Characteristics of liquid, solid and gaseous biofuels effects on the energetic use

Characteristics of liquid, solid and gaseous biofuels effects on the energetic use 1, 2,* Rolf Strenziok 1 Senior Experten Service-SES, Bonn, Germany 2 University, Rostock, Germany * Corresponding email: rolf.strenziok@uni-rostock.de ABSTRACT Liquid, solid and gaseous biofuels are well-known, important and versatile energy sources for engines, turbines and combustors. Biofuels are replacing conventional fuels such as fossil diesel, gasoline and natural gas. In this paper, these biofuels were assessed whether their physical and chemical properties are suitable for energetic use. In addition, properties of biofuels were compared with each other. The comparisons showed that some engines and fuel systems must be adapted to be suitable for biofuels. Sustainably produced biofuels can play an important role in the energy mix of the future. First-generation, vegetable oil-based biofuels will be partly substituted by second-generation biofuels (BtL), leading to new scientific and technical challenges for energetic use. KEYWORDS Biomass, biofuel, biodiesel, sustainability, engines INTRODUCTION The use of bioenergy is on the rise for the application in thermal engines and combustors. Important reasons for this rise are the increase in costs of fossil fuels, decrease in their availability and efforts to reduce greenhouse gas emissions. Almost all types of biomass can be modified for the use as biomass fuels. Many biogenic solid fuels can be produced without expensive physical or chemical processing. For example, wood, straw and bagasse can be modified through drying or crushing to obtain firewood or chips that can then be used to generate heat and power. These cost-efficiently produced biogenic solid fuels can also be transformed into Biomass-to-Liquid (BtL), Pyrolysis oil, bioethanol and biogas for direct use in engines and turbines. The most important biogenic liquid fuels are bioethanol, pure plant oil and biodiesel, which are mostly used in vehicle engines and must therefore meet certain existing standards. Moreover, biogenic gases such as digester gas (Biogas, Sewage Gas, Landfill Gas), Biomethane, SNG (Synthetic Natural Gas) and Biohydrogen are used for gas engines. Biofuels are efficient and sustainable. In the present paper, some of these physical and chemical properties of biofuels were evaluated for their effects on combustion and emission. METHODS Liquid and gaseous biofuels are the most important sources of future bioenergy production. However, they have to be adjusted for the use in engines because of some of their physical and chemical properties. The most important of these characteristics will be discussed in the first part of this paper, whereas they will be evaluated for their energetic potential in the second part of the paper. Possible damage of engines from the use of liquid and gaseous biofuels will also be discussed. Page 1 of 8

Characteristics of liquid biofuels The liquid biofuels (for diesel engines, gas engines and gas turbines) include: 1 st Generation bio-fuels Pure Plant Oil (PPO) Biodiesel: Rape Methyl Ester (RME), Palm Methyl Ester etc. Ethanol 2 nd Generation bio-fuels Biomass to Liquid (BtL). This Fischer Tropsch process is used to produce synfuels from gasified biomass. Characteristics of Pure Plant Oil PPO is: Oil from plants obtained through pressing or extraction, but chemically unmodified Produced from rape oilseeds etc., 2.5 kg oilseeds => 1 l PPO Produced on agricultural land: 1.3 t oilseeds per ha yearly Energy content: 1 kg PPO = 0.87 kg diesel fuel Figure 1. Rape as raw material for PPO and Biodiesel (B. Bastian 2010, canola fields Germany, www.bebersee.com) Table 1. Comparison between diesel fuel and PPO (Rape seed oil) Properties diesel fuel (EN 590) PPO (Hassel et al. 2004) Density at 15 C [kg/m³] 854 920 Kin. Viscosity at 40 C [mm²/s] 3.8 30 40 Cetane number > 51 > 40 Carbon residue [%] 0.1 0.1 CFPP [ C] - 21 + 22 Flashpoint [ C] 70 230 Calorific value [MJ/kg] 42.4 36.8 S-Content [%] 0.05 0.3 TAN [mg KOH/g] (EN 14104), - 4 max. 2, (Remmele, 2002) Water [%] 0.02 0.6 PPO is an alternative fuel for diesel engines, turbines and oil burners. For engines that are designed to burn diesel fuel, the viscosity of pure plant oil must be lowered to allow for proper atomization of the fuel, otherwise incomplete combustion and carbon build up will damage the engine. Page 2 of 8

