Energy demand
The energy demand of the ethanol process and the heat losses that occur have been studied in detail in previous studies [27, 38]. The ethanol process includes three energy-demanding steps: steam pretreatment, distillation and evaporation. Preheating the SSF broth also contributes significantly to the overall energy demand. The steam consumption in pretreatment is highly dependent on the moisture content of the raw material, while in distillation and evaporation the feed concentrations (of ethanol and water-soluble non-volatile substances, respectively) have a major effect on the energy demand. The drying step in scenarios B and C requires high-pressure steam, but also generates low-pressure secondary steam that can be utilized in other process steps. Consequently, the net steam demand of the drying step is low.
The process heat duty, i.e. the heat in the form of steam provided to the ethanol process by the steam boiler, varies between 31.6 and 32.3 MW (15.3-15.6 MJ/L ethanol) for the base case scenarios. The Y- cases have a slightly lower total heat duty than that in the base cases, while the total heat duty in the Y+ cases are somewhat higher. This is due to differences in the energy demand of the distillation and evaporation steps. It should be noted that although the energy demand is higher, the cases with higher ethanol yield result in a lower cost per litre of ethanol. A reduction in the amount of water used in the process decreases the energy demand in distillation and evaporation. As a result, the heat duties of the Q- cases are reduced to 26.5-27.0 MW (12.8-13.1 MJ/L ethanol).
While the energy demand of the ethanol production process in the different scenarios (A-E) is similar, there are differences in the utilization of waste heat. Heat losses (defined as unutilized heat streams relative to a state at ambient temperature and water in the liquid phase) occur in the condensers following the rectification column and the last evaporator. In scenario E the heat losses are reduced by utilizing the latent heat leaving the evaporation system for district heating. For utilization of the heat that is removed by cooling in the condenser of the rectification column, a heat pump is required to raise the temperature. Heat pumps were not considered in the current study. The high moisture content of the fuel used in the steam boiler results in a considerable amount of heat leaving the combustor in the form of water vapour in the flue gases. In scenarios D and E the heat losses of the steam generation system are reduced by the implementation of a flue gas condenser. A significant amount of heat is also unutilized in waste water treatment, but as the process step is associated with great uncertainties it will not be further discussed here.
Energy efficiency
The energy efficiency can be defined in many ways, which sometimes makes it difficult to compare the results of different studies. The energy efficiency in the current study, as presented in Figure 2, was defined as the energy output in the products (ethanol, pellets, excess electricity and/or heat for district heating) divided by the energy input, comprised of the raw material, a minor contribution from the addition of enzymes and molasses and, in scenario B, the electric power requirement (recalculated as the fuel necessary to produce this electricity, assuming an electricity-to-fuel ratio of 0.4). The calculations were based on the LHVs of the raw material, pellets, ethanol, etc. The energy output is comprised of different energy sources, which will be used for different purposes. For instance, ethanol will be used as a transportation fuel, the pellets for heat and power generation and heat for district heating, while there is a multitude of applications for electricity. Hence, caution should be exercised when making comparisons of the energy efficiency between the different scenarios.
The base case overall energy efficiencies varied from 53% (scenario A) to 92% (scenario E). The major difference was that the heat removed by cooling water in scenario A was utilized for district heating in scenario E. The pellet-producing scenarios B and C are similar in terms of overall energy efficiency. The energy output is higher in scenario B but, as electricity has to be imported from an external source, the energy input is also higher.
