A study on growth, fermentation and thermochemical conversion of two microalgae species

Abstract

Butanol is a four carbon alcohol commonly used in the chemical industry and also as a transportation fuel additive. Acetone-butanol-ethanol (ABE) fermentation is one of the processes used to produce butanol through biological conversion. The current fermentation process that uses lignocellulosic materials or food-based feedstock is not favourable because it requires high energy for the pretreatment due to the presence of lignin in lignocellulosic materials. Therefore, fermentation using an alternative feedstock such as microalgal biomass, which is a non-food based material and contains low lignin content, is considered as one of the approaches to overcome these issues. This study, therefore, was undertaken to evaluate the potential of ABE fermentation using microalgal biomass from two microalgae species, a freshwater microalgae Chlorella sp. and a marine water microalgae T. suecica. This research involves the investigation of the entire process consisting of biomass production, pretreatment, and enzymatic saccharification for reducing sugar production and ABE fermentation. A preliminary study on the thermochemical conversion of microalgal biomass was also carried out in this research. A microalgal cultivation and carbohydrate accumulation study indicated that microalgal growth rate and carbohydrate content were significantly influenced by the cultivation conditions such as light intensity, temperature, pH, salinity and carbon dioxide concentration (CO2). The maximum biomass production, specific growth rate (µ) and carbohydrate content for Chlorella sp. were 0.567 gL-1, 0.252 d-1, and 32.41% of dried biomass respectively, attained at 2000 lux, 30°C in a medium with initial pH of 7 without addition of NaCl. The maximum biomass production, µ and carbohydrate content for T. suecica of 0.54 gL-1, 0.22 d-1, and 20.6% of dried biomass respectively, were attained at 3000 lux, 30°C in a medium with initial pH of 7 and 30 gL-1 of NaCl. This study also indicated that both microalgae were able to grow in a medium supplied with 15% CO2. Comparison of indoor and outdoor microalgal cultivation was performed at two different temperature ranges, low temperature: 10 - 20°C and high temperature range: 20 - 32°C. It was observed that higher microalgal biomass production and growth rate were obtained from the indoor cultivation compared than that of outdoor cultivation. The results suggested that the ambient temperature and natural light intensity fluctuation have a significant influence on the microalgal growth in outdoor cultivation. The biomass obtained from the cultivation was pretreated prior to hydrolysis and ABE fermentation. Dilute alkaline pretreatment, which is a less harsh and more environmentally friendly approach compared to acid pretreatment, was applied to pretreat the microalgal biomass and the process was optimised. The pretreatment conditions (alkaline agent, alkali concentration, temperature and reaction time) were found to influence the pretreatment performance. A quadratic model that describes interaction of pretreatment conditions was developed and successfully fitted to the experimental results (r2=0.92 for Chlorella sp. and r2=0.96 for T. suecica). This pretreatment method is able to disrupt the microalgal cell structure and preserve the chemical compound of the microalgal cell. The results also demonstrated that the dilute alkaline pretreatment was able to enhance the enzymatic saccharification of microalgal biomass. The enzymatic saccharification condition for reducing sugar production was optimised by varying the temperature, pH, enzyme concentration and biomass concentration in order to obtain the maximum sugar concentration from microalgal biomass. It was found that ≈90% saccharification yield of both pretreated microalgal biomass was achieved from the saccharification at the optimum conditions (temperature: 40°C, pH: 4.5 and biomass concentration: 5-10 gL-1). A high amount of glucose (50%) and xylose (45%) in both microalgal hydrolysate indicates that it can be used as chemical platform for biofuel production through the fermentation process. This study also demonstrates that a combination of dilute alkaline pretreatment followed by enzymatic saccharification can be applied to pretreat microalgal biomass prior to ABE fermentation. Subsequently, the ABE fermentation of microalgal biomass was performed using four different forms of these two microalgal biomass; (1) untreated, (2) alkaline pretreated, (3) lipid extracted, and (4) lipid extracted followed by alkaline treated biomass. Each of the samples was subjected to enzymatic saccharification for reducing sugar production prior to the ABE fermentation. The highest ABE concentration was obtained from the fermentation of the dilute alkaline pretreated Chlorella sp. (0.161 gL-1) and T. suecica (0.126 gL-1) biomass. It was found that the butanol conversion yield from the fermentation of alkaline pretreated Chlorella sp. and T. suecica was 0.3% and 0.7% dried biomass respectively. A preliminary study on thermochemical conversion of both microalgal biomass was also undertaken through pyrolysis and gasification process. The lipid extracted microalgal biomass exhibited low activation energy, which is favourable to be used in thermochemical conversion. In addition, the gasification of microalgae at 800°C and time of around 20 min were suitable conditions to complete the conversion in a thermogravimetric analyser. The findings from this study generate significant information on the production of biofuel in an environmentally friendly manner. This has the potential to be applied not only for butanol production, but also for the production of various types of microalgal carbohydrate-based biofuel such as bioethanol, biohydrogen and biomethane.