The number of studies on economic aspects of ethanol production from biomass is rather limited. This depends to a large extent on the fact that the ethanol production from biomass has not yet been demonstrated on commercial scale. The ethanol production cost varies between the studies performed from about 0.13 to 0.81 US$ per liter ethanol, see Table 1. Dur­ing 2006 the selling price of bioethanol, produced from starch or sugar-based materials, has fluctuated around 0.65 US$ per liter of ethanol with a peak at 1.12 US$/liter [21]. The future selling price will be dependent on demand and availability, which is influenced by political decisions, such as the EU direc­tive mentioned before. Also tax exemptions, e. g. exemption of CO2 tax and energy tax adopted in Sweden, and protective duty, as that applied in the EU, will impact the pricing for customers.

The large differences in ethanol production costs in Table 1 can be ex­plained by variations in the process design and in the assumptions underly­ing the techno-economic evaluations. Process variations are due to different conversion technologies, e. g. an enzymatic process with SHF, SSF or SSCF or the use of various types of raw materials. Thus, for meaningful compari­son the actual differences have to be identified. The discrepancies that arise due to various assumptions, in many cases, overshadow the actual differences.

Typical examples are the raw material cost (even if the same raw material is used), plant capacity and investment parameters, e. g. pay-off time and in­terest on capital. Also, the country in which the proposed plant is assumed to be located is of importance. One of the main influences on the produc­tion cost originates from the assumed annual capacities of the ethanol plants, which varied from 196 000 to 3 110 000 tons of raw material. This has a con­siderable influence on the total production cost (Fig. 4). Another difference is found in the overall ethanol yields assumed, e. g. if pentoses are converted to ethanol or not. Also, changes in process configurations, or a change in the equipment included in the ethanol production process also influences the overall cost, e. g. whether utilities such as process steam and electricity are in­cluded. Therefore, care must be taken when comparing ethanol production costs from different studies. However, this does not mean that the economic studies are without value. They give important information about which parts of the process are most costly, and where bottlenecks, which need to be ad­dressed by further research, can be expected.

Most economic studies performed on the enzymatic bioethanol process during the past ten years have been for a configuration using some kind of

steam pretreatment employing an acid catalyst. NREL has for many years been conducting detailed techno-economic evaluations of ethanol produc­tion from lignocellulosic materials. In the 1999 report [16] hardwood (poplar) was considered as the raw material and the proposed annual capacity was 700 000 dry metric tons. The following process configuration was assumed. The raw material is pretreated with dilute sulfuric acid at 190 ° C for 10 min­utes. The liquid hydrolyzate is detoxified by ion exchange and overlimed, after which an SSF step is employed. Some of the slurry following pretreatment is used for enzyme production. In the SSF step the remaining cellulose is con­verted to glucose and both hexoses and pentoses are considered fermentable. The ethanol is removed from the mash through stripping and the stillage is dewatered by means of centrifugation. The solids, together with the con­centrated liquid from the evaporation step, are transferred to a boiler for steam and electricity production. The estimated ethanol production cost was 0.38 US$ L-1. Especially the database of physical properties [22], but also sev­eral of the unit operation models from this study, has been used by many other investigators.

Lynd et al. [23] evaluated a process based on dilute acid hydrolysis, pentose fermentation and SSF using hardwood as the raw material, assuming the fol­lowing procedure. Distillation bottoms are centrifuged and the solid residue, together with methane and sludge from the anaerobic digester, is sent to the boiler where process steam and electricity are generated. The plant capacity was assumed to be 658 000 dry tons per year. The ethanol production cost was estimated to be 0.31 US$ L-1. A techno-economic evaluation based on the same process concept and the same raw material as the above, with an annual capacity of 640 000 tons, was conducted by Stone et al. [24]. This re­sulted in an estimated production cost of 0.34 US$ L-1. In the study presented by Lynd et al. [23] a more advanced scenario was also evaluated where fu­ture improvements in conversion technology were included. These include, but are not limited to, a higher overall ethanol yield, the use of a microorgan­ism capable of not only fermenting sugars to ethanol but also of hydrolyzing the cellulose (direct microbial conversion), and shorter residence times in the process steps. With these improvements, together with an increased cap­acity (2 738 000 dry tons per year) the projected ethanol production cost was 0.13 US$ L-1.

A comparative study of an SSF-based process and a process using dilute acid hydrolysis was performed by So and Brown [25]. The SSF process used the same conversion technology as the process evaluated by Lynd et al. [23], while the dilute sulfuric acid step was assumed to be carried out at 180 °C with an acid concentration of 5 gL-1. Estimated production costs at a plant capacity of 25 million gallons of ethanol per year (equivalent to around 260 000 tons of dry raw material per year) were 0.34 US$ L-1 for the SSF-based plant and 0.36 US$ L-1 for the process employing a dilute sulfuric acid process.

