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Different processes have been proposed to meet the global need for renewable fuel. The Biomass to Liquid process (BtL) converts biomass via the Fischer-Tropsch route to hydrocarbon chains that can be refined to transport fuel. With the addition of electrolytic hydrogen to the Power and Biomass to Liquid process (PBtL), the carbon efficiency can be increased relative to the BtL process. It was shown in previous studies that the PBtL concept has an economic edge over BtL when cheap electricity is available to maximize the fuel yield. In this study, a techno-economic analysis is conducted for a hybrid process concept which can switch operation modes from electrolysis enhanced to only biomass conversion. In case studies the effect of the Fischer-Tropsch conversion, H2/CO ratio of the Fischer-Tropsch feed and the biomass feed rate in the electrolysis enhanced mode are analyzed. Every process configuration is modeled based on experimentally validated unit models from literature in the commercial software Aspen Plus and analyzed using DLR’s software tool TEPET. For a 200 MWth biomass input plant, production costs of 1.08 €2019/L for the hybrid concept with a carbon efficiency of 53.3% compared to 0.66 €2019/L for BtL with 35.4% and 1 €2019/L for PBtL with 61.1% were found based on the Finnish day-ahead market for the base case. The net production cost for the hybrid concept can be decreased by 0.07 €2019/L when a Fischer-Tropsch H2/CO ratio of 1.6 instead of 2.05 is used.
With the European Green Deal, the European Union (EU) aspires to become carbon neutral by 2050. To that end, the share of renewable fluctuating electricity production is aimed to be ramped up from 32% today to 65% by 2030 (
The increasing share of renewables on the energy market has displaced already installed infrastructure. Especially, in Northern Europe the combined heat and power (CHP) plant infrastructure is under financial pressure competing on the power market (
At the same time, the EU aims to reduce the carbon emissions from the transport sector. Here, the electrification of light-duty vehicles is only one step. Heavy-duty transportation, especially aviation and shipping, will continue to rely on liquid fuels for their higher energy density. Therefore, the European commission states that the technology development and deployment for renewable, low-carbon fuels has to be achieved by 2030 (
The process concept proposed in the EU-project FLEXCHX offers a solution for the three fields of the energy transition: The fuel process converts biomass to liquid hydrocarbons via the Fischer-Tropsch (FT) route. Whenever cheap renewable electricity is available from the grid, an electrolysis unit is operated to enhance the fuel yield. In an adjacent CHP plant the process off-heat is used to generate district heating and electrical power (
Operation modes for the hybrid process concept.
The conversion of biomass to liquid fuels has been widely discussed in literature under the acronym BtL. The term includes all conversion routes, i.e. methanol, ethanol or DME (
Processes with the addition of electrolytic hydrogen to a BtL plant are referred to as power biomass to liquid (PBtL) (
Process concepts with flexible electricity sourcing have also gained attention in literature. Müller et al. show that it is experimentally possible to integrate H2 from a wind park profile into an FT-BtL process (
Hybrid processes have higher investment costs than steady-state processes because part of the equipment is inactive or only used in part-load. The advantage of a hybrid system lies in the lower operation costs. The PBtL concept can produce fuel at lower cost and with lower biomass consumption compared to BtL, if inexpensive electricity is available (
The concept studied here describes a process operated in two modes: In the biomass alone mode (BA), biomass is converted to fuel via the FT synthesis. In the electrolysis assisted mode (EA), hydrogen is produced by a grid-connected electrolyzer, which is used to enhance the fuel output of the process. For both modes, off-heat is converted to electricity and district heating in a CHP plant. A schematic flow diagram of the two operation modes is depicted in
Schematic flow diagram for the biomass alone operation mode (BA)
The biomass alone mode is depicted in a schematic flow diagram in
In the gas cleaning section water, CO2 and trace components are removed from the syngas. The clean syngas then reacts over the FT catalyst to hydrocarbon chains. Here, hydrocarbon chains with a chain length higher than five are considered product and are separated from the shorter hydrocarbons. Further upgrading steps such as cracking of longer chains is not considered in this study. The separated tail gas consisting of short hydrocarbons and unconverted syngas is partly recycled to the reformer. The remaining tail gas leaves the process and is burned. The energy content of the off-gas is used in the CHP.
