Energy balance and techno-economic assessment of algal biofuel production systems

There has been considerable discussion in recent years about the potential of micro-algae for the production of sustainable and renewable biofuels. Unfortunately the scientific studies are accompanied by a multitude of semi-technical and commercial literature in which the claims made are difficult to substantiate or validate on the basis of theoretical considerations.To determine whether biofuel from micro-algae is a viable source of renewable energy three questions must be answered:a. How much energy can be produced by the micro-algae?b. How much energy is used in the production of micro-algae?c. Is more energy produced than used?A simple approach has been developed that allows calculation of maximum theoretical dry algal biomass and oil yields which can be used to counter some of the extreme yield values suggested in the 'grey' literature. No ready made platform was found that was capable of producing an energy balance model for micro-algal biofuel. A mechanistic energy balance model was successfully developed for the production of biogas from the anaerobic digestion of micro-algal biomass from raceways. Preliminary calculations had suggested this was the most promising approach. The energy balance model was used to consider the energetic viability of a number of production scenarios, and to identify the most critical parameters affecting net energy production. These were:a. Favourable climatic conditions. The production of micro-algal biofuel in UK would be energetically challenging at best.b. Achievement of ‘reasonable yields’ equivalent to ~3 % photosynthetic efficiency (25 g m-2 day-1)c. Low or no cost and embodied energy sources of CO2 and nutrients from flue gas and wastewaterd. Mesophilic rather than thermophilic digestione. Adequate conversion of the organic carbon to biogas (? 60 %)f. A low dose and low embodied energy organic flocculant that is readily digested, or micro-algal communities that settle readilyg. Additional concentration after flocculation or sedimentationh. Exploitation of the heat produced from parasitic combustion of micro-algal biogas in CHP unitsi. Minimisation of pumping of dilute micro-algal suspensionIt was concluded that the production of only biodiesel from micro-algae is not economically or energetically viable using current commercial technology, however, the production of micro-algal biogas is energetically viable, but is dependent on the exploitation of the heat generated by the combustion of biogas in combined heat and power units to show a positive balance.Two novel concepts are briefly examined and proposed for further research:a. The co-production of Dunaliella in open pan salt pans.b. A 'Horizontal biorefinery' where micro-algae species and useful products vary with salt concentration driven by solar evaporation.<br/

Energy balance of algal biogas production

A mechanistic energy balance model was successfully developed for the production of biogas from the anaerobic digestion of micro-algal biomass from raceways. The energy balance model was used to consider the energetic viability of a number of production scenarios, and to identify the most critical parameters affecting net energy production. The output of the model demonstrated that no single method of harvesting studied (centrifugation, settlement or flocculation), produced a sufficiently greater energy output over operational energy inputs to make algal biogas production viable. Combinations of harvesting methods produced energy outputs 2.3 to 3.4 times greater than the operational energy inputs. Electrical energy to power pumps, mixers and harvesting systems is 5 to 8 times greater than the heating energy requirement. If the energy to power the plant is generated locally in a combined heat and power unit a considerable amount of “low grade heat” will be generated that is not required by the algal biogas process and for the plant to be efficient must be exploited. It is concluded that the production of micro-algal biogas may be energetically viable, but is dependent on the exploitation of the heat generated by the combustion of biogas in combined heat and power units to show a positive balance

Liquid ‘hold-up’ on stainless steel surfaces: II — Effect of surfactant concentration

The effect of surfactant concentration on the ‘hold-up’ of a representative food liquid (sucrose solution) on various stainless steel surfaces was studied.Surface finish was found to have a significant effect on ‘hold-up’, but no correlation was found between Ra value and ‘hold-up’.Although surfactant concentration was found not to be a significant independent variable it did have a significant interaction with surface finish, the differences in ‘hold-up’ between the surface finishes being reduced by an increase in surfactant concentration.Differing surfactant concentrations in food soils is proposed as a possible explanation of the conflicting reports in the literature on the cleanability of stainless steel.An explanation of the effects of surface finish and surfactant concentration on ‘hold-up’ is proposed in terms of the effects on wetting and contact angle of surfactant and the nature of the surface.<br/

Liquid ‘hold-up’ on stainless steel surfaces: I — Effect of surface finish

The effect of stainless steel surface finish on the ‘hold-up’ of a representative food liquid (sucrose solution) was studied for drainage angles between 0·46° and 3·7°. Surface finish was found to have a significant effect on ‘hold-up’, but no correlation was found between the Ra value and ‘hold-up’.An explanation of the results is proposed based on the effect of the nature of surface roughness rather than its magnitude (as measured by the Ra value) on wetting and contact angle.The orientation of grain direction had only a small effect on ‘hold-up’ and was dependent on the surface finish.It is suggested that electro-polishing may give the most cost-effective finish for stainless steel food contact surfaces. Drainage angle was found to have a considerable effect on ‘hold-up’ and it is recommended that the minimum drainage angle for food plant should be 2°.<br/

Commercial application of microalgae other than as biofuels: a brief review

There has been considerable discussion in recent years about the potential of micro-algae for the production of sustainable and renewable biofuels, but there may be other more readily exploitable commercial opportunities for microalgae. This paper briefly reviews the current and potential situation for the commercial application of the growth of microalgae for products other than biofuels. <br/