4. R ESULTS AND D ISCUSSION
4.4. C OMPARISON OF THE DIFFERENT PRETREATMENT PROCESSES
The main purpose of this study was to investigate the different pretreatment process mainly in the view of enzymatic hydrolysis.
The enzymes are of vital importance in the breakdown of cellulose to glucose, however the pretreatment is essential to make cellulose accessible to the enzymes. In Chapter 4, detailed results following different type of pretreatments are presented, and it seemed, that many of them were able to increase significantly the enzymatic convertibility of cellulose
Table 4.19. Summarzing table: Results following the best pretreatment methods. The mass balance based on 100 g (DM) untreated corn stover (containing 41 g cellulose, 28 g hemicellulose and 22 g lignin).
XYLOSE (G) CELLULOSE (G) ECC (%) RELEASED GLUCOSE (G) RECOVERY (%) OF
Released from
the pretreatment in the solid fraction following pretreatment
Enzymatic
convertibility From
hydrolysis From hydrolysis and pretreatment
Hemicellulose Cellulose
UNTREATED CORN STOVER 41 18.1 8.2
CHEMICAL PRETREATMENT 1: ALKALINE PRETREATMENT
1% NaOH 14.5 28.6 79.5 25.0 33.3 85.9 89.4
5% NaOH 17.9 24.0 82.0 21.6 32.3 85.1 84.0
10% NaOH 20.2 19.3 84.4 17.9 31.5 84.1 79.7
CHEMICAL PRETREATMENT 2: ACIDIC PRETREATMENT
1% H2SO4 17.5 25.8 51.9 14.7 25.1 84.7 87.5
5% H2SO4 19.3 29.5 38.4 12.5 20.5 83.0 90.9
CHEMICAL PRETREATMENT : TWO-STEP PRETREATMENT
1% NaOH and 1% H2SO4 18.9 18.5 95.7 19.5 36.7 82.9 79.6
STEAM EXPLOSION
190°C/2% H2SO4/5min 21.3 24.2 81.1 21.6 34.3 72.0 83.0
200°C/2% H2SO4/5min 19.0 20.0 83.6 18.4 31.3 64.0 74.0
210°C/2% H2SO4/5min 16.8 13.9 78.8 12.0 28.3 55.1 73.0
WET OXIDATION
195°C/15min/alkaline 19.5 34.9 83.1 31.9 35.3 60.1 92.6
195°C/15min/acidic 20.3 36.2 73.7 29.3 31.6 57.2 93.4
74
The highest enzymatic cellulose conversion, nearly theoretical (95.7%), was achieved following two-step chemical pretreatment, where 1% NaOH was used after 1% H2SO4. Although the convertibility of the pre-treated cellulose is very important in characterisation of the pretreatment, but a good fractionation method is also able to retain the cellulose in the pre-treated solid fraction, which makes the further processing more easier. During this two-step chemical pretreatment nearly half of the original cellulose and around 75% of the original hemicellulose was solved. The purification and/or utilisation of the solved sugars from the acidic and the alkaline solution and also from the washing-water is a difficult technical problem and makes a shock in process economy. The separation and the washing following both steps of this pretreatment are necessary to remove the chemicals from the fibers, because it has a negative effect to the enzymes.
Satisfactory enzymatic conversions were also achieved using concentrated, 5 and 10%
NaOH, resulting 73 and 79% conversions respectively. The disadvantages of this chemical pretreatment are the same as in the case of the two-step pretreatment. Acidic chemical pretreatment, even when concentrated acid was used, at the applied relative low temperature (100-120°C) gave only poor enzymatic conversion. Thus it is not proposed for pretreatment of lignocellulosic materials, however it could be used effective when the aim of the work is the hemicellulose solubilization.
To make the pretreatment more environmental friendly and find an economically feasible method, steam pretreatment and wet oxidation process were tested for treating of corn stover. Both processes run at higher temperature and with fewer amounts of applied chemicals, compared to previous mentioned chemical pretreatments.
Steam pretreatment and wet oxidation showed several common features. Both processes increased the enzymatic conversion significantly, the highest conversions was 83.1 following wet oxidation and 83.6% after steam pretreatment. The selected “best”
pretreatment conditions in the view of enzymatic conversion, were 195°C, 15 min, alkaline addition in wet oxidation and 200°C, 5 min, 2% H2SO4 in steam pretreatment. The distinctive feature of an ideal pretreatment, is the high recovery of both polysaccharides. In comparison of the hemicellulose recovery of this two physico-chemical pretreatments, the results were also the same around 60%, however cellulose recovery was 15% lower following steam pretreatment. The wet oxidation was also much better in retaining cellulose in the solid fraction after reaction, than steam pretreatment. Following the above selected pretreatment conditions the cellulose content in the solid fraction was 30% higher after wet oxidation, than after steam pretreatment. In lignin content the difference was even bigger. The original lignin content decreased by 75% following wet oxidation while it was only 10% after steam pretreatment.
