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Storing renewable energies in a substitute of natural gas
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Storing renewable energies in a substitute of natural gas G. Spazzafumo Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Via G. Di Biasio, 43, I-03043, Cassino, FR, Italy
article info
abstract
Article history:
This paper proposes a way to obtain valuable electric power and valuable fuel starting from
Received 26 February 2016
renewable variable electric power plus biomass and/or waste products.
Accepted 23 May 2016 Available online xxx
Biomass/biofuel can be oxyburned using electrolytic oxygen to generate electric power. Gas turbines or internal combustion engines are suitable to such a task, but there is the problem of very high temperatures connected to oxycombustion. In the case of gas turbine
Keywords:
the inlet temperature could be controlled by adding steam and/or carbon dioxide, while in
Renewable electric power
the case of internal combustion engines only carbon dioxide could be used. In such a way
Energy storage
the exhaust gas continues to be formed by carbon dioxide and steam which can be easily
Electrolysis
separated by condensation. Carbon dioxide is fed to a Sabatier process together with
Biomass
electrolytic hydrogen to generate a gas with characteristics similar to natural gas.
Sustainable natural gas substitute
Although electrolytic hydrogen could be used directly both in internal combustion engines and fuel cells, significant problems to hydrogen distribution and on-board storing still exists. Therefore the substitute of natural gas could be a real bridge solution for the short/medium term. A simulation has been carried out and the resulting efficiencies range from 0.52 to 0.58. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In Italy, and maybe also in other countries, the diffusion of systems based on renewable energy sources (RES) has not been correctly planned. The result is that the electric power production from run-of-river hydropower plant, wind and especially photovoltaic, sometimes exceeds the demand on the grid and a system for energy storage has not yet been realised. The consequence is that the price of electric power drops and some combined cycles powerplant fed by natural gas (NG) could be shut down because their operation is more expensive than that of coal powerplants, which are much
more pollutant. This reduces the environmental benefits of the RES diffusion. Moreover RES are irregular and the stability of the grid poses a limit to the amount of electric power which can be absorbed. A third issue is that, with the exception of biomass, all RES are currently converted to electric power, while in most of the countries the final use of energy is strongly dominated by fuels (usually >75%) which are used for transportation via land, sea and air, for heating, for cooking and for industrial processes. Finally we can observe that even biomass (and wastes) is often used to produce electric power: this is due to the fact
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2016.05.209 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
that they are initially converted to a gas characterised by a low heating value. As a consequence such a gas is not suitable for distribution as a fuel and it is burned on-site to generate heat or electric power. Electrolytic hydrogen could certainly be a long term solution to store electric power generated by RES. Unfortunately nowadays there are still critical aspects in storage and distribution of the hydrogen itself, especially when considered as a clean fuel for urban mobility. At the beginning fossil fuels were biomass. What made them energetically valuable is the progressive reduction of oxygen which percentually enriched them of carbon. Such a change required millions of years and particular thermodynamic conditions, but hydrogen can have on biomass the same effect very quickly because it is able to subtract oxygen from biomass:
combustion. The exceeding amount of oxygen could be addressed to the industrial sector or to hospitals. In this perspective it could be possible to generate a mix of a clean fuel and stable electrical power close to the actual local requirement. Using electrolytic oxygen allows to obtain, after gas cleaning, a combustion product containing only carbon dioxide and steam, which are easily separable by condensation. Carbon dioxide is then used together with electrolytic hydrogen as a reactant for the Sabatier reaction (e.g. Ref. [4]). The same process could be realised also using fossil fuels, and coal in particular, as a carbon source [5e7]. And finally it could be realised also mixing coal, biomass and waste products as a carbon source. The oxygen produced by electrolysis exceeds the amount required by the process and therefore could be sold for several different uses being of particular high purity.
