In-situ biodiesel production from a municipal waste water clarifier effluent stream
Van Tonder, Gert Cornelius
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This study investigated In situ biodiesel production with supercritical methanol. A micro-algae based feedstock was used and obtained from a local water treatment plant situated just outside of Bethal, South Africa (S 26° 29’ 19.362” E 29° 27’ 11.552”). The wet feedstock was used as harvested with only the excess moisture being removed. Characterisation of the feedstock showed that a wide variety of macro-algae, micro-algae, cyanobacteria and bacterial species were present in the feedstock. The main algal species isolated from the feedstock were Nostoc sp. and Chlamydomonas. The feedstock was found to have a higher heating value (HHV) of 22 MJ.kg-1 and a lower heating value (LHV) of 16.03 MJ.kg-1 with an inherent moisture content of 270g.kg-1 feedstock. The protein and fat content of the feedstock was determined by the Agricultural Research Council (ARC) and found to be 370.1 g.kg-1 and 61.6 g.kg-1 on a moisture free basis respectively. The high protein and fat content gives a theoretical bio-yield of 430 wt%. The low lignin content and high cellulose and hemi-cellulose content indicated that the feedstock would be suitable for energy production. Three experimental sets were performed to determine the effect certain reaction parameters will have on the bio-char, bio-oil and biodiesel yields. The first set entailed hydrothermal liquefaction without the addition of methanol. The second set involved in situ biodiesel production with supercritical methanol, while both supercritical methanol and an acid catalyst were used during in situ biodiesel in the third set. For the first set of experiments the effect of temperature (240°C to 340°C in intervals of 20°C) on the crude bio-oil and bio-char yields were investigated. The highest bio-char yield was found to be 336g g char.kg-1 biomass at 280°C, while the highest crude bio-oil yield was 470.7 g crude bio-oil per kg biomass at 340°C. In the second set of experiments the dry biomass loading was kept constant at 500 g.kg-1 and the temperature varied (240°C to 300°C in intervals of 20°C) along with methanol to dry biomass ratio (1:1, 3:1 and 6:1). The optimum bio-oil yield of 597.1 g bio-oil per kg biomass for this set was found at 500 g.kg-1 biomass loading, 300°C and 3:1 methanol to dry biomass ratio. The highest bio-char yield was found to be 382.6 g bio-char.kg-1 biomass for a 1:1 methanol to dry biomass weight ratio set with 500 g.kg-1 biomass loading at 280°C. An increase in methanol ratio also led to an increase in crude bio-oil yields however the 3:1 methanol to dry biomass mass ratio was found to give the highest bio-oil yield and the purest biodiesel, with less unsaturated FAME. The 6:1 methanol to dry biomass mass ratio did however increase the FAME yield, which tends to show completion of the in situ production of biodiesel. This was also seen in the amount fatty acid methyl esters (FAME) present in the crude bio-oil as the degree of transesterification starts to increase with an increase in methanol. The FAME content was determined using gas chromatography (GC) and gas chromatography coupled to mass spectrometry (GC-MS). During the last set of experiments the temperature (260°C to 300°C in intervals of 20°C) and methanol to dry biomass ratio (1:1, 3:1 and 6:1) was varied at a constant catalyst loading of 1 wt% of the dry biomass. The optimum yields achieved were 627 g crude bio-oil per kg biomass and 376 g bio-char per kg biomass at 300°C and 280°C, respectively. These yields were achieved at 500 g.kg-1 biomass loading and 6:1 methanol ratio. Compared to the experiments where no catalyst was used, a slight increase in the yield was observed with the addition of an acid catalyst. This might be due to the base metals present in the feedstock that can lead to saponification during transesterification without the addition of an acid catalyst. An overall improvement in the extraction of crude bio-oil was observed with in situ production compared to hydrothermal liquefaction. During in situ liquefaction, the bio-oil yield increased by 150 g crude bio-oil per kg biomass higher, while the bio-char yields did not significantly vary at the optimum point of 280°C this finding has a significant value for green coal research. The highest HHV for the bio-char of 27 MJ.kg-1 +/- 0.17 MJ.kg-1 was found at 280°C and a 3:1 methanol ratio. The HHV of the bio-char decreases with an increase in temperature as more of the hydrocarbons are dissolved and form part of the bio-crude make-up. The highest HHV recorded for the crude bio-oil was 42 MJ.kg-1 at a 6:1 methanol ratio, a temperature of 300°C and an acid catalyst. The crude bio-oil HHV, which increased with an increase in temperature, is well within the specifications of the biodiesel standard (SANS, 1935). The highest FAME yield of 39.0 g.kg-1 was obtained using a 6:1 methanol ratio and a temperature of 300°C in the presence of an acid catalyst. The crude oil contained 49.0 g.kg-1 triglycerides with alkenes (C13, C15 and C17) making up the balance. The purest biodiesel yield was achieved at 3:1 methanol to dry biomass mass ratio, as it had the lowest yield unsaturated methyl esters. The overall FAME yield increased with an increase in methanol ratio. The derivatised FAME yields were the highest during hydrothermal liquefaction (55.0 g.kg-1 biomass). The in situ production of biodiesel from waste water clarifier effluent stream was found to be possible. Further investigation is needed into sufficient harvesting methods, including the optimum harvesting location, as this will result in fewer impurities in the stream and subsequent higher yields.
- Engineering