Biofuels

   Renewable….if there is sufficient net gain in energy (minimal fossil-fuel inputs for planting, fertilizing, harvesting, processing)

   There are two main reasons to go to biofuels:

• Greater energy independence (from imports), via domestic production: they can be grown almost anywhere that the climate allows.  Many areas of the US qualify for growing field crops for biofuels (corn, sorghum, switchgrass); these crops can go dormant or be reseeded and need not be winter-active or even winter-hardy.  Southern regions of the US hold promise for algae, with the limitation that the culture can't be allowed to freeze.  Heating is very expensive and seriously reduces the net energy yield, so that siting is very important.
• Relatively low potential for air pollution: coal, in contrast, is the dirtiest fuel imaginable, as it contains mercury in significant amounts, as well as sulfur as another major pollutant.  Biofuels contain no more than background levels of such pollutants, so that burning them causes little impact.  Nitrogen oxides form in any combustion, but these are controllable in vehicles with catalytic converters.  At powerplants, the picture is mixed.
• Mitigation of climate change: net reduction in greenhouse gas emissions.  Burning any fuel emits CO₂ to the air, with attendant contributions to climate change.  Regrowing the crop recaptures CO₂ in a sustainable cycle.  Carbon neutrality is not guaranteed, however; see below. 

   There are concomitant problems of:

• Very high land usage per unit of net energy produced (energy yield is on the order of 1 to 3 W m^-2 annual average, vs. the other distributed direct solar sources, photovoltaics or solar thermal, at 35 W m^-2), or even traditional fossil-fuel extraction and processing (on the order of kW m^-2, given the spatial footprint of mines, wells, roads relative to the very high energy density of the fuels).   Heuristic calculations are given in Appendix I below.
• Raising of food crop prices from competition for land and for resource inputs (fertilizer, water)
• Water use - for irrigation, or for evapotranspiration of a good fraction of what would have otherwise become runoff.  See Appendix II below.
• Habitat conversion at the expense of wild species that provide diverse ecosystem services (host plants for insects and nesting sites for birds that provide pest control; flood control; etc.)
• Pest and disease problems: for the biofuel crops themselves (if they are extensive monocultures) or for other crops, humans, livestock, and wild fauna and flora, if the biofuel crops are hosts for pests and disease vectors.   Appendix III below notes some problems that could develop very rapidly.
• Fossil-fuel inputs, noted earlier…and, for algal biofuels, especially biodiesel, there is a high energy/ fossil-fuel input in fabricating the massive containment systems.  See Appendix IV below.
• Nutrient mining from soil - net extraction of N, P, K, micronutrients.  Fortunately, in many designs these nutrients can be returned to the soil (or tank), if the process waste containing the nutrients is returned to the growth site. This makes the nutrient-mining problem  far more amenable than with food crops, where the ultimate disposal is in sewage streams that are often mixed with industrial streams that contain toxic loads (sewage sludge never made it as a fertilizer).   
• Whether organisms are grown in soil or tanks, the demand for fertilizers adds a significant marginal expense to the product.  See Appendix V below.
• Water pollution from N and P fertilizer excesses carried into waterways by runoff and deep drainage.  Ordinary food crops in the Mississippi River basin and other basins are already responsible for massive N and P loads that lead to very large (multiple tens of thousands of km²) coastal dead zones; the N and P support algal blooms, which, upon death, deplete dissolved oxygen in water to essentially zero.  Biofuel cropping has the potential to add to this problem.

   A closer look at the two goals, energy independence and GHG reduction, which are mixed together in any decision system:

• Energy independence:
• Biofuels are often projected to be indefinitely renewable (barring land degradation from soil erosion or nutrient mining, increasing pest and disease problems, or economic and political problems from interference with the food crops market).  The multiple tradeoffs - economic, environmental, social - against using depletable fossil fuels need project-specific evaluation.
• There are cheaper ways to go, such as using coal where it is abundant, as long as one can continue to externalize the costs of climate change, air pollution, and water pollution).   Of course, the cheaper ways have negative effects on GHG emissions.
• GHG emission reduction, viewed over the life cycle of the system:
• Huge qualifier: what type of land was converted to biofuel production?  Several recent studies show that conversion of some cover types (e.g., tropical forest) to biofuels production releases massive amounts of GHG, mostly CO₂, from decay or burning of original cover, plus oxidation of soil organic carbon.  In some cases, the land conversion emitted more than 90 times the annual C capture by the biofuel crop.
• Another qualifier: what fossil-fuel inputs were used, particularly in processing the biomass?  In some egregious cases in the US, coal was used for distilling the alcohol.  The net carbon impact was adverse, because coal generates twice the CO2 per unit energy delivered compared to common hydrocarbons (oil or natural gas).   Why use coal for distillation in biofuel production, vs. biomass itself, or why not use coal for direct synthesis of liquid transportation fuels, such as by the Fischer-Tropsch synthesis (used by South Africa under apartheid during embargoes imposed by other nations)?
• Coal is cheap
• Coal is an abundant internal resource in the US and several other nations.
• Subsidies for ethanol production are largely political, and do not account for GHG impact
• Subsidies are essentially absent (research funds only) for direct coal conversion to liquid fuels

