Challenges include supply of feedstock at commercial scales, and the cost of the conversion processes, according to An Overview of Second Generation Biofuel Technologies by Ralph E.H. Sims, Warren Mabee, Jack N. Saddler and Michael Taylor.

First generation biofuel production – usually ethanol – is expected to rise to around 100,000 Ml in 2014, and it is widely used in countries such as Brazil. But there are a number of limitations:

• Competition for land and water – often with competing with food and fibre production;
• High production and processing costs – often requiring government subsidies; and
• Widely varying assessments of net greenhouse gas reduction when land use is taken into account.

Although biomass crops take up less than 2% of the world’s arable land, estimates about biomass’ contribution to total food price increases typically range from 15% to 25%, with some estimates reaching 75%.

A wider range

2nd generation biofuels do not have the same direct competition for food crops, as ligno-cellulosic feedstocks include by-products such as cereal straw, sugar cane bagasse and forest residues; waste including organic components of municipal wastes; and dedicated feedstocks such as purpose-grown vegetative grasses, short rotation forests and other energy crops. Many dedicate crops can be grown on poorer quality land than food crops, although higher yields are likely if grown on quality land.

The technologies used for 2nd generation bioenergy are still relatively immature, which is why both public and private sources are placing significant investment in RD&D. Ligno-cellulosic biofuels are at the later stages and several demonstration plants are under construction. The US Department of Energy’s Biomass Program has invested heavily in bioenergy demonstration plants. Biotechnology and oil companies are also active investors.

However, even if 2nd generation biofuels are advancing, it is unlikely that there will be a clean break in the switch from 1st to 2nd generation. In the near to medium term, the biofuel industry is likely to encompass both generations. This is could continue the next one or two decades. But when 2nd generation biofuel technologies are fully commercialised, they would likely be favoured over many 1st generation alternatives due to policies designed to reward national objectives such as environmental performance or security of supply.

Main drivers for biofuels to date are:

• Energy supply security;
• Support for agricultural industries and rural communities;
• Reduction of dependence on oil imports; and
• The potential for greenhouse gas mitigation.

Expensive option

Although biofuels have great potential to meet the above, bioenergy remains an expensive option for reducing greenhouse gas emissions. Except sugarcane ethanol, there is a relatively limited scope for cost reductions and its perceived competition with increasing global demand for food and fibre, little improvement in these areas can be expected in the short term.

Uncertainty has also been raised about greenhouse gas savings if direct and indirect land-use change is taken into account.

Feedstocks and supply

The supply of biomass feedstock could be a real issue for the biofuels industry. In order to reduce project investment risks for the developer, supplies need to be contracted and guaranteed in advance over a prolonged period.

Feedstocks from residues and waste, where available, could enable biofuels production with virtually no additional land requirements or impacts on food and fibre crop production. The drawback with this option is that many regions only have limited supplies of these biomass feedstocks, necessitating the growing of vegetative grasses or short rotation forest crops to meet demand.

There is also the issue of transporting the biomass, and as many forms of biomass have relatively low energy density, numerous vehicle movements are inevitable (see Table 1).

Supply logistics will become increasingly important as development accelerates and competition for biomass feedstocks arises. The costs of feedstock delivery and storage need to come down as they are a significant component of the total cost.

Conversion Routes

Typically, 2nd generation biofuel production from ligno-cellulosic feedstocks can be divided into two main different processing routes:

• Biochemical – Where enzymes and other micro-organisms are used to convert cellulose and hemicelluloses to sugars prior to fermentation to produce ethanol; and
• Thermo-chemical – Where pyrolysis/gasification produces a synthesis gas (CO+H2) from which a wide range of long carbon chain biofuels such as synthetic diesel, aviation fuel or ethanol can be reformed, based on the Fischer-Tropsch conversion.

At the moment, there is no clear advantage of one above the other and both have significant technical and environmental barriers to overcome.

The biochemical route needs to improve feedstock characteristics, reduce cost by perfecting the pre-treatment process, improve enzyme efficacy, lower production costs, and improve overall process integration. The advantage is that the cost reductions for the biochemical processes have proved reasonably successful – for example, enzyme recycling.

The thermo-chemical route is the more mature process having been in operation for decades in coal-to-liquid and natural gas-to-liquid projects. However, its relative maturity means there is perhaps less opportunity for cost reductions.

One of the main problems for this route is securing large enough quantities of feedstocks at a reasonable cost. The gasification of biomass reliably and at a reasonable cost has also yet to be achieved.

A key difference between biochemical and thermo-chemical processes is that the lignin component of the biomass is a residue of the biochemical enzymatic hydrolysis process and can therefore be used for heat and power generation. In the thermo-chemical process the lignin is converted into synthesis gas along with the cellulose and hemicelluloses biomass components.

Both can potentially convert one dry tone of biomass to around 6.5 GJ/t of energy in the form of biofuels, giving an overall conversion efficiency of around 35%. Overall efficiencies can improve if surplus heat, power and co-product generation are included in the total system, bringing overall cost down.

The difference comes in litres per ton of feedstock occurring in practice.

The enzyme hydrolysis process can be expected to produce up to 300 l of ethanol (6 GJ) per dry ton of biomass, whereas the thermo-chemical route can yield up to 200 l (4 GJ) of synthetic diesel per dry tone (see Table 2).

