Using solar energy to enrich methane and generate electricity

Solar thermal decomposition technology may eventually help rid the world of its dependence on fossil fuels. In the meantime, some of these technologies may be integrated with existing infrastructure to help lower CO2 emissions. Scientists at the University of Rome proposed and modeled the integration of solar steam reforming technology with a steam turbine and existing natural gas (NG) infrastructure to create a tube and shell reactor (De Falco et al). Following decompression, natural gas with a volumetric hydrogen concentration of 17% (HCNG-17) can be fed straight into a low or medium pressure NG grid. Hydrogen enriched NG produces less CO2 when burned because a portion of the energy comes from hydrogen: hydrogen produces water, not CO2, when burned. The model found that concentrating a solar plant with an area of 16,000 m2 coupled with a tube and shell reactor with 4 reformers is capable of supplying the enriched methane and electricity demands of about 2930 domestic users. —Tim Fine
  De Falco, D., Giaconia, A., Marrelli, L., Tarquini, P., Grena, R., Caputo, G., 2009 Enriched methane production using solar energy: an assessment of plant performance. International Journal of Hydrogen Energy 34 98–109.

 Marcello De Falco and colleagues at the University of Rome and the ENEA Research Center modeled the integration of solar thermal methane reforming to enrich natural gas coupled with a steam turbine to generate electricity. In the model, a field of solar collectors concentrates sunlight onto solar receivers filled with molten salt. The molten salt is used as a heat transfer fluid and is transferred into a storage tank. From the tank the salt is pumped to either a steam reformer, where it heats the feedstock and drives the reformer, or to a steam turbine, to generate electricity. The model assumed an exit temperature of 550 ºC for the molten salt: the flow rate would be adjusted to meet this depending on the intermittency of solar radiation. The salt storage tank allows for the flow of salt to the reformers to be kept constant at 4 kg/s. Allowing longer residency times in the reformers and steam turbine increased their thermal efficiency but reduced the rate at which enriched methane was produced. The residence time would need to be set based on the enriched methane and electricity requirements, as well as the intermittency of solar radiation as measured by the availability of molten salt at the required temperature. Pressure was found to adversely effect the production of enriched methane while slightly increasing the electrical output from the steam turbine. While the reformers themselves don’t consume large amounts of heat, vaporizing their feedstock does, which puts constraints on the number of reformers that can be used on the same salt circuit. Examining the space requirements for the number solar collectors, the study found that the proposed plant would work for small municipalities: the space requirement could be a drawback for towns larger than 20,000 inhabitants.

Using molten salts may help solve solar thermal reforming’s intermittency problem.

As with many other alternative energy sources, solar thermal reforming suffers from intermittency problems. Tatsuya Kodama and his colleges have come up with a new way to reduce the impact temporary disruptions–such as clouds drifting across the sun–have on the efficiency of a solar thermal reformer. They lined the outer tube of a double-walled tubular reformer with a mixture of Na2CO3– a salt–and MgO. While molten salt has a high heat and latent heat capacity, it conducts heat poorly. Mixing it with Mg– which has high conductivity–allows the heat from the salt to transfer to the methane in the reactor efficiently. The integration of Na2CO3 and MgO into a solar thermal reformer allowed it to continue running at high efficiency–methane conversion of 90% or more–for 22 minutes longer than without the salts. —Tim Fine  
Kodama, T., Gokon, N., Inuta, S., Yamashita, S., 2009  Molten-Salt Tubular Absorber/Reformer (MoSTAR) Project: The Thermal Storage Media of Na2CO3–MgO Composite Materials. Journal of Solar Energy Engineering  131.4 041013.

