1. Introduction
Access to efficient mobility is an important need for billions of people all over the world, and freight of goods has been increasing very substantially parallel to the process of globalization. Both person mobility and freight are cornerstones of economic activity and thereby improve quality of life. Liquid transportation fuels play a significant role in enabling the mobility of people and goods in our society. In just the last century, fossil-based resources have grown to command a lion’s share of the liquid transportation fuels in use today. Decades of research and development next to incremental innovation alongside heavy investments have ensured the availability of fossil resources while providing efficient technologies that convert these resources into affordable transport fuels. However, the finite nature of fossil resources puts a question mark on their future availability and ability to provide affordable fuels. In addition to this uncertainty, the use of fossil-resource-based transport fuels is also responsible for negative environmental impacts such as climate change due to greenhouse gas emissions, species destruction due to global warming, ocean acidification, and oil spills. Human-induced climate change threatens to unleash a wide range of other adverse effects including sea level rise, extreme weather events, and volatile crop yields that will have a direct impact on our quality of life [
1]. The development of technologies that enable the utilization of renewable biomass resources to produce affordable and environmentally beneficial transportation fuels can contribute to minimize these adverse effects. Current first-generation renewable fuels, e.g., ethanol and biodiesel, face significant limitations, such as competition with food supplies [
2]. Use of widely available lignocellulosic resources [
3,
4] can help overcome such limitations.
Pyrolysis represents an important technological route for conversion of lignocellulosic resources into second-generation liquid transportation fuels that can be blended with gasoline [
5,
6,
7,
8]. The process of pyrolysis involves conversion of solid biomass to liquids either using only heat (thermal pyrolysis) or in combination with catalysts (catalytic pyrolysis). A variety of literature studies have looked into the economic viability and environmental impacts [
5,
6,
9,
10,
11,
12] of producing biofuels via thermal pyrolysis. These studies provide an essential insight into the viability and challenges for practical utilization of thermal pyrolysis. Significant technology developments have been reported in literature for use of catalysts in the biomass pyrolysis process [
13,
14,
15,
16,
17]. A combined economic and environmental analysis that provides a holistic perspective on these technology developments is needed. Furthermore, existing reported studies are one-off assessments that analyze the potential of the route based on existing technological developments and typically do not provide feedback on development of catalysts for use in different sections of the process for conversion of lignocellulosic resources to usable transportation fuels via pyrolysis.
Against this backdrop, the two main goals of this study are to understand the potential of the catalytic pyrolysis route and to provide specific feedback on essential requirements (or boundary conditions) for catalytic and process developments. In addition to these goals, a key aim is to identify data gaps and uncertainties associated with this route. Working towards these goals, we have developed assessments for catalytic and thermal pyrolysis processes for conversion of waste pinewood chips to biofuel. The complete study relies on laboratory experiments, process simulation models, technical, economic, and lifecycle environmental assessments.
This article follows up on previous technical catalyst and process development work. Patel et. al. [
7] reports analysis based on laboratory experiments and process simulation models, and provides feedback from a technical (including product quality) and energy perspective. This article follows up on the previous work and provides holistic economic analyses and lifecycle environmental impact assessment. The outcome of economic analysis is a minimum viable price (MVP) for the biofuel, while the outcomes of environmental analysis are the cumulative energy demand (CED) and greenhouse gas emissions (GHG) associated with the biofuel. Apart from comparison of the different pyrolysis routes, these outcomes also have been used to compare similar values for gasoline. The MVP and GHG have been combined to estimate the CO
2 abatement cost with reference to gasoline. Four different product scenarios for each of the catalytic and thermal process cases have been taken for analysis in this study. These scenarios highlight different potential process configurations that can utilize novel catalysts and thereby also help to provide feedback on future developments for these routes.
3. Results and Discussion
In this section, we first present the results in detail for one process variation, which leads to biofuel with a potentially blendable quality. Thereafter we present the comparative results for all the process cases and scenarios.
The results are presented in detail for the biofuel produced using the Cs/ASA catalyst and complete hydro-treating to reach 3.7% oxygen content in the final fuel product (HT_Oil_Cs/ASA). Based on a simulated plant capacity to process 480 metric tonnes of pinewood chips per day the plant is expected to produce 15.4 metric tonnes (670 gigajoules) per day of fuel product that can be blended with gasoline/diesel. Hydrogen, electricity, and steam are the other main co-products from the process.
Table 1 below shows the expected capital investment associated with the plant. As expected, based on stream flows, the pyrolysis and the combined heat and power (CHP) sections entail the most capital investment. In the case of the pyrolysis section, the pyrolyzer is the most expensive unit. The cost for the CHP section includes the combustor, which is also used as catalyst regenerator unit for catalytic pyrolysis. The cost of the combustor is higher for the catalyst cases to account for the extra capacity needed to process the catalysts.
