Issues Magazine

Biofuels Get Moving with Catalysis

By Rebecca Lesic and Thomas Maschmeyer

Catalysts and supercritical fluids can be key players in biofuel technology and in green chemistry.

Currently we are in the middle of a growing energy debate fuelled by the twin scenarios of increasing resource scarcity and greenhouse gas emissions, with governments, industry and academia around the world considering how best to address these. Although debate continues over when our fossil-based resources will peak, this point is undoubtedly approaching. Both recorded and projected global oil and gas availability highlight the real need for alternatives to crude oil when viewed simply from a supply-side perspective. Couple this issue of peak oil with the rapidly increasing cost and environmental impact of traditional fossil fuels in terms of greenhouse gas emissions and climate change, and the need for alternative fuels becomes greater than ever.

As the search for new fuel options continues, it is evident that not just any fuel will do. Ideally, alternative fuels will be not only economical and sustainable, but also efficient and environmentally friendly.

Despite arguments about the best technology for centralised power generation (coal, gas, nuclear, solar-thermal, wave, wind, hydro, geo-thermal) it is clear that transport fuels will be liquid organics for the foreseeable future. Current technology simply does not allow for the complete replacement of today’s vehicle fleet with electric or hydrogen-powered vehicles. Therefore, fuels derived from biomass are an obvious solution because they can be sustainable (particularly when looking at salt-tolerant, high-fibre algae or seaweed grown offshore) and greenhouse-neutral (or even greenhouse-negative by burying carbon when using fuel gases from agricultural char production). Developments in this area fall under the general term “green chemistry” (see box).

Catalysis and Biofuels

Catalysis has a key role to play in both green chemistry and the development of alternative fuels. A catalyst is capable of increasing the rate of a reaction without being consumed or incorporated into the final product. It can also increase the energy efficiency and selectivity of a given reaction.

Today, catalysis is important in most industrial processes, particularly those of the fuel industry. By increasing product selectivity and turnover frequency and decreasing energy consumption, catalysts are important not only financially but also in green chemistry by reducing waste and resource consumption. Catalysts make a reaction system work smarter, not harder, thereby maximising the outcome of the reaction.

Based on their physical properties, catalysts can be divided into two broad categories: homogeneous and heterogeneous. A homogeneous catalyst exists in the same phase as the reaction mixture, while a heterogeneous catalyst exists in a different state to the reaction mixture.

Heterogeneous catalysts have a vital role to play in industry because they are typically easy to remove from a reaction mixture using filtration or centrifugation, thereby often allowing them to be recycled.

Biodiesel

Biodiesel is gaining interest throughout the world and is already well-established in many countries, particularly in mainland Europe and increasingly in the US. A major advantage of biodiesel is that it is derived from renewable materials such as vegetable oil or animal fat. It is biodegradable, has low toxicity (less than table salt) and has much lower overall emission profiles than its petroleum counterpart. Biodiesel can also be used directly in most current diesel engines without the need for engine modifications.

The use of biodiesel has at times been widespread in Australia, with both ferries and trucks running on this biofuel, and various service stations selling blends of biodiesel/diesel to the general public. However, a sudden change in the taxation of this emerging fuel has rendered biodiesel in Australia largely uncompetitive for mainstream use.

The concept of using vegetable oil as an engine fuel dates back to 1895 when Dr Rudolf Diesel developed the first diesel engine, which ran on peanut oil. Today, a process called transesterification is used to convert the triglycerides present in vegetable oil into methyl esters and glycerol. The methyl esters can then be used as an additive to or substitute for fossil-based diesel.

Currently, the production of biodiesel typically involves a two-step homogeneously catalysed process, whereby the vegetable oil is treated with an acid catalyst followed by an alkaline catalyst. An acid catalyst is used first to treat any free fatty acids present within the low grade feedstock – direct use of an alkaline catalyst encourages soap formation. Subsequently, the alkaline catalyst is introduced to convert the triglycerides into methyl esters and glycerol.

Considerable disadvantages are associated with this process: the homogenous catalysts are corrosive and they are consumed during processing, so they are not reusable or recyclable. What initially appears to be a relatively straightforward chemical reaction is clearly a much more complicated process requiring the use of some clever catalysis.

Supercritical Fluids in Biodiesel Synthesis

The current process for biodiesel synthesis relies on homogeneous catalysis. With growing pressure for industry to adopt environmentally friendly processes, the approach to the production of biodiesel has changed somewhat, with a heavy focus on the use of “green” components such as continuous processing, supercritical fluids and heterogeneous catalysts. Supercritical fluids are highly compressed gases whose molecules collide with substances so frequently that the gas appears liquid-like. The interactions of these molecules are frequent enough that supercritical fluids can dissolve compatible liquids and solids.

