Transesterification of plant, animal and waste oils and fats to methyl esters
●
(biodiesel) is a first-generation biofuel.
Oil accumulated by some microalgae, extracted and transesterified into biodiesel,
●
is a third-generation biofuel.
biomass using an air-blown circulating fluidized bed operating at 900°C, but as the gas formed is not clean the system requires a catalytic reformer to remove many of the contaminants (Fig. 7.3). The gas from the fluidized bed gasifier contains H2, CO, CO2, H2O and considerable amounts of hydrocarbons such as CH4, benzene and tars (Table 7.1). The second option is entrained flow gasification where higher tempera- tures (1300°C) are used. This system requires a supply of very small particles to burn
53%
23%
11%
4%
8%
1%
Ammonia H2 refineries Methanol
Elec GTL Other
Fig. 7.2. Present industrial uses of syngas:
ammonia production, hydrogen for refineries, methanol production, electricity generation and GTL which is the conversion of gas to a liquid fuel. (From van der Drift and Boerringter, 2006.)
Fig. 7.3. The processes that can be used to prepare syngas made from biomass for the Fischer-Tropsch synthesis of fuels. (From van der Drift and Boerrigter, 2006.)
Fluidized bed gasifier
900°C
Flash pyrolysis
500°C
Slow pyrolysis
500°C
Torrefac- tion 250–300°C
Catalytic reformer
Entrained flow gasifier 1300°C Biomass
Liquid fuels
Methanol
Dimethyl ether
Hydrogen Filter
Water shift
Cleaning
Trace removal
Pre-treatment Syngas
production
Syngas conditioning
Fuels produced
correctly so that any material used has to be milled, which is energy-intensive and makes handling difficult.
No matter which method is used to produce the gas, extensive syngas cleaning and conditioning are required before the FT process can be used to produce liquid fuels as the contaminates inhibit the catalyst. The syngas also needs to have a H2/CO ratio of 2:1. The concentration of CO and H2 can be adjusted in the water shift reactor which converts CO to H2 and CO2. The reverse can also be carried out as the syngas com- position varies depending on the feedstock.
Forward (<250°C)
CO+ H2O= CO2+ H2 (7.1)
Backward (>500°C)
H2+ CO2= CO + H2O (7.2)
Fischer-Tropsch process
Figure 7.3 gives an overall view of the methods that can be used to produce biofuels from coal and biomass, and Table 7.2 gives the maximum concentration of impurities that syngas should have in order to be suitable for FT synthesis. Too high a concen- tration of impurities will poison the cobalt catalyst in the process.
The exothermic FT synthesis combines H2 and CO when passed over a cobalt catalyst at a temperature of around 260°C producing a mixture of hydrocarbons including petrol (C8—C11) with an average of C8H18 and diesel (C11—C21) with an overall hydrocarbon average of C16H34.
2H2+ CO= CH2+ H2O (7.3)
The FT synthesis unit operations are given in Fig. 7.4 when using dried biomass. The dried biomass is gasified in an entrained flow gasifier at 900–1300°C in the presence
Table 7.1. Typical gas composition produced by a fluidized bed gasifier using biomass. (Adapted from van der Drift and Boerrigter, 2006.)
Main constituents Vol % dry wt
Lower heating value (LHV%) Carbon monoxide (CO) 18 27.8
Hydrogen (H2) 16 21.1
Carbon dioxide (CO2) 16 –
Water (H2O) 13 –
Nitrogen (N2) 42 –
Methane (CH4) 5.5 24.1
Acetylene (C2H2) 0.05 0.4 Ethylene (C2H4) 1.7 12.4
Ethane (C2H6) 0.1 0.8
BTX 0.53 10.5
Tars (total) 0.12 2.8
BTX, benzene, toluene, xylenes.
Table 7.2. Maximum concentrations for impurities allowed in syngas. (Adapted from van der Drift and Boerrigter, 2006.)
Impurity Specification
H2S, COS, CS2 <1 ppmv
NH3, HCN <1 ppmv
HCL, HBr, HF <10 ppbv Alkali metals (Na, K) <10 ppbv
Soot, ash Complete removal
Organic (tar) Not condensing Hetero-organic components (S,N,O) <1 ppmv
Fig. 7.4. Outline of the Fischer-Tropsch process using biomass to produce a mixture of hydrocarbons including petrol and diesel.
Dried biomass
Gasifier
Gas cleaning
Water gas shift
Fischer- Tropsch reactor
Mixture of gaseous & liquid fuels Methane C 1, ethane C 2, LPG C 3–4, naphtha C 5–11, diesel C12–20, waxes C 30 +
CO, H2 Ratio of 1:2 CO, CO2 H2 CO, CO2 H2, H2S Steam
Oxygen
Ash
Sulfur
Steam or carbon dioxide
Carbon dioxide or water
Cobalt catalyst 260°C
900–1300°C
of steam and oxygen. In some cases, the biomass may be pretreated by pyrolysis or torrefaction (Fig. 7.3) or even taken from a fluidized bed gasifier. The ash is removed and the gas is cleaned of sulfur-containing compounds, and then the CO/H2 ratio is adjusted by the water shift reaction. The cleaned gas is then passed over a cobalt cata- lyst in the FT reactor producing a range of hydrocarbons from CH4 to waxes. The alpha factor shown in Fig. 7.5 describes the proportion of the various products formed, and this is affected by the catalyst used and process conditions. Maximum diesel production is around 30% of the total products at an alpha value of 0.85–0.9.
The lower-temperature conditions which favour diesel production are 260°C, with cobalt-based catalyst at a pressure of 15–40 bar.
The process of gasification, gas cleaning and FT synthesis is a complex chemical process where the larger the scale, the more economic the process (Fig. 7.6). As the size increases, the conversion costs reduce, levelling out at around 1800 MWth (mega- watts thermal) while the other costs remain static.
Thus, the production plant, using biomass to produce syngas and FT products, will be much larger compared to other biomass processes because of the increased efficiency
0.75 0.80 0.85 0.90 0.95
0 20 40 60 80 100
Weightfraction(%)
Probability of chain growth alpha
Heavy wax C > 30 Light wax C 21–30 Diesel C 12–20
Naphtha C 5–11 LPG C 3–4 Ethane C 2
Methane C 1
Fig. 7.5. The effect on the products formed in the Fischer-Tropsch process of the alpha factor, the probability of chain growth.
of the FT process at the larger scales. The fossil fuel-based FT plants are huge, above 1000 MWth. With biomass, there may be a problem in supplying such a large process without extensive transport of biomass from distant sources. This may mean that any biomass-based plant is likely to be smaller at 100 MWth. However, there are ways to treat biomass to reduce its volume, so that it can be transported easily to the large central FT plant. The first is torrefaction, where biomass is heated at 250–300°C, which turns it into a brittle, solid mass that can be treated like coal. The second option is pyrolysis at 500°C that converts the biomass into oil/char slurry (Fig. 7.3).
At present syngas is mainly used by the chemical industry (Fig. 7.2), but some 8%
(500 PJ per year) is used to produce fuels called GTL. FT processes are operated by Sasol in South Africa, and Shell in Bintulu, Malaysia. These are large plants of 1000 MWth due to the economies of scale and in one case use natural gas (CH4).
To supply the EU-25, ten large plants of 1000 MWth would be required. At present, small- to medium-scale gasification systems of biomass are used for distrib- uted heat and power (CHP) production. The larger scale of the GTL production also allows for the possibility for CO2 capture and storage.