   A potential role/niche for our consulting consortium: evaluation of biofuel options (design, operation, governmental policies) by site, region, or political entity

• Providing the full context (all the issues above); such context is highly unlikely to be provided by potential vendors of various biofuel production systems.
• Providing evaluation methods and quantifying all the criteria.
• Projecting these to the specific system design(s): crop species, climate, soils, economic conditions, processing methods, GHG treaties, etc.
• Providing optimization models with mixed objectives and constraints (a very interesting area of applied mathematics, and with great promise in this thorny problem).

 

Appendix I: estimates of land use

    Vince Gutschick, GCCC director

• Simple fact: photosynthesis has a low energy efficiency.  There are very good reasons for this, including metabolism required to make organisms self-maintaining / self-repairing and also the drop in atmospheric CO₂ over millions of years that plants themselves caused by being partly rot-resistant and thus causing the burial of huge amounts of carbon.
• Normal green plants capture energy in light, using about 120 photons to make one molecule of glucose sugar.  The energy in the photons is more than 4 times larger than the energy captured in making sugar, so that the efficiency of capturing energy in sunlight is less than 25% at best.  That's in the initial steps.  The plant then has to use part of the energy in sugars to make other things – proteins, fats, etc., and also to remake damaged parts.  The highest efficiency attained in field crops is 6% over a season in sugar cane.  The average efficiency is more like 2%.  For the globe as a whole, it is 0.3%, because of dormant seasons, etc.  Algae are no better than land plants.  However, they can be grown in contained cultures that use high levels of CO₂, such as in power-plant exhaust (oops – that means taking up mercury and such if it's a coal-fired plant).  They might hit 6% on a continuing basis.
• What does this mean for land use?  The other consideration is that sunlight is a dilute resource.  We can hit 1 kW per square meter at high noon in clear western US climates, but the average is about 180 W per square meter (W m^-2), accounting for day and night, winter and summer, clear and cloudy conditions.  So, at 6% efficiency in capturing energy, algae might average 10 or 11 W m^-2. 
• That's as raw biomass.  They have to be processed, and drying either is really slow (using more sun and more land, or, worse, using some of the energy provided by the rest of the crop).  If the system hits 60% efficiency in going from raw biomass energy to final electric or fuel energy, we're getting about 6.5 W m^-2. 
• Now let's say we want to replace a typical 1000-MW power plant, which actually consumes about 2500 MW of thermal (fuel) power, converting it at only 40% efficiency.  This is 2.5 billion watts.  We will need about 0.4 billion square meters.  This is 400 square kilometers, or over 150 square miles.  Of course, you don't have to make one big plant, but the total area of many smaller plants has to be this large. 
• There are much better ways of using land, even cheap land.  Solar cells, or photovoltaics, easily hit 11% efficiency, and new ones, not yet commercial, are exceeding 15%.  Let's say that 13% is a decent figure for plants that will be built.  This is 13% from sun to electricity – no other conversion inefficiencies.  This gives us 0.13 * 180 W m^-2 = 23.4 W m^-2.  To get 1000 MW of power, we still need a large area, almost 43 million square meters.  This is 43 square kilometers or about 16 square miles…slightly more than 10% of what the algal pond would take.  Solar thermal power is about as efficient.  Both technologies are proven to a high level.  I attended the American Geophysical Union meetings in 2007 and 2008 and the recent Energy Summit in the Sandias.  The reality is that these technologies have only a short way to go to large-scale commercialization.  They are far ahead of algal biofuels.  They don't readily make liquid fuels for transportation, but they have a clear role in the energy economy.  They could even be pushed, with some conversion losses, to make liquid fuels, as by making hydrogen gas from electricity to combine with CO₂ in the air, generating alcohols and then hydrocarbons.

