Chapter 2: Literature study
2.10 Methods for physical characterisation
In order to use bio-oils as heating fuels and oil refinery feedstock, fuel standards are needed. Based on feedback from customer end-users and other research institutes the following physical properties have been suggested to specify: solids, stability, homogeneity, water, and flash point (Peacocke et al., 2003). These properties can be influenced during bio-oils liquid production. Physical properties such as density, heating value and viscosity which cannot directly be influenced by the pyrolysis process are important for liquid end-use customers.
2.10.1 Water content
Water is believed to be chemically dissolved in bio-oils. A change in water content indicates a change in moisture of feedstock, process operating conditions, or an oxygen leak into the system. The water content can be easily adjusted by adjusting the initial feedstock moisture levels. Water content in the bio-oils affects other properties for example viscosity, heating value and density of the product (Asadullah et al., 2008). Scholze (2002) recommended water content of the oils to be analysed by Karl-Fischer titration according to the standard ASTM E 203.
2.10.2 Solids and its components
The solids content of the bio-oils originate from feedstock initial ash, pyrolysis biochar, and sand from reactor fluidising bed or from dirt in the feedstock. From a rice straw biomass of < 5 mm particle size, particles with sizes of 10-100μm were captured by cyclones and solid content in bio-oil was about 0.03 wt. % (Park et al., 2004). In contrast, the hot filter could catch particle size around 0.1μm (Park et al., 2004). Solid content can be influenced e.g. using homogeneous feedstock size, reducing fines particles, efficient cyclones, or effective solids separation technology such as hot vapour filtration. Oasmaa and Kuoppala (2003), Oasmaa et al. (2009) and Roy et al. (1990) recommended that solid content of bio-oils to be analysed as insoluble material in methanol dichloro methane solution (1:1).
2.10.3 Homogeneity
Homogeneity of bio-oil is a very important property for its end-use. The amount of water in the liquids has a negative effect on the homogeneity of bio-oils. During production the homogeneity of the oils can be controlled by visual observations. Microscopic determination
40
gives possible phase-separation or presence of solid material, e.g. extractive crystals or inorganics, in the liquid. A 7 day test is recommended for homogeneity verification. The method allows a homogeneous sample to stand for a week at room temperature and the water content from different depths are determined by Karl-Fischer titration (Scholze, 2002).
2.10.4 Stability
Stability of bio-oils can be monitored by changes in viscosity and average molecular mass. These properties are related (Oasmaa et al., 2003a). The use of accelerated aging test (24 hours at 80 0C, viscosity at 40 0C) is recommended as a quick test for measuring the stability of oils. The accelerated aging test relates very well with the chemical changes in the liquid (Oasmaa and Kuoppala, 2003). Stability tests should be performed each time, in exactly the same manner. If the weight loss is > 0.1 wt. % during the test, the results should be discarded. Stability testing is recommended for comparison of bio-oils from one specific pyrolysis process. The best comparisons can be obtained when the differences in the amount of water of the samples is negligible.
2.10.5 Flash point
Flash point is the lowest temperature at which the application of an ignition source causes the ignition of vapours under specified test conditions. The test method ASTM D 93 covers the procedure for the determination of flash point of petroleum products by manual Pensky-Martens closed cup apparatus. The method is applicable to all petroleum products with flash point above 40 0C and below 360 0C, except fuel oils. This method has been used with pyrolysis liquids. However, the flash point cannot be measured for bio-oils at 70-100 0C, where the evaporation of water suppresses the ignition (Oasmaa et al., 1997).
2.10.6 Viscosity and pour point
Viscosity of bio-oils can be affected indirectly by changing the water content or by solvent addition (Oasmaa et al., 1997). Viscosity of bio-oil is recommended to be determined as kinematic viscosity according to the standard method ASTM D 445. Dynamic viscosity by rotating viscometers can also be used for measuring the viscosity of the pyrolysis oils. However, it is not as accurate as kinematic viscosity (Oasmaa and Meir, 2000). The lowest temperature at which movement of sample is observed is recorded as the pour point (Li
41
ASTM D 97. When measuring the pour point of bio-oils pre-heating of the sample should be excluded due to thermal instability.
2.10.7 Heating values
Heating values are defined as the amount of energy contained in a fuel. The heating values are dependent on the phase of water/steam in the combustion products. If H2O is in liquid form, the heating value is called Higher Heating Value (HHV). When H2O is in vapour form, the heating value is called Lower Heating Value (LHV). The heating values are measured by DIN 51900 using a bomb calorimeter and depend mainly on the elemental composition of the material (Oasmaa et al., 1997; Oasmaa et al., 2002). The high water content of bio-oils may lead to poor ignition. The information on heating value determination of bio-oil with high water content is currently unavailable. TAPPI (2011) reported that vacuum distillation to remove part of the water content before analysis can be used. Due to the high volatility of bio-oil lighter components, vacuum distillation can be used to reduce the water content at low temperatures. The bio-oil heating value is a function of water content of the liquid (Czernik and Brigdwater, 2004). The extractive group of compounds contains high energy content and their dissolution in the whole product is beneficial to the product energy content (Oasmaa et al., 2003a). The heating values of the bio-oils can also be determined from the chemical analyses (ultimate analyses) using a correlation by Channiwala and Parikh (2002) (Equation 12).
( )
Equation 12
Where C is the carbon, H is the hydrogen, S is the sulphur, O is the oxygen and N is the nitrogen.
2.10.8 Density
The density of bio-oils can be determined with a digital density meter according to the standard method ASTM D 4052. The standard method covers the products which can be handled in the liquid state between 15 and 35 0C. Vapour pressure of the samples should be lower than 80 kPa and kinematic viscosity below 0.015 m2/s. The method is based on the effect of change in the mass of the sample tube in oscillatory frequency. The density of bio- oil liquids correlates well with the amount of water in the liquid (Oasmaa et al., 2004). The
42
lower the water content the more viscous and denser is the bio-oil and the higher the water content the less viscous and dense is the pyrolysis oil.