SYNFUEL: DIVERSIFICATION OF PRIMARY ENERGY
SK ANSARI, SEEMA ANSARI
Aug 16 - 22, 2010
Synthetic fuel is regarded as sustainable future alternative fuel for the transport sector. It can be produced from natural gas as well as from almost all types of gasified biomass including garbage and sewage sludge.
The diesel combustion process is undergoing constant development to further reduce emissions and increase efficiency as well as driving fun. Another underlying challenge is depletion of conventional fuel sources.
Synthetic fuel is free from sulphur and aromatics, which lead to considerable reduction in emissions. Furthermore, it can be optimised for new engine requirements that allow emissions to be reduced further and also increase efficiency. CO2 emissions are almost completely avoided when biomass is used to produce synfuel (BTL).
Sulphur compounds and aromatics contribute to the formation of soot and nitrogen oxides when fuel are combusted in engines. The automobile industry has therefore been calling for the reduction of these fuel components and decades. Today low sulphur fuels have established in several regions. Synthetic fuel goes one decisive step further however.
As the synthetic fuel is free of aromatics, its density is slightly lower than commercially available diesel, which contains around 20 per cent aromatic compounds. The density of synfuel is therefore outside the European specification for diesel fuel (EN 590).
In cities with air quality problems, for example, Bangkok and Athens, the addition of Shell GTL to diesel fuels has helped reduce summer smog problems. GTL alone can significantly improve the air quality in cities without expensive and sophisticated technical modifications to vehicles and infrastructure.
Daimler Chrysler investigated the emissions reduction potential of synfuel additions to diesel. The soot and nitrogen oxides emissions were measured on a bench dynamometer at a characteristic partial load point. The rate of exhaust gas cycling was varied and the start of the pilot injection phase and main injection phase was kept constant during the test. From a 20 per cent production of synfuel, considerable improvements could be achieved. Once a synfuel fraction in the diesel reached 50 per cent, the reduction in emissions almost reached the level of pure GTL.
COMBINED COMBUSTION SYSTEM (CCS)
A significant reduction in diesel engine emissions requires intensive, continued development of the diesel combustion process and the use of homogenisation effects. Zones with oxygen deficiency, which cause soot formation, and local peek temperatures, which are responsible for NOx emissions, are then avoided or reduced. Thus, both the NOx and soot emissions can be reduced with internal engine measures.
Further development of both the engines and the fuel is necessary. The diesel combustion process has great potential for reducing emissions if the engine and fuel are tailored to each other. Synfuel will make this possible to a significant extent for the first time.
The new generation of Combined Combustion System engines, which combine the low emission of Otto (gasoline) engines with the low consumption of diesel engines, is based on the use of homogenisation effects that can be achieved with engine and fuel measures. Carefully directed changing of fuel properties is a key issue to master homogenization in diesel engines. One fuel that has been optimised for the CCS combustion process is CSS fuel.
From the viewpoint of the automobile industry, synthetic fuel provides solution for the three big challenges of future mobility:
* Further emissions and CO2 reductions
* Reliable supplies, i.e., diversification of primary energy
* Use of existence infrastructure.
Well-to-Wheels (WTW) and Life-Cycle Analysis (LCA) are the most frequently used methodologies for comparing the Green House Gas (GHG) emissions associated with transportation fuels. Both aim to quantify the environmental impact of producing and consuming one or more (alternative) oil products, in terms of grams of CO2 equivalents produced (including during the fuel production phase) per kilometer driven by a standard vehicle.
WTW focuses on production of automotive fuels only such as gasoline and diesel. However, the production process of certain fuel often involves coproduction of several other products as well. This implies that fractions of the total amount of GHG produced need to be allocated to the individual products.
LCA considers the full systems including all products of the two technologies comparing for example the GTL process with a refinery process. However, the GTL and refinery processes are not directly comparable as a result of their different product slates, which serve different functions. In the case of the refinery, all products are not equal in terms of market desire and economic value. In contrast, the GTL system only provides products valued by the market.
