Patent Grant
7332146
This invention relates to liquid hydrogen (LH2) production processes, and in particular to methods, systems and devices for liquid hydrogen production from methane, landfill gas (LFG), gob gas or natural (NA) that combines gaseous hydrogen (GH2) production and cryogenic hydrogen liquefaction in a single process, producing high purity liquid hydrogen (LH2) with zero emissions.
A typical conventional liquid hydrogen (LH2) production process can be separated into three steps: (1) gaseous hydrogen production; (2) separating hydrogen from a gaseous mixture; and, (3) liquefying gaseous hydrogen to liquid hydrogen (LH2). As shown in
The disadvantages of the Pressure Swing Adsorption (PSA) process include, but are not limited to, low process efficiency, low hydrogen recovery from a gas mixture, and the production of off-gases, containing high concentrations of carbon monoxide (CO); H2 and methane (CH4) that are generally burned to recover only the fuel value of the combustible gases, thereby wasting CH4 and CO as H2 production components. Another disadvantage of PSA systems related to the level of hydrogen purity obtainable. Normally, H2 purity greater than 99.9995% is required for use in aerospace, as a space travel propellant. In case of hydrogen fuel cells, such applications do not tolerate contaminants such as carbon monoxide (CO) and hydrocarbons at levels exceeding a few parts per million (ppm), because the contaminants poison the platinum catalysts utilized in fuel cells. Thus, GH2 production through PSA purification alone cannot meet high purity standards. In order to do so an H2 purification procedure must be accomplished prior to the liquefaction.
The present invention provides methods, systems and devices that overcome the disadvantages of the conventional process shown in
The first objective of the present invention is to provide cryogenic systems, methods and devices to produce liquefy hydrogen directly from methane or landfill gas at the same time to separate a mixture of hydrogen rich gases.
The second objective of the present invention is to provide methods, systems and devices for increasing the process efficiency for the production of liquid hydrogen.
The third objective of the present invention is to provide methods, systems and devices to improve the level of hydrogen recovery from its admixture with other gases.
A fourth objective of the present invention is to provide methods, systems and devices to decrease the high carbon dioxide (CO2) to hydrogen (H2) ratio of conventional processes for LH2 production.
A fifth objective of the present invention is to provide methods, systems and devices to produce liquid hydrogen with zero green house gas emissions.
The preferred embodiment of the invention is a process for liquid hydrogen (LH2) production comprising the steps of: partial oxidation of methane containing gas producing heat; thereafter pyrolyzing the remaining methane to produce hydrogen and carbon as primary components of a resulting gas mixture containing hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), water (H2)) and trace amounts of hydrocarbons; cryogenically separating said gas mixture yielding H2O, liquid carbon dioxide (LCO2), and low temperature gaseous H2; recycling the CO and CH4 separated from the said gas mixture through a steam methane reforming (SMR) reactor to further produce hydrogen; cooling the gaseous hydrogen until it reached its dew point; and, liquefying said cold gaseous hydrogen into LH2 by applying small amounts of additional cooling energy.
A first embodiment of the invention is a cryogenic method for hydrogen cryogenic liquefaction and hydrogen separation for liquid hydrogen (LH2) production comprising the steps of: (a) producing a hydrogen rich gas mixture containing H2, CO, CO2, CH4, H2O and trace amounts of hydrocarbons; (b) cryogenically separating the gas mixture of step (a) resulting in H2O, and liquid carbon dioxide (LCO2); (c) separating low temperature gaseous H2 and gas mixture of CO and CH4; (d) recycling the CO and CH4 of the gas mixture to further produce hydrogen rich gas; (e) mixing the hydrogen rich gas from step (d) with the hydrogen rich gas mixture in step (a) to form a closed loop for CO and CH4 gas mixture; (f) further cooling the low-temperature gaseous hydrogen until it is liquefied; and (g) producing LH2 without CO2 emission.