Characteristics of Biodiesel (RME) By esterification of rapeseed oil with methanol, the fuel will be adjusted to the engine. RME is: Produced from vegetable oil; vegetable oil is produced from rape, sun flowers etc. Energy content: 1 kg biodiesel = 0.89 kg diesel fuel Applicability: Mixing with fossil diesel is possible in every proportion Usage in diesel engines is possible with some technical adaptations Figure 2 shows the use of biodiesel in fishing boats in ecologically sensitive areas in Germany. RME can decrease the environmental burden in these areas. Exhaust emissions of biodiesel can be significantly decreased by small adjustments of a standard engine. Figure 2. Use of RME in ecologically sensitive areas (Strenziok et al. 2004) Table 2. Comparison between diesel fuel and RME Properties diesel fuel (EN 590) RME (Strenziok et al. 2004) Density at 15 C [kg/m³] 854 822 Kin. Viscosity at 40 C [mm²/s] 3.8 4.4 Cetane number > 51 > 51 Carbon residue [%] 0.1 0.024 CFPP [ C] - 21-14 Flashpoint [ C] 70 150 Calorific value [MJ/kg] 42.4 37.9 S-Content [%] 0.05 0.01 TAN [mg KOH/g], max. 0,5-0.5 (Remmele, 2002) Water [%] 0.02 0.11 These biodiesel properties are determined by the chemical structure: density, kinematic viscosity, calorific value, sulfur content. Biodiesel is used in blends up to 7% in conventional diesel engines. For higher blends and pure biodiesel operation the motor must be adapted for biodiesel operation. One problem is the biodiesel entry into the lubrication oil. Characteristics of gaseous biofuels (biogas) Biogas is produced from organic substances (agricultural waste, maize, organic waste) by fermentation under anaerobic conditions. 5 kg maize silage => 1 Nm³ biogas, Page 3 of 8

Energy content: 1 Nm³ biogas = 0.6 Nm³ natural gas Biogas: 60% methane, 40% CO 2, ~ 6 kwh/nm³ Upgraded biogas: 98% methane, 2% CO 2, ~ 10 kwh/nm³ Biogas is predominantly converted into electricity using combustion engines. Combined Heat and Power Plants use either Gas Otto Engines or Pilot Injection Gas Engine. Gas Otto engines have been specifically developed for biogas. Pilot Injection Gas Engines work on the same principles as diesel engines. Since biogas does not ignite spontaneously when it is compressed, an ignition oil (<5 %), (Herdin, 2001), is injected to produce a self-igniting mix of gases. Average composition of biogas: Methane (CH 4 ) 40-75% Carbon dioxide (CO 2 ) 25-55% Hydrogen sulfide (H 2 S) 50-5000 ppm Ammonia (NH 3 ) 0-1% Water (H 2 O) 0-10% Nitrogen (N 2 ) 0-5% Oxygen (O 2 ) 0-2% Hydrogen (H 2 ) 0-1% RESULTS Characteristics of liquid biofuels effects on the energetic use for diesel engines PPO in diesel engines The use of PPO in diesel engines requires the following engine adjustments: Installation of an additional automatic starter to avoid starter problems Heating of the PPO pipes and tanks More frequent oil changes to avoid thinning of the motor oil, damaging (particularly at part load) Replacement of materials that are not compatible with PPO Replacement of fuel injection nozzles and pumps Page 4 of 8