In contrast to the BA mode, the electrolysis assisted mode features a CO2 recycle. As can be seen in
Overall, the EA mode requires less biomass feedstock to produce an equal amount of FT product compared to the BA mode. Yet, the higher biomass conversion has to be weight against the additional cost for the electrolysis power demand.
The flowsheet for the FLEXCHX process concept is depicted in
Process flowsheet–blue signifies equipment only operated during BA mode, orange during EA mode.
A CFB gasifier is used to convert the dried biomass into syngas. This gasifier type is suitable for the FLEXHCX operation strategy because of its high load flexibility. Warnecke reports an operation rage of 50%–120% relative to standard load for a CFB gasifier (
The catalytic tar reformer not only lowers the tar content but also the hydrocarbon gas content produced in gasifier and FT reactor. With the addition of oxygen, both component types undergo an autothermal reformation reaction (
A cryogenic ASU is used for the oxygen production for gasifier and reformer. This production method is reported to be the most economical option for large scale oxygen production (
To adjust the syngas H2/CO up to a stoichiometric ratio of ∼2 (
The addition of steam in the shift reactor entails the reaction of CO to CO2, which decreases the overall carbon efficiency of the process. To avoid the additional formation of CO2 in the EA mode the sour shift reactor can be bypassed. In this mode the H2/CO is adjusted by adding hydrogen from the electrolyzer.
The gas cleaning train consists of a water scrubber, syngas compression, Selexol scrubber and a guard bed (
The FT reaction can be characterized as a polymerization reaction. The reactants H2 and CO form hydrocarbon molecules of different chain lengths. The production of paraffines and olefins can be described with the chemical reactions in
Although a multitude of reactor types have been presented in the past, the most notable designs are the slurry bubble column reactor and the fixed bed reactor (
At the outlet hydrocarbon products C5+ are separated from water and tail gas. Tail gas, consisting of short chained hydrocarbon gases C1–4 as well as unconverted syngas, is partly recycled to the filter unit.
In this study the alkaline electrolysis AEL technology is chosen for the production of hydrogen and oxygen in the EA mode. Compared to other technologies, specifically proton exchange membrane electrolysis (PEMEL) and solid oxide electrolysis (SOEL), AEL is the most mature technology with the lowest investment costs (
To give reference points, a BtL and PBtL process are simulated and techno-economically evaluated. Both processes rely on the same equipment and flowsheet layout as the hybrid concept. The hybrid plant requires additional investment costs compared to BtL and PBtL. This is due to units that are only active in one mode and over-dimensioned units.
The BtL plant is comprised of all units operated in the BA mode, i.e. no electrolyzer and CO2 compressor are needed for this process. For the PBtL, on the other hand, only equipment types that are used in the EA mode are required. Therefore, ASU and sour shift reactor are excluded.
One hybrid operation mode defines the equipment size. If e.g. the syngas stream in the EA mode is larger than in the BA mode, a larger water scrubber is needed for the EA mode. Therefore, the water scrubber is over-dimensioned for the BA mode. For the comparison cases, BtL and PBtL, no equipment has to be over-dimensioned.
