The application of combined heat and power (CHP) run on biomass for district heat­ing has been applied widely in Scandinavia and Austria. CHP has the advantages of higher efficiency of electricity generation and lower costs, and these systems have also encouraged the development of a biomass market. The burning of straw for CHP has seen the development of more complex boilers and pre-treatment of the straw. An example of a system using biomass to produce electricity is shown in Fig. 4.6. Short rotation coppice or perennial grasses are chipped or made into pellets and dried using some of the waste heat from the turbine before use. The flue gas can be treated before release and the only waste that needs disposal is the slag from the boiler.

Co-firing

The addition of biomass to coal for use in power stations occurs in the UK and many European countries. The advantages are that overall emissions of GHGs are reduced by incorporating biomass, the efficiencies are high due to the large scale, and invest­ment costs are low making this an attractive GHG mitigation option. Co-firing with coal is used for the generation of electricity and has been shown to reduce emissions of sulfur dioxide and NOx. Normally the co-firing proportion is around 10% which changes the processes very little. The biomass that has been used includes straw, short rotation coppice (SRC), sawdust and other wastes such as sewage sludge, manure and municipal solid waste (Sami et al., 2001).

The grinding of lignocellulose materials into small particles increases the surface area and allows subsequent enzyme or acid treatment to hydrolyse the cellulose. The pro­cess requires a considerable energy input and is not as effective as other treatments.

Ammonia explosion

The lignocellulose is milled and the ground lignocellulose, with a moisture content of 15-30%, is placed in a pressure vessel with ammonia (1-2 kg/kg biomass) at pressures of 12 atmospheres for 30 min. No sugars are released but the hemicellulose and cel­lulose are opened up to enzymatic digestion.

Acid treatment

Acid treatment can use sulfuric, hydrochloric, nitric and phosphoric acids although sulfuric is used most widely. Acid treatment converts the hemicellulose to sugar (xylose) (80-95%) and furfural, and increases cellulose digestibility. The treatment time is short (minutes), and depending on the substrate between 80 and 95% of the hemicellulose sugar can be recovered.

Biogas, hydrogen and DME have been proposed as gaseous replacements for fossil fuels used in transport and electricity generation.

Gaseous fuels have problems of storage and supply not encountered with either solid or liquid fuels. Storage of gas at atmospheric pressure is not practical, and so the gas has to be either compressed at high pressure or liquefied at low temperatures to reduce its volume. Compression to pressures of 200 bar and liquefaction, which for hydrogen requires a temperature of -253°C, expends a considerable amount of energy, and subsequent storage has to be in strong pressure vessels or in well-insulated tanks. The lower energy density of the gaseous fuels compared with liquid fuels means that larger fuel tanks are required in vehicles. There is a small number of modified internal combustion engines using gases derived from fossil fuels such as liquid natural gas (LNG), liquid petroleum gas (LPG) and compressed natural gas (CNG). After treat­ment biogas is the same as natural gas, and therefore could be used as a replacement for liquid and CNG. Biogas has been used as a fuel for boilers, dual fuel engines and the generation of electricity. DME has a boiling point of -24.9°C and so can be lique­fied and stored easily. DME has lower energy content than diesel, 28.6 MJ/kg com­pared to 38-45 MJ/kg, but it has a higher cetane value of 55-60 compared with diesel at 40-55. DME could be used as a replacement for diesel but its properties are similar to propane and butane so that it could also replace these fuels for distributed power generation, heating and cooking. Hydrogen is a high energy clean fuel producing no carbon dioxide on combustion and has been used as a fuel for internal combustion engines and fuel cells. There are problems with the sustainable production of hydro­gen, its storage and distribution which may require a completely new infrastructure. The number of alternative-fuelled vehicles in the USA from 1993 is shown in Fig. 5.10. The number of CNG and liquefied natural gas vehicles has remained static, whereas the numbers of liquefied petroleum gas vehicles have declined. Thus, there must be some doubt about the introduction of gaseous biofuels as transport fuels.

The use of microalgae to sequester carbon dioxide was proposed in the past by a number of authors (Benemann, 1997; Sheehan et al., 1998; Chisti, 2007; Skjanes et al., 2007).

Microalgae have been proposed as systems for the sequestration of carbon diox­ide (Sawayama et al., 1995; Zeiler et al., 1995; Ono and Cuello, 2006; Cheng et al., 2006; de Morais and Costa, 2007a, b) and the production of biofuels (Chisti, 2007). The biofuels include biogas (methane) by anaerobic digestion of the biomass (Spolaore et al., 2006), biodiesel from microalgal oils (Nagle and Lemke, 1990; Minowa et al., 1995; Sawayama et al., 1995; Miao and Wu, 2006; Xu et al., 2006; Chisti, 2007), and biohydrogen (Fedorov et al., 2005), and the direct use of algae in emulsion fuels (Scragg et al., 2003).

