D. Hess, K. Napan, B. McNeil, J.C. Quinn
Utah State University,
Keywords: microalgae, biofuel, flue gas, scalability
Summary:Large scale biodiesel production from microalgae is expected to be integrated with point source CO2 sources, such as coal fired power plants. Flue gas integration represents a required nutrient source for growth while concurrently providing an environmental service. Heavy metals inherent in coal will ultimately be introduced to the culture system. Introduction of heavy metals have the potential to bind to the microalgae cells, impact growth due to toxicity, and negatively impact the quality of biofuel and other microalgal derived products. Heavy metals As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, V and Zn were added to microalgae growth medium at a base concentration estimated to be representative of concentrations expected from 7 day growth periods using coal flue gas. Experimentation was conducted with Nannochloropsis salina cultivated in photobioreactors at a light intensity of 1000 µmol m-2 s-1. Heavy metals negatively impacted the growth with the average productivity being 0.54 ± 0.28 g L-1 d-1, corresponding to an average decrease of 52% in biomass yield compared to control growths. Heavy metals analysis performed using inductively coupled plasma mass spectrometry (ICP-MS) shows significant binding of the majority of the heavy metals to the biomass. Lipid content analysis performed using gas chromatography (GC) on the control and heavy metal contaminated biomass measured a decrease in lipid content from 38.8 ± 0.62 to 31.58 ± 0.50 (percent dry biomass) respectively. Control and heavy metal contaminated biomass were processed into biodiesel through one of two different in-situ transesterification techniques, being either an acid-catalyzed or supercritical methanol conversion. The effects of the heavy metals on biofuel production and lipid content were quantified for both conversion types. For the acid-catalyzed conversion, average crude biodiesel production decreased from 0.31 ± .03 grams biodiesel/gram microalgae for the control growths to 0.28 ± .02 grams biodiesel/gram microalgae from the heavy metals growths, representing a 9.7% reduction. For the supercritical methanol conversion, average crude biodiesel production decreased from 0.38 ± .03 grams biodiesel/gram microalgae for the control growths to 0.32 ± .01 grams biodiesel/gram microalgae from the heavy metals growths, representing a 15.8% reduction. Compared to the control the total production of biofuel from the contaminated system was decreased by 51% for the acid catalyzed conversion and 55% for the supercritical methanol conversion. Heavy metal analyses of the biofuel and particulates present in the crude were performed using the ICP-MS. Results are extrapolated to a national scale illustrating the impact microalgae can have on 2030 DOE renewable fuel targets.