To obtain high ethanol yield, the efficient enzymatic hydrolysis is necessary, however the fermentability of the pre-treated material is also essential. The wet oxidised corn stover
195°C, for 15 min, both at alkaline and acidic conditions. It is worth to mention, that while alkaline wet oxidation resulted the highly digestible cellulose, the highest ethanol yield was achieved after acidic wet oxidation. The fermentability of steam pre-treated material was also tested, and these results were also attractive, the achieved ethanol yield was above 80%.
Based on the comparison of wet oxidation and steam pretreatment, it could be concluded, that both processes are able to enhance significantly the enzymatic conversion, and the pre-treated material is highly fermentable. However because of the higher cellulose recovery, and the higher cellulose content of the pre-treated material, wet oxidation process (under the above selected conditions) is proposed for pretreatment of corn stover.
4.5. E
STIMATED PRODUCTION COST OF FUEL ETHANOL4.5.1. Production cost of starch based bioethanol
For a rough cost calculation, approximately 3.4 kg corn is needed for production of 1 L EtOH. While the production cost of corn is on average 73-98 $/t, the total production cost of 1L starch based ethanol is 0.25-0.35 $ in general.
The use of corn for producing EtOH in the world shows significant regional variations with respect to the energy consumed. The major differences are to be found in the use of fertilizers and in the irrigation techniques employed. In most cases, the energy inputs are coal, natural gas and fuel oil, i.e., all of fossil origin. Energy is consumed both in the corn production process (primarily in the form of energy used for the production of fertilizers and as fuel for the agricultural machines) and in the process of converting corn into ethanol (grinding, enzymatic hydrolysis, fermentation and distillation).
4.5.2. Production cost of lignocellulose based bioethanol
A detailed engineering and economical analysis of a process employing steam explosion as pretreatment followed by enzymatic hydrolysis, performed by Nysrtom et al. (1985) gave an ethanol production cost of 1.02 $/L, employed a capacity of 410 metric tons of dry hard wood per day.
Another cost estimate made by Clausen and Gauddy (1983) for a dilute acid hydrolysis process resulted in an ethanol production cost of 0.24 $/L. The process capacity was about 630 metric tons of dry oak per day.
Lambert et al. (1990) have presented data for hardwoods and corn stover as feed stock for dilute and concentrated acid hydrolysis processes, based on pilot-scale studies. Both
processes were based on 500 metric tons of raw material per day, treated in two hydrolysis steps. The ethanol production cost was estimated to 0.45 $/L in the concentrated acid process, and 0.48 $/L in the dilute acid process.
A flow sheet program and an economic evaluation program were used for simulations and economics evaluations in the cost estimate study of Söderström (2004). The production plants considered were designed to utilise spruce as raw material with a capacity of 550 tons per day, employing steam explosion as pretreatment followed by enzymatic hydrolysis. Based on these assumptions, the ethanol production cost was 0.57 $/L.
The variation in the production cost can be explained by different assumptions made in the technical and economic calculations. Differences are found not only in the raw materials used (softwood, hardwood, straw, corn stover, etc.) and the type of process utilised (enzymatic hydrolysis, dilute or concentrated acid hydrolysis), but also in the design of the process (separate hydrolysis and fermentation, simultaneous saccharification and fermentation, pentose fermentation, etc.) and the assumptions concerning yields, capacity and costs (Von Sievers and Zacchi, 1996). There are other factors that have to be taken in consideration; e.g. collection cost of corn stover would depend on the amount of stover collected per unit area, the number of operations, machine efficiency in each operation and bulk density, but it also depends on the delivering distance from a harvested field (Sokhansanj et al. 2002).
The differences of the estimators are, to a certain extent, unavoidable, as there are no full-scale plants from which information on yields and other crucial data are available. The accuracy in the calculations also varies between different investigators. A rough estimate of the poduction cost on starch and cellulose based bioethanol is shown in Figure 4.19.
0 25 50 75 100
starch base d ce llulose base d
Fuel ethanol cost (%) Variable operating
costs
By-products
Labor, supplies and overhead Feedstock
Figure 4.20. shows the contribution of major cost elements to overall ethanol production cost (Wyman, 1999). The feedstock in this case is poplar and the applied pretreatment method is a dilute acid hydrolysis at elevated temperature. This figure shows that the feedstock is still the single most costly element, at approximately 39%
of the total, but as mentioned above, it is diffcult to impact feedstock costs substantially for eventual large-scale bioethanol production. However, the costs of the processing steps can be reduced further, and the most expensive of these steps is for pretreatment, representing almost one-third of the total processing costs. The second most costly operation is the SSF process, accounting for approximately 28% of the total. Thus, a better pretreatment technology could have an impact both by lowering the cost to break down hemicellulose and by improving the rates and yields in the SSF process. The third most costly operation is product recovery, but at a far lower 12.6% of the total processing cost with a similar contribution by the remaining process steps, including waste recovery. The costs for pentose conversion and cellulase production are about half the cost of distillation.
0.0 10.0 20.0 30.0 40.0
Feedstock
Pretreatmen t
SSF
fermentatio n
Distil latio
n
Othe
r processin g
Pentose conversion Cell
ulase pro duction
Power c ycle
Percent total cost
Figure 4.20. Contribution of major cost elements to overall ethanol production costs.