The process was simulated using the commercial software Aspen Plus® (V10). The Soave-Redlich-Kwong equation of state is chosen (
The biomass properties are taken from Hannula et al. (
Properties of biomass feedstock (
Proximate analysis, wt% dry basis | |
Fixed carbon | 25.3 |
Volatile matter | 70.8 |
Ash | 3.9 |
Ultimate analysis, wt% dry basis | |
Ash | 3.9 |
C | 53.2 |
H | 5.5 |
N | 0.3 |
Cl | 0 |
S | 0.04 |
O (difference) | 37.06 |
Other properties | |
HHV, MJ/kg | 20.67 |
Initial moisture content, wt% | 50 |
The initial moisture content of 50 wt% is reduced in a belt dryer to 12 wt%. For the dryer an electrical power consumption of 32 kWh/t based on dry feedstock mass and a heat demand of 1,300 kWh/t based on the evaporated water mass is assumed (
The CFB gasifier is modeled as a combination of an RYield reactor and an RGibbs reactor (
Gasifier yield model (
Biomass | Conversion [%] | Selectivity [%] | Product |
---|---|---|---|
Nitrogen | 98 | 12.8 | N2 |
86.7 | NH3 | ||
0.5 | HCN | ||
Sulfur | 100 | 98 | H2S |
2 | COS | ||
Carbon | 2 | 100 | Fly ash |
0 | Bottom ash | ||
Ash | 100 | 1 | Fly ash |
99 | Bottom ash |
The yield for the carbon species formed during gasification can be taken from the
The gasifier is assumed to have a heat loss of 1% of the dry biomass input LHV (
Gasification ash is completely removed from the syngas. Bottom ash can be removed from the gasifier directly. For fly ash the filter unit is required. At a high syngas temperature filter blinding may occur due to the sooth formation tendency of the tar components (
The ASU is assumed to have an energy demand of 1 MWe/(kg/s) with an output pressure of 1 bar (
The tar reformer is modeled as an adiabatic RGibbs reactor with an operation temperature of 900°C. In the autothermal reformer oxygen is added to attain this temperature level. The steam to oxygen feed mass ratio is set to 1 (
In the subsequent sour-shift reactor steam at 4 bar is added to attain a defined H2/CO ratio in the syngas. It is modeled as an REquil reactor in which only the water gas shift reaction is taking place (cf.
The water scrubber is modeled as two flash units with an outlet temperature of 60°C for the first and 30°C for the second stage. The syngas is cooled to 200°C at the scrubber inlet by the HRSG system in both modes (
The subsequent syngas compression is modeled as a five-stage compressor with equal pressure ratio and intercooling to 80°C (
For a 90% CO2 removal rate the energy consumption for the Selexol process is assumed to be 74 kJ/kgCO2,removed (
As the reduction of H2S to an acceptable level for the FT reactor, below 10 ppb (
The AEL unit is modeled as a splitter with a system energy demand of five kWh/Nm3, which amounts to a system efficiency of 70.8%HHV (
In this study, the kinetic reaction model proposed in Todic et al. (
The model describes the production rate of n-paraffins and 1-olefins up to a carbon length of 30 as a differential-algebraic system of equations. The system has six input variables reactor temperature, pressure and total molar feed rate as well as the partial pressure of H2, CO and H2O at the reactor output and one design parameter, catalyst loading. Further, the kinetic reaction model assumes that the slurry bubble column reactor can be idealized as a continuously stirred tank reactor (CSTR) (
In this study, the operation conditions for the FT reactor are set to 230°C and 25 bar. Higher pressure level has been shown to increase the selectivity and reaction rate for the FT Co. catalyst (
To avoid a large recycle stream the FT reactor should be designed to maximize the CO conversion and the product selectivity C5+ i.e. the selectivity for hydrocarbons with a chain length higher than 4. Given the reactor’s operation conditions, the gas hourly space velocity (GHSV) can be adjusted to maximize product output. Lowering the GHSV leads to an increased CO conversion and product selectivity (
For the reactor simulation the H2 conversion is set to a value as defined in the case studies cf.
Since the FT reactor is operated in two modes, BA and EA, one requires a lower catalyst mass. It is therefore assumed that the reactor consists of two modules, of which one can be by-passed. For the cost analysis the larger catalyst mass is considered.
