to edible crops if appropriately designed and optimised in growing systems (in equivalently well designed and costly agricultural systems). Microalgea are actually very difficult to extract from water and further, the photosynthetic efficiency is not likely greater than for multicellular plants (ie conventional crops), the main difference is that some algea are efficient at making lipids from photosynthesis, with less 'other' biomass production (ie cellulose or carbohydrate), and are not actually higher in photosynthetic efficiency, since they suffer photosaturation in the same way and cannot scatter excess light as can plants. The problem for conventional crops is at least 50% of their energy goes into stems, stalks and roots, and this can be reduced by breeding (ie controlling height), permaculture and hydroponics. Effectively, the cost of this would be lower than for microalgea due to ease of extraction of the product in normal plant agriculture. The main driver for algea-fuel at present is the production of jet fuel, which they are suited to do due to the type of hydrocarbons they can manufacture. In reality though, aircraft in the future would be much more economically powered by hydrogen, even with cryocooling costs, since a) cryocooling energy at up to 30% of fuel energy value, can be partly (or largely) reclaimed on the aircraft through compressor cooling which reduces compressor energy requirement and enables a more aerodynamic engine / turbofan design and b) the cost of hydrogen in the first place is likely going to be much cheaper than other fuels due to recent breakthroughs (ie at Univerity of East Anglia) in photo-hydrolysis efficiency eclipsing (and prospectively at much lower cost), the efficiency per kW yielded per m2 as compared to PV panels. This technology means that almost anywhere where aircraft typically fly, it can become powered by solar hydrogen. Local PV's can be used to provide cryo-cooling energy. C) Hydrogen is much lighter than hydrocarbon fuel for equivalent energy although more bulky, which has important impacts. Within 20 years, which it will take to commercially develop biofuels, job done. There is no reason to invest in it for aviation energy given the promise of other energy systems.
But, pyrolysis of open field ie switch grass or meadow grass and forest leaf and branch growth, and as well the remaining biomass from food production does have potential, if the costs and mass conversion efficiency is good enough. However, here, not only is there prospect to reduce this yield (from crops) but also that many of these wastes could be turned to useful human nutrition via fungal protein grown on lignin rich wastes - ie 'Quorn' extends life span in animals, whereas normal animal meat does not. The use of biomass for energy from trees would presume we dont want a material product, and the biomass from switchgrass assumes it isn't competing with food production - which it may well be as open land is normally farmed by ruminants. The ultimate solution is SynFuel production and the technology to do this with much less land compared to biofuels is virtually upon us. An overall efficiency of around 30% land area solar insolation to H2 and easy atmospheric extraction of CO2 and fixing CO2 to carbon or Carbon Monoxide, is possible so that a range of hydocarbons or methanol - and methanol to gasoline can be used subsequently, can be generated, and at a total land efficiency several times higher than in agriculture, even algea (maybe by 6 fold to 12 fold more fuel per unit of biomass land), and the chemistry in producing a pure end product is much easier, at minimum energy cost, compared to biomass pyrolysis.