Bioethanol – status report on Lignocellulosic biomass is seen as an attractive bioethanol production from wood feedstock for renewable fuels, particularly ethanol. and other lignocellulosic feedstocks Lignocellulosic feedstocks include agricultural residues, wood, municipal solid waste and dedicated CHRIS SCOTT-KERR3, TONY JOHNSON1, energy crops which have significant advantages over BARBARA JOHNSON2, JUKKA KIVIAHO4 first generation feedstocks for ethanol production. The net energy balance of lignocellulosic ethanol, in terms 1 General Manager – Forest Industries NZ, Beca of energy in/energy out, has been shown to be AMEC, P.O. Box 903, Tauranga, New Zealand significantly lower than ethanol produced from 2 Snr Process Engineer, Beca AMEC, P.O. Box 903, sugarcane and starch feedstocks (2). Additionally, life- Tauranga, New Zealand cycle emissions of green house gases are reported to be 3 Snr Consultant, Forest Industries Consulting Group, 50-85% lower for lignocellulosic ethanol than those AMEC Americas Ltd, Vancouver, Canada from gasoline, with corn ethanol providing a 25-40% 4 Director of Projects & Technology, AMEC, Jose reduction (2,3). Lignocellulosic ethanol presents a Domingo Canas 2640, Nunoa-Santiago, Chile means of satisfying demand for ethanol without further pressuring food supply. Marginal land, not suitable for ABSTRACT food crops can be used, with less intensive use of water Lignocellulosic biomass is seen as an attractive and fertilisers. Production of cellulosic ethanol can feedstock for future supplies of renewable fuels, also utilise ‘waste materials’ such as agriculture and reducing the dependence on imported petroleum. forest residues as feedstocks. However, there are technical and economic impediments to the development of commercial TECHNOLOGIES processes that utilise biomass feedstocks for the Lignocellulosic biomass can be converted to ethanol production of liquid fuels such as ethanol. Significant using either a biochemical or thermochemical platform. investment into research, pilot and demonstration plants is on-going to develop commercially viable Biochemical conversion processes utilising the biochemical and thermochemical In biochemical conversion the plant fibre is separated conversion technologies for ethanol. This paper into its component parts; cellulose, hemicelluloses, and reviews the current status of commercial lignocellulosic lignin hence the term lignocellulosic or cellulosic ethanol production and identifies global production ethanol. The cellulose is then further broken down to facilities. simple sugars that are fermented to produce ethanol. Typically the process is carried out in 4 stages (Fig. 1): INTRODUCTION 1. Physical or chemical pretreatment of the plant Escalating petroleum prices, green house gas emissions fibres to expose the cellulose and reduce its and the threat to fuel security are strong drivers in the crystallinity, search for sustainable fuel alternatives. 2. Hydrolysis of the cellulose polymer, with enzymes Governments around the world have recognised the or acids, to simple sugars (glucose), role that biofuels will play in a renewable fuels 3. Microbial fermentation of these simple sugars to portfolio and have introduced minimum targets for ethanol, and their implementation in the future (1). 4. Distillation and dehydration to produce 99.5% pure alcohol. Fig. 1 Schematic of a biochemical cellulosic ethanol production process (4). Lignin is a byproduct of this process, and can be used Many pretreatments are currently being explored, as a boiler fuel or processed into specialty chemicals. ranging in chemistries from very acidic to mildly Hydrolysis and fermentation can be conducted alkaline, such as dilute acid, ammonia fibre expansion simultaneously in one stage but simultaneous (AFEX), wet oxidation, solvent based pulping (i.e. saccharification and fermentation (SSF) is yet to be organosolv) and steam explosion. The ideal implemented commercially, significant advances are pretreatment liberates hemicellulose, exposes the being made in this area. cellulose and allows the lignin to be separated and must also minimise the formation of degradation products Thermochemical conversion that can inhibit the subsequent hydrolysis and Thermochemical conversion transforms the fermentation processes. lignocellulosic feedstock into carbon monoxide and hydrogen (syngas) by partial combustion (Fig. 2). Lignin – As lignin is mainly responsible for These gases can be converted to liquid transportation lignocellulosic recalcitrance, particularly in softwoods, fuels or commodity chemicals by catalytic or biological studies have shown its separation during pretreatment pathways. The biological process converts carbon greatly enhances cellulose accessibility and enzyme monoxide