An idealized complete separation is assumed for the reaction water, tail gas and product C5+. Part of the longer hydrocarbon products accumulate in the FT slurry and have to be removed by a filter unit (
To increase the process carbon efficiency, tail gas containing hydrocarbon gases C1–4 and the unconverted syngas is recycled to the reformer. A recycle rate of 95% of the total tail gas is modeled here. Various studies point out that to avoid the accumulation of inert gas content the recycle ratio has to be below 100% (
The CHP plant is modeled as a steam cycle fed by the process off-heat. In addition, the heat from burning FT off-gas, which is not recycled, is counted as a source for the CHP plant. It is assumed that 90% of the off-gas’s LHV can be recovered. The electrical efficiency for the CHP system is set to 40% relative to its heat input (
Case definition for the simulation.
Case | 1.1 (base) | 1.2 | 1.3 | 1.4 | 2.1 | 2.2 | 2.3 | 2.4 |
H2 conversion [%] | 70 | 55 | 70 | 55 | 70 | 55 | 70 | 55 |
H2/CO [−] | 2.05 | 2.05 | 1.6 | 1.6 | 2.05 | 2.05 | 1.6 | 1.6 |
Biomass feed rate BA [MWth] | 200 | 200 | 200 | 200 | 200 | 200 | 200 | 200 |
Biomass feed rate EA [MWth] | 200 | 200 | 200 | 200 | 100 | 100 | 100 | 100 |
Lowering the H2/CO ratio to 1.6 has several positive effects on the process. Firstly, a lower H2/CO is associated with a higher product selectivity (
The conversion limit for the FT reactor is lower for operation points with an under-stoichiometric H2/CO ratio (
Feeding 100 MW instead of 200 MW biomass in the EA mode is advantageous in two aspects: For the smaller syngas stream less hydrogen is needed to attain the defined H2/CO ratio. Therefore, the electrolyzer, which is not operated for a part of the year, can be designed with a lower capacity. On the other hand, the plant is over dimensioned for the EA mode. All cases with 100 MWth biomass input are listed under case 2.1–2.4 in
Three performance indicators are used to evaluate the simulated process performance: carbon efficiency
Carbon efficiency
The energetic fuel efficiency is stated in
The energetic plant efficiency (
The economic analysis is conducted with the DLR software tool TEPET. The tool retrieves stream and unit dimension data from Aspen Plus. By linking the modelled units with according cost data within the TEPET database, a transparent cost estimation can be obtained. The calculation method is described in depth by Albrecht et al. (
In this study investment costs are updated using the Chemical Engineering Place Cost Index (CEPCI) for the year 2019 taken from (
The investment cost
The net production costs (NPC) are calculated according to
All utility prices are listed in
Utility prices.
Utility | Prices | Source |
---|---|---|
Wet biomass | 42.232 €/t |
|
Electricity selling price | 50.4 €/MWh |
|
Demineralized water for electrolysis | 2 €/m3 |
|
Fresh water | 0.434 €/m3 |
|
District heating | 40 €/MWh |
|
FT catalyst |
33 €/kg |
|
Selexol |
4.346 €/kg |
|
Waste water | 0.907 €/m3 |
|
Catalyst replacement rate 0.5%/day (
Selexol makeup 0.00018 kgmakeup/kmolsyngas (
For the hybrid capital expense estimation, the characteristic size
The operation costs are defined by each mode independently. Therefore, net production costs (NPC) can be calculated assuming that one mode is active for all 8,100 h. This is subsequently denoted as NPCBA/EA. The NPChy for the hybrid operation of both modes follows from
To calculate the hours spent in BA and EA mode according to the electricity price profile on the Finnish day-ahead market, the electricity price for which BA and EA mode have the same NPC has to be found. Days with electricity prices below this threshold are operated in EA, above in BA mode. For further calculations, the average electricity price in BA and EA mode operation have to be determined. It is assumed that the hours the plant is not operated do not affect these average electricity costs.
The cases defined in
Correspondingly, the PBtL will require lower investment costs compared to the EA mode. Sour shift reactor and ASU are not included. This extends to cases were the oxygen from the electrolyzer not sufficient. For the small oxygen stream only the energy demand for its production is included in the calculation.