CxHyOz þ (2x y/2 þ z) H2 / xCH4 þ zH2O This is what directly happens, in theoretical conditions, if you hydro-gasify biomass. But it also happens, indirectly, in other processes. For example when biomass or the humid fraction of wastes are subjected to anaerobic digestion followed by methanation with hydrogen. In this case carbon is first converted to methane and carbon oxides, and subsequently also carbon oxides are converted to methane:
CO þ 3H2 / CH4 þ H2O
CO2 þ 4H2 / CH4 þ 2H2O In real conditions such reactions are not complete, but it is possible to push them towards a high level of advancement by supplying over-stoichiometric hydrogen. The result, after steam condensation and water separation, is a gas very rich of methane and containing a residual amount of hydrogen, while carbon oxides are almost completely absent. Therefore such a gas has characteristics very similar to those of NG and can be distributed, stored and used exactly as we do with NG. Actually the possibility to distribute hydrogen up to a concentration of 20% through NG pipelines was already proved [1] and such a mixture can be used directly into internal combustion engines resulting in performance improvement [2]. Such a solution could be very interesting especially for developing countries which have no fossil resources, but have a good availability of biomass and other RES. A typical example could be Costa Rica [3]. Thus coupling exceeding electric power from RES as a source of hydrogen with biomass or waste products as a source of carbon, could allow to store energy from RES in the best possible way as far as for the current energy system, that is as chemical energy of an already widely utilised fuel. Another possibility is to generate electric power for the grid before producing the substitute of NG (SNG). It implies that combustion be carried out using oxygen rather air. However it is not a problem because electrolysis generates also oxygen and the amount of oxygen available exceeds the demand for
System lay-outs and modelling Two possible configurations of the system have been investigated. A first one is based on a gas turbine, while the second one is based on an internal combustion engine. In both cases critical issues are the increase of flame speed and the very high temperature obtained with oxycombustion and it is necessary to add a diluent capable to replace the nitrogen of the air: as the final products should be only water and carbon dioxide they are the unique two candidates, though they are not really diluent, but products. It means that they are able to moderate the temperature, but they could obstacle the combustion. Adding steam to a gas turbine unit is an already developed technology: STIG (STeam Injected Gas). In this case, however, the steam flow required could be very high. It is also possible to add both steam and carbon dioxide. The two extreme cases (only steam or only carbon dioxide) have been analysed and have been identified as ST and GT. In the case of internal combustion engines it is not possible to add a large amount of steam and therefore the addition of carbon dioxide is the unique reasonable solution and has been identified as ICE. Starting from a lignocellulosic biomass (see Table 1 for composition) and electrolytic hydrogen and oxygen, a simulation has been carried out using a thermochemical software (AspenONE® v8.4). Fig. 1 shows a simplified schematic of the general layout. A flow of oxygen coming from the electrolysis unit is fed to a biomass gasifier which carries out a partial oxidation (POX) of the biomass. As electrolysis is assumed to be carried out at 30 bar, also the POX reactor operates at such a pressure and supplies fuel to the gas turbine or to the internal combustion engine. The biomass treatment includes also ash separation (cyclone) while a clean-up unit is not required because
Table 1 e Biomass composition (weight %). H2O
Ash
C
H
N
O
FC
VM
20
0.24
40.32
4.88
0.08
34.48
14.89
64.87
Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 1 e Simplified schematic of the general layout.
Fig. 2 e Aspen schematic of the biomass treatment.
lignocellulosic biomass does not contains significant amount of sulphur or chlorine compounds. The syngas obtained enters the power unit together with a second flow of oxygen and with a flow of a diluent which allows to control the combustion temperature. Exhaust gas from the power unit consists
3
almost completely of steam and carbon dioxide which are recompressed and separated. The separator supplies three different flows: the above mentioned diluent flow, a water reintegration flow to the electrolyser, and a carbon dioxide flow to the Sabatier process. This last one receives also an over-stoichiometric flow of hydrogen to generate SNG. The POX reactor, schematised in Aspen as a combination of a Yeld reactor and a Gibbs reactor, and the cyclone are common elements for all the layouts (Fig. 2), just as the part of the plant devoted to the Sabatier process (Fig. 3). This is realised with three units. The first unit consists of a Gibbs reactor (MET1) and two heat exchangers. REX1 is a regenerative heat exchanger having the task to reheat the syngas to 300 C before entering in the subsequent unit. HEX1 has a double task: to recover the exceeding heat and to allow a partial condensation of the water generated by the Sabatier reaction. The reactor is supplied with electrolytic hydrogen and carbon dioxide recovered from the exhaust gas downstream of the power unit. Moreover there is a partial recycle of the outlet gas according to the typical layout of methanation process from Haldor Topsoe [8]. The second unit is almost identical to the first with a Gibbs reactor (MET2), a regenerative heat exchanger (REX2) and a heat exchanger (HEX2), but there is not a recycle and the reactor is supplied only with the syngas coming from the first unit. Finally, the third unit receives the syngas from the second unit and has only a Gibbs reactor (MET3) and a heat exchanger (HEX3), without a regenerative heat exchanger. The part of the plants with the power unit and the carbon dioxide recovery are quite different. ST layout (Fig. 4) requires a flow of steam at 30 bar to be injected into the combustion chamber (CC) of the turbine together with the syngas from the cyclone and stoichiometric oxygen. Combustion products are expanded into the turbine and then cooled into a heat exchanger (HEX0). Carbon dioxide is separated from condensed water and re-compressed to 30 bar inside a multistage intercooled compressor which also
Fig. 3 e Aspen schematic of the Sabatier process. Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 4 e Aspen schematic for ST layout.
