Fossil-based fuels, in particular oil, dominate the transport sector. Alternatives to fossil-based fuels will become necessary as the number of vehicles increases, especially in countries with rapidly growing economies such as India and China. Furthermore, the world faces problems with greenhouse gases and diminishing oil resources. The use of biofuels, such as bioethanol and biogas, can decrease the production of greenhouse gases and reduce dependence on oil.
Ethanol can be produced from sugar, starch or lignocellulosic materials. Ethanol is currently mainly produced from sugar (sugar cane) or from starch (corn or wheat). Ethanol from lignocellulosic materials is only produced in pilot plants and demo plants. Using lignocellulosic materials, such as wood and agricultural residues, has the advantage over using sugar and starch that it minimises the conflict between using land for food production or for energy feedstock production . Corn stover is a low-cost agricultural residue that is available in large quantities. Corn stover and other lignocellulosic biomasses consist of three main components: cellulose, hemicellulose and lignin. Ethanol can be produced from lignocellulose, by fermenting monomeric sugars, liberated from the cellulose and hemicellulose. Enzymatic hydrolysis is one method that can be used to convert cellulose and hemicellulose to monomeric sugars. The conversion is, however, very slow, since the cellulose is surrounded by hemicellulose and lignin, and some type of pretreatment is required . Many different types of pretreatment method are used. These include pretreatment with dilute acid, steam pretreatment, wet oxidation, ammonia fibre explosion and alkaline pretreatment .
Steam pretreatment, also known as “steam explosion”, has been extensively investigated and tested in several pilot plants and demo plants worldwide . An additional acid catalyst can be used to increase the effectiveness of the steam pretreatment, in which case hemicellulose recovery and the enzymatic hydrolysis of the solids both increase . Sulphuric acid and sulphur dioxide are commonly used as acid catalysts. The pretreatment of corn stover using steam explosion with no catalyst [5, 6], sulphuric acid [6, 7] or sulphur dioxide [8, 9] has been studied using different concentrations of the catalyst and different residence times and temperatures. However, sulphuric compounds such as sulphuric acid and sulphur dioxide must be handled in the downstream processing, and a process without sulphur is therefore preferred, if it can give similar yields and process economics as a sulphur-containing process.
The yeast strain Saccharomyces cerevisiae is well suited for the fermentation of pretreated and hydrolysed lignocellulosic material. Naturally occurring strains ferment glucose and mannose, but not pentoses such as xylose and arabinose. Corn stover consists of large amounts of xylose in addition to glucose, and a process that can ferment pentose sugars is essential. Several alternatives have been investigated; the use of genetically modified microorganisms to ferment pentose to ethanol [10, 11], production of hydrogen [12, 13] or biogas [12, 14–16]. Biogas production through the anaerobic digestion (AD) of activated sludge is commonly used. The biogas can be used to produce heat or electricity, or it can be upgraded to transportation fuel . Microorganisms degrade organic material to biogas during AD. Almost all organic material can be biodegraded: one exception is complicated material such as lignin . Some other organic materials can be hard to degrade due to the toxic or inhibitory effects of products, resulting from previous process steps, on the organisms from, for example, phenols and some types of long-chain fatty acid . Sulphide, which is produced when sulphate is reduced, can also inhibit biogas production. The main cause of inhibition is competition between sulphate-reducing bacteria and other microorganisms, in particular methane-producing organisms, for substrates. Sulphide itself is also toxic to many organisms . The level of sulphides that causes inhibition has been reported to lie in the range 100–800 mg/l dissolved sulphide, and 50–400 mg/l undissociated hydrogen sulphide , which makes it difficult to predict the effect of pretreatment with dilute sulphuric acid or sulphur dioxide. Thus, a process that does not require sulphurous compounds is preferred, both due to the possible inhibitory effect of sulphurous compounds and due to the need to handle sulphur in the downstream processing.
The aim of the work presented here was to investigate the influence on ethanol and biogas production of steam pretreatment with or without sulphuric acid. The time, temperature and catalyst concentration during pretreatment were varied and the sugar yield determined in each case. The ethanol production by simultaneous saccharification and fermentation (SSF) and biogas production by anaerobic digestion (AD) were then studied for material that had undergone pretreatment in the conditions, both with and without acid, that gave the highest glucose yields.