The aim of this study is to analyze a hybrid process over a broad range of operation conditions. To that end, a sensitivity study over the electricity price and the capacity factor of each mode is conducted. Here, only the electricity price for the energy input is varied. Further, to provide a reference point to BtL, the electricity price for which the hybrid concept and the BtL process have equal NPC is calculated.
The production costs are estimated for a specific set of economic parameters which may change in the future. To account for this possible change, a sensitivity analysis is conducted for the largest cost contributors biomass price, electrolyzer investment cost and investment costs for the BtL plant.
The investment costs for the BtL plant entail all equipment except the electrolyzer and CO2 recycle. Haarlemmer et al. find investment costs in the range of 300–1200 M€2011 for 400 MW BtL plants (
The biomass cost contributes substantially to the overall NPC in the BtL and PBtL concept (
Lastly, the electrolyzer investment costs are predicted to fall in the coming years (
This section presents carbon and energy efficiency alongside the production costs for all simulation cases, as shown in
In the base case a carbon efficiency of 35.4% was found for the BA mode. As depicted in the Sankey diagram in
Carbon flow Sankey diagram for case 1.1 BA mode
An advantage of the EA mode over the BA mode is the higher carbon efficiency. In the base case, 61.1% of the biomass carbon instead of 35.4% is converted into FT product. The higher carbon efficiency is mainly due to the amount of carbon converted to CO2—a similar share of carbon leaves the process in the form of ash and FT off-gas in both modes. For the BA mode almost twice the amount of CO2 is produced in the WGS reaction (cf.
Efficiency values and key process results for all simulated cases.
Case number | 1.1 | 1.2 | 1.3 | 1.4 | 2.1 | 2.2 | 2.3 | 2.4 |
H2 conversion [%] | 70 | 55 | 70 | 55 | 70 | 55 | 70 | 55 |
H2/CO [−] | 2.05 | 2.05 | 1.6 | 1.6 | 2.05 | 2.05 | 1.6 | 1.6 |
Biomass feed rate BA [MWth] | 200 | 200 | 200 | 200 | 200 | 200 | 200 | 200 |
Biomass feed rate EA [MWth] | 200 | 200 | 200 | 200 | 100 | 100 | 100 | 100 |
|
||||||||
Fuel Efficiency [%] | 57.6 | 55.6 |
|
56.0 | 57.6 | 55.6 | 58.4 | 56.0 |
Process Efficiency [%] | 77.4 | 78.5 | 77.8 |
|
77.4 | 78.5 | 77.8 | 79.0 |
Carbon Efficiency [%] | 35.4 | 34.2 |
|
34.5 | 35.4 | 34.2 | 35.9 | 34.5 |
CO2 produced [kg/s] | 14.00 | 14.11 | 13.72 | 13.84 | 14.00 | 14.11 | 13.72 | 13.84 |
FT C5+ Selectivity [%] | 83.2 | 84.4 | 87.3 | 87.5 | 83.2 | 84.4 | 87.3 | 87.5 |
Per-pass FT CO Conversion | 67.2 | 52.7 | 53.1 | 41.6 | 67.2 | 52.7 | 53.1 | 41.6 |
FT product output [kg/s] | 2.62 | 2.53 | 2.66 | 2.56 | 2.62 | 2.53 | 2.66 | 2.56 |
Electricity output [MW] | 2.1 | 3.4 | 1.8 | 3.3 | 2.1 | 3.4 | 1.8 | 3.3 |
District heating output [MW] | 37.5 | 42.3 | 37.1 | 42.6 | 37.5 | 42.3 | 37.1 | 42.6 |
|
||||||||
Fuel Efficiency [%] | 55.2 | 53.6 |
|
54.2 | 55.2 | 53.6 | 56.1 | 54.2 |
Process Efficiency [%] | 73.6 | 74.3 | 74.4 |
|
73.6 | 74.3 | 74.4 | 75.3 |
Carbon Efficiency [%] |
|
60.4 | 56.0 | 54.5 | 61.1 | 60.4 | 56.0 | 54.5 |
CO2 produced [kg/s] | 7.83 | 7.67 | 8.83 | 8.80 | 3.91 | 3.84 | 4.41 | 4.40 |
FT C5+ Selectivity [%] | 83.5 | 84.6 | 87.5 | 87.7 | 83.5 | 84.6 | 87.5 | 87.7 |
Per-pass FT CO Conversion | 67.2 | 52.7 | 53.1 | 41.6 | 67.2 | 52.7 | 53.1 | 41.6 |
Power input AEL [MW] | 187.9 | 198.7 | 145.2 | 151.8 | 93.9 | 99.4 | 72.6 | 75.9 |
FT product output [kg/s] |
|
4.48 | 4.15 | 4.04 | 2.26 | 2.24 | 2.07 | 2.02 |
District heating output [MW] | 66.2 | 76.3 | 59.3 | 69.0 | 33.1 | 38.2 | 29.6 | 34.5 |
Bold values signify best process performace within case 1.
