Energy and greenhouse balance of photocatalytic CO 2 conversion to methanol

Within the Leading-Edge Cluster “Forum Organic Electronic”, the research project “Solar2Fuel” funded by the German Ministry of education and research (BMBF) (2009 – 2012), EnBW, BASF, Karlsruhe Institute of Technology and Ruprecht-KarlsUniversity of Heidelberg aim to develop a future solar powered CO2 to methanol conversion technology. CO2 from stationary sources such as power plants shall be catalytically converted together with water to a product such as methanol by use of solar irradiation. For this purpose a catalyst shall be developed. EnBW investigates the required boundary conditions to make such a principle interesting with respect to energy and greenhouse gas balance as well as economic evaluations. The assessment of boundary conditions includes the analysis of the whole chain from power generation, CO2 capture and transport, a virtual photocatalytic reactor, the product purification and use in the traffic sector. Most important technical factors of the process such as CO2 conversion efficiency is presented. CO2 capturing and liquefaction are the most energy intensive process steps, CO2 transport in pipeline is highly energy efficient and depending on energy need of the photoconversion step and the product purification, the overall greenhouse gas balance is comparable with the underground storage of the captured CO2.


Principle of photocatalytic CO 2 conversion into methanol
The research project "Solar2Fuel" pursues the future vision of photocatalytic CO 2 conversion, to use direct sunlight as energy resource for reduction of CO 2 .Thus, CO 2 can be used as feedstock for renewable fuel and energy storage.CO 2 captured from flue gas of a power station is liquefied and transported to the location of a photocatalytic reactor field, which converts CO 2 with sunlight to methanol.The latter can be used as fuel in cars and therefore substitute oil based gasoline and subsequently avoid emissions from fossil gasoline.Photocatalytic reduction of CO 2 is a redox-reaction with water as reducing agent.The net chemical reaction is shown in equation (1).EnBW investigates the required boundary conditions for an environmentally friendly and economic operation of the "Solar2Fuel" process, if captured CO 2 is transported far from Germany to sunny regions in South Africa (Figure 1).

Figure 1:
The "Solar2Fuel" system.Captured CO 2 in Europe, transport to sunny regions in South Africa for photocatalytic conversion The assessment of boundary conditions includes the analysis of the whole chain from power generation, CO 2 capture and transport, a virtual photocatalytic reactor the product purification and use in the traffic sector.It is evaluated, if it is energetically and economically feasible to transport huge amount of CO 2 from Germany to sunny areas in South Africa.

The process chain and data of photocatalytic CO conversion into methanol
In the following, data for CO 2 supply to the reactor field, beginning with CO 2 capture and liquefaction in fossil fuelled power plants in Germany, subsequent pipeline transportation to the place of delivery in In Salah (South Africa) are presented.
The cost effectiveness and the efficiency of CO 2 pipeline transport at far distance increases strongly with larger inner diameter of the pipeline.Half inner diameter gives rise to 16 fold increase in friction loss (Hagen Poiseuille theory).Thus, the pipeline is simulated with a large inner diameter of 1.1 meter and 50 million tons annual capacity.This annual amount of captured CO 2 corresponds to 6.397 GW el coal fired power stations with a net efficiency of 34.3 % after CO 2 capture and liquefaction and CO 2 transport.
We assume that 90% of the delivered CO 2 is converted to methanol, 10% is lost to the atmosphere during the conversion process.Oil based gasoline can be substituted by methanol on basis of lower heating value.The higher efficiency of methanol combustion compared to gasoline combustion is not considered in this study.

Carbon capture and liquefaction in fossil fuelled power stations
As explained above, the annual transportation capacity of the CO 2 pipeline is set to 50 million tons per year.The total thermal power of power stations with carbon capture is set to 18.653 GW th , to operate the pipeline with full capacity, 8760 hours per year.That gives rise to a total net power of 6.397 GW el of power stations after losses due to carbon capture, liquefaction and transport.
To compare the capture case ("Solar2Fuel system) with the today´s situation of power stations without carbon capture and oil based gasoline fuelled cars, the capacity of the power stations without carbon capture is set to 6.397 GW el as well (