 Kodama and colleges at Niigata University’s Department of Chemistry and Chemical Engineering tested the effects of integrating Na2CO3 and MgO into a reformer. The intent of the experiment was to examine the feasibility of this approach for solar thermal reforming. The experiment used a conventional reformer reactor. The tubes were heated until the catalyst bed reached 920ºC. After 100% methane conversion was observed the power to the reactor was intermittently turned on and off to simulate clouds drifting over the sun.
Clouds drifting over the sun cause a greater decrease in the efficiency in solar thermal reforming than in traditional photovoltaic panels. Solar thermal reformers have to re-heat to the temperature necessary to breakdown their feedstock. Using molten Na2CO3 as an energy reservoir can circumvent this problem by providing heat when the solar energy is temporarily disrupted. This study tested double walled tubes lined with Na2CO3, 90% Na2CO3 and 10% MgO, and 80% Na2CO3 and 20% MgO. All three combinations retained heat more efficiently when the power was off than tubes without Na2CO3. The tube containing 90% Na2CO3 and 10% MgO proved most effective at maintaining the reactor temperature after the power supply had been interrupted. Thirty minutes after powering off the reactor, the temperature had dropped to 770ºC: the methane conversion rate had only marginally decreased to 95%. The findings presented in this study could greatly reduce the problems caused by solar radiation in solar reformers, so long as they can be replicated in field tests. 

Hydrogen production using middle-temperature solar thermal reforming.

Middle temperature solar thermal methanol steam reforming reactors have been shown to be very effective at efficiently producing hydrogen (Liu et al. 2009).  Higher solar flux values lead to an increase in methanol conversion as more energy is available to drive the reaction.  The reactors decomposed over 90% of the injected methanol at a solar flux of 580 W/m2 with an injection rate of 3.0 kg/h and 750 W/m2 with an injection rate of 4.3 kg/h. The volumetric concentration of hydrogen found in the product gas was between 66–74%: within 1% of the theoretical maximum hydrogen concentration. Hydrogen was produced in a 3:1 ratio with CO2, with trace amounts of CO, CH3OH and H2O also found in the product gas. The maximum hydrogen yield produced per mole of methanol was 2.90 mole, which was 0.10 mol less than the maximum theoretical yield of 3.00 mol per mol of methanol. The observed thermochemical efficiency of 30–50% is competitive with other high-temperature thermochemical processes.­—Tim Fine  
Liu, Q., Hong, H., Yuan, J., Jin, H., Cai, R., 2009. Experimental investigation of hydrogen production integrated methanol steam reforming with middle-temperature solar thermal energy. Applied Energy 86.2, 155–162. 

  Lui and colleges at the Chinese Academy of Sciences’ Institute of Engineering Thermophysics examined the effect of solar radiation and mole ratio of water/methanol on the reactivity and hydrogen yield in a methanol steam reformer. The mole ratio of water to liquid methanol was set from 1 to 2.5. The reactor laden with Cu/ZnO/Al2O3 was driven by solar energy at 150–300º C.

The study showed that increased solar flux values raised the reactor temperature and increased methanol conversion. Methanol conversion rates were found to be higher than 90% for solar flux values of 580 W/m2 and 750 W/m2. More than 40% thermochemical efficiency can be achieved with two different mass flow rates. The observed 3.0 kg/k injection rate had a maximum thermochemical efficiency of 46%; the 4.3 kg/h injection rate was had a maximum thermochemical efficiency of 50%. However, the thermochemical efficiency of the reactors decreased above 580 – 630 W/m2, indicating that more solar energy was being lost through heat radiation from the reactor. The study found that hydrogen concentrations of up to 66–74% were found using solar driven methanol steam reforming, which is larger than the 58–63% concentrations produced by methanol decomposition. It is important to note that the maximum theoretical hydrogen concentrations obtainable for each technology are 75% and 66% respectively. With a solar flux of about 600 W/m2, the hydrogen yield ranged from 2.56–2.90 mol per mol of methanol, which is very close to the maximum theoretical hydrogen yield of 3.0 mol per mol methanol. Solar thermal methanol reforming can produce a hydrogen yield 70% greater than solar methanol decomposition because hydrogen is obtained from the water as well as the methanol.

Increasing Temperature and Residence Time Increases the Efficiency of A Solar Chemical Reactor

Decarbonizing energy will help avoid further CO2 emissions and provide a more sustainable energy solution in the near to medium-term future (Rodat et al. 2009). Solar decomposition technology takes carbon based energy sources such as natural gas and fossil fuels and converts them to carbon-black and H2 There is a variety of different types of solar decomposition reactors in development. This study examined the effects of temperature, methane concentration, and residence time on methane decomposition in a 10 kW solar chemical reactor prototype. The study found that temperature and residence time significantly affect both methane conversion and H2 yield. Higher temperatures cause the decomposition reaction to go to completion, eliminating the ethane, ethane and acetylene byproducts. An increase in residence time—the amount of time the methane spent in the reactor—also resulted in a higher methane conversion and H2 yield.—Tim Fine  
Rodat, S., Abanades, S., Sans, J., Flamant, G., 2009. Hydrogen production from solar thermal dissociation of natural gas: development of a 10 kW solar chemical reactor prototype. Solar Energy 83, 1599—1610.

 Sylvain Rodat and colleagues at the Processes, Materials and Solar Energy Laboratory calculated the effects of temperature, methane concentration, and residence time on methane decomposition in a 10 kW solar chemical reactor prototype. The reactor was insulated with three layers of different insulation materials to retain heat. The functional parts of the prototype consisted of three double graphite tubes. Each tube consisted of two tubes, one inside the other, through which a mixture of argon and methane gases was pumped. The gas entered through the inner tube and exited through the outer tube. Because of the way the solar reactor was designed, the outer tube is slightly hotter. This prevents the carbon generated by methane decomposition from depositing on the pipe. The reactor retained 60% of the solar energy as ambient heat to drive the reaction. Thirty-five percent of the energy was lost through the walls of the reactor and the remaining 5% was lost through the gas flow.
Increasing the concentration of methane pumped into the reactor was found to have no significant effect on either the percent decomposition of methane or the H2 yield. This indicates that the efficiency of the reactor can be greatly increased by increasing the concentration of methane present in the gas source. The temperature inside the reactor was found to have positive correlation with the efficiency of the reactor. Increasing the temperature from 1670 K to 1740 K resulted in a 22% increase in methane conversion and a 3% increase in the H2 yield. Increasing residence time of the gas also increased the efficiency of the reactor. Increasing the residence time from 12 ms to 35 ms increased the methane conversion by 36% and the H2 yield by 40%. The three aspects examined—residence time, methane concentration, and temperature—are basic components of a solar thermal decomposition reaction suggesting that these results may be useful in the development of other solar reactors.

Using small-scale solar thermal reforming in conjunction with a hydrogen fuel cell.

Solar steam reforming of bioethanol offers a sustainable way to domestically produce hydrogen for use in a hydrogen fuel cell. However, the intermittency of solar energy within a given day prevents the solar reformer from operating at full efficiency, affecting the rate of hydrogen production. A study in Japan found that even with the intermittency of available solar energy a domestic hydrogen reformer—running on a combination of solar and electrical power—operated at above 40% for both cloudy and sunny days. The GHGs emitted during the reformers’ operation were found to be 19% lower than conventional commercial power generation. Furthermore, the percent utilization of solar energy by the 2 m2 collecting area of solar reformer was superior to that of photovoltaic cells.— Tim Fine
 Shin’ya, O., 2009 Hydrogen production characteristics of a bioethanol solar reforming system with solar isolation fluctuations. International Journal of Hydrogen Energy 34, 5347–5356.

 Shin’ya Obara at the Kitami Institute of Technology’s Department of Electrical and Electronic Engineering studied the efficiency of a domestic bioethanol reformer used in conjunction with a hydrogen fuel cell. Two solar collectors measuring 1 m2 were used to collect the solar energy for vaporizing the bioethanol feedstock and operating the solar reformer. Gaps between the solar energy available and the energy required for reforming were met with power from the grid. The results were obtained using the meteorological data from March 1 and August 23, 2007.
The uneven heating of the catalyst in the solar reformer leads to a drop in the efficiency of the reactor, as it prevents the decomposition reaction from reaching completion. Consequently, the changes in solar energy available due to cloud coverage can have a significant effect on the efficiency of a solar reformer. The study looked at the theoretical operating results of both a cloudy and sunny day in Sapporo, Japan by using the meteorological data from March 1 and August 23 of 2007. The reforming component operated with an efficiency of 47% on the cloudy day, March 1, and 42% on the sunny day, August 23. While efficiency of the reactor was lower for the sunny day, the longer daylight hours—there were nearly 2 more hours of sunlight on the 23rd—meant that the reactor produced 17g hydrogen and 0.5 kWh more than it did on the cloudy day—the 1st. Considered as a fraction of the power demand for each day, the reactor produced 21.4% and 25.3% of the energy required to run the reactor for March 1 and August 23rd respectively.

While running the reforming component required a significant portion of energy from the grid, adding a solar thermal component to the reformer was found to be a more efficient way to capture solar energy—thereby reducing the energy needed from the grid—than adding a photovoltaic cell array of the same size. Domestic implementation of solar thermal reforming, in conjunction with a hydrogen fuel cell, presents a possible way to reduce GHG emissions; the emissions per unit of power from the reforming process are less than those generated by conventional power production. 

Steam Reforming of Methanol May Provide an Economical Way to Produce Hydrogen

With the impending shortage of fossil fuels and the concerns over climate change, many new alternative sources of power are being considered. Solar hydrogen technologies promise to help alleviate the world economy’s dependence on fossil fuels, many of the new solar hydrogen technologies currently being tested use very high temperatures to obtain hydrogen. A new study demonstrates the potential feasibility of solar hydrogen production at temperatures as low as 150–300º C (Hong et al. 2009). The study examined a new process involving the steam reforming of methanol using light as the energy source. Methanol conversion ratios of 90% and 98% were achieved with a reactor temperature of 220–280ºC. The hydrogen obtained with the process reached purities of 99.99% with hydrogen recovery rates between 80% and 90 %.­ —Tim Fine 
Hong, H., Liu, Q., Jin, H., 2009. Solar Hydrogen Production Integrating Low-Grade Solar Thermal Energy and Methanol Steam Reforming. Journal of Energy Resources Technology 131, 012601–012611.

Hui Hong and colleagues at the Chinese Academy of Sciences tested a new process for producing hydrogen using low temperature steam reforming of methanol. The process was made up of three parts: a tracking parabolic trough concentrator with a concentration ratio of about 30–100, a middle and a low temperature solar reformer laden with Cu/ZnO/Al2O3 catalysts, and a pressure swing adsorption (PSA) unit. The PSA unit was used to extract the hydrogen from the products of the solar reformer. However because the PSA unit works at 35 – 40ºC the product gases need to be cooled. The heated cooling water used to cool the product gas was fed into the preheater – the feedstocks are heated before being injected into the reformer –, thus increasing the efficiency of the process. 
The conversion of methanol increased with an increase in the average solar energy, reaching a conversion rate of more than 90% at 580 W/m2.  The heat to chemical conversion efficiency peaked at 45%, and a heat to chemical conversion efficiency of over 40% was achieved with the average solar energy ranging from 580 W/m2 to 720 W/m2. At average solar energy values above 720 W/m2, the heat to chemical conversion efficiency started to decrease. This indicates a higher radiance heat loss from the reactor. The 35–46% energy conversion efficiency obtained in this study is competitive with the 30% energy conversion efficiency found in high-temperature solar reforming of natural gas.
Because the process investigated in this study uses a solar reformer, it has a higher H2 yield than conventional methane reformation: H2 is obtained from H2O in addition to the feedstock. Both methanol and water are decomposed to produce H2 and GHGs, the majority of which were CO2. Because of the high efficiency of the PSA at extracting pure H2 from the product gas, the CO2 produced can easily be captured for sequestration.

The low heat requirement of this process greatly reduces one of the largest obstacles to the large-scale implementation of solar hydrogen decomposition technologies. The low heat requirement lowers the energy required to obtain the hydrogen, which means that any power plants built using this process may be smaller than their high temperature counter parts. Smaller plants will cost less and have the potential to increase the hydrogen production capacity of an area: several low temperature reactors can be built on the area required a single high temperature reactor. The decrease in reactor costs and size associated with low temperature methanol reformation may help hydrogen become a viable power source

Testing a 5 kW solar thermal cracking reactor

Possibly the biggest obstacle to stopping climate change is the world’s fossil fuel based economy. Economic growth means increased fossil fuel consumption and the accompanying rise in greenhouse gas emissions. Solar cracking presents a way to continue to use fossil fuels without generating the greenhouse gases such as CO2 and CO that cause global warming. While solar cracking reactors are still in their prototype phase, the results generated by the 5 kW reactor tested in this study show great promise; the reactor generated a methane conversion ratio of 98.8% and a hydrogen yield of 99.1% (Maag et al. 2009). Furthermore, the results indicate that increasing the amount of methane pumped into a reactor as a fraction of the total gas could increase the energy efficiency of the reactor beyond the maximum 16.1% solar-to-chemical energy conversion rate observed. Tim Fine
Maag, G., Zanaganeh, G., Steinfeld, A., 2009 Solar thermal cracking of methane in a particle-flow reactor for the co-production of hydrogen and carbon. International Journal of Hydrogen Energy 34, 7676–7685.

G. Maag and colleges at the Department of Mechanical and Process Engineering in Zurich tested a 5 kW Solar partial-flow solar chemical reactor in a solar furnace over a 1300 – 1600 K range. The reactor was equipped with a continuous flow of methane laced with µm sized carbon black particles. The effect of flow rate on reactor efficiency was examined.
The key to solar thermal cracking is the heating of the target feedstock. This study found that the carbon-black particles injected into the methane acted as a catalyst, as they amplified the radiative heat transfer: converting the light into heat that is absorbed by the methane. In doing so, the carbon black increased the rate at which the reaction proceeded, improving the efficiency of the reactor. Higher concentrations of methane led to increased gas temperature and cooler reactor walls because the higher concentration of methane absorbed more light before it reached the reactor walls. There appears to be a trade-off between reaction completion—how much methane is converted—and the efficiency of the reactor. The reaction completion decreases when the concentration of methane is increased. While the temperature of the gas increased with a higher concentration, the energy per molecule went down, resulting in a lower conversion ratio. The solar-to-chemical energy conversion efficiency increases with an increase in methane concentration: more energy is being absorbed by the gas instead of the reactor walls, resulting in a higher energy conversion efficiency. Modeling simulations suggest that using pure methane could increase the efficiency of the reactor by a factor of 2–4.

Solar decomposition technology could prolong the lifespan of fossil fuel reserves

Hydrogen was once touted as the alternative energy source that would end the civilized world’s dependence on greenhouse gas-emitting fossil fuels. However, while hydrogen is the most abundant element on earth, it is almost always bonded to other elements and large amount of energy is required to obtain elemental hydrogen. This energy requirement has made using hydrogen as a fuel prohibitively expensive. Solar decomposition technology puts hydrogen back on the table as a potential alternative fuel. Solar decomposition technology can take a variety of feedstocks, including fossil fuels, water, and biomass, and release hydrogen and commercially usable carbon-black without producing any harmful greenhouse gases. The resulting hydrogen can be used in internal combustion engines and fuel cells. Additionally, using solar decomposition with a fossil fuel feedstock would prolong the lifespan of the fossil fuels, which would give more time for new alternative power sources to be developed (Ozalp et al. 2009).—Tim Fine
Ozalp, N., Kogan A., Epstein M., 2009. Solar decomposition of fossil fuels as an option for sustainability. International Journal of Hydrogen Energy 34.2, 710-720.