Although supercritical fluids have been used in industry for decades (particularly the food industry for extracting caffeine or vitamins), only recently have they been actively applied to the area of biodiesel production. The inherently higher concentrations of acid and base achievable when using supercritical alcohols as solvents as well as reactants makes such systems an ideal processing option because no catalysts are needed. When used together with heterogeneous catalysts, the effectiveness can be increased further.

The basis of these considerations is research conducted by Ayhan Demirbas (Energy Sources 2002), which demonstrated that different types of vegetable oils could successfully be converted into their methyl esters in the presence of supercritical methanol alone. Later work carried out by Diribar Madras and colleagues (Fuel 2004) confirmed the use of supercritical methanol and supercritical ethanol as both catalyst and solvent in the transesterification of vegetable oil.

Enzymatic Catalysis

Another approach to the production of biodiesel involves the use of enzymatic catalysis. Although at present biological catalysts have not been used industrially to produce biodiesel due to the low speed of conversion, it has been suggested by Keith Harding and colleagues (A Life Cycle Comparison between Processes Using Either Inorganic or Biological Catalysis for the Production of Biodiesel 2005) that such an approach may offer distinct advantages over the traditional inorganic route in regard to product purification and energy savings. Similar findings of Edward Shay (Biomass Bioenergy 1993) and of Nick Nagle and Peter Lemke (Applied Biochemistry and Biotechnology 1990) also support the potential value of microalgae in the production of biodiesel. Although the use of high-lipid microalgae offers a novel approach to biodiesel synthesis, this alternative may not be viable for use in industry due to high costs (the current price of 1 litre of algae oil is about $23) and possible upscaling problems.

Ethanol

Ethanol is currently the most widely used biofuel, and is commonly used as an additive to or substitute for petrol. It can be blended with petrol and safely used in unmodified engines or, after some simple engine modifications (e.g. seal replacement), used as a fuel replacement.

Traditionally, ethanol is obtained from sugar cane, wheat or corn by enzymatic conversion of starch into sugars, yeast fermentation to alcohol and finally distillation. This approach often causes much debate as the feedstock for this process is also a source of food and the energetics are very poor. In light of this debate, research is currently being directed towards utilising lignocellulosic biomass derived from crops such as bagasse (sugar cane waste left after sugar cane processing) and sawdust. These waste products can be converted into ethanol or into other value-added chemicals, thus gaining maximum value out of the crop without competing with the food supply.

The cellulose present in the lignocellulosic biomass is hydrolysed to produce sugars, which are then fermented to give ethanol. At present there are no commercial operations using this approach, but the US government has set aside A$500 million to be matched by industry with another A$500 million over the next 5 years to solve this issue, indicating the potential importance of this technology.

A key problem in terms of catalysis is the efficient removal of the lignin from the celluloses without compromising the biocatalysts in subsequent steps due to the generation of bio-inhibitors.

Syngas

In the biomass gasification process, biomass is converted to synthesis gas (syngas), a mixture of gases including carbon monoxide, carbon dioxide, methane, hydrogen and nitrogen. These components can then be reacted further to give a variety of value-added chemicals including alternative fuels. Choren Industries, a Germany-based alternative fuel company, is converting syngas into a hydrocarbon-based fuel suitable for use in the automotive industry. In this process, biomass is broken down into simple gaseous components, which can then be reassembled, in the presence of a cobalt catalyst, to give long-chain hydrocarbons. These resulting hydrocarbons are known as Sundiesel, and offer yet another sustainable and biomass-based alternative transport fuel. However, the use of energy in this process is very significant and the overall energy balance is poor due to the high energy costs.

Hydrothermal Upgrading

This approach is another example of supercritical (or near-supercritical) processing using water, rather than methanol, as the solvent. The temperatures are lower than those used in the gasification/pyrolysis regime.

Although this process can produce bio-oil from biomass, the oil is unstable and still contains a significant amount of oxygen. The oxygen content makes it immiscible with some components of fossil diesel, causing blending problems. In other aspects it represents a perfect solution to biomass pre-processing due to its low degree of complexity and independence of bio-catalysts and fermentation to ethanol.

However, an Australian start-up company, Licella Pty Ltd, has just announced a world breakthrough: it has produced the first stable bio-oil from woody waste. This can be blended with crude oil to make all the common refinery products sustainable (by the same percentage as bio-oil is blended), without the need to change the existing refinery infrastructure and allowing the current chemical feedstocks to remain the same. The authors view this announcement as the beginning of a global paradigm shift for transport fuels and the petrochemical industry, since it allows the introduction of a renewable source without the costly and time-consuming rebuild of global infrastructure.

Conclusions

A multitude of approaches exist to the fuels problem, each with merits and disadvantages. Biomass is part of the answer to sustainability issues and helps to combat the problems of greenhouse gases. By adopting the 12 principles of green chemistry, one of which includes the use of catalysts, we are well on our way to producing alternative fuels for a sustainable future.