In the embodiment, the input hydrogen concentration of step (a) is greater than 70% resulting in a cryogenic separation efficiency which exceeds 90%, the hydrogen recovery is as high as approximately 99.99% with a purity of approximately 99.9999%, the high purity LCO2 obtained in step (b) is sequestered to avoid emission of greenhouse gas and the high purity LCO2 obtained in step (b) is served as a co-product. The producing step includes reacting CH4 and water to produce the gas mixture containing H2, CO, CO2, CH4, H2 or reacting CH4 and oxygen to produce the gas mixture containing H2, CO, CO2, CH4, H2O. The recycling step includes low temperature reaction of gas mixture of CO and CH4, to convert the CO into CO2 and H2 and high temperature reaction of gas mixture of CO and CH4, to convert the CH4 into CO2 and H2 or alternatively, reacting the CO and CH4 mixture with water to convert both CO and CH4 into CO2 and H2 simultaneously at one reactor or separating CO and CH4 mixture into CO and CH4, reacting the CO with H2O at a water shift gas reactor to convert the CO into CO2 and H2 and reforming the CH4 with H2O at a steam methane reforming reactor to convert the CH4 into CO2 and H2.
Several typical methods can be applied to produce hydrogen from methane. These are steam reforming (SMR), partial oxidation, pyrolysis, and autothermal pyrolysis (ATP) and autothermal SMR processes. In addition, the intermediate product, CO, can be converted to hydrogen via a Water Gas Shift Reaction (WGSR) to produce hydrogen and carbon dioxide. By combining these methods with a cryogenic separation process we can form a number of flow diagrams for LH2 production directly from CH4. One embodiment of the present invention uses the CH4 autothermal pyrolysis (ATP) process to produce H2 and carbon in order to reduce the production of CO2. The energy required for CH4 pyrolysis is derived from the partial combustion of CH4 in the feedstock, resulting in a zero energy requirement for this step. The outlet steam of the autothermal reactor is a gaseous mixture containing CH4, H2, CO, CO2 and H2O. The gaseous mixture is then separated into its components by cryogenic distillation. The separated portion containing CH4 and CO are passed through a High Temperature Gas Conditioning Reactor (HTGCR) or a Low Temperature Gas Conditioning Reactor (LTGCR) to produce hydrogen. The recycled steam is then mixed with the main steam from the CH4 autothermal pyrolysis reactor and redirected or recycled into the separation process.
Further objectives and advantages of this invention will be apparent from the following detailed description of the presently preferred embodiments which are illustrated in the accompanying drawings.
Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.
A glossary of frequently used terms is provided below.
The novel and innovative design of the present invention is based on the concept of using an existing cryogenic system, such as the cryogenic cooling system, to liquefy hydrogen and at the same time to separate a mixture of gases.
During the separation, hydrogen is cooled step by step, gradually reaching its dew point. Applying small amounts of additional cooling energy, cold gaseous hydrogen is liquefied into LH2. In this process, methane is completely converted into hydrogen and LCO2 because CO and remaining CH4 are recycled in the system without outlets.
Starting materials for the present invention can include methane, landfill gas (LFG), gob gas or natural gas to produce a gaseous hydrogen (GH2) mixture. The gas mixture is separated through the use of a cryogenic purification process, while at the same time hydrogen is separated from its mixture and further liquefied into liquid hydrogen.
Five embodiments have been developed through the use of the chemical engineering simulation software HYSYS™ and Aspen Plus™ to confirm the process efficiencies and to calculate carbon dioxide to hydrogen ratios.
The total energy requirements for LH2 production from a gas mixture (hydrogen, carbon monoxide, methane, carbon dioxide, water and hydrocarbons) via a cryogenic process is expressed in Equation 1 below:
ΔHTotal=(ΔHCooling+ΔGSeparating+ΔHRecovering)Mixture+ΔHLiquefaction (1)
While the first-law efficiency of the cryogenic separation is defined in Equation 2:
A HYSYS™ 3.0 simulation flow diagram of hydrogen separation and liquefaction is illustrated in
In
Based on
Several factors, discussed below, have a bearing on separation efficiency. The input concentration of hydrogen plays an important role in the determination of efficiency. Equation (2) above predicts that for two extreme conditions, inlet H2 concentrations of 100% and 0%, the efficiencies should be 100% and 0% respectively.
