Evaluation of suitability of water hyacinth as feedstock for bio-energy production

MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2014The suitability of water hyacinth (Eichornia crassipes) as a viable feedstock for renewable energy production was investigated in this project. Water hyacinth used in this study was harvested from the Vaal River near Parys in the northwest region of the Free State province, South Africa (26°54′S 27°27′E). The wet plants were processed in the laboratory at the North-West University by separating the roots from the leaves and the stems, thus obtaining two separate water hyacinth feedstock. Characterisation of the feedstock showed that the stems and leaves are more suitable for bio-energy production than roots, due to the higher cellulose and hemicellulose content and very low lignin content of the stems and leaves. Water hyacinth was evaluated as feedstock for the production of bio-ethanol gel, bio-ethanol, bio-oil and bio-char. The recovery of water from the wet plants for use in bio-refining or for use as drip-irrigation in agriculture was also investigated. Cellulose was extracted from water hyacinth feedstock to be used as a gelling agent for the production of ethanol-gel fuel. A yield of 200 g cellulose/kg dry feedstock was obtained. The extracted cellulose was used to produce ethanol-gel with varying water content. The gel with properties closest to the SANS 448 standard contained 90 vol% ethanol and 10 vol% water, with 38 wt% cellulose. This gel was found to ignite readily and burn steadily, without flaring, sudden deflagrations, sparking, splitting, popping, dripping or exploding from ignition until it had burned to extinction, as required by SANS 448. The only specifications that could not be met were the viscosity (23,548 cP) and the high waste residue (32 wt%) left after burning. The other major concern is the extremely high costs involved with the manufacturing of ethanol-gel from water hyacinth cellulose. It can be concluded that ethanol-gel cannot be economically produced using water hyacinth as feedstock. Chemical and enzymatic extraction of water from the feedstock, which is stems and leaves or roots, showed that the highest yield of water was obtained using a combination of Celluclast 1.5 L, Pectinex Ultra SP-L and additional de-ionised water. A yield of 0.89 ± 0.01 gwater/gwater in biomass was realised. This is, however, only 0.86 wt% higher than the highest yield obtained (0.87 ± 0.01 gwater/gwater in biomass) using only Pectinex Ultra SP-L and de-ionised water. It is recommended to use only Pectinex Ultra SP-L and de-ionised water at a pH of 3.5 and a temperature of 40°C. Using one enzyme instead of two reduces operating costs and simplifies the chemical extraction process. The extracted water, both filtered and unfiltered, was not found to be suitable for domestic use without further purification to reduce the total dissolved solids (TDS), potassium and manganese levels. Both the unfiltered and filtered water were, however, found to be suitable for industrial and agricultural purposes, except for the high TDS levels. If the TDS and suspended particle level can be reduced, the extracted water would be suitable for domestic, industrial and agricultural use. The potential fermentation of the sugars derived from the water hyacinth, using ultrasonic pretreatment, was investigated. Indirect ultrasonic treatment (ultrasonic bath) proved to be a better pretreatment method than direct sonication (ultrasonic probe). The optimum sugar yield for the ultrasonic bath pretreatment with 5% NaOH was found to be 0.15 g sugar/g biomass (0.47 g sugar/g available sugar) using an indirect sonication energy input of 27 kJ/g biomass. The optimum sugar yield is lower than those reported in other studies using different pretreatment methods. Theoretically a maximum of 0.24 g ethanol can be obtained per g available sugar. This relates to an ethanol yield of 0.08 g ethanol/kg wet biomass. The low yield implies that ethanol production from water hyacinth is not economically feasible. The production of bio-oil and bio-char from water hyacinth through thermochemical liquefaction of wet hyacinth feedstock was investigated. An optimum bio-char yield of 0.55 g bio-char/g biomass was achieved using an inert atmosphere (nitrogen) at 260°C and the stems and leaves as feedstock. With the roots as feedstock a slightly lower optimum yield of 0.45 g bio-char/g biomass was found using a non-reducing atmosphere (carbon monoxide) at 280°C. The bio-oil yield was too low to accurately quantify. As water is required during thermochemical liquefaction, it was found unnecessary to dry the biomass to the same extent as was the case with the pretreatment and fermentation of the water hyacinth, making this a more feasible route for biofuel production. Bio-char produced through liquefaction of roots as the feedstock and leaves and stems as the other feedstock had a higher heating value (HHV) of 10.89 ± 0.45 MJ/kg and 23.31 ± 0.45 MJ/kg respectively. Liquefaction of water hyacinth biomass increased the HHV of the feedstock to a value comparable to that of low grade coal. This implies a possible use of water hyacinth for co-gasification. The most effective route for bio-energy production in the case of water hyacinth was found to be thermochemical liquefaction (12.8 MJ/kg wet biomass). Due to the high production costs involved, it is recommended to only use water hyacinth as a feedstock for biofuel production if no alternative feedstock are available.Master