  

Appendix II. Possibly large water use

   Vince Gutschick, GCCC director

• Certainly, field crops use a great deal of water.  This is a physiological necessity that absolutely cannot be altered.  Some gains in what we term water-use efficiency are possible, but they are marginal.  It takes about 500 masses (pounds, kilos, whatever) of water to make one mass of plant matter.  I have done theory and experiments on this.
• Algae can be used in closed systems, with only minimal water loss.  Some can also grow well in brackish water.  These plusses are balanced against some minuses. 
• First, closing the system means that the containment vessel has to be built.  This is expensive in both dollars and materials.  The materials cost a significant amount of energy to make and deploy.  (The same is true of, say, photovoltaic cells, but their energy payback is only 1.5 to 2 years, in a 20 to 30 year lifetime…a bargain).
• Second, the standing volume of water is large.  A reasonable depth for growth is at least 0.5 meter or about 1.5 feet.  Let's go back to that algal pond that replaces a 1000-MW power plant (and that needs such a traditional plant nearby, to get high efficiency, by growing at high CO₂).  The area is 400 million square meters, so the volume is 200 million cubic meters.  This is about 7.2 billion cubic feet, or 54 billion gallons, or about 1.65 million acre-feet.  We don't have this to spare in fresh water, so we'd go to brackish water.  We do have a lot of this.  We'd have to be careful not to spill much of it and salinize the land.  We'd also have to reuse it very thoroughly.  This is where my knowledge is nearing its limit.  The water does accumulate metabolic byproducts of the algae, which have to be removed.  I'm not sure how readily this is done by purification vs. partial disposal.

 

Appendix III. Potential for rapid loss of biofuel cultures

   Vince Gutschick, GCCC director

All crops, conventional field crops or unconventional crops like algae, are subject to losses.  These can come from the abiological environment, in the form of high or low temperatures.  Controlling high or low T in an algal farm is likely to be quite expensive (fuel or electricity for heating; water for evaporative cooling).  The threats can also come from biological sources.  In the ocean, most algae and other plankton die in a fraction of a day to several days, primarily from viral infections and the like.  In a contained culture, this threat might be reduced.  However, a realized threat would spread with considerable speed, and decontamination is difficult and costly.  With ordinary crops, soil microorganisms help control pathogens, if not completely.  In algal farms, there is no such help.  The risk is probably not insuperable, but it cannot be costed out currently for large operations.

 

Appendix IV. Algal biofuels may require large inputs of energy and funds for construction

   Vince Gutschick, GCCC director

This has been brought up in the other topics above.  Containment vessels are costly, in both dollars and energy.  Let's say that the vessel is polycarbonate (it has to be strong and tough, resistant to impact), and that it's about ¼" or 6 mm thick, and on top and bottom, for a total of about ½" or 12 mm. I haven't done detailed calculations on the energy it takes to make this plastic,  nor on dollar costs for purchases in volume.  I'll make a preliminary estimate here.  The embodied energy (energy needed to manufacture an item, including the energy value left in the product as a potential combustible fuel) in plastic is likely to be at least 3 times the energy in the same amount of hydrocarbons, which is about 40 kJ per gram.  It is 1.2 times as dense as water, so that it weighs about 1200 kg per cubic meter.  This huge pond container has 400 million square meters x 0.012 m depth, or 4.8 million cubic meters of polycarbonate, weighing about 5.8 million tons (5.8 trillion grams). The embodied energy, at 120 kJ per gram, is about 7 x 10¹⁷ Joules (J, or watt-seconds).   A MW-year is 3.1x10¹³ J.  Thus, it takes about 22,000 MW-years of energy.  The 1,000-MW plant pays back its energy cost in 22 years!  No viable commercial plant can bear this.  The payback time has to be far less than half the useful lifetime.

 

Appendix V. Major operating cost of providing fertilizer

   Vince Gutschick, GCCC director

The most costly nutrient to provide, in dollars and in energy to produce fertilizer forms, is nitrogen.  Ammonia manufacture for fertilizer consumes 3% of world energy currently.  For high-value food crops, the cost is sustainable by farmers, even if only in more developed nations or subsidized regions.  For fuel, which is worth much less per mass, the cost of this input is very high.  There are some cyanobacteria that fix nitrogen from the air and that might be co-cultured with true algae.  This can be tricky.  There is also the possibility of recovering a substantial amount of the nitrogen in the algal biomass, if the whole organisms are not dried and burned as fuel.  Of course, if the whole organism is not used – say, only 40% that is lipid (oil) – a very generous figure, then this reduces the efficiency of algal growth for fuel even more and increases the land use and water use.