This is a guest post from Professor David B. Benson (retired), who is a regular commenter on The Big Biofuels Blog.
David's contact details: email@example.com
Photosynthesis uses only a very small fraction of the available sunlight. This means that growing biomass to make biofuels will require considerable land (and water) resources. Even so, these are available and some biofuels offer unique advantages.
As a gas, biogasses and biomethane, biofuels can replace the use of natural gas. Indeed, sufficiently pure biomethane can be, and is to a limited extent, directly introduced into existing natural gas pipelines.
As a liquid, biofuel can, and does, replace liquid fuels derived from fossil oil. Currently ethanol is used to replace some gasoline; possibly butanol will do so in the future as it has greater energy density. Similarly biodiesel is already replacing some diesel and pilot projects to replace some jet fuel with a bio equivalent are under way.
Various techniques produce other grades, such as heating oils, from biomass.
As a solid, biofuels can replace some or all of the fossil coal used in coal reactors used to generate electricity or provide space and process heat. An older technique, being revitalized, is to torrefy wood. Newer pyrolysis methods produce a combination of liquids and solids; the solids are usually called biochar. The biochar is, in effect, extremely high grade coal for burning purposes; in a later section we will touch on what may well be a more important use of biochar.
There are several different ways to turn biomass into biofuel. The algal techniques require minimal land to sit upon, indeed just rock will do. Considerably more equipment is required. Unfortunately, none of these methods, algal or otherwise, as yet preserve NPK for later reuse; we touch on this most important subject in a later section.
Food and Fuel
A recent FAO report states there are, world-wide, about 5 billion hectares of agricultural land. Of this total, about 30% is defined as arable land and another 20% is currently not in production. The arable land grows mostly foods; the unused lands, often degraded, could be used to grow biofuel stocks. Most of these currently unused lands are in the Global South, South America and Africa. Additional lands are governmental agriculture program set-asides; with better irrigation, additional lands could become available in Central Asia and elsewhere.
Much farmland in Russia is currently unused.
So through 2030, according to a recent analysis, there is plenty of land to grow biomass for biofuels without competing with lands currently used for growing foods. In addition, much of the arable lands are used inefficiently to grow animal feed for meat animals; beef, mutton and pork are probably the worst offenders for inefficiency and concomitant release of carbon dioxide; beef and mutton animals the worst for release of methane. So the future may bring less animal protein and more vegetable protein, much more efficient. This could, in turn, release arable land for growing biomass for biofuels, relieving the supposed 2030 date for serious competition between food and fuels.
For some farmers in developing countries, growing food crops for biofuel feedstocks appears wise. Tubers such as sweet potatoes and cassava are food crops, but not preferred ones. The advantage to such tubers is the ability to grow on somewhat degraded soils. Some of these farmers may well wish to grow their primary food crops on good soils and tubers on the poorer ones. If the primary harvest is successful, the tubers provide a cash crop, perhaps for biofuel feedstaocks. If the primary harvest is less than adequate, some or all of the tubers can be eaten; a form of food security.
The result, as I now see it, is that there will be no serious competition for land resources between food and fuel crops through about 2050, provided the quantity of meat in diets goes down, on average.
Eating less meat is considered to be more healthful, by the way.
I haven't considered the competing needs for fibre such as wool and hemp, and construction woods, nor parks and other land set-asides.
Since so much of the land appropriate for growing biofuel feedstocks is in the global south, the concept of energy independence for many countries in the northern hemisphere is a chimera; it will not be possible via biofuels and so not possible to be completely energy independent. What could occur, I suppose, is energy independence by hemisphere; the Americas on the one hand and the rest of the world on the other. Given the extent of investment in Africa by countries and corporations in Asia and Europe, with almost none from North America, such may become the defacto arrangement of the future, with various smaller degrees of cooperation between, say, Brazil and African countries.
Potassium, chemical symbol K, is in ample supply.
Phosphorus, chemical symbol P, is currently being mined at a rate of 0.8% of reserves per year; the reserve base is not(currently) economic to mine. This rate may seem small, but the unused, degraded lands to be devoted to biofuel production will require some; suppose doubling to 1.6% per year. Then the reserves are depleted in 62 years, 2070.
Worse, this assumes that world reserves are not overstated. Analysis suggests that reserves are overstated. If so, the end may come in, say, 2050. Whatever, agriculture, biofuel production, waste management and so on needs to start conserving phosphorus for reuse; don't waste phosphorus.
Nitrogen, chemical symbol N, is in short supply only in that it needs converting from diatomic nitrogen in the air into a biologically useful form in the soil. Some micro-organisms do just that; these are often associated with legumes. For example, it was locally the practice to alternate soft white winter wheat one year with dry peas and lentils the next. This practice meant that less chemical nitrogen fertilizer had to be applied to the growing wheat.
The chemical nitrogen fertilizer is fixed from the air via the Haber process, steam reforming natural gas to start the process. The price of these fertilizers varies with that of natural gas, thought to generally increasing over time. Obviously biomthane could replace the natural gas, but this may not be the best use of biomethane.
Producing biologically useful nitrogen could well be something that addtional micro-organisms, including genetically modified ones, could play an increasing role, lessening dependence upon the Haber process.
Civilizations end when the topsoil is used up. Avoiding this requires conservation and soil building. Building soils includes adding compost but now also some biochar. This later amendment then competes with simply burning the biochar as a fuel. So to the list of competing uses for agricultural land, using some to grow biomass for biochar to simply build topsoil has to be added.
Vast amounts of biomass simply go to waste. While crop wastes left in the field replenish the soil, some is collected with the crop, so could be used as biomass feestock for biofuels. Other concentrations of wastes abound: animal feedlot wastes, abattoir and fish offal, other food processing wastes, biomethane from landfill operations.
Now that half of humanity lives in cities and towns, municipal wastes are an important source of biomass which are underutilized. A few municipalities use some: Dayton, Ohio, ferments to biogasse and burns this to generate electricity; some municipalities in The Netherlands generate enough extra biomethane to support CNG filling stations; San Diego, California, generates enough high quality biomethane to supply some to the natural gas pipeline. But there is much more which can and should be done to note only improve the quality of wastewater discharge but also efficiently capture the energy currently often just wasted.
All this involves quite substantial infrastructure development and improvement.
An estimate of current world energy consumption, from all sources not including foods, is 420 exajoules per year. With increased energy efficiency, but also a larger and wealthier population, an estimate for the year 2050 is 800 exajoules. At the same time the peak in traditional fossil fuels will have come (and according to some, gone).
While various means of producing electricity and process heat, including space heating, will surely be non-biological, a reasonable estimate for a biofuel contribution is between 200 and 400 exajoules, depending upon the competing demands for land, water and other resources. The higher figure requires substantial development of supporting infrastructure and other equipment. This is surely possible, in some amount, so that biofuels will supply some, not all, energy needs between 2030 and 2050.