Of the 106.1 MWLHV originating from the raw material, 43.7 MWLHV or 41% ends up as ethanol assuming the base case ethanol yields. In the cases with higher yields the figure is increased to 48.7 MWLHV (46%). (Theoretically, i.e. if all carbohydrates in the raw material are converted to ethanol, the ethanol output would be 71.2 MWLHV, corresponding to 67% of the lower heating value of the feed-stock.) The effect of changes in the ethanol yield on the overall energy efficiency differs between the scenarios. For scenarios B and C a change in the ethanol yield is compensated for by a change in the amount of pellets produced, resulting in reasonably similar overall energy efficiencies. In the other scenarios, a change in the ethanol yield affects the amount of fuel used in the steam boiler; for example, if the ethanol yield is lower the amount of heat and electricity generated will be higher. For scenarios D and E this results in a higher overall energy efficiency when the ethanol yield is lower, while the opposite result is obtained in scenario A, due to the low degree of utilization of the residues in this case.
A reduction in the energy demand of the ethanol process increases the energy output in the form of co-products. Consequently, the Q- cases result in a significant increase in the overall energy efficiency, especially for scenarios B, C and D. The efficiency in scenario E is only slightly higher, as the reduced energy demand results in a lower amount of heat available in the condenser following the last evaporator. The increase of a little more than two percentage units reflects energy savings that occur mainly in the distillation step. Regarding scenario A, the ethanol yield has a greater effect on the efficiency than energy savings in the process, due to the reason mentioned above.
The above results can be summarised as follows:
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The dominant contribution from ethanol to the overall energy efficiency in scenario A makes the efficiency of this scenario sensitive to changes in the ethanol yield.
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For scenarios B, C and D changes in the energy demand have a greater effect on the overall energy efficiency than changes in the ethanol yield.
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The high degree of waste heat utilization in scenario E leaves the overall energy efficiency almost unaffected by changes in yield and energy demand.
Production cost
The demand for district heating is highest during the winter and least in the summer. This was taken into account in the economic evaluation. In a typical Swedish district heating system there is a factor of roughly ten between the minimum and maximum heat demand. In scenario E, 42.9 MW heat is produced. For this to be delivered all-year round, a system with a maximum capacity of 430 MW and an annual delivery of approximately 1000 GWh is required. There are currently less than 10 systems of this size in Sweden. Scenario D, with a heat production of 26.1 MW, may be somewhat easier to implement, as there are around 30 Swedish district heating systems of adequate size. The most likely scenario is that a district heating system will be provided with heat from the plant during part of the year. In the economic evaluation it was assumed that heat is delivered to a district heating system during a period of time equivalent to 4500 hours of maximum capacity annually. Cooling water is used during the remaining 3500 hours to remove the heat, i.e. the process operation is similar to scenario A during this period. An example of how heat from the ethanol plant can be delivered to a district heating system, according to the assumptions discussed above, is shown in Figure 3.
Annual cash flows
The annual income and costs associated with each scenario are presented in Figure 4. The main costs are those for feedstock and capital. The estimated investment cost ranges between 1200 (scenario B) and 1340 million SEK (scenario E) (126-141 MEUR), with the pretreatment and steam generation systems, together with the SSF fermentors, being responsible for the major contributions to the cost. There are only small differences in the total cost between the scenarios. Scenario B, with the lowest capital cost, has the highest total annual running cost (342.1 MSEK, 36.0 MEUR), due to the higher utility cost as electricity has to be purchased in this case. The main difference between the scenarios lies in the annual income from selling the co-products. This figure varies from 59.9 MSEK in scenario A to 110.9 MSEK in scenario E (6.3-11.7 MEUR). However, the income from selling the ethanol (327.6 MSEK, 34.5 MEUR) is much higher, and, although the co-product revenue is important for the overall economics, Figure 4 clearly shows that ethanol is the main product.
