Bio-ethanol and biodiesel are being hailed by many as part of the solution to climate change, energy security and as an economic opportunity to develop domestic industries. Both North American and European jurisdictions are supporting biofuels, with some of them even requiring quotas for mixing biomass derived fuels with conventional gasoline or diesel. More often than not we have heard about the controversy around these issues: should we really use our agricultural land to cultivate biomass only to burn it afterwards, instead of growing food or other high value agricultural products? What about the energy balance of these processes? Do some of them even require more fossil fuels over their life cycle than what they displace when they are used in transportation?
As both residual biomass and land for energy crops is limited, it is worthwhile considering how to use it most efficiently. Our research team wanted to know: what are the greenhouse gas (GHG) emission reductions one can obtain from biomass in various uses, and which ones should therefore be preferred? Adapting several existing studies examining life-cycle emissions of various biomass processing chains, we endeavoured to compare the emissions displaced by biomass-based fuels both on a per-ton, as well as on a per-hectare basis. It hadn’t been done before, and the results were startling.
Energy Intensive Bio-Ethanol
Making automotive grade ethanol from biomass requires large amounts of energy, mainly due to the distillation process that is involved. Depending on whether residual biomass (waste) or bioenergy crops are used, varying amounts of energy are consumed and related emissions are created due to farming, fertilizer use, transportation, processing, and storage. Mainly due to the high energy demand of the distillation process, the energy balance of bio-ethanol from corn and wheat is barely positive – and for lignocellulosic feedstocks, such as straw or hay, it is negative. However, we found that GHG emission reductions will always be created. A negative energy balance shows that more than 50 percent of the biomass used in a process is used up on the way, i.e. if I put straw with a heating value of 100 MJ into the process, I may only get 40 MJ worth of ethanol, whereas the remainder of the energy is used up for various processing steps.
However, as 100 percent of my input is biomass, no net GHG emissions result from oxidising this biomass. This means that one will still get emission reductions for the fossil fuels displaced by the climate-neutral biofuels. So the energy balance is not the best indicator for environmental performance, although it helps distinguish energy intensive biomass utilization pathways. Keep in mind, though, that biomass generally has far higher moisture contents than fossil fuels, such as coal, which means the energy inputs for drying will always mean that the biobased process is more energy intensive.
Which Ethanol Feedstock?
We examined wheat, straw, switchgrass, corn, corn stover and hay as ethanol feedstocks. Based on tons of input, their emission reduction benefits are very similar: about 0.5 tons of CO2 per ton of biomass. However, when we compare the energy crops only, this picture changes: now high-yielding species, such as switchgrass, take the lead over wheat and corn. The math is relatively simple – switchgrass is assumed to yield about 11.5 tons of biomass per hectare, whereas wheat only yields 3, and corn about 7 tons. This means I can displace a lot of extra fuel per hectare simply because I grow a plant that gives me more biomass each year, thus more ethanol as well. What is the conclusion? You can always use residual biomass to make ethanol, but if you want to grow energy crops, look for the one that has the highest yield per hectare and therefore the highest amount of ethanol produced for the land you are setting aside for biofuel production.
Biodiesel – Go for Cooking Oil!
The results for biodiesel are very similar. Both canola/rape and soy – preferred crops to produce the vegetable oil used for biodiesel production – have very low yields per hectare (Canola: 1.3 tons per hectare). Consequently, alternative uses of agricultural land can result in much higher emission reductions. If farmland is used to grow biodiesel energy crops, the amount of land required to contribute to national emission reduction targets is multiplied as the crops simply don’t yield a lot of oil. On the other hand, waste cooking oil or animal fat should be collected and used for biodiesel production wherever possible. In urban centers, the collection of yellow grease (another name for used cooking oils) is cost-effective and even now biodiesel can be had at a competitive price to conventional diesel in Ontario, albeit with some preferential tax treatment.
The report also looked at other options to use biomass: electricity generation, combined heat and power, and hydrogen were added to the analysis. What was learned is that we need to use biomass in applications where it displaces a high amount of fossil fuels. This means, for example, that burning biomass to make electricity, which then displaces low-emission sources, such as large hydropower, or even natural gas-based generation, is not a good option. This can be improved when efficiencies are increased in combined heat and power applications. On the other hand, displacing a transportation fuel or also coal, when biomass is co-fired in power stations, can yield significant emission reductions – up to about one ton of CO2 per ton of biomass invested for some of the applications.
It seems that the fixation of wheat and corn as ethanol feedstocks is mistaken, at least from a GHG perspective. Land set aside for bioenergy crops could be used much more efficiently if other crops were grown. In addition, trees or switchgrass can grow on marginal agricultural land and generally require less energy and water inputs than many other energy crops. The best land can thus still be used for food crops. If biomass residues are used, it is imperative to avoid electricity-only applications (apart from biomass co-firing in coal power stations), but to concentrate on combined heat and power wherever possible.
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Source: SolarAccess.com News vom 2004-08-16.