In the NREL report of 2002 [26] the raw material was changed to corn stover. Several changes were also made to the model from 1999. Instead of running SSF, an SHF configuration (including pentose fermentation) was employed, where the saccharification was carried out separately prior to fer­mentation. The reason for this was to be able to carry out saccharification at a higher temperature than in the fermentation step. The enzyme produc­tion step was also removed and it was assumed that the enzymes had to be purchased from an enzyme-producing company at an estimated cost of 0.10 US$ per gallon of ethanol. This represents a projected future cost rather than the present cost. Changes were also made to the separation of solids from the stillage stream. Instead of centrifuges, as suggested in the 1999 report, a horizontal belt filter (of the type manufactured by Pneumapress Filter Corporation, CA, USA) was employed and the concentration of wa­ter insoluble solids (WIS) in the filter cake was assumed to be about 50%. The overall ethanol yield increased significantly compared to the 1999 report (from 73.6% to 85.5% concerning cellulose, while 85% yield was assumed from all hemicellulose sugars). The estimated production cost was reduced to 0.28 US$ L-1.

According to a techno-economic evaluation of ethanol production from biomass performed by “SRI Consulting’s Process Economic Program” (PEP) [27], the capital investment required for a plant producing ethanol from 2000 metric tons of straw per day would be around 260 million US$. The plant is assumed to produce 190 million liters of ethanol per year, which gives an investment cost of around 1.37 US$ per liter ethanol and 370 US$ per ton raw material. This is about 2.5 times higher than the investment cost of a corn-based ethanol plant with the same feed capacity. On the basis of the ethanol produced, the ratio would increase to above 3 as the yield of ethanol per ton raw material is higher for corn than for lignocellulosic materials.

The highest contribution to the capital cost, 45% of the total, was equip­ment for the production of heat and electricity for the process and for sale to the grid, wastewater treatment and other utilities. This is not a direct cost of the ethanol production equipment, and in our opinion is often underesti­mated in most studies on ethanol production cost. Another cost that differs widely between studies is the cost of raw material. This depends on both dif­ferences in the type of raw material (agricultural residues, forest residues or energy crops) and on the location of the raw material. According to the Road Map for Agricultural Biomass Feedstock Supply in the US presented by the DOE [28] the goal is to reach a feedstock cost of 30 US$ per dry ton. On the basis of this figure the net raw material cost, i. e. after by-product credit, in the PEP study would be about 0.07 US$ per liter of ethanol, which corres­ponds to 20% of the total production cost of 0.34 US$ L-1. This production cost is, however, without any profit. The cost of biomass in Sweden, and other European countries is much higher, exceeding 90 US$ per metric ton of dry matter [29], which results in a net raw material cost of about 60 US$ per met­ric ton. This would have increased the total production cost in the PEP study to about 0.44 US$ L-1. However, according to the PEP study, the main reason for the higher investment cost for biomass-produced ethanol is due to the cost of conditioning and pretreating the biomass to make the cellulose accessible to enzymatic hydrolysis, which was estimated to represent 27% of the total fixed capital.

Eggeman et al. [30] investigated the pretreatment cost in ethanol produc­tion from corn stover for five different pretreatment methods: dilute acid, hot water, ammonia fiber explosion (AFEX), ammonia recycle percolation (ARP) and lime. The pretreatment design was based on experimental data from various research groups [31] and was implemented in the Aspen Plus model for a full-scale bioethanol plant previously developed by NREL [26]. The model was based on a corn stover feed rate of 2000 dry metric tons per day. The process configuration was based on pretreatment, SSF, ethanol re­covery and internal production of heat and electricity from the syrup and solid residue from the process. The process configuration was identical for all processes except for the pretreatment step. The dilute acid pretreatment process resulted in the lowest ethanol production cost, 0.26 US$ L-1 for the base case alternative where oligomers released in the pretreatment and hydro­lysis steps were not considered for ethanol production. The production cost includes depreciation, but no income tax or return on capital, to make it com­parable to the other costs presented in this review. The total investment cost was estimated to be 185.8 million US$, of which the pretreatment step consti­tuted 25 million US$, i. e. 13.5% of the total. The largest investment cost was for steam and power, 41.8 million US$, which represents 22.5% of the total investment cost.