Table 3. PPO parameter and their effects on engine operation (Remmele, 2002), (Biofuel specification MAN), (DIN 51605), (www.fnr.de) Density The density is a typical characteristic of PPO and is used for characterization. PPO has a higher density than diesel fuel. The density is dependent on the C- content. Cetane number The low cetane number results in a lower ease of ignition and a higher ignition delay of PPO. Ignition aids improve cold start performance. Higher exhaust gas and noise emissions may arise. Kin. Viscosity The kinematic viscosity is a characteristic property of PPO and is determined by the composition of the fatty acids. It is temperature and pressure dependent. The viscosity of PPO is about 10 - fold higher than that of diesel fuel. This results in a poorer atomization, combustion and soot formation in the engine and increased particulate emissions. Thus, the vegetable oil, reaches the viscosity approximately to diesel fuel, it must be preheated. There is a 1 - or 2-tank system. Flashpoint PPO is less flammable than diesel fuel and therefore easier to storage and transport. Oxidation stability Oxidation stability is a measure of the aging. It also depends on the storage conditions. Admixtures of diesel fuel and RME reduce the oxidation stability. Sulfur content PPO is almost sulfur-free. The low sulfur content of PPO is to be regarded as an advantage over diesel fuel. Coke residue During combustion residues are formed in the combustion chamber. This can lead to deposits and restrictions on engine operation. Water content The presence of water may cause hydrolysis. With an increasing water content, the activity of microorganisms increases. Iodine number The Iodine number describes the average number of double bonds of the max. 125 g fatty acid molecules. A high Iodine number can cause problems in the Iodine/100 g injection system. Deposits will occur in the form of polymerization and (www.fnr.de) resin and the oxidation stability decreases. Total pollution The total pollution is an important quality criterion; high total pollution represents a too high proportion of contaminants and can lead to deposits Total Acid Number max. 2 (Remmele, 2002) Phosphor content max. 15 [mg/kg] (Remmele, 2002) Boiling characteristics Calorific value and clogging of fuel pipes, filters and nozzles. Free fatty acids lower the flash point, which can lead to safety problems during storage and transport. In addition, free fatty acids cause corrosion in the engine. The Total Acid Number is a criterion for the aging of the rapeseed oil. The phosphorus oxides produced during combustion; form deposits on the cylinder and are very difficult to remove. Thus, the operation of the engine is limited. The Boiling curve of PPO is up to 100 C higher than that of diesel fuel. The calorific value characterizes the energy content in the fuel. The lower calorific value of PPO results in a reduction of engine power. Under equal conditions, the fuel consumption increases. Possible malfunctions of diesel engines with PPO: Malfunction of fuel injection pumps and nozzles Reduced output during rapeseed oil operation Seized exhaust valves, soot deposits Increased rapeseed oil entry into the lubricating oil Page 5 of 8

Viscosity [%] Rape Methyl Ester in diesel engines Table 4 RME parameter and their effects on engine operation (DIN 14214), (www.profi.de) Density The density of RME is between rapeseed oil and diesel fuel. Cetane number Biodiesel has a higher cetane number compared to rapeseed oil. This corresponds to the ignition behavior of diesel fuel. Methanol in biodiesel reduces the ignition behavior. Viscosity The viscosity of biodiesel is slightly higher than that of diesel fuel. No problems occur during injection. Flashpoint Biodiesel has a flash- point of about 120 C, it is therefore not dangerous. Storage and transportation are unproblematic. Sulfur content Biodiesel contains little sulfur. Sulfur damages the oxidation catalyst (Acrolein). Coke residue Deposits can occur in the injection pump and the piston rings. Water content The water content of biodiesel is lower than that of rapeseed oil. Total acid number The Total Acid Number is low. Storage of biodiesel increases the TAN. max. 0,5 (Remmele, 2002) Calorific value Biodiesel has almost the same calorific value as rapeseed oil. The fuel consumption is only slightly higher compared to diesel fuel for the same power. Filtrability Biodiesel without an additive has a CFPP value of -10 C. Ash content Deposits are similar to those of the diesel fuel. Methanol content The methanol content is max. 0.2%. Methanol lowers the flash point and deteriorates the ignition performance in the biodiesel (there are also problems with the storage and transport). Glycerin Glycerin forms during the combustion the harmful Acrolein. The max. content is 0.24%. Carbon deposits in the fuel injectors and piston rings occur. Alkali content A high alkali content, max. 5 mg/kg, leads to filter problems and deposits, [mg/kg], EN 14108 caused by the catalytic converter during the biodiesel process. Painted parts Due to its solvent behavior biodiesel should not come in contact with painted parts. Lubrication oil At partial load, RME can cause dilution of the engine lubrication oil (decrease in viscosity). A very common problem of the RME- operation is the dilution of the lubrication oil. The biodiesel leaks into the lubrication oil in two ways: together with the blow-by gases or it is slipped off from the cylinder wall together with the lubrication oil by the piston rings. 120 100 80 100 87 85 84 75 72 68 60 40 20 0 0 10 20 30 40 50 60 Operation Hours Figure 3. Viscosity drop of the lubrication oil caused by RME-content (Strenziok, 2004) Page 6 of 8