As highlighted in
In the EA mode the highest carbon efficiency is found for case 1.1. at 61.1%. The combination of high FT yield and low CO2 production lead to the highest carbon efficiency. In both modes, the EA biomass feed amount has no influence on the carbon efficiency.
For all analyzed cases fuel and process efficiency is found to be higher in the BA mode compared to the EA mode cf.
Energy flow Sankey diagram for case 1.1 BA mode
The highest fuel efficiency is found for cases with high H2 conversion and a low H2/CO ratio i.e. case 1.3 and 2.3. In the BA mode a fuel efficiency of 58.4% and 56.1% in the EA mode is reached as shown in
When including the by-products district heating and electricity for the process efficiency, the highest process efficiency values can be found for case 1.4 and 2.4. With the lower H2 conversion of 55% and an H2/CO ratio of 1.6 a process efficiency of 79% BA and 75.3% EA can be attained (cf.
For the base case NPC of 1.08 and 1.04 €2019/L for continuous operation in BA and EA mode are found. The average electricity price of 55.49 €/MWh is used to determine the continuously operated EA NPC. BtL and PBtL, in contrast, have NPC of 0.66 and 1 €2019/L (cf.
Net production cost NPC and fixed capital investment FCI for all studied cases.
Case number | 1.1 | 1.2 | 1.3 | 1.4 | 2.1 | 2.2 | 2.3 | 2.4 |
H2 conversion [%] | 70 | 55 | 70 | 55 | 70 | 55 | 70 | 55 |
H2/CO [−] | 2.05 | 2.05 | 1.6 | 1.6 | 2.05 | 2.05 | 1.6 | 1.6 |
Biomass feed rate BA [MWth] | 200 | 200 | 200 | 200 | 200 | 200 | 200 | 200 |
Biomass feed rate EA [MWth] | 200 | 200 | 200 | 200 | 100 | 100 | 100 | 100 |
NPCBA [€2019/L] | 1.08 | 1.13 |
|
1.03 | 0.85 | 0.87 | 0.80 | 0.82 |
NPCEA [€2019/L] | 1.04 | 1.07 |
|
1.01 | 1.29 | 1.33 | 1.26 | 1.30 |
FCI hybrid plant [M€2019] | 535 | 554 | 482 | 500 | 390 | 401 | 367 | 376 |
FCI AEL [M€2019] | 224 | 236 | 176 | 184 | 118 | 124 | 93 | 97 |
NPC BtL [€2019/L] | 0.66 | 0.66 | 0.65 | 0.66 | 0.66 | 0.66 | 0.65 | 0.66 |
FCI BtL relative to hybrid plant [%] | 50.9 | 49.8 | 56.8 | 55.7 | 50.9 | 49.8 | 56.8 | 55.7 |
NPC PBtL [€2019/L] | 1.00 | 0.99 | 0.94 | 0.97 | 1.00 | 0.99 | 0.94 | 0.97 |
FCI PBtL relative to hybrid plant [%] | 92.2 | 92.8 | 91.8 | 92.0 | 50.9 | 49.8 | 56.8 | 55.7 |
Electricity price for equal NPC BA-EA [€2019/MWh] | 61.0 | 62.9 | 56.1 | 57.5 | −0.13 | 1.63 | −13.1 | −12.3 |
cfBA [%] | 30 | 24 | 50 | 42 | 100 | 100 | 100 | 100 |
Average electricity price during EA operation [€/MWh] | 50.65 | 51.43 | 47.51 | 48.69 | — | — | — | — |
NPChy [€2019/L] | 1.02 | 1.06 | 0.95 | 0.99 | — | — | — | — |
Carbon efficiency hybrid concept [%] | 53.5 | 54.0 | 46.0 | 46.0 | — | — | — | — |
Bold values signify lowest production costs for case 1.