To enable an objective like-for-like comparison, the GTL system has to be expanded to meet the same range of market functions as products from the refinery system. This implies that products and processes should be added together with their associated energy and environmental burdens. The expansion of the GTL system is performed on the basis of existing complementary technologies. The choice of product for fulfilling the missing functions is based on market analysis and industry trends.
This LCA approach is different from the WTW where only the diesel and gasoline line streams from a refinery are taken into account. The WTW comparison ignores the fact that a refinery has a range of products associated with the diesel stream, some of which are not so desirable from an environmental and economical point of view, specifically the heavier carbon-rich residues, whereas GTL produces only desirable products.
GTL produces similar amounts of GHG as crude-based fuels; CTL produces about twice the amount; and BTL produces less than 20 per cent of the amount, illustrating the significant potential of BTL for reducing GHG emissions.
GTL has additional environmental benefits over crude-based fuels. GTL is very selective in producing only desired products as gasoil whereas crude based refineries inevitable produce much less desired "bottom-of-the-barrel products" such as heavy fuel oil and residue. In addition, GTL has considerably lower local emissions in the product application phase (compared with crude-based diesel fuel), owing to a number of key properties, including its very high cetane number and to being virtually free from sulphur and aromatics. This has important benefits for air quality in burden environments, in particular congested "megacities".
Diesel engine combustion occurs when a diesel engine intakes air, compresses it, injects fuel into the compressed air and ignites. Diesel engines convert fuel from a liquid state to a vapor using a mechanical high-pressure injection process as well as the heat of compression because only vaporised fuel will burn. Modern analysis using different devices for computational fluid dynamics (CFD) code and advanced visualisation techniques can be used to better understand this process. When diesel fuel is injected into a cylinder under high-pressure, it is injected through a multi-hole nozzle with several orifices. The fuel atomizes into small droplets and begins to vaporize as it moves away from the injection nozzle. As the fuel vaporizes into the hot air created by compression, it begins to react with the oxygen molecules in the hot air (oxidation). This reaction of fuel and oxygen is necessary for combustion. As more fuel vaporizes and mixes with air, the number and rate of the oxidation reactions increase until the end of the ignition delay period, when ignition occurs. For diesel combustion applications the mechanisms of spray atomization and mixing are of primary importance. The conversion of the high-speed liquid columns injected into air forms a multitude of individual fluid drops. This occurs via a multi-stage process, including the fluid mechanics instabilities, followed by their nonlinear stage, appearance of flat fluid sheets, annular and conical sheets, jets, etc.
Synthetic diesel provides numerous economic and environmental benefits over typical petroleum diesel. Synthetic diesel is significantly cleaner, cleaner-burning, and can be formulated for superior cold weather performance and fuel system lubricity. Because synthetic diesel has fewer contaminates, it is lower in toxicity.
* Some remote natural gas can now be economically converted through a GTL process into an ultra-clean fuel for diesel engines. At times, this fuel can be economically blended with conventional petroleum diesel fuels to improve refinery capacity of cleaner diesel fuels.
* An opportunity exists to use GTL fuels to reduce the emissions from old diesel vehicles especially school buses. One plant in South Africa (Mossgas) and Shell's Indonesia plant both produce GTL fuels suitable for use in heavy-duty diesel applications.
* Discussions are underway to develop a GTL production facility in Alaska to produce 40,000 barrels per day with a goal to produce 300,000 bbl/d. However, with existing technology, oil pipeline capacity and North Slope gas reserves over 1,000,000 bbl/d could be produced.
* Converting natural gas to a liquid through a Fischer-Tropsch technology provides an opportunity to expand the use of the natural gas and lower the transportation cost from remote sources of low-cost gas.
* Fischer-Tropsch is a gas-to-liquid process that can produce a high-quality diesel fuel from natural gas, coal and biomass resources. Shell refers to the GTL process as a middle distillate synthesis (MDS). In all cases, the middle distillate produced from this process can be blended with today's diesel fuel.
