Interest in bioliquids as fuels derived from biomass has grown recently due to a number of reasons, but mainly for their positive effect in reducing carbon emissions. The main advantages of bioliquids when compared to raw (solid) biomass are their higher energy density and their availability on demand, thus making them easier to store and transport.
Fast pyrolysis liquid or bio-oil has been used in a variety of engine configurations with limited success due to the poor combustibility and adverse effects on engine components. It requires a pilot fuel and/or an additive for successful engine combustion and there are issues with materials and liquid properties. It is immiscible with all conventional hydrocarbon fuels.
Biodiesel, a product of the esterification of vegetable oil with an alcohol, is widely used as a renewable liquid fuel, usually as an additive to diesel at up to 20%. There are however limits to its use in conventional engines due to poor low temperature performance and variability in quality. Biodiesel is also seen as an alternative transportation fuel. The composition of blends of diesel and biodiesel are defined by the “B” prefix, where B100 is 100% biodiesel, and thus B5 refers to a blend of 5%vol. biodiesel and 95%vol. conventional petro diesel. The latter is currently widely used in the European transportation market.
Blends of bio-oil and biodiesel were evaluated at Aston University and its properties tested, aiming at overcoming the disadvantages of using either fuel by itself. A blend could allow for mixtures to be created with improved properties to enable matching of the blend to the requirements of the application, such as a desired viscosity or a target minimum heating value. The main objective was to assess their macroscopic visual homogeneity using different alcohols as co-solvents. A 100% renewable liquid fuel could take advantage of a range of incentives available for renewable power.
The macroscopic results were used to build a three phase diagram depicting the blend stability and identifying regions of miscibility and non-miscibility. The role of the various bio-derived alcohols as co-solvent in the blend is ideally low, thus the alcohol content should be minimised in order to achieve the optimum result, however; in reality this should not compromise the homogeneity of the blend. The blend long term stability was assessed adapting an accelerated test following the procedure established by Oasmaa et al (2011). The blend pH was also measured to document the variation with a range of bio-oil proportions.
Sample preparation and proposed test
The bio-oil was weighted in the container first, followed by the biodiesel. The alcohol was then added and the container was sealed and lightly shaken. All samples were prepared at room temperature. The blend sample size was fixed at 0.02kg and prepared in clear glass containers.
The blends were labelled according to their weight proportions, e.g.: sample 10 (30, 30, 40) 1-butanol is sample No 10 made out of 30%wt bio-oil, 30%wt biodiesel and 40%wt 1-butanol. A photograph was taken 48 hours after the blend was prepared to document its appearance.
An adapted accelerated long term stability test was completed on selected samples. The procedure involved measuring viscosity and/or water content before and after the sample was subjected to a 24h continuous heating at 80°C. This is believed to mimic the stability over 12 months when stored at ambient conditions. The sample weight is also recorded in order to make sure no evaporation took place during the test.
In addition, pH was measured for all homogeneous samples. A variation in pH is expected, however, the extent of the change cannot be calculated by the equations of average weight distribution.
Different alcohols were tested; however, 1-butanol showed the best performance as it is required in lower concentrations to prepare homogeneous blends. The resulting chart for 1-butanol is shown below, in which it can be seen that a blend containing less than 30% 1-butanol is likely to be phase separated. Blends prepared with 1-butanol can tolerate a maximum of 60%wt bio-oil.
Three phase chart for bio-oil, biodiesel and 1-butanol blends
The results for the accelerated stability test for selected blends is shown in the table below. The weight loss is less than 0.1% for these samples; in addition, the variations in both water content and viscosity are negligible. Therefore, the stability of the blends can be described as remarkably good, especially when compared to fast pyrolysis oils. The blend is expected to remain unaltered for up to one year while in storage.
Accelerated stability test results
|Test||Value before||Value after||Variation|
|10 (30,30,40) 1-butanol|
|Water content (%wt)||7.38||7.64||0.03|
|60 (45,18,37) 1-butanol|
|Water content (%wt)||11.02||11.86||0.08|
As expected, the measured pH values for the one phase blends showed an intermediate value between the biodiesel and the bio-oil. The increase in pH relative to the bio-oil pH is not significant and depends on the amount of bio-oil present in the mixture.
Homogeneous and non-homogeneous blends
The experiments showed that it is possible to prepare homogenous blends of bio-oil and biodiesel in the presence of an alcohol. Furthermore, the miscibility of blends of bio-oil and biodiesel depends on the type and amount of alcohol employed.
- In the particular case of 1-butanol, a blend containing less than 30%wt 1-butanol is likely to be phase separated. Blends prepared with 1-butanol can tolerate a maximum of 60%wt bio-oil.
- The adapted accelerated stability test showed negligible changes in water content and viscosity for the selected blends. It is therefore expected that these blends will remain stable and homogeneous after one year in storage.
- In relation to the pH, its increase in the blend relative to the original bio-oil pH value is not significant and depends on the amount of bio-oil present in the mixture. It still represents a challenge to be solved due to the implication on handling materials.
- Blends of bio-oil and biodiesel can overcome some of the disadvantages of using either fuel by itself. Improved properties will enable matching the blend to the application requirements, thus creating synergies and allowing for wider fields of application.