When applying the Finnish day-ahead price profile, the NPC for the hybrid process in the base configuration is found to be 1.02 €2019/L. If the electricity price is lower than 61 €/MWh, the hybrid process is operated in EA mode. The remaining 30% of the year the process is operated in BA mode. The resulting electricity price for all hours operated in EA mode amounts to 50.65 €/MWh. Under this operation regime the hybrid process has a carbon efficiency of 53.5%.
The lowest production costs are found for case 1.3.
Breakdown for net production cost NPC for the FT product in case 1.1 and 1.3 and corresponding fuel (▲) and carbon efficiency (◆).
The hybrid process in case 1.3 operated under the conditions of the Finnish day-ahead market has the lowest NPC of 0.95 €2019/L—0.07 €2019/L less than in case 1.1. With 50% operation in EA mode, the average price for electricity is 47.51 €/MWh. The overall carbon efficiency amounts to 46%.
The reduced EA biomass feed rate in cases 2 leads to a decrease in the BA NPC at the expense of the EA NPC. The cost reduction for the BA mode can be attributed predominantly to the lower capital investment for the electrolyzer, as less hydrogen is needed for 100 MW biomass input. On the other hand, the higher NPC in the EA mode is due to the lower product output for all cases with 100 MW input.
On the Finnish day-ahead market cases 2 could not be sensibly applied. The NPC of the EA mode are only lower than BA NPC, when electricity is available at negative prices. Since this is not the case for 2019, the process would only be operated in the BA mode. Consequently, on the present energy markets of Finland, the electrolyzer would have to be inactive for the entire year.
To assess under what conditions on the energy market the hybrid operation principle is economical, a sensitivity analysis is conducted for the electricity price.
Variation of electricity price for case 1.1 EA and BA mode in comparison to BtL and PBtL. The light blue line (50-50) signifies the production costs if the process is operated half a year in BA mode.
The PBtL and BtL comparator production costs stay below those of EA and BA mode respectively. This is due to investment costs of inactive equipment. The inactive electrolyzer accounts for most of the price spread between BA mode and BtL comparator.
The EA mode would have to be operated for the entire year at a price of 8 €/MWh to reach the same production costs of a BtL plant. If the EA mode is only operated for half a year electricity prices of below—40 €/MWh would have to be available for the same time period to reach the BtL price level.
In
Electricity price required to attain equal NPC to BtL as a function of time operated in BA mode for case 1.1 (blue) and case 2.1 (orange).
It can be seen that none of the cases reaches an equal electricity price above—40 €/MWh at 50-50 operation. For a lower EA operation share the required electricity prices asymptotically approach negative infinity. Further, it can be taken from
In
Variation for key economic parameters applied to case 1.1 and 2.1.
The investment costs for the BtL plant, which entail all equipment except the electrolyzer and CO2 recycle, are estimated as 272 M€2019 for case 1.1 at 200 MW biomass input. Therefore, the BtL investment costs increased by + 100% would be 544 M€ at 200 MW biomass input. This estimate is in line with the upper range of the cost estimates of 1200 M€2011 for 400 MW input reported by (
Increasing the biomass price by +100% has a positive effect on the hybrid process. For case 1.1 and 2.1 it reduces the electricity price to around—30 €/MWh. Further, the electrolyzer investment costs reduction decreases the electricity price to around—10 €/MWh for case 1.1 and—20 €/MWh for case 2.1. The effect on case 1.1 is stronger, because in this case a larger AEL is required (cf.