Table 2 above describes the operating costs and by-product credits associated with this plant. In this case, the by-product credits cover a significant fraction of the operating expenses. This is mainly due to the low non-water-soluble bio-oil yield and also higher char and gas yields. The capital and operating expenses and discounted cash flow analysis lead to a minimum viable price (MVP) of 75.3 EUR/GJ for the fuel product (HT_Oil_Cs/ASA).
Table 3 shows the comparison of this minimum viable price with the prices of gasoline and first-generation ethanol in the European Union. As shown in the table, the HT_Oil_Cs/ASA is expected to be about four times more expensive as compared to gasoline. The MVP, however, is significantly dependent on the yield of non-water-soluble bio-oil. As shown in the first work, the oil yield is only about 20% of the theoretical maximum and thus there is significant room for improvement based on novel catalysts. Moreover, it is also important to take into account the environmental impacts associated with the production of fuels.
Table 4 shows the lifecycle environmental impacts associated with production of HT_Oil_Cs/ASA and gasoline in the form of cumulative energy demand (CED) and greenhouse gas emissions (GHG). These results take into account potential credits from co-products due to the replacement of conventional methods of production in the Netherlands. The negative greenhouse gas emissions mean that introduction of this process can potentially lead to a net reduction in greenhouse gas emissions by replacing fossil-based products. The bio-based process has a higher CED due to its high renewable energy use, and requires a correspondingly high land use of about 0.5 m
2 annually per mega joule (m
2a/MJ) of HT_Oil_Cs/ASA. The land use associated with gasoline (0.00015 m
2a/MJ) is negligible in comparison. However, it is important to note that the bio-based process also can lead to a significant net reduction in non-renewable energy use (see
Table 4).
In the European Union, ethanol is currently blended with gasoline using subsidies to pay for the difference in prices. The main goals behind this blending are to increase the share of renewable fuels and to reduce greenhouse gas emissions. In the case of of HT_Oil_Cs/ASA the savings in greenhouse gas emissions translate to an abatement cost of about 138 EUR per metric tonne of CO
2. This value compares with an abatement cost of about 270 EUR per metric tonne of CO
2 in the case of ethanol produced in the EU [
42].
Figure 3 and
Figure 4 respectively show the sensitivity of MVP and abatement cost to 20% variation in some of the key data inputs. Given the high sensitivity to capital costs, it is important that more detailed vendor estimates are used for the most expensive process units like the pyrolyzer. The internal rate of return or the discount rate can also have a significant effect on the outcome and it can vary depending on the prevalent economic situation at the location and company policies. The income tax rate can also affect the outcome and is dependent on government tax policies that can vary on a case-specific basis. In this case, a 0% income tax rate would reduce the MVP by 11% to 67 EUR/GJ and the abatement cost by 15% to 106 EUR per metric tonne of CO
2. In this study we also assume that the plant is 100% equity financed. If, instead, the plant is financed completely by a loan with an 8% interest rate for repayment in 10 years, the MVP and abatement cost would increase respectively by 21% to 91 EUR/GJ and 28% to 159 EUR per metric tonne of CO
2.
With woodchips being a low-energy-density feedstock, the source of woodchips and their transportation to the facility can have a major impact on price of feedstock and also the abatement cost. Since steam and hydrogen are major co-products, their prices will also have a significant influence. According to the present simulations, the process uses inexpensive catalysts and hence the catalyst price does not seem to have a major effect. However, the changes in the catalyst performance and lifetime (denominated as catalyst charges required per year) will have a high impact on the outcomes. Along these lines, the use of more expensive metals or catalyst supports coupled with increased catalyst losses can have a high impact on the final outcomes. As can be observed from
Figure 4, the abatement cost also has a high sensitivity to the GHG emissions associated with the steam that will be replaced by the steam produced from this process. In this model it is assumed that the steam being replaced is being produced from natural gas via CHP. However, in areas where steam is being produced from sources like oil and coal using inefficient boilers, significant further reductions in GHG emissions and thereby abatement costs can be achieved. Factors like CED of steam, woodchips, and woodchip transportation distance have a major effect on the CED of the biofuel.