Microalgae should be considered to have the following features:

• Higher photosynthetic efficiency than terrestrial plants.

• Rapid growth rate, doubling times of 8-24 h.

• High lipid content 20-70%.

• Direct capture of carbon dioxide, 100 t algae fix ~183 t carbon dioxide.

• Can be grown on a large scale.

• Will not compete with terrestrial plants in food production.

• Produce valuable products.

• Freshwater and marine species.

• Have a much better yield of oil per hectare: oil palm 5000 t/ha, algae 58,700 t/ha (Chisti, 2007).

Figure 3.12 shows a possible system for carbon dioxide sequestration and biofuel production using carbon dioxide from a stationary carbon dioxide source such as a power station. Microalgae can fix large quantities of carbon dioxide but it is likely that only a proportion of the carbon dioxide in the flue gases will be removed. Also the flue gases from power stations contain other gases which may affect the growth of microalgae. A number of studies have been carried out on the effect of flue gases on microalgae. Nannochloris sp. was shown to grow in the presence of 100 ppm

Fig. 3.12. Possible sequestration of carbon dioxide from a power station and use of Biodiesel algal biomass to produce biodiesel.

nitric oxide (NO) (Yoshihara et al., 1996). Tetraselmis sp. could grow in flue gas containing 185 ppm sulfur oxides, 125 ppm nitrogen oxides and 14.1% carbon dioxide (Matsumoto et al., 1995). A Chlorella sp. was also found to grow in the pres­ence of various combinations of sulfur and nitrogen oxides (Maeda et al., 1995).

The biological production of hydrogen has been known since the early 1900s and the enzymes involved were discovered in the 1930s. Hydrogen production has been found in many prokaryotes, green microalgae, and a few eukaryotes as shown in Table 5.5 (Das and Veziroglu, 2001; Happe et al., 2002).

The production of hydrogen is due to the presence of two enzymes either nitro — genase or hydrogenase in the organism. Nitrogenase has the ability to use ATP and electrons to reduce substrates including protons to hydrogen gas and has been found in photoheterotrophic bacteria such as Rhodobacter sp.:

Hydrogenases have been found in a large number of green microalgae such as Chlamy- domonas reinhardtii and Chlorococcum littorale, anaerobic bacteria such as Clost­ridium sp. and Cyanobacteria sp. Hydrogenases can be either uptake or reversible hydrogenases and can be divided into three classes based on their metal composition. These classes are Ni/Fe, Fe and metal-free where the Fe hydrogenase has a unique active centre giving the enzyme a 100-fold higher activity:

hydrogenase

H2 « 2H+ + 2e

These hydrogenases and nitrogenase are responsible for hydrogen production by a number of microorganisms.

Fast pyrolysis of biomass in the absence of air at atmospheric pressure and 450-550°C will produce a mixture of gas, liquid and char (Fig. 4.7). The liquid is known as bio­oil, pyrolysis oil or bio-crude with yields as high as 80% depending on conditions. Bio-oil has a low calorific value at 16-18 MJ/kg but has the advantage of being a renewable fuel with low levels of sulfur and low net emissions of CO2 (section ‘Pyrolysis’, Chapter 4). However, bio-oil is acidic, has a high viscosity, and is thermally unstable, and therefore requires processing before it can be used as a fuel. As a conse­quence bio-oil is regarded as a second-generation biofuel as it is not produced com­mercially at present. Bio-oil properties are compared with heavy fuel oil and diesel in Table 7.3, where the differences in viscosity and energy content are clear.

Bio-oil is a complex mixture containing some 300 compounds including acids, alcohols, aldehydes, esters, ketones, sugars, phenols, guaiacols, syringols, furans and lignin-derived compounds. Most of the compounds identified are phenols with alde­hydes and ketones attached, which gives a high oxygen and highly hydrated content. The oxygen content needs to be reduced before the bio-oil can be used and the fol­lowing methods have been used.

Another possible source of biodiesel are the microalgae, which will not grow on agri­cultural land. Microalgae are more photosynthetically efficient than land plants and the consensus for microalgal production and carbon dioxide fixation are 1.7 g/l/day and 25.7 g CO2/m2/day, respectively. For example a 10,000 l microalgal bioreactor

193 I

with a depth of 0.15 m would cover an area of 6670 m2 or 0.67 ha. The bioreactor would yield 51,000 kg biomass within a 300-day year. If the oil yield from the micro­algae was 30%, this would yield 15,300 kg oil per year which is equivalent to 22,930 kg oil per hectare or 22.93 t/ha which is considerably better than rapeseed at 1 t/ha. In the data put forward by Chisti (2007), the yield of oil was 30,000 kg from

100.0 kg of biomass from an area of 7828 m2. This is a yield of 38.32 t/ha which is somewhat higher than the figures used above. Therefore, to replace 100% diesel,

23.989.0 t, using the lower yield figure would require 1,046,184 ha, which repre­sents 5.8% of the agricultural land (Fig. 8.26).

