Inevitably, an essential facet of the public discussions on costs and subsidies of biodiesel production has been that of its potential amelioration of greenhouse gas emissions. If significant, this would augment the case for production and consump­tion incentives to offset higher production costs than for conventional diesel. At the scientific level, this debate has mirrored that for bioethanol (chapter 1, section 1.6) and has proved equally contentious and acrimonious.

In the early 1990s, net energy balance (NEB) values of up to 3.8:1 were calculated for rapeseed-derived biodiesel, depending on how the coproducts and crop straw were assessed in the calculations (figure 6.6).4 Unpublished reports and communica­tions quoted in that report were from 1.3 to 2.1 without coproduct credits and from 2 to 3 if thermal credits for the meal and glycerol coproducts were included. Radi­cally different conclusions were reached in a 2005 publication: biodiesel production

2.0

from soybean oil required 27% more fossil energy than the biodiesel energy content, whereas sunflower oil was even less viable (requiring 118% more fossil energy than in the product).49

Midway (in time) between these conflicting estimates was a report from the National Renewable Energy Laboratory whose main conclusion was that biodiesel (from soybean oil) yielded 3.2 units of fuel product energy for every unit of fossil energy consumed in its life cycle, whereas conventional diesel yielded only 0.83 unit per unit of fossil fuel consumed, that is, that biodiesel was eminently “renew­able.”55 Direct comparison of these conflicting results shows that the disagreements are major both for the stages of soybean cultivation and biodiesel production (fig­ure 6.6). As so often in biofuel energy calculations, part of the discrepancy resides in how the energy content of coproducts is allocated and handled in the equations (chapter 1, section 1.6.1). As the authors of the 2005 study pointed out, if the energy credit of soybean meal is subtracted, then the excess energy required for biodiesel production falls to 2% of the biodiesel energy content.49 A paper posted on the Uni­versity of Idaho bioenergy site suggests other factors:56 [60]

• Even adding in energy requirements for oil transport and transesterification as well as biodiesel transport produces a favorable energy balance of 2.9:1.

The energy balance, in any of these scenarios, is highly dependent on viewing the process as a biorefinery producing coproducts as well as biodiesel. If the energetic (and economic) value of the soybean meal cannot be realized, then the balance will be negative — even using the soybean meal as a “green manure” spread on the soybean fields would only partially offset the major loss of replaced fossil energy in the total process. If biodiesel production from oil seed crops is viewed as an “opportunity” over their alternative uses as, for example, foodstuffs, then the NEB can be recalculated to be favorable.57 As discussed previously (chapter 1, section 1.6.1), this is a contentious argument, and much media comment in Europe dur­ing 2006-7 has pointed to increased areas of arable land being devoted to oilseed rape as demand for the crop as a source of biodiesel increases, thus fulfilling the prediction of a subsidized cash crop.4 A focus of future attention may be that of realizing an economic return on the greatly increased amounts of seed meal and of finding a viable use for glycerol — refining the glycerol coproduct to a chemically pure form is expensive, and alternative uses of glycerol for small — and medium — scale biodiesel facilities are being explored, for example, its use as an animal feed supplement.58

As the number of industrial units producing biodiesel increases, assessments of energy balances should be possible from collected data rather than from calculations and computer simulations. A report on activities in six Brazilian and Colombian biodiesel facilities using palm oil as the agricultural input attempted precisely this.59 NEBs were in the range of 6.7-10.3, with differences arising because of

• Different rates of fertilizer application

• Different uses of plant residues as fertilizers or as boiler fuel for electric­ity production

• On-site electricity generation at some sites, whereas others were entirely dependent on purchased electricity

• Differing efficiencies in the generation of coproducts and the recovery of unused palm oil

Taken together as a group, these palm oil biodiesel producers were assessed as being more energy efficient than reference manufacturers in Europe or the United States — the most recent (2006) detailed estimate of biodiesel from soybean oil in the United States arrived at a NEB of 1.93:1, but this was critically dependent on full credits being taken for soybean meal and glycerol coproducts (without them, the balance decreases to only 1.14:1).60

The energy balance is an important parameter that defines the extent of the biodiesel’s capacity to reduce greenhouse gas emissions, because, in the extreme case, if biodiesel requires more fossil energy in its production than can be usefully recovered in the product, no savings could possibly accrue.49 With a favorable energy balance for soybean biodiesel, its use could displace 41% of the greenhouse gas emissions relative to conventional diesel.60 As headline statements, the National

Renewable Energy Laboratory study on biodiesel use for public transport concluded the following:55

1. Substituting 100% biodiesel (B100) for petroleum diesel reduced the life cycle consumption of petroleum by 95%, whereas a 20% blend (B20) reduced consumption by 19%.

2. B100 reduced CO2 emissions by 74.5%, B20 by 15.7%.

3. B100 completely eliminated tailpipe emissions of sulfur oxides and reduced life cycle emissions of CO, sulfur oxides, and total particulate matter by 32%, 35%, and 8%, respectively.

4. Life cycle emissions of NO* and hydrocarbons were higher (13.4% and 35%, respectively) with B100, but there were small reductions in methane emissions.

Earlier assessments indicated that only 55% of the CO2 emitted from fossil diesel could be saved if biodiesel were to be used because of the CO2 emissions inher­ent in the production of biodiesel and that, other than a marked reduction in sulfur oxides, effects on CO, hydrocarbons, NO*, and polyaromatic hydrocarbons were inconsistent.4 As the use of biodiesel has widened globally, the number of publi­cations exploring individual pollutants or groups of greenhouse gas emissions has expanded, especially after 2000 (table 6.5).61-68 The report of increased mutagenicity in particulate emissions with a biodiesel is unusual as two earlier reports from the same research group in Germany found reduced mutagenicity with rapeseed oil — and soybean-derived biodiesels.66 6970 A high sulfur content of the fuel and high engine speeds (rated power) and loads were associated with an increase in mutagenicity of diesel exhaust particles. This is in accord with the desirability of biodiesels because of their very low sulfur contents, zero or barely detectable, as compared with up to 0.6% (by weight) in conventional diesels.4 There are suggestions that exhaust emis­sions from biodiesels are less likely to present any risk to human health relative to petroleum diesel emissions, but it has been recommended that the speculative nature of a reduction in health effects based on chemical composition of biodiesel exhaust needs to be followed up with thorough investigations in biological test systems.71