Minimum ethanol selling price
The estimated MESP for each case is presented in Figure 5. For the base cases the MESP varied between 3.87 (scenario E) and 4.73 SEK/L ethanol (scenario B) (0.41-0.50 EUR/L). This variation is due to different ways of utilizing the residue streams. The income from selling the electricity certificates makes it beneficial to generate electricity. As the revenue for electricity is higher than for pellets, the MESP of scenario A is lower than in scenario B. Scenario C with a similar energy efficiency to scenario B (see Figure 2), but with some of the output in the form of electricity, results in a lower MESP (4.56 SEK/L, 0.48 EUR/L). At an electricity spot price of 429 SEK/MWh (45 EUR/MWh) (with the certificate price maintained at 200 SEK/MWh) the same MESP (4.51 SEK/L) is obtained for scenarios A and C. Without the extra revenue of 200 SEK/MWhel from selling the electricity certificates, the base case MESP of scenario A would be 0.36 SEK/L higher. The increase in scenarios D and E would be 0.34 SEK/L, and for scenario C 0.12 SEK/L. The MESP of scenario B remains unaffected as no electricity is generated.
The scenarios with district heating (D and E) are by far the most profitable alternatives. Compared to scenario C, the MESP is 0.36 (scenario D) and 0.69 (scenario E) SEK lower per litre of ethanol. District heating revenues of 97 and 68 SEK/MWh for scenarios D and E, respectively, give an identical MESP to that in scenario C, i.e. a considerably lower price than the 280 SEK/MWh assumed in this study. With an 8000-hour delivery of district heating, the MESP of scenarios D and E would be 3.79 and 3.18 SEK/L (0.40/0.33 EUR/L), respectively. However, finding a location where this could be implemented would be difficult.
Effect of variations in ethanol yield and/or energy demand
The cases with lower ethanol yield result in a lower total annual cost, mainly due to the reduced enzyme cost, and a higher co-product revenue compared with the base cases, which to some extent compensates for the smaller amount of ethanol produced. Accordingly, the higher the impact of co-product revenue on the economics, the lower the negative effect of a reduced yield. In scenario E the Y-case actually results in a lower MESP than in the base case. However, with the assumed ethanol selling price, the annual revenue decreases with a lower yield. A higher yield is beneficial for the overall economics, although it results in reduced co-product revenue, which affects scenarios D and E to a greater extent. In the Y+ cases the MESP is reduced by 0.38 SEK/L in scenario A, but only by 0.22 SEK/L in scenario E compared with the base cases.
The results presented in Figure 5 clearly show that the ethanol yield has a high impact on the process economics, which makes it a key factor for the successful full-scale introduction of ethanol production from lignocellulosic feedstock. The yield dependency is reduced in scenarios D and E, which may make the implementation of these scenarios economically less risky.
As a consequence of the reduced energy demand, the Q-cases result in lower MESP for all scenarios due to lower investment costs and higher co-product revenue. The effect on the production cost of a reduced energy demand is higher for scenarios B and D. For scenarios A and E a higher WIS concentration does not affect the production cost as much, due to the same reasons as mentioned regarding the effect on the energy efficiency. In scenario C, the reduced demand for steam results in lower electricity production which, to some extent cancels out the positive economic effects of a more energy-efficient process. For a plant configured according to scenario C, a further reduction in steam demand will, at a certain point, result in a need for electricity import.
In scenarios A, B and C the Y+Q- case results in a reduction in MESP almost equal to the sum of the reductions obtained for the Y+ case and the Q- case. In other words, the positive effects on the economics of a higher yield and a lower energy demand are almost additive. In scenarios D and E the reduction of MESP in the Y+Q- case actually exceeds the sum of the reductions of the Y+ and the Q-cases. The reason for this lies in the process conditions set, in particular keeping the inlet and outlet temperatures of the flue gases in the flue gas condenser fixed at 150 and 50°C in all cases, and the fact that a large proportion of the steam generated in the steam boiler was withdrawn for use in the ethanol process. As a result, the entire flue gas stream could not be utilized in the flue gas condenser in the base cases and Y+ cases, as this would result in a temperature crossover (the constraint was set at a minimum temperature difference anywhere within the flue gas condenser of 1°C). For the other cases this was not a problem. Consequently, the amount of heat available for district heating was somewhat reduced in the base cases and Y+ cases in comparison with the other cases.