Required data for evaluation of the system balances
For sound comparison of today´s situation and the "Solar2Fuel" system, both cases need to supply the same amount of electricity to the grid as well as the same amount of fuel (measured in lower heating value) to combustion engines in the traffic sector.Data for lower and higher heating value for gasoline and methanol is listed in Table 3.To analyse the greenhouse gas balance of both systems, the upstream emissions of coal mining and supply, gasoline supply and the construction of the power stations as well as of the CO 2 pipeline are taken into account.Specific values for the commodities, energy carrier and the infrastructure (power station and pipeline) are listed in Table 5. CED of steel for pipeline and its construction 2 kWh/kg 7.0 -GHG of steel for pipeline and its construction 3 kg CO 2 -eq/kg 2.7 [09] CED of power station construction 4 Wh/kWh el (without/with carbon capture) 5.7/7.5 -GHG of power station construction 5 g CO 2 -eq/kWh el (without/with carbon capture) 2.2/2.9 [10] according to Hondo CED for CO 2 capture facility 6 Wh/kWh el 7.5 -GHG for CO 2 capture facility 6 g CO 2 -eq/kWh el 2.9 -1 = "CO 2 -eq", add on "-eq" denotes CO 2 -equivalent 2 = estimation by dividing GHG with specific emission factor of hard coal (2.7/0.3864).
46.4 g CO 2 -eq/kWh origins from coal supply chain, 340 g/kWh from combustion 3 = data from Probas for steel; 2.0 kg CO 2 -eq/kg steel plus 15% for manufacturing of tube segments and another 20% for pipeline construction itself 02006-p.5 4 =estimation by dividing GHG for construction with 386.4 g CO2eq/kWh primary energy (340 g CO 2 /kWh LHV + 46.4 g CO 2 -eq from coal supply chain) 5 =data taken from [10] according to Hondo, adapted to capacity utilization 6 =estimated in the same order of magnitude as the power station itself

Results about annual greenhouse gas emissions
Table 6 shows the greenhouse gas emissions for the today´s situation and the "Solar2Fuel" system including the contribution of the different steps of the process chains.Relative reduction of the "Solar2Fuel" system [%] -39.6% 1 = 35 years pipeline lifetime assumed 2 =upstream chains of construction of photocatalytic reactor is not considered as well as the methanol purification process.Final relative GHG emission reduction will be lower.
The relative annual emission reduction of the "Solar2Fuel" system compared to the today´s situation is about 39.6%.It is to notice, that the emissions for the construction of a reactor field for the photocatalytic reaction is not included in the calculation as well as the purification of the methanol to fuel grade.

02006-p.6
To estimate the effect of methanol purification to fuel grade, we assume that the heat input for distillation is about 25% of the lower heating value of methanol and that the required heat is supplied with steam supplied via parabolic trough collector.
Table 7 shows the CED and GHG for solar thermal electricity supply from solar concentrating power plants with parabolic trough [11, 12].Supposed that the efficiency of steam/heat conversion into electricity is 30%, it can be concluded, that the CED for one kWh steam is only 0.14*0.3= 0.042 MJ or 11.6 Wh/kWh.Considering GHG emission, that means 4.2 g CO 2 -eq/kWh steam.This gives rise to GHG emissions of 23.2 kg CO 2 -eq/t purified methanol or 760,124 t CO 2 -eq every year.
The "Solar2Fuel" system has a relative GHG reduction of 38.84% compared with today´s situation.It can be concluded, that purification will have only little effect, as long as the required heat is supplied with solar energy.

Results about annual primary energy consumption
Table 8 shows the annual primary energy consumption of today´s situation and the "Solar2Fuel" system.8 does not include the CED of the upstream chain for construction of parabolic trough field for steam generation of methanol distillation.The supply of 45.29 TWh steam for distillation does roughly cause a CED for construction of annual 0.525 TWh and would decrease the relative advantage of "Solar2Fuel" system to 48.8%.
Here is to mention again, that CED for construction of reactor field for photocatalytic conversion process is not included as well as the energy for operation of such a field.

Comparison of the relative emission reduction of "Solar2Fuel" system with carbon capture and storage (CCS)
Under environmental aspects, the "Solar2Fuel" system does only make sense, if there is an advantage compared to carbon capture and storage of CO 2 in underground formations.
In [13] is described, that the greenhouse gas emissions for the production of one kWh el including the upstream processes can be reduced by 71.4%.In this case, cars will further use oil based gasoline, while in the "Solar2Fuel" system, the captured CO 2 is not stored in underground formations, but converted back to methanol and substitutes gasoline in cars.With data from Table 6 it can be concluded, that GHG emission reduction in the system with CCS and oil based gasoline consumption for cars, is about -32.2% compared to the today´s situation.The comparison of the relative greenhouse gas emission of the "Solar2Fuel" system with carbon capture and storage is shown in Figure 2. On axis of abscissa, the CO 2 conversion rate is depicted.It describes the percentage of CO 2 supplied to the reactor field, which is converted to methanol.The rest is lost to the atmosphere.The graph shows, that greenhouse gas balance of "Solar2Fuel" system can be better than CCS, if CO 2 conversion rate of supplied CO 2 to the reactor field is above 77%.The blue line describes the relative GHG reduction of "Solar2Fuel" system compared to the today´s situation.The red line is independent of CO 2 conversion rate and shows the relative GHG reduction of CCS case compared to the today´s situation.The blue line of "Solar2Fuel" system in Figure 2 does again not include the upstream chain of photocatalytic reactor field construction and methanol purification.Thus, blue line will shift down/to the right side, if these two factors are considered.To compete with the CCS case, the minimum CO 2 conversion rate needs to be higher than 77%.
02006-p.8The economic feasibility of solar CO 2 conversion to methanol depends of a variety of parameters.Table 9 lists some important parameters that do influence the profitability of photocatalytic methanol production.The more annual solar irradiation, the higher is the area specific methanol yield and thus generates higher cash flows.In all subsequent calculations, a solar irradiation of 2,000 kWh/m²/year is assumed.