Fig. 5 e Aspen schematic for GT layout. Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
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Fig. 6 e Aspen schematic for ICE layout.
contributes to reduce the residual water content. At the end of the compression line the flow of carbon dioxide is fed to the previously described Sabatier process. The water recovered from HEX0 and from the intercoolers (IC1 and IC2), together with an external flow of water, is pumped into a heat recovery boiler which receive heat from HEX0, HEX1, HEX2 and HEX3 to generate the required steam at 30 bar and 250 C. GT layout (Fig. 5) differs from the previous simply because the combustion chamber does not receive a flow of steam, but a flow of carbon dioxide recirculated from the last
Table 2 e Basic assumptions. Ambient temperature Turbine inlet temperature Steam temperature Electrolysis efficiency (HHV) Pump efficiency Compressor efficiency Turbine efficiency ICE efficiency ICE heat losses Electric efficiency
20 C 1500 C 300 C 0.70 0.70 0.80 0.85 0.32 0.20 0.985
Fig. 7 e Power and temperature vs oxygen supplied.
compression stage. Such a concept is similar to that of the Matiant Cycle [9]. The water recovered is not used directly, but could be supplied to the electrolyser together with the water recovered from the Sabatier Process.
Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Though the systems analysed are suitable for different biomass flow rate values, all simulations have been referred to 1 Mg/h in order to have a direct comparison. The composition of the biomass considered is reported in Table 1 and its HHV is 14,078 MJ/kg. Stoichiometric oxygen is 9.702 mol/s and has to be split between partial oxidation and combustion. The more oxygen is supplied to the POX reactor, the higher is the temperature
while the HHV of the syngas decreases. Fig. 7 shows the changes of thermal and chemical power of the syngas and its temperature when varying the flow of oxygen supplied to the POX, while Fig. 8 shows how the gas composition varies when varying the flow of oxygen supplied. At low rates of oxygen supplied, temperature is rather low and methane and carbon dioxide are the main components of the syngas. Increasing the oxygen available results in higher temperatures and the methane gradually disappears while hydrogen and carbon monoxide become predominant. Over 3 mol/s the exothermicity of combustion is no more counteracted by the endothermicity of methane reforming and the temperature starts to increase rapidly. Therefore a flow of oxygen of 3 mol/s has been chosen to feed the POX reactor obtaining a syngas at about 890 C. The maximum turbine inlet temperature has been assumed equal to 1500 C and it requires a steam flow of 48.39 mol/s with the result that the steam content of combustion products is higher than 86% by volume. Actually it is more similar to a steam powerplant than to a STIG one and the turbine could require a specific design. However steam at 1500 C is properly a gas and only the last stages have to expand a vapour. The heat required for steam production is 2484 kW and absorbs most of the useful heat recoverable from the heat exchangers. The DT at pinch point is 25 C. The heat exchange diagram shown in Fig. 9 has been obtained cumulating the heat available at proper temperature: it is evident that the net residual heat available (which has not been reported completely in the diagram and account for more than 4000 kW) is scarcely useful. Only a little more than 1000 kW could be used, for example, for adding a second level of pressure or as a heat source for an Organic Rankine Cycle. About double (1974 kW at T > 60 C) could be used for direct thermal use to obtain warm water. In the other cases the recovered heat available is much higher because there is no production of steam. However, though the temperatures are suitable for a good conversion to electric power, for example with a steam powerplant, the size of the plant could not allow a high efficiency. On the other hand, thinking to a close Brayton cycle with air it is impossible to obtain the maximum power and the maximum
Fig. 9 e Heat exchange diagram (ST).
Fig. 10 e Heat exchange diagram (GT).
Fig. 8 e Syngas composition vs oxygen supplied.
Finally, ICE layout is shown in Fig. 6. The ICE is supposed to be a naturally aspirated engine, even there is the possibility to supply syngas and oxygen at 30 bar. Therefore mechanical work is recovered by means of two expanders (EXP1 and EXP2) and the expanded syngas is cooled into a heat exchanger (HEX4). The exhaust gas from ICE is cooled and partially dried into another heat exchanger (HEX0) before being split in two flows, one directly fed to the ICE and the other re-compressed and further dried inside a multistage intercooled compressor, like in the previous layouts, to be supplied to the Sabatier process. Also in this case all the water recovered could be supplied to the electrolyser. Table 2 shows the main basic assumptions.
Results and discussion
Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
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hcog ¼ (Ech,SNG þ Eel,produced þ Qrecovered)/ (Ech,biomass þ Eel,consumed)
Conclusions
Fig. 11 e Heat exchange diagram (ICE).
Table 3 e Energy balance.
Power for electrolysis Biomass (HHV) SNG (HHV) Electric power generated Residual heat available (T > 60 C) Maximum temperature of residual heat Cogeneration efficiency System efficiency
Unit
ST
GT
ICE
kW kW kW kW kW
16044 3911 8929 2548 1974
1417 4134
9067 1299 3872
164.5
784.5
777
0.676 0.575
0.727 0.519
0.714 0.520
C
temperature at the same time as shown in Fig. 10 for GT layout and in Fig. 11 for ICE layout. Table 3 shows the energy balance resulting from the simulation carried out. The difference in chemical power of the SNG generated depends on the different amounts of water separated which slightly affect the final composition. And the average composition by volume is:
CH4 79.89% e H2 19.70% e N2 0.07% e H2O 0.33% e CO2 0.01% with very small differences. To compare the layouts analysed, two different efficiencies could be defined: - a system efficiency which considers the electrical (Eel) and the chemical (Ech) outputs divided by the inputs:
hsys ¼ (Ech,SNG þ Eel,produced)/(Ech,biomass þ Eel,consumed) - a cogeneration efficiency which considers also the direct use of recoverable heat (Q), e. g. to heat water or generate steam:
The proposed system allows to convert low grade energy inputs (coal, biomass, waste products, unstable electric power) into high grade energy outputs (methane þ hydrogen mixtures, oxygen, stable electric power). From a quantitative point of view efficiency is lower than that of other energy storage system (e.g. hydro pumped storage power plants), however this system makes use also of biomass and enhances its utilisation. Moreover it seems very important that the main final product is a clean gaseous fuel. When used in place of gasoline it allows better efficiency and lower emissions. Therefore it would be proper to extend the comparison to the global energy chain. Finally, when the system should be applied to waste products there would be further advantages. This is a preliminary analysis. Next steps will be the improved analysis of ICE layout using a specific software, the analysis of alternative layouts including further power recovery, for example through ORC, and the analysis of alternative layouts based on high temperature fuel cells.
references
[1] Melaina MW, Antonia O, Penev M. Blending hydrogen into natural gas pipeline networks: a review of key issues. Technical Report NREL/TP-5600e51995. March 2013. [2] Akansu SO, Dulger Z, Kahranman N, Veziroglu NT. Internal combustion engines fueled by natural gas-hydrogen mixtures. Int J Hydrogen Energy 2004;29:1527e39. http://dx.doi.org/ 10.1016/j.ijhydene.2004.01.018. [3] Roldan C, Spazzafumo G, Zamora JP. Costa Rica: potential production of renewable methane/hydrogen mixtures for (Costa Rica); vehicles. In: Proc. Of HYPOTHESIS IX, San Jose December 2011. [4] Hoekman SK, Broch A, Robbins C, Purcell R. CO2 recycling by reaction with renewably-generated hydrogen. Int J Greenh Gas Control 2010;4:44e50. http://dx.doi.org/10.1016/ j.ijggc.2009.09.012. [5] Spazzafumo G. South Patagonia: wind/hydrogen/coal system with reduced CO2 emissions. Int J Hydrogen Energy 2013;38:7599e604. http://dx.doi.org/10.1016/ j.ijhydene.2012.08.152. [6] Buceti G, Capobianco D, Spazzafumo G, Tosti S. Wind & coal to generate a substitute of natural gas by hydro-gasification. In: Book of extended abstracts HYPOTHESIS XI, Toledo (Spain); September 2015. available on, www.hypothesis.ws. [7] Buceti G, Capobianco D, Spazzafumo G, Tosti S. Wind & coal to generate a substitute of natural gas using electrolytic hydrogen and oxygen. In: Book of extended abstracts HYPOTHESIS XI, Toledo (Spain); September 2015. available on, www.hypothesis.ws. [8] From solid fuels to substitute natural gas (SNG) using TREMP™”, https://www.netl.doe.gov. [9] Iantovski E, Ph Mathieu. Highly efficient zero emission CO2based power plant. In: Third intern. conf. on carbon dioxide removal (ICCDR-3), Boston; 1996.
Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Storing renewable energies in a substitute of natural gas G. Spazzafumo Department of Civil and Mechanical Engineering, University of Cassino and Southern Lazio, Via G. Di Biasio, 43, I-03043, Cassino, FR, Italy
article info
abstract
Article history:
This paper proposes a way to obtain valuable electric power and valuable fuel starting from
Received 26 February 2016
renewable variable electric power plus biomass and/or waste products.
Accepted 23 May 2016 Available online xxx
Biomass/biofuel can be oxyburned using electrolytic oxygen to generate electric power. Gas turbines or internal combustion engines are suitable to such a task, but there is the problem of very high temperatures connected to oxycombustion. In the case of gas turbine
Keywords:
the inlet temperature could be controlled by adding steam and/or carbon dioxide, while in
Renewable electric power
the case of internal combustion engines only carbon dioxide could be used. In such a way
Energy storage
the exhaust gas continues to be formed by carbon dioxide and steam which can be easily
Electrolysis
separated by condensation. Carbon dioxide is fed to a Sabatier process together with
Biomass
electrolytic hydrogen to generate a gas with characteristics similar to natural gas.
Sustainable natural gas substitute
Although electrolytic hydrogen could be used directly both in internal combustion engines and fuel cells, significant problems to hydrogen distribution and on-board storing still exists. Therefore the substitute of natural gas could be a real bridge solution for the short/medium term. A simulation has been carried out and the resulting efficiencies range from 0.52 to 0.58. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In Italy, and maybe also in other countries, the diffusion of systems based on renewable energy sources (RES) has not been correctly planned. The result is that the electric power production from run-of-river hydropower plant, wind and especially photovoltaic, sometimes exceeds the demand on the grid and a system for energy storage has not yet been realised. The consequence is that the price of electric power drops and some combined cycles powerplant fed by natural gas (NG) could be shut down because their operation is more expensive than that of coal powerplants, which are much
more pollutant. This reduces the environmental benefits of the RES diffusion. Moreover RES are irregular and the stability of the grid poses a limit to the amount of electric power which can be absorbed. A third issue is that, with the exception of biomass, all RES are currently converted to electric power, while in most of the countries the final use of energy is strongly dominated by fuels (usually >75%) which are used for transportation via land, sea and air, for heating, for cooking and for industrial processes. Finally we can observe that even biomass (and wastes) is often used to produce electric power: this is due to the fact
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2016.05.209 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
that they are initially converted to a gas characterised by a low heating value. As a consequence such a gas is not suitable for distribution as a fuel and it is burned on-site to generate heat or electric power. Electrolytic hydrogen could certainly be a long term solution to store electric power generated by RES. Unfortunately nowadays there are still critical aspects in storage and distribution of the hydrogen itself, especially when considered as a clean fuel for urban mobility. At the beginning fossil fuels were biomass. What made them energetically valuable is the progressive reduction of oxygen which percentually enriched them of carbon. Such a change required millions of years and particular thermodynamic conditions, but hydrogen can have on biomass the same effect very quickly because it is able to subtract oxygen from biomass:
combustion. The exceeding amount of oxygen could be addressed to the industrial sector or to hospitals. In this perspective it could be possible to generate a mix of a clean fuel and stable electrical power close to the actual local requirement. Using electrolytic oxygen allows to obtain, after gas cleaning, a combustion product containing only carbon dioxide and steam, which are easily separable by condensation. Carbon dioxide is then used together with electrolytic hydrogen as a reactant for the Sabatier reaction (e.g. Ref. [4]). The same process could be realised also using fossil fuels, and coal in particular, as a carbon source [5e7]. And finally it could be realised also mixing coal, biomass and waste products as a carbon source. The oxygen produced by electrolysis exceeds the amount required by the process and therefore could be sold for several different uses being of particular high purity.
CxHyOz þ (2x y/2 þ z) H2 / xCH4 þ zH2O This is what directly happens, in theoretical conditions, if you hydro-gasify biomass. But it also happens, indirectly, in other processes. For example when biomass or the humid fraction of wastes are subjected to anaerobic digestion followed by methanation with hydrogen. In this case carbon is first converted to methane and carbon oxides, and subsequently also carbon oxides are converted to methane:
CO þ 3H2 / CH4 þ H2O
CO2 þ 4H2 / CH4 þ 2H2O In real conditions such reactions are not complete, but it is possible to push them towards a high level of advancement by supplying over-stoichiometric hydrogen. The result, after steam condensation and water separation, is a gas very rich of methane and containing a residual amount of hydrogen, while carbon oxides are almost completely absent. Therefore such a gas has characteristics very similar to those of NG and can be distributed, stored and used exactly as we do with NG. Actually the possibility to distribute hydrogen up to a concentration of 20% through NG pipelines was already proved [1] and such a mixture can be used directly into internal combustion engines resulting in performance improvement [2]. Such a solution could be very interesting especially for developing countries which have no fossil resources, but have a good availability of biomass and other RES. A typical example could be Costa Rica [3]. Thus coupling exceeding electric power from RES as a source of hydrogen with biomass or waste products as a source of carbon, could allow to store energy from RES in the best possible way as far as for the current energy system, that is as chemical energy of an already widely utilised fuel. Another possibility is to generate electric power for the grid before producing the substitute of NG (SNG). It implies that combustion be carried out using oxygen rather air. However it is not a problem because electrolysis generates also oxygen and the amount of oxygen available exceeds the demand for
System lay-outs and modelling Two possible configurations of the system have been investigated. A first one is based on a gas turbine, while the second one is based on an internal combustion engine. In both cases critical issues are the increase of flame speed and the very high temperature obtained with oxycombustion and it is necessary to add a diluent capable to replace the nitrogen of the air: as the final products should be only water and carbon dioxide they are the unique two candidates, though they are not really diluent, but products. It means that they are able to moderate the temperature, but they could obstacle the combustion. Adding steam to a gas turbine unit is an already developed technology: STIG (STeam Injected Gas). In this case, however, the steam flow required could be very high. It is also possible to add both steam and carbon dioxide. The two extreme cases (only steam or only carbon dioxide) have been analysed and have been identified as ST and GT. In the case of internal combustion engines it is not possible to add a large amount of steam and therefore the addition of carbon dioxide is the unique reasonable solution and has been identified as ICE. Starting from a lignocellulosic biomass (see Table 1 for composition) and electrolytic hydrogen and oxygen, a simulation has been carried out using a thermochemical software (AspenONE® v8.4). Fig. 1 shows a simplified schematic of the general layout. A flow of oxygen coming from the electrolysis unit is fed to a biomass gasifier which carries out a partial oxidation (POX) of the biomass. As electrolysis is assumed to be carried out at 30 bar, also the POX reactor operates at such a pressure and supplies fuel to the gas turbine or to the internal combustion engine. The biomass treatment includes also ash separation (cyclone) while a clean-up unit is not required because
Table 1 e Biomass composition (weight %). H2O
Ash
C
H
N
O
FC
VM
20
0.24
40.32
4.88
0.08
34.48
14.89
64.87
Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Fig. 1 e Simplified schematic of the general layout.
Fig. 2 e Aspen schematic of the biomass treatment.
lignocellulosic biomass does not contains significant amount of sulphur or chlorine compounds. The syngas obtained enters the power unit together with a second flow of oxygen and with a flow of a diluent which allows to control the combustion temperature. Exhaust gas from the power unit consists
3
almost completely of steam and carbon dioxide which are recompressed and separated. The separator supplies three different flows: the above mentioned diluent flow, a water reintegration flow to the electrolyser, and a carbon dioxide flow to the Sabatier process. This last one receives also an over-stoichiometric flow of hydrogen to generate SNG. The POX reactor, schematised in Aspen as a combination of a Yeld reactor and a Gibbs reactor, and the cyclone are common elements for all the layouts (Fig. 2), just as the part of the plant devoted to the Sabatier process (Fig. 3). This is realised with three units. The first unit consists of a Gibbs reactor (MET1) and two heat exchangers. REX1 is a regenerative heat exchanger having the task to reheat the syngas to 300 C before entering in the subsequent unit. HEX1 has a double task: to recover the exceeding heat and to allow a partial condensation of the water generated by the Sabatier reaction. The reactor is supplied with electrolytic hydrogen and carbon dioxide recovered from the exhaust gas downstream of the power unit. Moreover there is a partial recycle of the outlet gas according to the typical layout of methanation process from Haldor Topsoe [8]. The second unit is almost identical to the first with a Gibbs reactor (MET2), a regenerative heat exchanger (REX2) and a heat exchanger (HEX2), but there is not a recycle and the reactor is supplied only with the syngas coming from the first unit. Finally, the third unit receives the syngas from the second unit and has only a Gibbs reactor (MET3) and a heat exchanger (HEX3), without a regenerative heat exchanger. The part of the plants with the power unit and the carbon dioxide recovery are quite different. ST layout (Fig. 4) requires a flow of steam at 30 bar to be injected into the combustion chamber (CC) of the turbine together with the syngas from the cyclone and stoichiometric oxygen. Combustion products are expanded into the turbine and then cooled into a heat exchanger (HEX0). Carbon dioxide is separated from condensed water and re-compressed to 30 bar inside a multistage intercooled compressor which also
Fig. 3 e Aspen schematic of the Sabatier process. Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
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Fig. 4 e Aspen schematic for ST layout.
Fig. 5 e Aspen schematic for GT layout. Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
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Fig. 6 e Aspen schematic for ICE layout.
contributes to reduce the residual water content. At the end of the compression line the flow of carbon dioxide is fed to the previously described Sabatier process. The water recovered from HEX0 and from the intercoolers (IC1 and IC2), together with an external flow of water, is pumped into a heat recovery boiler which receive heat from HEX0, HEX1, HEX2 and HEX3 to generate the required steam at 30 bar and 250 C. GT layout (Fig. 5) differs from the previous simply because the combustion chamber does not receive a flow of steam, but a flow of carbon dioxide recirculated from the last
Table 2 e Basic assumptions. Ambient temperature Turbine inlet temperature Steam temperature Electrolysis efficiency (HHV) Pump efficiency Compressor efficiency Turbine efficiency ICE efficiency ICE heat losses Electric efficiency
20 C 1500 C 300 C 0.70 0.70 0.80 0.85 0.32 0.20 0.985
Fig. 7 e Power and temperature vs oxygen supplied.
compression stage. Such a concept is similar to that of the Matiant Cycle [9]. The water recovered is not used directly, but could be supplied to the electrolyser together with the water recovered from the Sabatier Process.
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Though the systems analysed are suitable for different biomass flow rate values, all simulations have been referred to 1 Mg/h in order to have a direct comparison. The composition of the biomass considered is reported in Table 1 and its HHV is 14,078 MJ/kg. Stoichiometric oxygen is 9.702 mol/s and has to be split between partial oxidation and combustion. The more oxygen is supplied to the POX reactor, the higher is the temperature
while the HHV of the syngas decreases. Fig. 7 shows the changes of thermal and chemical power of the syngas and its temperature when varying the flow of oxygen supplied to the POX, while Fig. 8 shows how the gas composition varies when varying the flow of oxygen supplied. At low rates of oxygen supplied, temperature is rather low and methane and carbon dioxide are the main components of the syngas. Increasing the oxygen available results in higher temperatures and the methane gradually disappears while hydrogen and carbon monoxide become predominant. Over 3 mol/s the exothermicity of combustion is no more counteracted by the endothermicity of methane reforming and the temperature starts to increase rapidly. Therefore a flow of oxygen of 3 mol/s has been chosen to feed the POX reactor obtaining a syngas at about 890 C. The maximum turbine inlet temperature has been assumed equal to 1500 C and it requires a steam flow of 48.39 mol/s with the result that the steam content of combustion products is higher than 86% by volume. Actually it is more similar to a steam powerplant than to a STIG one and the turbine could require a specific design. However steam at 1500 C is properly a gas and only the last stages have to expand a vapour. The heat required for steam production is 2484 kW and absorbs most of the useful heat recoverable from the heat exchangers. The DT at pinch point is 25 C. The heat exchange diagram shown in Fig. 9 has been obtained cumulating the heat available at proper temperature: it is evident that the net residual heat available (which has not been reported completely in the diagram and account for more than 4000 kW) is scarcely useful. Only a little more than 1000 kW could be used, for example, for adding a second level of pressure or as a heat source for an Organic Rankine Cycle. About double (1974 kW at T > 60 C) could be used for direct thermal use to obtain warm water. In the other cases the recovered heat available is much higher because there is no production of steam. However, though the temperatures are suitable for a good conversion to electric power, for example with a steam powerplant, the size of the plant could not allow a high efficiency. On the other hand, thinking to a close Brayton cycle with air it is impossible to obtain the maximum power and the maximum
Fig. 9 e Heat exchange diagram (ST).
Fig. 10 e Heat exchange diagram (GT).
Fig. 8 e Syngas composition vs oxygen supplied.
Finally, ICE layout is shown in Fig. 6. The ICE is supposed to be a naturally aspirated engine, even there is the possibility to supply syngas and oxygen at 30 bar. Therefore mechanical work is recovered by means of two expanders (EXP1 and EXP2) and the expanded syngas is cooled into a heat exchanger (HEX4). The exhaust gas from ICE is cooled and partially dried into another heat exchanger (HEX0) before being split in two flows, one directly fed to the ICE and the other re-compressed and further dried inside a multistage intercooled compressor, like in the previous layouts, to be supplied to the Sabatier process. Also in this case all the water recovered could be supplied to the electrolyser. Table 2 shows the main basic assumptions.
Results and discussion
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hcog ¼ (Ech,SNG þ Eel,produced þ Qrecovered)/ (Ech,biomass þ Eel,consumed)
Conclusions
Fig. 11 e Heat exchange diagram (ICE).
Table 3 e Energy balance.
Power for electrolysis Biomass (HHV) SNG (HHV) Electric power generated Residual heat available (T > 60 C) Maximum temperature of residual heat Cogeneration efficiency System efficiency
Unit
ST
GT
ICE
kW kW kW kW kW
16044 3911 8929 2548 1974
1417 4134
9067 1299 3872
164.5
784.5
777
0.676 0.575
0.727 0.519
0.714 0.520
C
temperature at the same time as shown in Fig. 10 for GT layout and in Fig. 11 for ICE layout. Table 3 shows the energy balance resulting from the simulation carried out. The difference in chemical power of the SNG generated depends on the different amounts of water separated which slightly affect the final composition. And the average composition by volume is:
CH4 79.89% e H2 19.70% e N2 0.07% e H2O 0.33% e CO2 0.01% with very small differences. To compare the layouts analysed, two different efficiencies could be defined: - a system efficiency which considers the electrical (Eel) and the chemical (Ech) outputs divided by the inputs:
hsys ¼ (Ech,SNG þ Eel,produced)/(Ech,biomass þ Eel,consumed) - a cogeneration efficiency which considers also the direct use of recoverable heat (Q), e. g. to heat water or generate steam:
The proposed system allows to convert low grade energy inputs (coal, biomass, waste products, unstable electric power) into high grade energy outputs (methane þ hydrogen mixtures, oxygen, stable electric power). From a quantitative point of view efficiency is lower than that of other energy storage system (e.g. hydro pumped storage power plants), however this system makes use also of biomass and enhances its utilisation. Moreover it seems very important that the main final product is a clean gaseous fuel. When used in place of gasoline it allows better efficiency and lower emissions. Therefore it would be proper to extend the comparison to the global energy chain. Finally, when the system should be applied to waste products there would be further advantages. This is a preliminary analysis. Next steps will be the improved analysis of ICE layout using a specific software, the analysis of alternative layouts including further power recovery, for example through ORC, and the analysis of alternative layouts based on high temperature fuel cells.
references
[1] Melaina MW, Antonia O, Penev M. Blending hydrogen into natural gas pipeline networks: a review of key issues. Technical Report NREL/TP-5600e51995. March 2013. [2] Akansu SO, Dulger Z, Kahranman N, Veziroglu NT. Internal combustion engines fueled by natural gas-hydrogen mixtures. Int J Hydrogen Energy 2004;29:1527e39. http://dx.doi.org/ 10.1016/j.ijhydene.2004.01.018. [3] Roldan C, Spazzafumo G, Zamora JP. Costa Rica: potential production of renewable methane/hydrogen mixtures for (Costa Rica); vehicles. In: Proc. Of HYPOTHESIS IX, San Jose December 2011. [4] Hoekman SK, Broch A, Robbins C, Purcell R. CO2 recycling by reaction with renewably-generated hydrogen. Int J Greenh Gas Control 2010;4:44e50. http://dx.doi.org/10.1016/ j.ijggc.2009.09.012. [5] Spazzafumo G. South Patagonia: wind/hydrogen/coal system with reduced CO2 emissions. Int J Hydrogen Energy 2013;38:7599e604. http://dx.doi.org/10.1016/ j.ijhydene.2012.08.152. [6] Buceti G, Capobianco D, Spazzafumo G, Tosti S. Wind & coal to generate a substitute of natural gas by hydro-gasification. In: Book of extended abstracts HYPOTHESIS XI, Toledo (Spain); September 2015. available on, www.hypothesis.ws. [7] Buceti G, Capobianco D, Spazzafumo G, Tosti S. Wind & coal to generate a substitute of natural gas using electrolytic hydrogen and oxygen. In: Book of extended abstracts HYPOTHESIS XI, Toledo (Spain); September 2015. available on, www.hypothesis.ws. [8] From solid fuels to substitute natural gas (SNG) using TREMP™”, https://www.netl.doe.gov. [9] Iantovski E, Ph Mathieu. Highly efficient zero emission CO2based power plant. In: Third intern. conf. on carbon dioxide removal (ICCDR-3), Boston; 1996.
Please cite this article in press as: Spazzafumo G, Storing renewable energies in a substitute of natural gas, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.209