Chapter 12 - Biogas from microalgae

Abstract

Microalgae have attracted increasing interest as a source of biomass in recent years and their use for biofuels, replacing traditional fuels, has been widely studied. The anaerobic digestion (AD) of microalgae is one option that can be explored to produce renewable biofuel. Biogas produced by AD can be used to generate heat or electricity in cogeneration engines, be injected into the natural gas grid, or be used as fuel for vehicles after going through an upgrading process. However, the AD of microalgae can be limited by their morphological characteristics, such as microalgal cell wall composition, and the unbalanced carbon/nitrogen ratios. Different pretreatment methods have been studied in attempts to break down the cell wall. These methods include thermal, chemical, mechanical, and enzymatic pretreatments. Another approach to improving biogas production is to increase the carbon content within the media by using different co-substrates. This measure can balance the carbon/nitrogen ratio and prevent accumulation of ammonia in the media that could inhibit the digestion process. The use of an ammonia-adapted inoculum has also been studied. Microalgae can be used at different points of the AD process. They can be digested for biogas production, grown in anaerobic digestates where they utilize nitrogen, phosphorous and the remaining nutrients, or even be grown by utilizing the CO2 of the biogas, reducing the environmental impacts and costs of biogas upgrading and microalga growth. This chapter summarizes the state of the art of microalgae AD, including the study of different strategies for improving it.