Photo conversion efficiency (PCE)
[%] Relative energy of solar irradiation, that is stored as chemical energy in methanol (based on HHV) The cost of CO 2 supply to the photocatalytic reactor field will strongly influence methanol production cost

Carbon dioxide conversion rate [%]
Part of delivered amount of CO 2 , that is converted into methanol

Energy demand for operation of photocatalytic reactor field [TWh]
Energy consumption for reactor field operation will influence energy and greenhouse balance.Furthermore energy has a price and has to be 02006-p.9financed by sale of the product (methanol)

Capital expense for reactor field construction [€]
Depending on the PCE, solar CO 2 conversion needs huge area.The area specific invest together with interest rates will have strong influence on production cost Heat requirement for methanol purification [kWh/t] It is expected, that methanol leaves the reactor in low concentrated in a mixture with water.The higher the heat requirement, the higher the production cost of solar methanol Table 10 shows the underlying assumptions in the economic evaluation of photocatalytic methanol production from CO 2 .Solar irradiation [kWh/m²/a] 2,000 1 = photocatalytic reactor field operates only during daytime.CO2 arriving during the night period needs to be stored.Storage capacity is set to 14 hours pipeline capacity with pipe storage of 1.4 m diameter and specific invest of 3,000 €/m.
Figure 3 shows the methanol yield per hectare in dependence of photo conversion efficiency from 1% to 20%.This hectare specific yield is used for the following evaluation.
02006-p.10 Figure 4 shows the hectare specific monetary turnover from sales of the product methanol for different price.From this money flow, the construction and operation of the photocatalytic reactor field need to be financed.The construction has an important impact on the methanol production cost because the solar energy powered CO 2 conversion is a process with intensive land use.Subsequently, the hectare specific capital expenditure needs to be as low as possible.Figure 5 shows the decrease of the free cash flow that can support capital expenditure after considering the cost of CO 2 supply (46.0 € /t), electricity cost for reactor field operation (assumptions: 150 MWh/ha*year; 5 €ct/kWh el ) and cost for solar steam generation (1,383 kWh/t methanol for 2.0 €ct/kWh).These cost factors reduce the free cash flow about one third.The maximum allowed hectare specific invest, that can be financed with the free cash flow can be evaluated with the cash method.Figure 6 shows the hectare specific net present value of photocatalytic reactor field.The blue horizontal line introduces a benchmark to get an imagination of the minimum required capital expenditure for the construction of a photocatalytic reactor field.A photovoltaic field with a land cover ratio of 70% and panel efficiency of 20% has a hectare specific peak capacity of 1,400 kW p .With an assumed future photovoltaic system cost of cheap 500 €/kW, this gives rise to hectare specific capital expenditure of 700,000 €.It can be expected, that the construction of a reactor field for photocatalytic CO 2 conversion is rather more expensive than an easy and simple photovoltaic system.A photocatalytic reactor field need to have a transparent glass cover to prevent evaporation and CO 2 loss.Behind the transparent glass a catalyst absorbs the light.The field need to include a good network of pipes to handle liquid and gaseous flows together with sensors and pumps.Solar glass alone cost already 10 €/m² [15].That is equivalent to 100,000 €/hectare aperture area photocatalytic reactor field and already 14% of the benchmark of 700,000 €/hectare.
Therefore, it can be considered as likely, that the hectare specific net present value of a photocatalytic reactor field need to exceed the investment for photovoltaic field on one hectare.
02006-p.12The lowest line in Figure 6, considering the cost for CO 2 supply, electricity consumption and solar heat supply for methanol distillation, hits the "threshold" of 700,000 € at PCE of roundabout 12%.Furthermore, the cost assumptions for electricity and solar heat supply are rather optimistic/low.These circumstances imply, that the photocatalytic reactor field shall have a PCE above 12% to support itself in economically respect.Or, on the other hand, it has to be estimated, if the reactor field can be build for maximum 700,000 €/hectare.
350 €/t methanol is close to the Free On Board price of methanol in the harbour of Rotterdam in Netherlands in the first Quarter 2012 [16].To demonstrate the effect of lower methanol price, Figure 7 shows the hectare specific net present value like in Figure 6, with methanol price set to 250 €/t.
The "threshold" of 700,000 € per hectare is now exceeded at roundabout 18% PCE.Compared to a methanol price of 350 €/t the net present value drops substantially.

