An introduction to different methods of Biofuel Production. Biomass is an alternative renewable feedstock for the production of transportation fuels to the current feedstock which is fossil fuels. Biomass encompasses all materials synthesised by biological processes. It is currently the only renewable feedstock that exists.1 Fossil fuels are the current feedstocks for the majority of fuels. Fossil fuels, usually referring to coal, petroleum, and natural gas, are not renewable feedstocks because they take millions of years to form. Current reserves are being used at a much faster rate than new reserves are being made. The burning of fossil fuels for production of heat releases large amounts of CO2. The increase in this greenhouse gas has led to global warming and therefore there are sustainability issues with regard to the burning of fossil fuels. In light of human impact on the environment, new technologies that can help to reverse our affects are being explored, initially with the generation of renewable feedstocks for fuels. There is an important reason why oil has become the world's most important source of energy. The combustion of organic fuels generates a large amount of energy. This added to its high energy density, ease to transport, and relative abundance makes oil very convenient as a transportation fuel, for many kinds of vehicles.2 Nature has evolved to be very efficient at fixing carbon dioxide from the atmosphere and then synthesising sugar molecules via photosynthesis. In the long-term a synthetic mechanism that could mimic nature could supply us with all the energy we needed. In the short-term however such a mechanism does not exist and so at present we really on biomass as the alternative to our dependence on fossil fuels.
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Industrial biomass can be grown from many types of plants including; miscanthus, switchgrass, hemp, corn, poplar, willow, sorghum and sugarcane. It can also be grown from a variety of tree species, ranging from eucalyptus to oil palm. The particular plant or tree used is not important for determining the end products however it does affect the processability of the raw materials.8 Plant derived oils have the potential to substitute currently used petrochemicals, since a variety of chemical intermediates can be derived from these resources by straightforward reactions. Terpenes are found in many essential oils and represent a sustainable chemical feedstock.10
The main component of biomass is lignocellulose, which comprises of cellulose, hemicellulose and lignin. Lignocellulose is a second generation biofuel because it is non-food biomass. Biotechnology is interested in manipulating existing plants to make them more salt-tolerable so they are able to grow in soils, usually uninhabitable, so as not to be competing with plants that produce food.14 The processes required to synthesis hydrocarbon fuels from biomass include reducing the substantial oxygen content of the feedstocks, to improve energy density, and creating C-C bonds between intermediates to increase the molecular weight of the final hydrocarbon product. The presence of lignin in biomass contributes significantly to its recalcitrance and consequently increases the costs. Researchers are contemplating on new strategies to design the synthesis of lignin polymers so as to be more easily processed for biofuel production. Advances in analysis techniques and an increasing number of sequenced plant genomes are enabling researchers to engineer lignin and develop this optimal feedstocks.17
Bioethanol is mainly produced in Brazil and USA by means of the fermentation of sugar grains or starch. Corn and sugarcane are examples of first generation feedstocks. They will eventually be phased out for their use as a feedstock for bioethanol production. This is because they are food crops and as the world changes towards alternatives to fossil fuels there is a danger that farmers, especially in developing countries, will grow crops for production of biofuels instead of food because it's more valuable. This could create a food shortage and increase the price of foods.21 Corn stover and wheat straw are examples of inedible feedstocks that can be used for bioethanol productions. Renewable biofuels are produced via thermochemical and biochemical conversion technologies such as liquefaction, pyrolysis, gasification, hydrolysis and fermentation. Bioethanol can also be derived from biomass feedstocks such as wood and fruits wastes. The use of waste material for the production of energy also helps to clean the environment from excessive waste accumulation.
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Biodiesel is produced by the transesterification reactions of triglycerides with an alcohol (usually methanol). The products produced are fatty acid methyl esters (FAMEs), which make up biodiesel, and glycerol as a by-product of the reaction.
The sustainable feedstocks used for this process are generally plant oils such as rapeseed, soybean, palm and sunflower oil. Other low cost feedstocks include animal fats and waste cooking oils (WCO). A promising feedstock is the harvesting of lipids from algae to synthesis long hydrocarbon chains. Currently though the technology is limited and the production costs are not economical. The use of low quality triglyceride containing raw materials, which feature high water and free fatty acids (FFAs) content, strongly affects the behaviour of conventional homogeneous base catalysts. NaOCH3 is currently the best and most expensive commercially used homogeneous catalyst for transesterification. An appropriate solid acid catalyst which could simultaneously carry out esterification of FFAs and transesterification of triglycerides is preferred for biodiesel production. Additionally, a heterogeneous acid catalyst could be incorporated into a packed bed continuous flow reactor, simplifying product separation and purification and reducing waste generation.
