Issues Magazine

First- and Second-Generation Biofuel Technologies

By Stephen Schuck

How are biofuels produced from biomass, and can we produce enough for our energy needs?

Bioenergy covers the broad spectrum of heat and power, transportation fuels and chemical production. Biomass can take many and varied forms – from fairly dry (15% moisture content) to fairly wet (over 90% moisture content), including agricultural and forestry residues and food wastes. The focus here is on biofuels.

The technologies for manufacturing first-generation biofuels are generally mature and are commercially available. First-generation technologies essentially use only a small fraction of the standing biomass, such as oil seeds from crops such as canola and soya or, for ethanol, maize, sorghum grain and molasses. The first-generation ethanol processes may be categorised by the so-called six-carbon sugars, such as glucose. The more exotic five-carbon sugars, like arabinose and xylose, are associated with the hemicellulose fraction of the biomass and require alternative processing using second-generation technology.

There are huge benefits in moving towards the whole-of-biomass approach – second-generation fuels – because of the larger scales and more complete utilisation attainable. Much current development is focused on the conversion of all of the biomass to biofuels. One of the main routes being followed is the production of lignocellulosic ethanol via the fermentation of the five-carbon sugars.

First-Generation Biofuels

First-generation biofuels are:

  • ethanol derived from food crops such as grains and sugarcane; and
  • biodiesel from feedstocks of vegetable oils and animal fats.

Ethanol production from a grain feedstock entails:

  • milling, mashing, fermenting and distillation to increase the content of the ethanol in the product;
  • dehydration, to produce anhydrous ethanol (basically 100% ethanol); and
  • denaturing with petroleum petrols.

Ethanol blends mainly into petrol, but with suitable emulsifiers can also blend into petroleum diesel. Vegetable oils can be converted into bio-esters, which blend extremely well into diesel and at low blend levels can meet the Australian Diesel Standards.

First-generation ethanol technology has been used for several decades, particularly in the USA and Brazil.

The first-generation fuel biodiesel (methyl ester) is essentially produced from vegetable oils and animal fats. To produce biodiesel, vegetable oils and methanol (or ethanol) are combined in the presence of a base catalyst such as sodium hydroxide to produce the same amount of biodiesel as the vegetable oil feedstock. The by-product glycerol can itself be used as a fuel.

Second-Generation Biofuels

Interest in second-generation biofuels (from non-food feedstocks) has been driven by the need to find a broader range of feedstock and to allow production at a much greater scale to provide a greater proportion of future energy needs. The two main avenues for the production of second-generation biofuels are:

  • biochemical: using microbes to convert cellulose and hemicellulose in the biomass to sugars for fermentation; and
  • thermochemical: applying heat to gasify the biomass into a chemical feedstock that can be re-synthesised into fuels.

For second-generation ethanol, the biomass follows the biochemical pathway of pre-treatment, scarification (to liberate the sugars for fermentation to ethanol) and fermentation.

For the thermochemical pathway, the steps are drying (fresh biomass tends to be about 50% moisture) followed by gasification, conditioning the gas and various synthesis processes.

Some of this work is being driven by a desire to reduce greenhouse gas emissions.

Because of the high compression ratio of diesel engines, diesel is a moderately better fuel than petrol with regard to greenhouse gases. The assumptions made in life cycle analyses are important. Some studies of ethanol from grains (including corn) have shown that it is mildly negative in terms of greenhouse gas performance, but most show it is fairly positive. Of particular interest is that “biomass to liquid” can achieve about 90% greenhouse gas reduction, similar to ethanol from wood.

Sugar is quite a good performer as well as biodiesel. The benefit of the latter is quite variable, depending on where the feedstock has come from, distances travelled, and so on.

One of the greatest limitations of biomass and biofuels is the fairly low energy density, particularly if you are trying to transport it as wood chips. One way of overcoming this, in a decentralised manner, is to apply pyrolysis technology. This is a thermal process that operates in the absence of air. The biomass can be thermally fractionated into much the same chemical constituents as the original biomass, but in a liquid form known as bio-oil. Up to 75% of the energy in the dry biomass can be retained in this liquid fuel.

Bio-oil has an energy density of about 60% on a volume-for-volume basis as petroleum diesel. Although the process offers opportunities to produce various value-added products, its great attraction is in densifying the biomass so that existing infrastructure (fuel tankers etc.) can be used.

Can the World Produce Enough Biomass?

How much of the planet’s energy needs can be provided by biomass? Some relevant work in this area has been done for the International Energy Agency.

Under one scenario of high population growth and people moving to meat-intensive diets (something like 3.7 kg of vegetable protein is required to produce 1 kg of animal protein) there is little potential to produce substantially more biomass for fuel. In another scenario, moving from subsistence farming to industrial agriculture and without great competition for other things such as pure carbon sequestration in plantations, you can get a very high potential. A lot of work now needs to be done to refine the quantum between these two scenario extremes.

Adapted with permission from “What Now and What Next for Global Biofuel Technologies?”, presented at the Crawford Fund’s conference “Biofuels, Energy and Agriculture: Powering Towards or Away from Food Security?” (http://www.crawfordfund.org/publications/conference07.htm).