Conclusion
In this paper, the energy and greenhouse gas balance of photocatalytic CO 2 conversion to methanol was evaluated.The chain included the CO 2 capture and liquefaction in coal fired power stations, CO 2 transport via far distance of 3,743 km from Germany to Algeria, photocatalytic conversion and methanol purification to fuel grade for application in combustion engines in the traffic sector.It was figured out, that the CO 2 capturing and liquefaction step is a very energy intensive step in the process chain.CO 2 pipeline transport to Algeria is highly energy efficient and does nearly not influence the energy-and greenhouse gas balance.
Compared to the today´s situation of power stations without CO 2 capture and oil based gasoline fuelled cars, the "Solar2Fuel" system with CO 2 capture, liquefaction, transport and photocatalytic conversion to methanol can decrease the greenhouse gas emissions by about -39.6%.This is even better than the relative reduction of carbon capture and storage in underground formations where cars would be further fuelled with oil based gasoline.The CCS case would reduce the greenhouse gas emissions about -32.2% compared to the today´s situation.To compete with the CCS case, the CO 2 conversion to methanol in the "Solar2Fuel" system needs to exceed 77%.In terms of primary energy consumption, the "Solar2Fuel" system can offer a maximum reduction of -49.0%compared to today´s situation.It was shown, that CO 2 supply price from coal fired power stations inclusive transporation and overnight storage in Algeria is about 46.0 €/t.In case of a methanol value of 350 €/t, economic evaluations showed, that a hectare specific net present value of a photocatalytic reactor field can exceed a defined threshold of 700,000 €, if the photo conversion efficiency exceeds 12%, which means that 12% of the solar energy is stored in methanol in terms of higher heating value.
a e-mail : d.haumann@enbw.comEPJ Web of Conferences DOI: 10.1051/ C Owned by the authors, published by EDP Sciences,

Figure 2 :
Figure 2: Relative GHG emission reduction of "Solar2Fuel" system and CCS case (CCS and oil based gasoline) compared to today´s situation

Figure 3 :
Figure 3: Methanol yield per hectare in dependence on photo conversion efficiency (PCE)

Figure 4 :
Figure 4: Hectare specific monetary turnover for different methanol prices

Figure 5 :
Figure 5: Effect of CO 2 -, electricity-and solar heat cost on hectare specific free cash flow

Figure 6 :
Figure 6: Hectare specific net present value calculated with methanol price of 350 €/t in dependence on PCE and the influence of CO 2 -, electricity-and solar heat supply cost

Figure 7 :
Figure 7: Hectare specific net present value calculated with methanol price of 250 €/t in dependence on PCE and the influence of CO 2 supply, electricity-and solar heat supply cost

Table 1 :
Data about power stations and CO 2 capture & liquefaction ).

2.2 CO 2 pipeline transport from Germany to North Africa Data
for a CO 2 pipeline from Germany to Algeria via Italy, Sicily and Tunisia is listed in

Table 2 .
Analysis and evaluation was done from RBS wave GmbH and are taken from[02, 03]

Table 2 :
Data about CO 2 pipeline from Germany to Algeria

Table 3 : Higher and lower heating value from gasoline and methanol Category Unit Gasoline Methanol Reference
2 /kg methanol need to be delivered to the photocatalytic reactor field and 32.727 million tons methanol are produced (Table4).Based on lower heating value, the methanol can substitute 14.994 million tons gasoline.02006-p.4

Table 4 :
Data about photocatalytic CO 2 conversion

Table 5 :
Greenhouse gas emissions (GHG) and cumulated energy demand (CED) due to upstream chains

Table 6 :
Annual GHG emissions of today´s situation and the "Solar2Fuel" system

Table 7 :
CED and GHG of electricity supplied from solar thermal power station with parabolic trough

Table 8 :
Annual primary energy consumption of today´s situation and the "Solar2Fuel" system Table

Table 9 :
Parameters that influence the profitability of solar CO 2 conversion to methanol

Table 10 :
Parameters for economic evaluation of photocatalytic methanol production from CO 2