There has been some discussion about the provision of biofuels from microalgae. Chisti (2007) has proposed that biodiesel from microalgae is considerably better than the use of terrestrial crops. In contrast Reijnders (2008) has taken three studies on the life cycle of microalgal fuels and has concluded that the net energy yield of oil palm and sugarcane was better than microalgae (Table 8.5). The table contains raw energy data and net energy yields where the net energy gains for sugarcane and oil palm are better than microalgae, but in terms of raw energy they are very similar.

However, the study does not take into consideration the environmental effects of increasing both sugarcane and oil palm at the expense of rainforest, whereas micro­algae can be grown on non-agricultural land, in marine environments and in temper­ate climates. In a study of oil extraction, sunflower seeds were compared with oil extraction from microalgae (Bastianoni et al., 2007). The results show that sunflower oil extraction was more energy-efficient, but if extraction from microalgae can be improved it should be considered as a fuel source.

This chapter deals with the potential, the use and the future of solid biofuels.

Biological material in the form of wood, specific crops, crop residues and organic wastes fulfil both renewable and sustainable criteria as forests and crops can be replanted and, provided these are renewed, they are sustainable. These materials are often referred to as biomass which has been defined as the yield of organic matter that may be used as a source of energy and/or chemicals. Since biomass is largely of plant origin, it can be more correctly referred to as phytomass, although biomass is the widely accepted term. When biomass is burnt it can be regarded as carbon dioxide neutral as any carbon dioxide released during their combustion has previously been fixed from the atmosphere during photosynthesis. However, in the cultivation, har­vesting, preparation, transportation and processing of biomass, fossil fuels may be required, thus making them less than 100% carbon neutral. The world’s total annual energy use has been estimated to be 425 EJ and estimates of the contribution that biomass could make vary from 7.3 to 15.0% of this total (Boyle, 1996; Venturi and Venturi, 2003; Parikka, 2004; IEA, 2005a; Faaij, 2006) (Table 4.1).

Parikka (2004) estimates that the global use of woodfuel and firewood was 3271 Gm3/year (39.7 EJ) of which 55% is used a fuel (21.8 EJ) and 45% as round — wood (17.9 EJ). As can be seen the less developed countries obtain a higher propor­tion of their energy from biomass (16.4-27.2%), as do Sweden and Denmark where efforts have been made to exploit their forest resources.

Petrol engines will run on ethanol as the properties of ethanol are similar to petrol in many aspects (Table 6.3). Butanol has been included in the table as it is an alternative to ethanol as a petrol replacement with an energy content similar to petrol. The higher heat of vaporization of ethanol means that as the fuel is vaporized in the carburettor, the mixture is cooled to a lower temperature than for petrol. This means that more fuel enters the engine, in part compensating for the lower energy content, but the fuel inlet may need heating. The octane rating is a measure of the resistance of the fuel to pre-ignite when compressed in the cylinder of the engine. A low octane fuel will pre­ignite causing a condition known as ‘pinking’ and this will result in a loss of power. Ethanol has a higher octane number and higher oxygen content than petrol. The heat of combustion (or gross energy) is lower than petrol, which leads to some reduction in performance and a 15-25% increase in fuel consumption.

Ethanol has the disadvantage that it mixes with water and this type of mixture will corrode steel tanks. To avoid separation of an aqueous layer in cold weather the ethanol needs to be anhydrous as ethanol normally contains 4.5% water. At 95.6% ethanol the liquid and vapour have the same concentration, known as an ‘azeotrope’, so no further concentration is possible by simple distillation.

Biodiesel is defined as a mixture of fatty acid esters, normally methyl esters, produced from plant oils, animal fats and waste cooking oils. The mixture of fatty acid esters (biodiesel) has properties very similar to those of diesel and can be used with very little modification in a diesel engine. A comparison between conventional diesel, plant oils and biodiesel is shown in Table 7.13.

As can be seen, the main differences between diesel and the widely used plant oil from rapeseed are higher viscosity and higher flash point. The elevated flash point is an advantage as it makes the oil safer, but the higher viscosity makes the long-term use of the oil in engine difficult, especially in cold weather. However, by forming a fatty acid ester mixture, the viscosity is considerably reduced to a level where the biodiesel can be used in diesel engines without modification. The European standard for 100% biodiesel is EN 14214 and the values are given in Table 7.13.