There are a number of different technologies that can be used to convert biomass into energy and other products. In an assessment of the technologies two key questions need to be asked:
What is the optimal utilisation of the biomass?
How can energy and high-value products be produced in an integrated process?
In economic terms it is difficult for energy from biomass to compete with fossil fuels. There are large variations in the challenges facing the different technologies, but on an industrial scale it is difficult to produce bioenergy at a price and quality that can match that of fossil fuels.
For several of these technologies it is also a problem that the initial form of energy produced by the process is not readily usable and needs further transformation.
The idea behind a biorefinery is therefore to combine the production of energy with the production of a number of other products. The higher the value of these products, the better the overall economy of the production. The most promising products are currently animal feed, cellulose, fertilisers and polymers for the plastics industry. In the longer term, there is a large potential in the production of in-vitro meat and other food products.
Both in Denmark and at international level there has been an extensive research investment in the development of the different techniques in recent years, mainly with the primary aim of producing energy. The following section will briefly explain the main technologies that are the focus of research in Denmark. For each technology, the possibilities for integrated production of high-value products are reviewed.
Bioethanol (alcohol) is formed by the fermentation of biomass. First-generation bioethanol is made from sugar and starch crops (sugar beet, sugar cane, cereals and maize), while second-generation bioethanol is produced from residual products such as straw, wood chips or maize stover. First-generation bioethanol has been heavily criticised for increasing food prices, while an intensive cultivation of cereal, and others, is environmentally problematic.
Denmark has launched an intensive research effort in order to further develop the technology. DONG Energy has thus built the INBICON pilot plant near Kalundborg, which primarily produces bioethanol from straw. However, there are a number of challenges linked to the production of bioethanol. There is first and foremost a significant challenge in creating a viable production. Ethanol can be added to petrol but is often an intermediate product that needs further processing before use, which makes it more expensive. It is believed that its continued development will require large investments in research and facilities.
Another focus area is the development of techniques for the fractionation of straw, so that specific parts can be extracted for further processing before and after fermentation. One of the by-products of fermentation is C5 molasses, which has a certain value as an animal feed constituent.
Biogas consists primarily of methane (CH4) formed by methanogens (methane-producing Archaebacteria) under anoxic conditions. In principle, most biodegradable organic materials can be used for biogas production. In a number of countries, including Germany, there is an extensive cultivation of crops (especially maize) destined for biogas plants, which can be problematic in environmental terms and for the total energy balance. The technique is tested and proven, and there are 22 local biogas plants and 60 on-farm plants in Denmark. According to the biogas industry it is a major problem for the production that the politically fixed price for the biogas is too low.
In Denmark, the production is primarily based on manure and organic waste supplemented with cultivated crops. This makes sense in relation to environment and climate, since the processing of manure in biogas plants reduces greenhouse gas emissions and odour problems, etc. Danish politicians have previously aired plans that all Danish animal manure (about 30 million tonnes per year) should be processed into green energy.
The biogas process itself does not present any special opportunities for refinement of high-value products. The by-products from biogas production are, however, highly suitable as a fertiliser.
Another possibility is gasification. The gas is formed by a process whereby biomass is ignited but is fed too little oxygen to actually lead to a combustion. Instead, water is initially evaporated followed by a number of volatile compounds. The composition of the gas depends on the biomass feedstock, but typically consists of hydrogen and light hydrocarbons such as methane, carbon monoxide and carbon dioxide. The gas can also contain other hydrocarbons that may need to be removed before the gas can be used as motor fuel or the production of synthetic fuels.
One option is to convert the gas into methanol (wood alcohol) in a catalytic process. Methanol is a very flexible fuel – a chemical building block that can be used for a wide variety of purposes. In the transport sector it can replace gasoline and it can be used for fuel cells. Methanol can be used to produce DME (dimethyl ether) which is a very clean diesel fuel. This method has been the subject of intensive work over a long period, but there is as yet no large-scale functioning commercial plant to show for it.
The gasification process does not offer any direct opportunities for refinement of high-value products. However, there is an interest in returning the treated material to the ground for the purpose of soil improvement and CO2 storage. There are currently only a few small such facilities in Denmark.
When organic material is subjected to high pressures and temperatures, it can be converted into bio-oil or biodiesel. The method, called hydrothermal liquefaction, has been known since 1930, but has not yet become popular on a larger scale. The Department of Chemistry at Aarhus University is one of the organisations that has been working on improving the process in recent years.
Compared to the other technologies, hydrothermal liquefaction achieves the highest energy use efficiency of the biomass, at around 85 percent. It is relatively simple to convert the bio-oil to biodiesel, which can be used directly in the transport sector and for many other purposes.
It is interesting that in a hydrothermal process the nitrogen is a "waste product". In other words, the nitrogen in the biomass can be extracted from the biomass without impairing the energy conversion.
Scientists at Aarhus University have upscaled an HTL plant which is capable of converting biomass into crude oil. Read more about it here.
The simplest and most commonly used method for conversion of biomass into energy is combustion in a CHP plant. This method is useful when it comes to wood, but is not beneficial in relation to conversion of straw and agricultural crops. Combustion is, however, an excellent opportunity to exploit residual products from other bio-refining technologies.
It is possible to produce bio-oil/biodiesel by pressing and extracting the oil-bearing plants. In addition, this method is also used in connection with various forms of waste treatment. For now, the method will not be discussed further, since it is not related to the conversion of cellulose-rich biomass.