Overall, it can be seen that only a combination of the discussed parameter variations would increase the electricity price to a positive value. Seeing that negative electricity prices for half a year are not likely, it seems probable that a BtL plant is more economical than the presented hybrid process concept.
In this study a techno-economic analysis is conducted for a hybrid operation concept of an electrolysis enhanced biomass-to-liquid process. The electrolysis enhanced mode, which increases the overall product yield, is only activated when the prices on the Finnish day-ahead market for 2019 make it more profitable than feeding only biomass. To that end a cost calculation method for hybrid processes was applied within DLR’s software tool TEPET. Eight process design cases are analyzed to study the economic impact of FT conversion, H2/CO ratio and the biomass feeding rate in the electrolysis enhanced mode. To do so a FT kinetic model was implemented in Aspen Plus. All cases are compared to the steady-state alternatives BtL or PBtL. To gain a broader understanding of the process concept, a sensitivity analysis over electricity price and share of operation hours in each mode as well as key economic parameters is conducted. Based on the results presented here, the following conclusions can be drawn: • Production costs of 1.08 €2019/L for the hybrid concept compared to 0.66 €2019/L for BtL and 1 €2019/L for PBtL were found based on the Finnish day-ahead market for the base case. o Under these conditions, an overall carbon efficiency for the hybrid process of 53.5% is found compared to 35.4% for BtL and 61.1% for PBtL. o The production cost difference is mainly due to the lower capital investment requirement for the reference processes. Only 51% and 92% of the investment costs for the hybrid process are required for BtL and PBtL respectively. • The lowest NPC and highest fuel efficiency are found for cases with low H2/CO ratio (1.6 instead of 2.05) o Fuel efficiency can be increased by + 1% for BA/BtL and EA/PBtL for cases with equal H2 conversion. o The NPC for the hybrid concept can be decreased by 0.07 €2019/L • A 100 MWth biomass feed in the EA mode is sensible, if the process is predominantly operated in the BA mode. • The BtL concept appears to be the most economic process alternative given the current renewable electricity price. However, changing economic conditions, i.e. power and biomass prices, and technology development like the reduction of electrolyzer investment cost could make the hybrid concept economically feasible in the future.
The following points should be investigated further to either validate the assumptions made in this study or improve the efficiency and profitability of the two processes: The H2/CO ratio and FT conversion can improve the overall process performance. However, a correlation between H2/CO ratio and the conversion limit was not presented in literature so far. An experimental study on this correlation would help to better assess the optimal yield for the FT reactor.
Further, the CO2 recycling rate was not discussed in this study. With a higher recycling rate, the hydrogen demand of the process would be greater and a higher product yield can be expected. The amount of product yield can even be increased further when hydrogen is added to the reformer directly. This was simulated in (
The FT recycle was assumed to have a recycle ratio of 95%. Increasing this value comes at the cost of accumulation of inert gas content in the syngas. However, it also leads to higher fuel yield. An upper limit to the recycling rate should be found experimentally.
The original contributions presented in the study are included in the article/
The study is based on EK process design as studied in the EU project FLEXCHX. Modeling, simulation and analysis was done by FH under the guidance of R-UD and EK. The software tool TEPET was provided by SM. The authors FH and SM prepared the manuscript. R-UD, EK, and SM discussed and commented the manuscript.
This study is part of the FLEXCHX project, which has received funding from the European Union’s Horizon 2020 research and innovation Programme under Grant Agreement No 763919.
The author EK is employed by VTT Technical Research Centre of Finland Ltd.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Further, the authors would like to thank Julia Weyand, Moritz Raab and Dr. Marc Linder for their valuable inputs.
The Supplementary Material for this article can be found online at: