Ethanol production potential from AFEX™ and steam-exploded sugarcane residues for sugarcane biorefineries

Abstract

BackgroundExpanding biofuel markets are challenged by the need to meet future biofuel demands and mitigate greenhouse gas emissions, while using domestically available feedstock sustainably. In the context of the sugar industry, exploiting under-utilized cane leaf matter (CLM) in addition to surplus sugarcane bagasse as supplementary feedstock for second-generation ethanol production has the potential to improve bioenergy yields per unit land. In this study, the ethanol yields and processing bottlenecks of ammonia fibre expansion (AFEX™) and steam explosion (StEx) as adopted technologies for pretreating sugarcane bagasse and CLM were experimentally measured and compared for the first time.ResultsEthanol yields between 249 and 256 kg Mg−1 raw dry biomass (RDM) were obtained with AFEX™-pretreated sugarcane bagasse and CLM after high solids loading enzymatic hydrolysis and fermentation. In contrast, StEx-pretreated sugarcane bagasse and CLM resulted in substantially lower ethanol yields that ranged between 162 and 203 kg Mg−1 RDM. The ethanol yields from StEx-treated sugarcane residues were limited by the aggregated effect of sugar degradation during pretreatment, enzyme inhibition during enzymatic hydrolysis and microbial inhibition of S. cerevisiae 424A (LNH-ST) during fermentation. However, relatively high enzyme dosages (> 20 mg g−1 glucan) were required irrespective of pretreatment method to reach 75% carbohydrate conversion, even when optimal combinations of Cellic® CTec3, Cellic® HTec3 and Pectinex Ultra-SP were used. Ethanol yields per hectare sugarcane cultivation area were estimated at 4496 and 3416 L ha−1 for biorefineries using AFEX™- or StEx-treated sugarcane residues, respectively.ConclusionsAFEX™ proved to be a more effective pretreatment method for sugarcane residues relative to StEx due to the higher fermentable sugar recovery and enzymatic hydrolysate fermentability after high solids loading enzymatic hydrolysis and fermentation by S. cerevisiae 424A (LNH-ST). The identification of auxiliary enzyme activities, adequate process integration and the use of robust xylose-fermenting ethanologens were identified as opportunities to further improve ethanol yields from AFEX™- and StEx-treated sugarcane residues.Electronic supplementary materialThe online version of this article (10.1186/s13068-018-1130-z) contains supplementary material, which is available to authorized users.

Using steam explosion or AFEX™ to produce animal feeds and biofuel feedstocks in a biorefinery based on sugarcane residues

“…Low‐ and high‐severity AFEX™ pretreatment conditions were performed in a 3.8 L Parr reaction vessel (Parr Instrument Co, IL, USA) equipped with temperature and pressure sensors based on a previously published protocol . The bench‐scale AFEX™ pretreatments (conditions 1 and 3 in Table ) were performed with external heating to maintain the reaction temperature, and the ammonia cycle consisted of a three‐step sequence: NH 3 charging, soaking, and NH 3 vaporization.…”

“…Steam explosion (StEx) was performed at Stellenbosch University in an automated batch pilot‐scale unit (IAP GmBH, Graz, Austria) equipped with a 19 L StEx reaction vessel, a 100 L blow‐tank and a steam generator as previously described . Briefly, 500 g (dwb) of water‐impregnated biomass (bagasse or CLM) was top loaded into the preheated StEx reaction vessel and heated to the desired temperature using 30 bar (absolute pressure) saturated steam.…”

“…These enzymes were generously donated by Novozymes Inc. (Franklinton, NC, USA). Previously optimized ratios of CTec3, HTec3, and Pectinex Ultra‐SP were used for low and high solid‐loading hydrolysis of untreated, AFEX™‐ and StEx‐treated sugarcane bagasse, and CLM . A summary of the enzyme combinations used is available in Table .…”

“…Efficient and well studied biomass pretreatment technologies such as ammonia fiber expansion (AFEX™, trademark of Michigan Biotechnology Institute (MBI) International, USA) and steam explosion (StEx) have been demonstrated to enhance significantly the digestibility of low‐quality agricultural forages, including sugarcane residues, for biofuel production . Moreover, the same pretreatment technologies also have the potential to increase the digestibility and energy value of agricultural residues for animal feeds by enhancing the accessibility of the carbohydrates and proteins embedded in the plant cell wall matrix to rumen microbes and enzymes .…”

“…As a result, the removal of free soluble sugars from the forage or carrying out AFEX™ pretreatment at lower severity conditions (low temperature and lower ammonia‐to‐biomass loading) have been suggested as strategies to minimize the formation of imidazole‐derived compounds . However, reduction in either the pretreatment temperature or ammonia loading is likely to impact negatively on the improvement in digestibility of the pretreated forage . On the other hand, reducing the ammonia loading during AFEX™ pretreatment minimizes capital and operating costs associated with ammonia recovery …”