Apart from these factors it is also important to understand the effect of bio-oil yield (organic fraction) from the pyrolysis reaction on the overall outcomes. It is important to understand that although the bio-oil yield plays a major role, it does not have a directly proportional effect on the outcome, as every change in bio-oil yield results in a change in variety of other related factors. These other related factors, such as the quality of the oil produced and the composition of by-products, also have a major influence on the outcome. Hence, results from the different cases that have been analyzed in this study indirectly serve to highlight effect of change in yields on the outcomes. An ideal situation for catalytic pyrolysis [
7] is a catalyst that would convert the biomass completely into oil and remove the oxygen only as CO
2 without any other by-products. In the case of HT_Oil_Cs/ASA, if a catalyst were to function along these lines (conversion of biomass to 65% oil with only organic components and 35% CO
2) while producing oil of a similar combination, the final biofuel yield would increase to 21 wt% (of biomass). In this case natural gas would need to be burned in CHP to meet plant energy demands because char, coke and gases which fuel the CHP would not be produced in this situation and hence only the heavy oil fraction is available to be burned in CHP. At the current feedstock cost of 80 EUR per metric tonne of biomass (4.5 EUR/GJ) this situation would result in a biofuel with MVP of 31 EUR/GJ and an abatement cost of 52 EUR per metric tonne of CO
2. Also, a significant fraction of the bio-oil produced ends up as heavy oil, which is combusted in the CHP in this model. Depending on future developments in utilization of this heavy fraction to produce liquid biofuels and efficient transport systems, further cost reductions may be possible.
As was described in Patel et. al. [
7], a choice was made in the model to have high (98%) yields in the hydro-treating and hydrogen-production sections to ensure that the model is responsive to changes in the pyrolysis process. Hence the reported MVP and abatement costs represent a best-case scenario from hydro-treating and hydrogen-production perspectives. In the case of HT_Oil_Cs/ASA if the hydro-treating yield were 80% then the MVP would be 92.2 EUR/GJ with an abatement cost of 136 EUR/MT of CO
2. But the problem with this result is that almost 20% of the oil mass that is lost is not taken into account due to lack of experimental results. As an example, if these would lead to by-products that could be combusted in CHP, the costs could be reduced further. Hence, seamless experiments are needed to understand the practical yields, based on specific-input organic fractions, which are a result of the pyrolysis and separation sequences.
Figure 5 below shows the comparison of the four different pyrolysis cases and the relevant scenarios on the basis of minimum viable prices. According to these results there is still a substantial gap in proximity to economic viability between thermal and catalytic pyrolysis. However, it needs to be taken into account that catalytic pyrolysis is still in a clearly earlier development stage than thermal pyrolysis, which has a research and development (R&D) lead time of around 15 years. Detailed economic estimates showing individual equipment, capital and operating costs for all the scenarios are included in
Tables S4 and S5 in Supplementary Materials.
It is important to note that the quality of oil product in each of the above cases is quite different and not necessarily directly comparable to gasoline. Among the above cases, a product that is comparable to gasoline can be produced with existing catalysts for the HT_oil cases. However, since optimistic assumptions were made for all calculations (e.g., on the hydrogen/hydro-treating section as explained earlier in the article), the product cost is expected to be higher, at least for initial plants. With development of new catalysts [
7], oil with comparable qualities to gasoline can be obtained in the cases of SH_oil and SC_oil. In the case of DO_oil, the oil from thermal pyrolysis is not at all comparable to that of gasoline and needs further treatment for blending with motor fuels. In the cases of catalytic pyrolysis, the DO_oil products are better in quality than those from thermal pyrolysis, but still not comparable to that of gasoline. However, further innovations in catalysis can deliver an oil of higher quality that can be blended with gasoline [
7]. Comparing the MVP of SH_oil and HT_oil for thermal pyrolysis, it is evident that development of catalysts that can selectively hydro-treat the problem components in the input bio-oil can lead to a significant reduction (about 45%) in the minimum viable price of the biofuel.
The relatively small difference in MVP’s for catalytic pyrolysis in the DO_oil, SH_oil, and SC_oil scenarios indicates that the savings in capital costs in the hydro-treating section and excess hydrogen requirements for treatment of problem components have a minor effect on the MVP of the final product. The HT_oil scenario shows a significant increase in the MVP for both the catalytic and thermal processes. This is because of the higher operating and capital expenses that are associated with hydro-treating to an oxygen content of 3.7%. As one would expect, hydro-treating to a 0% oxygen content will lead to significant further increases in costs and thus the MVP of the fuel product. Due to the already higher quality of oil from the Cs/ASA catalyst as compared to other catalysis and thermal processes, there is a relatively small increase in the MVP from SH_oil to the HT_oil scenario. The significant increase in the case of HY-Zeolite is due to the high oxygen to carbon molar ratio (O/C) of the DO_oil product. The O/C ratio of DO_oil from HY-Zeolite is almost twice as high as compared to Cs/ASA and this leads to a significantly higher hydrogen requirement during hydrotreating. The MVP for fuel product from thermal pyrolysis in HT_oil and SH_oil is lower due to the high bio-oil yield obtained in comparison with catalytic processes.