The use of biodiesel can decrease the environmental burden resulting from combustion processes. However, in most applications of biodiesel the environmental potential is not used to its full potential since RME is used in unchanged standard diesel engines. The exhaust gas emissions of biodiesel can be significantly decreased by slight changes of the standard engine. The decreasing pollution potential of the use of RME in diesel engines is (Strenziok, 2004), (Jankowski A. Journal of KONES Internal Combustion Engines 2003, vol. 10, 3-4): o ~ 12 % for carbon monoxide o ~ 10-35 % for hydrocarbons o ~ 25-35 % for particles Depending from the driving cycle, the above mentioned exhaust components reduce mainly due to the oxygen content of RME. Characteristics of Biogas effects on the energetic use for gas engines The operation of gas engines with biogas; is fundamentally different from that of natural gas engines. In addition to differences in the biogas calorific value, methane number and flammability limits, two essential criteria of evaluation of gases for motor use exist: laminar flame speed and calorific value of the mixture. The laminar flame speed indicates how fast the flame propagates in laminar flow conditions. The second important criterion is the mixed calorific value. The requirement for the use of gases in gas engines make the following major gas properties: Calorific value > 5 kwh/m 3 Methane number > 70-80 Laminar flame speed > 5 cm/s Ignition limits Biogas must be cleaned and dried before it can be used in engines. Silicium components and hydrogen sulfide have to be removed prior to the use of biogas. The purification process of biogas depends on the raw material and S-containing trace gases. During combustion, intermediate components (SO 2, SO 3 ) are produced which are neutralized by the engine oil. Particularly critical in this context are Cu-containing materials. Copper pipes are not resistant to biogas. Experience has shown that Brass and Bronze pipes are suitable. Hydrogen sulfide in biogas cause: Decreased life time of lube oil Increased wear of metal components Indirectly affects availability Figure 4. SiO 2 deposits on a gas engine piston (Herdin, 2001) Solid biofuels- effects on the energetic use Solid biofuels have a high water and ash content, a low calorific value. They must be dried prior to combustion. Pressing increases the mass-based heating value. Figure 5 provides an Page 7 of 8

overview of common solid biofuels. Biomass ash consists mainly of the elements calcium (Ca), silicon (Si), magnesium (Mg), Potassium (K), phosphorus (P), and sodium (Na). Figure 6 shows the problem of straw combustion with high CL content. Figure 5. Solid biomass for heating www.wikipedia.de Figure 6. Combustion problems with ash deposits on the heat exchanger area. www.wikipedia.de DISCUSSION Biogenic fuels are currently thought to become the most important sustainable energy resources. As the price of natural oil continues to increase, the use of biofuels will become relatively less expensive. Today s engines and turbines are designed for conventional fuels. To use biofuels in these conventional engines- the physical and chemical properties of biofuels must be adjusted. These adjustments also affect the engines. PPO has properties that require significant modifications of thermal engines. RME has already been adjusted for the use in diesel engines. One important future challenge is the Methanization of biogas. CONCLUSIONS Using bioenergy has both advantages and disadvantages similar to area utility competition vs. food production. Country-specific guidelines need to be developed to ensure the sustainability of bioenergy production. 1 st Generation, sustainably produced biofuels will play an important role in the future whereas biofuels of the 2 nd Generation have to be adapted to meet the specific requirements of modern engines. Through the use and incorporation of BtL fuels, substitution of diesel fuels is at least partially possible in the coming decades. The availability of petroleum products can thus be significantly extended. The characteristics of liquid, solid and gaseous biofuels have a significant impact on the energy use. REFERENCES Hassel, E. Berndt, S., Flügge, E., Golisch, J., Wichmann, V. (2004). Use of Rapeseed Oil in tractors. Research project Herdin, G. (2001). State of the art of Gas engines. Jenbacher AG Remmele, E. (2002). Standardization of Rapeseed oil as fuel. Research project Strenziok, R. (2012). Bio-oil application in a small Gas Turbine. PyNe NL issue 32 Strenziok, R., Wendig, D. (2004). Bio fuel in Micro Gas Turbines. Workshop: Bio-fuelled Micro Gas Turbines in Europe, Brussel, Strenziok, R., Hansen, U., Schröder, T. (2004). Demonstration project for the use of RME in ecologically sensitive areas. Bioenergy conference, Montreal Strenziok, R. (2011) The Micro Gas Turbine in field trails with fermenter biogas. Primer Foro Internacional sobre Energías Renovables. Oaxaca, MX Bioenergy in Germany, facts and figures (2003), Nachwachsende-rohstoffe.de. www.fnr.de Page 8 of 8