     Nesrin Ozalp at Texas A&M University at Qatar, and his colleges at the Solar Research Facilities Unit at the Weizmann Institute of Science in Israel did a study examining the different solar decomposition processes currently available to produce hydrogen.
     Solar decomposition technologies are an interesting fusion of conventional and alternative power sources. Using the hydrogen generated by the decomposition reactions would eliminate greenhouse gases from the fuel life cycle, resulting in a huge step towards a carbon neutral economy. However, depending on which solar decomposition technology is used, some CO2 will be produced. The solar decomposition technologies that produce CO2 also produce more hydrogen than those that do not produce CO2. Producing hydrogen without simultaneously producing CO2 represents a tradeoff, less hydrogen for no CO2. The carbon-black—pure carbon—generated by solar decomposition of fossil fuels and biomass is used in the manufacture of rubber, batteries, nano-tubes, polymers, cars, and many other consumer items. Because the carbon produced by solar decomposition is marketable, the hydrogen produced has the potential to compete profitably on a per unit basis with gasoline and the sale of the carbon can offset the costs of using solar decomposition technology. Used in combination with fossil fuels, solar decomposition technology has the potential to prolong the lifespan of fossil fuels—buying time to research a permanent sustainable energy source.—Tim Fine

Solar decomposition technology could prolong the lifespan of fossil fuel reserves

Hydrogen was once touted as the alternative energy source that would end the civilized world’s dependence on greenhouse gas-emitting fossil fuels. However, while hydrogen is the most abundant element on earth, it is almost always bonded to other elements and large amount of energy is required to obtain elemental hydrogen. This energy requirement has made using hydrogen as a fuel prohibitively expensive. Solar decomposition technology puts hydrogen back on the table as a potential alternative fuel. Solar decomposition technology can take a variety of feedstocks, including fossil fuels, water, and biomass, and release hydrogen and commercially usable carbon-black without producing any harmful greenhouse gases. The resulting hydrogen can be used in internal combustion engines and fuel cells. Additionally, using solar decomposition with a fossil fuel feedstock would prolong the lifespan of the fossil fuels, which would give more time for new alternative power sources to be developed (Ozalp et al. 2009).—Tim Fine
Ozalp, N., Kogan A., Epstein M., 2009. Solar decomposition of fossil fuels as an option for sustainability. International Journal of Hydrogen Energy 34.2, 710-720.

Nesrin Ozalp at Texas A&M University at Qatar, and his colleges at the Solar Research Facilities Unit at the Weizmann Institute of Science in Israel did a study examining the different solar decomposition processes currently available to produce hydrogen.
Solar decomposition technologies are an interesting fusion of conventional and alternative power sources. Using the hydrogen generated by the decomposition reactions would eliminate greenhouse gases from the fuel life cycle, resulting in a huge step towards a carbon neutral economy. However, depending on which solar decomposition technology is used, some CO2 will be produced. The solar decomposition technologies that produce CO2 also produce more hydrogen than those that do not produce CO2. Producing hydrogen without simultaneously producing CO2 represents a tradeoff, less hydrogen for no CO2. The carbon-black—pure carbon—generated by solar decomposition of fossil fuels and biomass is used in the manufacture of rubber, batteries, nano-tubes, polymers, cars, and many other consumer items. Because the carbon produced by solar decomposition is marketable, the hydrogen produced has the potential to compete profitably on a per unit basis with gasoline and the sale of the carbon can offset the costs of using solar decomposition technology. Used in combination with fossil fuels, solar decomposition technology has the potential to prolong the lifespan of fossil fuels—buying time to research a permanent sustainable energy source.—Tim Fine