Biodiesel is mainly used as a transportation fuel to power diesel engine vehicles. Most diesel engines don't need to be converted and so can run using pure biodiesel or a blend with petroleum diesel. It has many advantages over conventional petroleum diesel, such as, natural lubricant, lower toxicity level, derived from renewable feedstocks, biodegradable and produces lower exhaust emissions.24 High purity glycerol has a number of various applications. It is extensively used in the food and pharmaceutical industry. Glycerol is also an important platform chemical and used as an intermediate in the chemical production industry. With the development of new catalysts glycerol could also be converted into biodiesel.
A significant portion of biomass sources like straw and wood are poorly degradable and cannot be converted to biofuels by microorganisms. The gasification of this waste material to produce syngas could offer a solution to this problem.31 Hydrogen production is very important in biorefining and strategies for managing its consumption are needed to ensure eventual independence from fossil fuels. Currently hydrogen production relies on processes involving fossil fuels however hydrogen can be produced from biomass. Hydrogen is needed in many organic chemical reactions that involve hydrogenation. This reaction is important in making long, energy dense hydrocarbons. Without a renewable supply of hydrogen the production of a wide range of liquid fuels and chemicals from biomass would be more limited.7 Hydrogen can be produced by the water-gas shift reaction. This industrial process involves reacting CO with H2O to produce CO2 and H2. This reaction is completed in two series of reactors. The first is a high temperature reactor at 350 - 500°C with a Fe-oxide-based catalyst which reduces the concentration of CO to about 2-3%. The second reactor is at a lower temperature of 200°C with a Cu-based catalyst which further reduces the CO concentration to approximately 0.2%.
Biomass can be used as a feedstock in a biorefinery to manufacture a wide range of products. The biorefinery concept is the production of platform chemicals from low-cost sustainable raw materials using green processing strategies. A biorefinery is a facility that uses equipment and processes to convert biomass into fuels, power, heat and chemicals. It is analogous to today's petroleum refineries, which produce products such as fuels and petrochemicals.3,4,5 The biorefinery takes advantage of the many compounds found in biomass to maximise the value of the feedstock. From biomass a biorefinery can produce low volume but high value products, such as pharmaceuticals, and it can produce low value but high volume products, such as biofuels. The process also generates electricity and heat which is used primarily to power the biorefinery but as efficiency levels increase perhaps enough electricity can be produced and sold to the national grid. The biorefinery business should become a successful business as the high value products will increase profit levels, the high volume products help meet increasing energy demands, and the electricity generation lowers energy costs and reduces greenhouse gas emissions. Biorefineries are likely to replace our dependence on petroleum and play the major role in producing chemicals and materials used in industrial processes.6,7
Section 2: Heterogeneous catalysts for the production of Biodiesel
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The use of heterogeneous catalysts for the production of biodiesel would result in much simpler downstream processing saving on the amount of water used and the processing costs as well as reducing the generation of waste. The main drawbacks are the high temperature and pressure, as well as the higher methanol to oil ratio needed for the transesterification reaction as compared with a homogeneous system. Conventional processes include the use of homogeneous alkaline catalysts, such as NaOCH3, under mild temperatures (60-80°C) and atmospheric pressure. FFA concentration in the oil feedstock is usually controlled below 0.5% wt, which avoids the formation of high soap concentrations as a result from the reaction between FFAs and the basic catalyst. This problem is usually overcome by a previous esterification step where FFAs are esterified with short-chain alcohols to FAMEs using a homogeneous acid catalyst. In this way, the FFA content is also converted into biodiesel without waste generation. Brønsted acids can catalyse the esterification of free fatty acids because of protonation of the acid group to give an oxonium ion which is readily attacked by an alcohol through an exchange reaction to give the corresponding ester after losing a proton. Once the acid catalyst has been removed, the transesterification of triglycerides is performed using an alkaline catalyst. Homogeneous acid catalysts, such as H2SO4, have been proposed to promote simultaneous esterification of FFAs and transesterification of triglycerides in a single catalytic step, thus simplifying the process when using feedstocks with high FFA content. However, these catalysts are less active for transesterification than alkaline catalysts and require higher pressures, temperatures, methanol to oil ratio and catalyst concentration to produce adequate reaction rates and product yields.