* GTL diesel has extremely low sulfur, aromatics, and toxics. GTL fuel can be blended with non-complying CARB diesel fuel to make a cleaner diesel fuel complying with stringent diesel fuel standards.
* Synthetic diesel fuel offers a new opportunity to use alternative fuels in diesel engines without compromising fuel-efficiency, increasing capital outlay, and impacting infrastructure or refueling cost.
* Further commercialisation of this fuel improves the prospects of new engines meeting proposed national 2007 heavy-duty diesel engine emission standards. In the near-term, this fuel can play a role reducing existing diesel vehicles exhaust and toxic emissions.
* Since the late-1990s nearly every major oil company including: ARCO, Chevron, Conoco, Exxon, Phillips, Mobile, Statoil, and Texaco announced plans to build pilot plants or commercial plants to produce synthetically derived diesel fuel through the improved GTL process.
* Stringent diesel exhaust emission standards and fuel specifications are compelling the petroleum industry to revisit the new, improved GTL process to competitively produce aromatic and sulfur complying diesel fuel.
Upon injection into the combustion chamber, fuel must quickly mix with air to form a flammable mixture and the mixture must ignite. Since there is normally no additional means for ignition (such as the spark plug in gasoline engines), the fuel must self-ignite (auto-ignition). This process is influenced greatly by the engine combustion system and by fuel properties. Diesel engines use the heat developed by compressing a charge of air to ignite the fuel injected into the engine cylinder. More specifically, the air is first compressed, then fuel is injected into the cylinder as fuel contacts the heated air, it vaporizes and finally begins to burn as the self-ignition temperature is reached. Additional fuel is injected during the compression stroke and this fuel burns almost instantaneously, once the initial flame has been established. A period of time elapses between the beginning of fuel injection and the appearance of a flame in the cylinder. This period is known as ignition delay and is a major factor in regards to the performance of diesel fuel. Cetane number is a measure of how readily the fuel starts to burn (auto-ignition) under diesel engine conditions. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; and therefore has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.
If the ignition delay is too long, the fuel will accumulate in the cylinder until it reaches ignition conditions and then will burn rapidly, causing a sudden pressure increase which may result in engine knocking, a decrease in engine efficiency, and the dilution of engine lubricating oil. Cetane number/ignition delay varies systematically with hydrocarbon structure. A reduction in ignition delay can be obtained by varying the chemical nature of the injected fuel.
The most important criteria that determine the suitability of gasoline components are the octane numbers (RON and MON), volatility (boiling properties and vapor pressure) and more recently chemical composition (aromatics, absence of benzene, olefins and sulphur).
STRAIGHT RUN GASOLINE
This gasoline is obtained by distillation is therefore easiest to produce and also longest in use. The composition of straight-run gasoline depends on the type of crude oil used (frequently high paraffin content). The octane quality is usually low.
Butane also belongs to the straight-run products but it has a high-octane level. It is to be added to gasoline, besides being used as liquefied petroleum gas (LPG). Because of its high vapor pressure, butane is good for engine cold-start. On the other hand, too high the vapor pressure can lead to hot-fuel handling problem (warm start problems, stumbling engine, vapor lock) and increased evaporative hydrocarbon emissions. Thus, addition of butane is limited by the vapor pressure specification.
CATALYTICALLY CRACKED GASOLINE
Catalytically cracked gasoline is more important for gasoline production. Catalytically cracked gasoline is usually separated into high-boiling and low-boiling fractions, which have different applications. The octane quality (especially the MON) of catalytically cracked gasoline is usually sufficient only for regular gasoline. In order to meet the legally defined olefin specification of gasoline the amount which can be used in gasoline production is limited. The high olefin content of these fuel components also requires special precautions to be taken against polymerisation and oxidation during their use.
Another variant of catalytic cracking is hydro cracking, a process in which hydrogen is added. The hydro cracked gasoline produce by this process contains practically no unsaturated components.