Not all biofuels are equal – some produce less greenhouse gases than others, some utilise greenhouse gases (such as methane) that would otherwise go into the atmosphere. Some biofuels are less environmentally friendly than others because they come with an overall high environmental cost (such as use of fertilisers, water, cultivation of soil, use of farm machinery). Some products are also intensive in their production and use only a small part of the plant to make biofuel.
Some biofuels are clean and green, and some are cleaner and greener than others, and some are very, very good with making gaseous fuels from anaerobic digestion of wastes. If you just leave that waste, it will decay in the atmosphere, and it will produce methane, which is quite a toxic greenhouse gas. So if you capture that material and anaerobically digest it, and capture the methane and then use it to create heat and electricity, then you've utilised the methane, reduced its greenhouse gas impact, and you've got the energy out of it, and you've cleaned up the wastewater, which has to go somewhere, so it is a really clean and green way of getting energy. Unfortunately, there is a relatively small biological resource for making that energy.
One of the biomass fuels which is possibly less clean and green than the others is making ethanol from corn because it’s a very intensive agricultural system to produce it. There is lots of tillage of the soil, lots of tractor use, lots of diesel burn, lots of fertiliser and, in some cases, irrigation, so there is a lot of energy input into growing it. And then you convert the small proportion of the plant – because when people talk about making maize into ethanol, they are only talking about the piece at the top of the plant, the bit that you eat, the starch – and they convert that piece into ethanol and discard all of the stalk. So you're only talking about a small proportion of the plant and that gets converted into ethanol and the energy return on the energy invested is about 1.4 to 1, so it’s not a great return.
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David Harding, Wanganui Aero Work Ltd