Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
Applying a biorefinery concept for algal biofule production requires that each component of the biomass, i.e., carbohydrates, lipids, proteins be exploited to products. The aim of this study was to evaluate different biorefinery concepts to produce syngas in a WoodRoll®
Gasifier using microalgae biomass as feedstock. Moreoever, mass culture of microalgae for commercially-viable yields requires CO2 addition above ambient air concentrations. Hence we at first compared growth of chlorophyta Dunaliella tertiolecta during exponential phase with continuous sparging of ambient, 3%, 7% and 12% CO2 in pre-moistened air. This series of experiments were conducted in order to deliver better understanding of CO2 tolerance and its effects on growth for this alga, ahead of a planned follow-on experiment (with this and other algae specie), which will use raw flue gas (i.e., a mixture of approximately 3% CO2 together withother natural gas combustion products such as NOx and Sox).There was no statistically significant difference in growth rate observed for treatments with ambient, 3% and 7% CO2 in air. However, a decrease in growth was observed for the 12% CO2 treatments. Furthermore, three microalgae, Dunaliella teriolecta, Scenedesmus dimorphus, and Chlamydomonas reinhardtii were evaluated for gasification using thermogravimetric analysis, to determine changes in weight relative to temperature applied. Thermogravimetric analysis enables observations to be made for moisture content, reactivity to different pyrolysis and gasification temperatures, and ash content of the biomass. Algal biomass slurries were prepared by centrifugation and lyophylization to approximately 5% moisture content, and samples for thermogravimetric analysis were weighed-out to 200 mg (±0.02g). Each sample was analyzed twice, first at a gasification temperature of 850° C for 90 minutes, and secondly at a gasification temperature of 1100 °C for 20 minutes. For all runs, three different pyrolysis temperatures were used 360 °C, 380 °C and 400 °C. After gsification at 1100 °C, ash content varied between 3-7% depending upon specie, compared to ash content in the range of 5-17% after gasification at 850 °C. The energy distribution after pyrolysis for S. dimorphus, and C. reinhardtii biomass fit perfectly into the energy and mass balances for a WoodRoll ® system, in that each showed high reactivity during gasification and low final ash content. Therefore we conclude that both specie are highly feasible potential fuels for the WoodRoll ® process. Dunaliella tertiolecta is also a good potential fuel for the WoodRoll ® gasification, especially when char yield is maximized through manipulation of process conditions.
In order to do a comparison study and to evaluate and rate different concepts, 24 different scenarios were designed in total; three different gasification capacities (0.5 MW, 5 MW, 50 MW), two different scenarios for biomass end-product, two different microalgal cultivation systems (Hyperlight ® and Open Pond Raceways) and two different methods for the total capital investment (Lang factor method & Dispersion method). With the delimitations and assumptions within this thesis, only one feasible scenario (5 MW, Open Pond Cultivation System, Scenario B, Dispensed Method) was achieved, a scenario with an internal rate of return (IRR) of 15% and pay back time of 4.9 years, which exceed the given data (IRR 11%).
microalgae, biorefinery, gasification, concentrated solar power, bioprocess, fluegas, CO2 capture