The Industrial Production of Biodiesel
The first industrial plant that used a heterogeneous basic catalyst to produce biodiesel was built in 2006 in the south of France. The plant uses the Esterfip-H process, which was developed by the Institute Francais de Petrole (IFP) and commercialised by Axens, to produce 160,000 tonnes per year of biodiesel at the European specifications.
The sustainable feedstocks used for this process are generally plant oils such as rapeseed, soybean and sunflower oil. These are generally quite expensive feedstocks but usually contain free fatty acid levels below 0.5%. Lower cost feedstocks, such as waste cooking oil, contain higher levels of free fatty acids and water and if used would lead to catalyst deactivation.
The catalyst used for the Esterfip-H process is a solid base catalyst which consists of a mixed oxide of zinc and aluminium. It is reported that none of the catalyst leaches from the fixed bed and so can be reused many times. A typical catalyst is expected to last for several years.
The catalysts performance isn't as efficient as a homogeneous catalyst but due to it being in a fixed-bed saves the cost of downstream processing of the products. The yield of biodiesel is more than 99% and the glycerol product also produced is of 98% purity.
Heterogeneous Biodiesel Process
(+) 99.8 Yield
(+) High glycerol quality
(+) Simplified purification steps
(+) No side-stream
(+) No chemical added
(+) Low catalyst cost
(-) High operating conditions
(-) High MeOH:oil ratio
Table shows the advantages and disadvantages of the heterogeneous catalyst
The reaction does require high temperatures and pressures (160-200°C, 40-70 atm) due to the lower activity of solid catalysts. The reaction also requires a high ratio of methanol to triglycerides. This is important to help to drive the equilibrium to the right and because methanol acts as a co-solvent in the reaction.
Figure shows simplified flow sheet of the Esterfip-H process. (Diagram was copied from Helwani, Z.; Othman, M. R.; Aziz, N.; Fernando, W. J. N.; Kim, J., Technologies for production of biodiesel focusing on green catalytic techniques: A review. Fuel Processing Technology 2009, 90 (12), 1502-1514).
In the continuous Esterfip-H process, the transesterification reaction is performed at high temperatures, with an excess of methanol. This excess is removed by vaporisation, recycled to the process and combined with fresh methanol. The reactants flow upwards through a tubular reactor containing a fixed-bed of catalyst in the form of extrudates. The desired chemical conversion is reached with two successive reactor stages, with glycerol separated to shift the equilibrium to the right. Excess methanol is removed after each reactor by a partial flash. Esters and glycerol are then separated in a settler. Biodiesel is recovered after the final recovery of methanol by vaporisation under vacuum, and then purified to remove traces of glycerol.
Esterification of free fatty acids
Sulphuric acid and alkyl sulfonic compounds are probably the most extended acid catalysts used for the esterification of free fatty acids. These acids show high acid strength and, unlike other mineral acids, can be found in high concentration, avoiding the addition of large amounts of water in the reaction media. However, the use of these strong acids leads to biodiesel with high sulphur content which is out of specifications. For this reason and many others, such as the impossibility to reuse mineral acids, heterogeneous acid catalysts are preferred over homogeneous ones. In the literature, several kinds of heterogeneous acid catalysts can be found to catalyse the esterification of free fatty acids to biodiesel, but two types are mainly reported: sulfonic acid-functionalized solids, both ion-exchange organic resins and inorganic supports, and inorganic metal-oxide based superacids.
Transesterification of triglycerides
Since esterification and transesterification reactions share a common molecular mechanism, acid catalysts showing activity in the former usually drive the latter. In this way, if the alkaline catalysts for the transesterification stage are changed by acid catalysts, the initial stage for FFAs removal from feedstock vegetable oil is not needed, therefore reducing the complexity of the biodiesel production process. Despite these acid catalysts showing a much lower catalytic activity in transesterification reactions, compared with alkaline catalysts, and therefore requiring higher operating temperatures, several recent studies have proved the technical feasibility and the environmental and economic benefits of biodiesel production via heterogeneous acid-catalysed transesterification. As in the case of the catalysts for esterification of FFA, most of the reported acid catalysts can be classified in two main groups: inorganic and organically-functionalised catalysts.