to ethanol using a non-yeast fermentation effectiveness (6). Pretreatments that minimise lignin microorganism (eg. Clostridium ljungdahlii). redeposition and condensation on the fibre surfaces are Alternatively, the syngas can be fed to a catalytic favoured. Separation of lignin and production of reactor where the carbon monoxide and water are specialty lignin co-products also has the potential to combined via a metal-catalysed process to produce improve the overall economics. methanol, ethanol, other higher alcohols or liquid fuels (Fischer-Tropsch liquids). Gasification is important Hemicellulose –Is composed primarily of 5 carbon because lignin, which constitutes about 25 – 30% of sugars, these may be liberated during the pre-treatment cellulosic biomass, is also converted to syngas and process or require further treatment with hemi-cellulase subsequently converted to fuel. enzymes. The C5 sugars may be fermented to ethanol or sold as a co-product. CURRENT STATE OF TECHNOLOGIES AND TECHNICAL CHALLENGES Hydrolysis - Cellulose is broken down into individual glucose units by cellulase enzymes, under mild Biochemical conditions. Research is on-going to find reduce the Pretreatment - the usefulness of cellulose as a costs of enzyme systems that produce high sugar yields feedstock has been limited by its rigid structure and at accelerated rates and without the formation of difficulty to breakdown into simple sugars. Cost- inhibitory byproducts. Currently, the per unit cost of effective pretreatments are needed to liberate the enzymes is considered to be a deterrent to the cellulose from the lignin/hemicellulose matrix and commercial success of the biochemical pathway. reduce its crystallinity. Pretreatments of increasing Alternative strategies to reduce enzyme cost include the severity are needed as feedstock recalcitrance increases recycling of enzymes and the use of polymers to reduce from nonwoods (agricultural residues) to hardwoods to the binding of enzymes to the substrate (7). softwoods. Fig. 2 Schematic of a thermochemical cellulosic ethanol production process (5). risk. Many starch-based ethanol producers are Fermentation - The hydrolysate contains both 5-carbon currently struggling to survive and many may go (pentose) and 6-carbon (hexose) sugars. The bankrupt due to the volatile swings in corn, wheat, conversion of pentose sugars into ethanol is less sugar and ethanol prices. efficient than conversion of hexose sugars. A system of mixed-sugar fermenting microorganisms is required A recent study of the production of Fischer-Tropsch to utilise the full range of sugars present and thus (FT) liquids from syngas has shown the maximise the production of ethanol. Metabolic thermochemical platform to be economically engineering is on-going to find low-cost, comparable to the biochemical platforms (8). microorganisms capable of C5 and C6 sugar co- fermentation that are also resistant to inhibitors (acetic CELLULOSIC ETHANOL PLANTS acid, furfural) that may be present. This section provides brief details on all publically announced bioethanol plants based on lignocellulosic Thermochemical feedstocks. The authors have endeavoured to be Contamination – various components of the biomass comprehensive but do not guarantee all facilities are feedstock can cause problems in the gasification and included. Table 1 summarises all known facilities as of catalytic synthesis stages. Contaminants such as tars February 2009, and these are shown geographically on and inorganic components (halides, alkalis, ash) present Fig. 4. in the syngas can deactivate the catalysts and must be removed prior to catalytic conversion. The formation of Table 1 Lignocellulosic Facilities as at February 2009. tars, and measures to deal with their removal, are significant challenges in biomass gasification. Pilot1 / Commercial3 Advances in gas clean-up and catalyst preparations are Demonstration2 also needed in order to make large-scale biomass to Biochemical 25 9 liquid facilities practical. Thermo Chemical 5 3 ECONOMICS Note: 1. Pilot Scale is R&D Both the biochemical and thermochemical pathways 2. Demonstration Scale is < 10 ML/yr 3. Commercial Scale is > 10 ML/yr require sophisticated processing steps that have higher operating costs and need significant capital investment United States compared with grain-based ethanol processes. Based Currently the US has a target of 136,260 million litres on the current state of technology, capital costs for per year (ML/yr) of renewable fuels production by biochemical cellulosic ethanol are estimated to be 2022. The US Renewable Fuels Standard calls for between US$4.03 and $5.60 per US gallon of annual 60,560 million liters to come from lignocellulosic capacity (8, 9). Operating costs are estimated to be