Two-step pretreatment has been suggested to improve the overall sugar yield in several studies [32-34]. The first step is performed at low severity to release hemicellulose sugars, which are then removed, followed by the sec­ond step at more severe conditions to make the cellulose more accessible to enzymatic attack. Wingren et al. [35] compared the ethanol production cost of two-step steam pretreatment of SO2-impregnated spruce with that for one — step pretreatment, based on experimental data for pretreatment at optimal conditions. The production plants considered were designed for a yearly cap­acity of 200 000 tons raw material, and the only difference between the two plants was the pretreatment step. The process was based on SSF of the pre­treated material. The two-step process resulted in a higher ethanol yield (see Table 2) and a lower requirement for enzymes. However, due to the higher energy demand and higher capital cost the estimated ethanol production cost was the same, 0.55 US$ L-1. In the most optimistic scenario, where the material from the first step was dewatered to 50% dry matter (DM) with­out reducing the pressure, and the overall ethanol yield was assumed to be the highest achieved in the experimental work, 77%, the production cost de­creased by 5.6% to 0.52 US$ L-1. This shows the potential of the two-step pretreatment process, which, however, remains to be verified in pilot trials.

Hamelinck et al. [36] investigated the effect of expected future improve­ments in the conversion of biomass to ethanol, with poplar as a model raw material. The paper contains detailed information on the technical and eco­nomic data used in the analysis. Three scenarios were investigated: short­term (5 y), middle-term (10-15 y) and long-term (> 20 y). Improvements were mainly expected to be in enhanced pretreatment and bioconversion steps, changing from SSF to SSCF of hexose and pentose sugars and finally Consolidated BioProcessing (CBP), resulting in higher ethanol yield and re­duced capital costs. This is based on improvements of both enzyme efficiency and fermentation microorganisms. For CBP a completely new microorgan-

ism, not yet known, was assumed. The production capacity is also assumed to increase, from a biomass input (in metric tons per year) of 620 000 to 1550 000 and 3 110 000 for the short, medium and long term, respectively. Also, the cost of the raw material was assumed to fall from about 58 US$ per metric ton for the short term to 48 and 39 US$ for the middle and long term.

The total investment cost for the short-term scenario was about 395 mil­lion US$, i. e. 475 US$ per ton raw material yearly capacity. The ethanol production cost was determined to be 0.51 US$ L-1. Forty-five percent was due to capital cost and about 35% to the net raw material cost, i. e. after credit for co-produced electricity. For the middle-term scenario the total invest­ment cost increased to around 465 million US$, corresponding to 300 US$ per ton raw material yearly capacity. The ethanol production cost decreased to 0.31 US$ L-1, of which about 44% arose from capital cost and about 31% due to net raw material cost. For the long-term scenario the total invest­ment cost increased to around 820 million US$, corresponding to 263 US$ per ton raw material yearly capacity. The ethanol production cost decreased to 0.20 US$ L-1, of which about 50% was due to capital cost and about 42% to the net raw material cost. The long-term ethanol production cost was still considerably higher than that predicted by Lynd et al. [23] who estimated the future ethanol production cost for a plant based on CBP to be between 0.1 and 0.16 US$ L-1. The main reasons for their lower cost are a higher conversion yield and lower capital costs. It must be pointed out that these are scenarios based on future projected improvements, which are very uncertain.

The number of studies on softwood is more limited. Fransson et al. [27] studied the potential for a two-stage dilute acid process using softwood as raw material. The plant is assumed to be co-located with a heat and power plant from which steam is purchased and co-products (solid residue) are sold. The first hydrolysis step is assumed to be run in co-current fashion, whereas the second reactor is working in counter-current mode to maximize sugar yield and reduce sugar degradation. A sulfuric-acid concentration of 5 g/L is used in both steps. The sugar stream is detoxified in an overliming step and then fermented to ethanol. The dilute ethanol is concentrated in a distillation step and the stillage is then evaporated. The plant is designed to process around 300 000 tons of raw material annually. The estimated ethanol production cost was 0.53 US$ L-1.

In another study by Wingren et al. [38], the production of fuel ethanol from spruce using the enzymatic process was investigated. The softwood was steam pretreated after impregnation with SO2. Two configurations, one based on SSF and the other on SHF, were evaluated and compared. The process conditions selected were based mainly on laboratory data and the processes were simulated using Aspen Plus, while the capital costs were estimated using Icarus Process Evaluator. The ethanol production cost was estimated to be 0.69 and 0.76 US$ L-1 for the SSF and SHF cases, respectively, based on a raw material cost of 63 US$ per dry ton. The main reason for SSF being less ex­pensive was due to the capital cost being lower and the overall ethanol yield higher. Improvements in the SSF process, by running SSF at 8% WIS rather than at 5%, and by recycling of process streams, were shown to result in a de­crease in production cost to 0.51 US$ L-1.

3.1