Highlighting the environmental impacts of the different processes and scenarios,
Figure 6 shows the changes in CED and GHG emissions for each case in reference to gasoline. The higher CED in the case of bio-based products is expected as the production processes are at a nascent development stage as compared to petroleum processes. In addition, one can expect the CED to be always higher in the case of bio-based fuel products, as they involve conversion of solid biomass to liquids, which was already done through natural processes over millennia in the case of crude oil. But as shown for the HT_oil_Cs/ASA case (
Table 4) the bio-based processes can entail a net reduction in the use of non-renewable energy resources. In line with this reduction we can also expect a corresponding net reduction in greenhouse gas emissions as evident from the
Figure 6 for all pyrolysis processes. The reduction in GHG emissions and increase in CED is almost constant over different scenarios for respective process variants. There are two main factors behind this: the first and dominant factor is that the CED and GHG emissions are influenced to a larger extent by the co-product credits from the potential replacement of conventional production routes electricity, hydrogen, and steam. The second factor is that since these values are compared on a per unit energy content basis, an increase in resource inputs while going from DO_oil to HT_oil is compensated by the increase in the energy content of the final oil product.
Figure 7 shows the annual land requirement for all the fuel options considered in this study. The land use follows the pattern observed for CED in
Figure 6 as it is reflected in the renewable energy use associated with each of the process options. Similar to other parameters, land use is also heavily dependent on the yield of oil and this is reflected in the lower land use for thermal cases, which have the highest oil yield. As evident from the example of thermal HT_oil scenario, pyrolysis processes could entail lower land use as compared to the first-generation ethanol in EU. However, these results are based on a number of technical processes and data uncertainties, which will have an influence on the actual land use when such processes are implemented in practice.
It is evident from
Figure 5 and
Figure 6 that for bio-based fuels, the benefits of GHG savings and increased reliance on renewable second-generation biomass resources are stacked against a higher price for fuel. Hence the abatement costs presented in
Figure 8 enables us to compare various routes based on the effectiveness of paying more for transportation to gain higher GHG savings in reference to gasoline. As discussed before, it is important to bear in mind the differences in product quality from different scenarios and variants. Considering the quality aspect, the DO_oil scenario from the thermal route is not a valid option. The graph shows that the other pyrolysis routes analyzed in this study have the potential to offer reductions in abatement costs as compared to ethanol, which is currently used for blending with gasoline in the EU. In the HT_oil scenario, which is more feasible with current technologies, the Cs/ASA catalyzed process and the thermal process show similar abatement costs. However, it is interesting to note that with future developments in catalysts, scenarios like SH_oil and SC_oil have the potential to significantly lower the abatement costs for these routes.
4. Conclusions
Catalytic and thermal pyrolysis fuels produced from a lignocellulosic resource like pinewood chips can be more expensive but can also provide environmental benefits. An efficient process can also lead to much lower abatement costs as compared to the current first-generation biofuels, like ethanol. One of the product scenarios of HT_oil is estimated to have 51% of the abatement cost of first-generation ethanol. Additional product scenarios such as SC_oil and SH_oil with future catalyst developments show the potential to reduce the abatement costs to less than 100 EUR per metric tonne of CO2 equivalents. It is, however, important to note that the outcome depends on several factors that are subject to variations in practice. The unknowns and uncertainties in the analysis need to be reduced through seamless experimental trials in which the oil from pyrolysis is separated and processed into hydrogen and fuel through hydro-treatment. More detailed environmental analysis is needed, which would consider other impacts associated with processing and the use phase of the fuels.
With regards to the location of the plant, a balance needs to be struck between being closer to the market for some of the outputs and the high economic and environmental cost due to the longer distance from resource. Optimized and efficient supply chains are critical for viable process operation. Shipping bio-oil directly after pyrolysis to a facility for further processing might not be a lucrative option in the catalytic case, due to high fraction of water shipping and reduction of integration opportunities. Further studies are needed to better understand such scenarios.
Fulfilling one of the key goals of this study, the results demonstrate the difference of performance between different catalytic and thermal pyrolysis processes. This ability enables the use of such a model to test a variety of different catalysts that need to be developed for different sections of the process. Results show that development of novel catalysts can significantly improve the economic and environmental aspects of pyrolysis-based fuels and could ultimately lead to practical implementation of such a process. Thus, novel catalysts are needed that can target specific molecules in the bio-oil.
In future, better utilization of aqueous bio-oil streams through chemical production needs to be explored. Further detailed analysis on the properties of the ash from pyrolysis and transportation logistics need to be considered to explore its fertilizer potential. In the current process, a significant fraction of the biomass ends up as heavy oil which is converted to heat and electricity. Valorization of this heavy oil stream into liquid fuels through hydro-treating needs to be studied further.
Overall results from this study indicate second generation or lignocellulosic bio-based resources can be utilized more effectively to meet environmental goals in an economically efficient manner. Effective and targeted development of such processes holds the key to adoption of more sustainable transportation fuels in future.