Enzymes as an environmental bottleneck in cellulosic ethanol production: Does on-site production solve it?

Highlights

• On-site enzyme production leads to lower environmental impact.

• Enzyme dosage and off-site inventory choice do not alter this previous conclusion.

• With on-site production, enzymes cease to be the major hotspot within the life cycle.

• On-site production leads to a 53% GHG emissions reduction, compared to off-site.

• There are trade-offs among impact categories for cellulosic ethanol production.

Abstract

With the progressive implementation of Low-Carbon Policies based on Life Cycle Assessment (LCA) methodologies to assess climate change mitigation, biofuels’ potential environmental advantages could finally be translated into economic revenues, especially for residue-based production chains. This implies, however, tackling environmental hotspots along the Life Cycle, such as the use of enzymes, an important input in cellulosic ethanol production. While relevant, however, aspects of enzyme manufacturing, crucial to the LCA results, are often oversimplified in databases or even undisclosed, which might render the accounting for this flow unreliable. A potential solution for this is to model enzyme production, disclosing aspects such as carbon source uptakes, energy consumption and purification steps, all of which may carry significant environmental burdens, especially for fossil-reliant procedures. Integrating these steps into the biorefinery process design as an on-site operation, however, would allow access to renewable energy and carbon sources for enzyme production, potentially reducing their burden in the final biofuel LCA profile, when compared with off-site supply. This work, then, aims to evaluate the effect of inserting on-site enzyme production within a stand-alone second-generation (2G) biorefinery, using sugarcane straw as feedstock, to produce ethanol and electricity, as opposed to off-site enzyme supply from manufacturers. The results show that adopting on-site production can reduce the overall environmental impact profile for cellulosic ethanol, in comparison to off-site supply, even though off-site data present significant variability among databases and literature. This trend persisted for varied enzyme dosages in the saccharification process and for different inventories to model off-site supply. The reference inventories for off-site supply lack transparency and details, while also displaying large discrepancies between impact indicators, indicating the need to pursue more consistent, reliable, and complete inventories for its modelling in LCA. Compared to first-generation (1G) sugarcane ethanol, 2G-ethanol (with on-site enzyme production) demonstrated a reduction in the Global Warming Potential category of around 80%. However, relevant trade-offs for 2G ethanol (with off-site enzyme supply) were observed – regarding ecotoxicity, mineral scarcity, eutrophication and even the potential GHG reduction – when different enzyme inventories are used.

Introduction

Bioenergy has been typically associated with energy security and a strategic mitigation instrument to climate change. Its use, in the last decades, has increased especially in the transport sector, a great contributor to the total anthropogenic greenhouse gases (GHG) emissions, accounting for 14.2% of the total emissions in 2018 (Climate Watch, 2021). Despite being a crucial mitigating alternative, biofuels corresponded to only 3.3% of the sector's total energy consumption in 2019 (IEA, 2021).

Among the biofuels in current use, ethanol holds a supply share of 62% on a global level (BP, 2020), mostly obtained from sugarcane and corn conversion routes. Such crops have been targeted by the “food versus fuel” debate and attributed with carbon emissions from direct and indirect land use change (Fargione et al., 2008; Lapola et al., 2010; Searchinger et al., 2008; van der Hilst et al., 2018), boosting research interest in residues-based biofuels.

Currently, most Low Carbon Policies (LCPs) are based on a life cycle approach, directly (or indirectly) supporting bioenergy production from residual feedstocks. In Europe, the Renewable Energy Directive (RED II) (European Union, 2018) has limited food/feed-based biofuels, promoting residues-based alternatives as more suitable options in Europe. In the United States, the Renewable Fuel Standard (RFS) (U. S. EPA, 2020) reports a large GHG emissions mitigation potential for cellulosic ethanol production from residues, in substitution to gasoline. This may even achieve negative net emissions, if land use change and avoided coproducts (i.e., electricity) are accounted for. In Brazil, with RenovaBio (ANP, 2018), residues-based pathways could also indicate lower global warming potential, when compared to food-based ones, since low or null emissions related to the feedstock procurement stages are attributed to the biofuel.

Despite its promising environmental advantages, ethanol production from lignocellulosic residues still faces challenges, especially regarding scale-up (Lynd, 2017) and operational costs (Bezerra et al., 2021), with LCPs playing a leading role to translate environmental performance into actual economic advantages that will stimulate the cellulosic ethanol (2G) industry and may finally overcome the barriers of technological and industrial learning curves. With this new perspective, tackling environmental bottlenecks will lead to a larger mitigation potential, and, thus, to higher revenues (Grassi and Pereira, 2019).

Several studies have reported the environmental performance of lignocellulosic ethanol production and use (Karlsson et al., 2014; MacLean and Spatari, 2009; Obnamia et al., 2019; Olofsson et al., 2017; Palma-Rojas et al., 2017; Papadaskalopoulou et al., 2019), mostly focusing on carbon emissions and energy use. Others have discussed the economic and environmental benefits for biofuel production comparing on-site enzyme production and off-site supply by manufacturers (Hong et al., 2013; Olofsson et al., 2017).

As observed in all these studies, the use of enzymes surfaced as an important hotspot in the overall Life Cycle Assessment (LCA) for all biorefinery products. Even so, enzymes are not always accounted as an input for cellulosic ethanol LCA (Gilpin and Andrae, 2017). The main reasons for that are the lack of reliable and transparent data, often associated with manufacturers’ bureaucracy and confidentiality regarding foreground process data (Jegannathan and Nielsen, 2013).

Some Life Cycle Inventories (LCI) are published based on primary information from enzyme suppliers (Inman, 2013; Nielsen et al., 2007; Novozymes, 2020). However, these inventories are not entirely transparent and do not disclose specific flows for energy consumption, macro and micronutrients used in the microbiological cultivation, water usage, among others. This severely limits their use, since no modifications can be made aside from elementary flows, which affects sensitivity analysis and also conceals process and inputs contributions to the final Life Cycle Impact Assessment (LCIA), which may mislead comparisons with other enzyme LCIs. On the other hand, some studies have disclosed more specific data regarding enzyme production environmental performance (Becker et al., 2021; Dunn et al., 2012; Gilpin and Andrae, 2017; Zhuang et al., 2007), which could be adapted within a specific biorefinery context.

This study aims to explore the enzyme contribution to 2G-ethanol's environmental performance, observing trends or possible trade-offs in different comparative scenarios. By a harmonized LCA, sugarcane straw ethanol production – with and without on-site enzyme production – was modelled and compared with first-generation sugarcane ethanol, assuming fifteen impact categories. The enzyme load contribution on the environmental performance was also evaluated, considering different dosages and eight different enzyme inventories – available on the literature or LCA databases – for off-site supply.