Dimethyl ether (DME)
A number of studies have been carried out on the emissions from a compression igni- tion engine (diesel) running on DME and DME blends. DME has been shown to pro- duce low noise, smoke-free combustion and reduced NOx when used in an internal combustion engine (Huang et al., 2006). DME, because of its high cetane number and low boiling point, has been used at 100% or as an oxygenated addition to diesel. When DME was used in a diesel engine, it reduced NOx and SOx emissions and was sootless (Semelsberger et al., 2006). Large motor manufacturers are developing truck and bus transport fuelled by DME. The emission levels from these development vehicles when run on DME show virtually no PM and low levels (0.5–2.0 g/kWh) of NOx.
Fig. 8.15. The combustion of the biomass produces 20 times less carbon dioxide than coal. When short rotation coppice (SRC) and Miscanthus are used to generate elec- tricity more carbon dioxide is formed per unit of energy than simple combustion.
Biomass can also be gasified and the gas used as fuel for gas turbines to produce electricity. Combustion and gasification as a source of energy have been compared using three biomass sources in terms of the amounts of carbon dioxide saved (Fig. 8.16) (Lettens et al., 2003). The perennial grass Miscanthus sp. gives the least carbon dioxide and mixed coppice the greatest, and there appears to be little signifi- cant difference between combustion and gasification.
Ethanol
Of all the first-generation biofuels, ethanol requires the most processing. As a con- sequence large quantities of carbon dioxide are produced and energy used during harvesting and preparation. For this reason bioethanol has been of most concern when the first-generation fuels have been evaluated in terms of energy use and car- bon dioxide emission. The substrate used to produce ethanol has a considerable
0 20 40 60 80 100 120 140
gCO2/MJ Coal Coke WillowSRC Miscanthus Switchgrass Reedcanary grass SRCelec Miscanthuselec
Fig. 8.15. Carbon dioxide emissions from bioenergy crops when burnt or used to generate electricity. SRC, short rotation coppice. (From Gustavsson et al., 1995;
Dubisson and Sintzoff, 1998; Matthews, 2001; Bullard and Elsayed et al., 2001;
Helleret al., 2001; Keoleian and Volk, 2005.)
0 20 40 60 80 100 120 140
gCO2/MJ
Miscanthus Willow Mixed coppice Gas Comb
Fig. 8.16. Greenhouse gases saved when biomass is either gasified or combusted in g CO2 equivalents/MJ. (From Lettens et al., 2003.)
influence on the energy input and the carbon dioxide released during its production.
The carbon dioxide released per megajoule of energy for a number of substrates is shown in Fig. 8.17. Compared with petrol, ethanol from all substrates produces less carbon dioxide with the greatest reduction when produced from cellulose and pro- duction from wheat showed the least reduction in carbon dioxide.
Biodiesel
The carbon dioxide produced during the synthesis of diesel and biodiesel combined with carbon dioxide produced when the biodiesel is burnt is given in Fig. 8.18. Diesel produces around 80 g CO2/MJ compared with 43.7 g CO2/MJ for rapeseed biodiesel which is a reduction of 45%. In the case of diesel produced by the FT process by the
0 10 20 30 40 50 60 70 80
gCO2/MJ Petrol Maize Wheat Sugarbeet Sugarcane Cellulose
Fig. 8.17. Mean values for carbon dioxide emissions (g CO2/MJ) from bioethanol production using a variety of substrates.
(From Elsayed et al., 2003; Gielen and Unander, 2005; Kim and Dale, 2005; Farell et al., 2006; Hill et al., 2006.)
0 50 100 150 200 250
gCO2/MJ
Diesel RME FT gas FT coal FT
biomass
Fig. 8.18. Carbon dioxide emissions when diesel is produced from different substrates.
RME, rapeseed methyl ester; FT gas, FT coal and FT biomass: FT diesel produced directly from natural gas and by the gasification of coal and biomass, respectively. (From Mortimer et al., 2003; Gielen and Unander, 2005; IEA, 2005a.)
gasification of coal, natural gas and biomass, the carbon dioxide produced varies considerably. Both natural gas and coal FT diesel produce more carbon dioxide than diesel, 98 g CO2/MJ and 233 g CO2/MJ, respectively. The use of biomass in the syn- thesis of FT diesel yields only 5 g CO2/MJ which represents a 94% reduction in car- bon dioxide compared with mineral diesel.
The carbon dioxide fixed during growth and its distribution in products during the production of biodiesel from rapeseed is shown in Fig. 8.19. The rapeseed plant
Fig. 8.19. The distribution of carbon dioxide in the production of biodiesel from rapeseed.