Another difference is that all the biochemical routes produce ethanol whereas the thermo-chemical route can produce a range of longer-chain hydrocarbons from the synthesis gas.

Although demonstration plants are under way for both routes, the first commercial plants are unlikely to be widely deployed before 2015 or 2020.

Lowering Costs

The production costs associated with both technology routes are uncertain and companies treat this information with a high degree of commercial property. The lack of published cost data makes comparisons difficult.

The International Energy Agency (IEA) has nonetheless developed projections on potential market penetration for 2nd generation biofuels to 2050. Future costs are assessed to range from US$0.80 to US$0.90/litre of gasoline equivalent (lge) for ethanol (see Table 3). The evaluations are based on a very ambitious goal of halving global CO₂ emissions by 2050.

For 2nd generation biofuel, costs would need to come down to around US$0.80/lge to compete with wholesale gasoline prices when crude oil is at around US$100/bbl. The widely fluctuating oil and gas prices therefore make 2nd generation biofuels at current production cost levels a risky investment – especially when alternatives such as heavy oils, tar sand, gas-to-liquid and coal-to-liquid can compete with crude oil at around US$65/bbl.

The potential for cost reductions is probably greater for ethanol from the biochemical route than the thermo-chemical route, as many of the processes for the latter are already mature.

If commercialisation succeeds in the 2012-2015 time frame, costs could decline to US$0.55-0.75/lge for ethanol by 2030.

Technology development challenges

In order to maximise feedstock conversion efficiencies, the idea characteristics of specific feedstocks must be identified. The differences between wood, straw, stover and vegetative grasses can pose particular challenges for bio-conversion in multi-feedstock plants. One of the areas that needs to be looked into is the area required to supply sufficient biomass to commercial-scale plants.

For the biochemical route, a major development area is feedstock pretreatment technologies, which are currently inefficient and costly. Diluted and concentrated acid processes are close to commercialisation and the AFEX process based around steam explosion using ammonia, could provide benefits. Furthermore, new and improved enzymes are being developed.

The recycling of enzymes is also a potential cost reduction route.

An important goal for efficient production of ligno-cellulosic ethanol is to ferment all C5 (pentose) and C6 (hexose) sugars released during the pre-treatment and hydrolysis steps into ethanol. No known natural organisms have the ability to convert both to produce high ethanol yields, although progress has been made in engineering micro-organisms for the co-fermentation of these sugars.

Another challenge is the need to accommodate the variability in biomass feedstocks and to manipulate ethanol and sugar tolerance to potential inhibitors generated in the pre-saccharification treatment. Pentose fermentation has been achieved on ideal substrates, but significant work remains to replicate this to actual ligno-cellulosic feedstocks.

For the thermo-chemical route the challenge is to develop a gasification process of biomass at commercial scale to produce synthesis gas to the exacting standards required for a range of ethanol and other biofuel synthesis technologies such as Fischer-Tropsch.

Cost effective and reliable methods for this remain fairly elusive.

Another cost reduction option is to develop new catalysts that are less susceptible to impurities and that have longer lifetimes. The production of valuable co-products during the production of 2nd generation biofuels can also increase overall revenue and improve the economics of the process. Co-products include heat, electricity and various chemicals.

The politics

When it comes to policies to support the development and commercialisation of 2nd generation biofuels, lessons need to be learnt from the experiences made with 1st generation – for example that the full environmental ramifications of deploying biofuels are thoroughly understood.

Policies to support either generation should ideally be part of a comprehensive strategy to reduce greenhouse gas emissions. If, for example, taking into account the environmental impacts of CO₂ emissions from petroleum products would mean biofuels that have a proven mitigation potential could compete on a more equal footing.

Continued investment in RD&D is essential for 2nd generation biofuels to reach commercialisation. And both public and private investment is needed and should be used to address issues such as:

• Crop productivity and ecosystem health;
• Evaluating land-use;
• Production cost effectiveness;
• Learning from 1st generation biofuels lessons; and
• Increasing the performance of conversion technologies.

To overcome some of the obstacles to commercialisation, a broad, international collaborative approach is needed, but this could be hampered by the unwillingness among commercial companies to participate due to the constraints of protecting intellectual property rights for commercial investments.

To encourage deployment, two main policy options are available: Mandatory Targets (which give certainty over outcomes, but not potential costs); and Tax Credits (which give certainty over potential costs, but not outcomes).

Policies should also ensure the continued progress in addressing and characterizing the environmental performance of biofuels through agreeing on standardisation and assessment methods, and harmonizing potential sustainable biomass certification methods.

These would need to cover the production of feedstock and potential impacts from land-use change.

If there is not a technical breakthrough that significantly reduces production costs and accelerates investment and deployment, the commercialisation of 2nd generation biofuels could take another decade or so.

This article is based on an article published in Bioresource Technology Vol 101 (2010), by Ralph E.H. Sims, Warren Mabee, Jack N. Saddler and Michael Taylor, An overview of second generation biofuel technologies, Page 1570-1580, Copyright Elsevier (2011).

doi:10.1016/j.biortech.2009.11.046

Bioresource Technology aims to advance and disseminate knowledge in all the related areas of biomass, biological waste treatment, bioenergy, biotransformations and bioresource systems analysis, and technologies associated with conversion or production.

About:

Kari Williamson is the Assistant Editor at Renewable Energy Focus.

Renewable Energy Focus U.S., May/June 2011.