(From Peterson and Hustrulid, 1998; Mortimer et al., 2003.) Plant
Rapeseed + atmospheric carbon dioxide
15,736 kg CO2/ha
13,336 kg CO2/ha (7850 kg/ha, 16 MJ/kg) Soil
Straw 2400 kg CO2/ha
8514 kg CO2/ha (5604 kg/ha, 36.4 MJ/kg) Seed
Meal 4800 kg CO2/ha
(2240 kg/ha)
Degrades in soil
2389 kg CO2/ha (1400 kg/ha, 16 MJ/kg) Oil
2451 kg CO2/ha (840 kg/ha, 37.7 MJ/kg)
Biodiesel 2227 kg CO2/ha
(778 kg/ha)
Combustion 2.85 kg CO2/kg biodiesel
2227 kg CO2
Glycerol 111 kg CO2/ha
(78 kg/ha)
fixes a total of 15,736 kg CO2/ha, where 13,336 kg was retained in the plant and the remaining 2400 kg stayed in the soil sequestered by the soil microorganisms. The yield of the seed was 2240 kg/ha, 28.5% of the total plant material, containing 4800 kg carbon dioxide/ha. The rest of the plant, the straw, contains 8514 kg CO2/ha, 63.8% of the carbon dioxide fixed, which is often ploughed into the soil.
The oil extracted from the seed contains 840 kg/ha which is an oil yield of 37.5%
leaving the meal or cake containing 2389 kg CO2/ha to be used as feed or fuel. On transesterification the oil is converted into 840 kg biodiesel and 78 kg glycerol. The biodiesel represents some 9.2% of the whole plant and yields 2227 kg CO2/ha when used as a fuel. Figure 8.19 also includes the energy content of the biodiesel and the co-products where it can be seen that the meal and straw contain more energy than the biodiesel. For these reasons, it may be more efficient to convert the whole plant or biomass into a biofuel rather than just the oil extracted from the seed. This should be the nature of the second-generation biofuels.
The unit operations used in the production of biodiesel have been evaluated for their carbon dioxide and GHG production and energy input (Fig. 8.20a,b,c). It is clear that the major carbon dioxide-producing stages were esterification, use of nitro- gen fertilizer and to a lesser extent solvent extraction of the oil. A second process has been included where solvent extraction has been replaced by cold pressing and low nitrogen growing conditions. This results in more than 50% reduction in energy input, carbon dioxide and GHG emissions. Thus, the processes of biofuel production can be made more environmentally suitable by modifications to the key stages of the process.
Gaseous and second-generation biofuels
The gaseous fuels DME and hydrogen can be produced using a number of routes including those from biological materials. The carbon dioxide produced per unit of energy (g CO2/MJ) is shown in Fig. 8.21 for hydrogen and DME. The production of DME from syngas, hydrogen from electrolysis, natural gas and coal is compared with petrol, CNG, LPG, coal and gas in terms of carbon dioxide produced per unit of energy. The amount of carbon dioxide produced by DME is similar to petrol and LPG. Hydrogen production by all three routes produces more carbon dioxide than petrol especially when coal is used. This indicates one of the problems of producing hydrogen from fossil fuels.
In a study by the Joint Research Centre EU, the cost of reducing carbon dioxide emissions for a number of biofuels was calculated and some of the data is shown in Fig. 8.22. The GHGs avoided in a life-cycle analysis, well-to-wheel (WTW), are related to the cost (€/t) of carbon dioxide equivalents avoided. A life-cycle analysis systemati- cally identifies and evaluates opportunities for minimizing the overall environmental consequences of using resources and releases into the environment. In terms of GHGs avoided, biodiesel is slightly less expensive than ethanol whether produced from either sugarbeet or wheat. The fuels DME, ethanol and FT diesel produced from wood bio- mass avoid the most GHGs because of using a sustainable source and are inexpensive at around €100–500/t CO2 avoided. Unfortunately the production of the biofuels from wood has yet to be commercialized. Electrolysis of water to produce hydrogen using sustainable electricity from nuclear and wind power avoids large quantities of GHGs.
0 1000 2000 3000 4000 5000 6000
MJ/t Esterif N-fert Solvent
Cultivation
Drying
Transport Refining
Distribution
Storage
Construction Normal
Low nitrogen
0 50 100
(a) (b)
150 200 250 300 350 400
kgCO2/t Esterification N-fert Solventextr Drying Transport Refining Distribution Storage Construction
Normal Low nitrogen
0 100 200 300 400 500 600 700 800
kgGHG/t Esterification N-fert Solventextr Cultivation Drying Transport Refining Distribution Storage Construction
(c)
Normal Low nitrogen
Fig. 8.20. (a) The energy input (MJ/t) for the processes used to produce biodiesel from rapeseed under normal and low nitrogen cultivation. (b) The carbon dioxide (kg/t) produced from the various stages of biodiesel production from rapeseed under normal and low nitrogen conditions. (c) Greenhouse gas (kg/t) for biodiesel production from rapeseed using a conventional and low nitrogen process. (Redrawn from Mortimer et al., 2003.)
CNG, hydrogen from natural gas and ethanol from wheat only avoid moderate amounts of GHGs. Hydrogen for use in fuel cells produced by on-board reforming of petrol is the most expensive option.