The efficiency calculation based on
The calculation above indicates that cryogenic separation technique is a highly efficient process because the cooling energies can be recovered. Based on fundamentals, five process diagrams shown in
Heater 1, Heater 2, Heater 3, Heater 4 can include, but not be limited to, an electric oven, gas oven and the like.
Cooler 1, Cooler 2 can include, but not be limited to, cryogenic coolers and the like.
Cooler 3 can include, but not be limited to, a water cooler and the like.
HX-1, HX-2, HX-3, HX-4, HX-5 can include, but not be limited to, a heat exchanger and the like.
H2O separator is used to separate water from hydrogen contended gas mixtures.
MIXER-1 and MIXER-2 mix incoming gases together.
Distillation Column can be a cryogenic distillation device that separates gas mixtures.
CH4 Auto Thermal Reactor decomposes methane into hydrogen and carbon.
CO WGSR is a carbon monoxide water gas shift reactor that shifts carbon monoxide (CO) to carbon dioxide (CO2) and shifts water to hydrogen.
CH4 SMR is a steam methane reforming reactor that uses methane and water to produce hydrogen and carbon dioxide.
In Embodiment 1, after cryogenic separation, methane and carbon monoxide are not separated in the process, and are passed through a high temperature gas conditioning reactor (HTGCR) and a low temperature gas conditioning reactor (LTGCR) as shown in
The flow diagram in
During cryogenic separation (stage 2), water and pure carbon dioxide are separated in their liquid forms, while the remaining gas mixture is distilled to separate the high purity and low temperature GH2 and a mixture of CH4 and CO. GH2 is then then liquefied to form LH2 while the mixture of carbon monoxide and methane is sent to gas conditioning reactors (HTGCR and LTGCR).
Stage 3 receives The mixture of carbon monoxide and methane is recycled for the production of hydrogen through HTGCR and LTGCR reactors. The results for these two reactors were calculated based on the HYSYS™ 3.0 equilibrium program and reactions of CO+H2O→H2+CO2 and CH4+H2O→3H2+CO2. The outlet steam R6 is then mixed with the main stream 1-0 from the methane autothermal pyrolysis reactor in stage 1 and then sent to stage 2 with water separator and cryogenic distillation column for component separation.
Energy flow and process efficiencies are calculated and given in Table 1. Compared to the current commercial hydrogen production through SMR plus PSA separation system, not only can the new process produce 99.9999% purity of LH2 with up to 99.99% of hydrogen recovery, but it can produce valuable co-products such as carbon and pure liquid carbon dioxide. In addition to high efficiency, the system is very stable with respect to temperature changes for HTGCR and LTGCR. While the temperature of the gas mixture increases from 250° C. to 900° C., the output of 1.1852 kgmole/hr of LH2 remains constant, indicating that the process has a high capacity to tolerate system disturbance. For example, in
Referring now to
In Embodiment 3, carbon monoxide (CO) and methane (CH4) are separated; CO passes through a WGSR while CH4 is sent to a SMR reactor, as shown in
Embodiment 4 is shown in
CH4(g)→C(s)+2H2(g) (Pyrolysis) (8)
CH4(g)+2H2O(g)→CO2(g)+4H2(g) (SMR) (9)
Energy requirements and efficiency calculations for Embodiment 4 are listed in Table 1. Since unreacted methane is recycled to the auto-thermal reactor where external energy is not required for SMR, the total energy input for Embodiment 4 is lower than for Embodiments 1, 2 or 3. In addition to the lower total energy input, the carbon dioxide to hydrogen ratio is the lowest among Embodiments 1-4. However, the ratio of the total energy required to LH2 produced is the highest because of more CH4 is converted into carbon. The first law efficiency of Embodiment 4 is the greatest of the four processes while the second law efficiency is the least.
In Embodiment 5, liquid hydrogen (LH2) is produced from landfill gas (LFG), as shown in
H2S removal component can include, but not be limited to, a column for eliminating hydrogen sulfide in the gas
Deoxygenation component can include, but not be limited to, a column for removing oxygen gas in the landfill gas
Compressor can include, but not limited to, a device used for compressing gas mixture to a high pressure.
HX-5 and HX-6 can include, but are not limited to, a heat exchanger and the like.
Cooler 1, Cooler and Cooler 3 can include, but are not limited to, cryogenic coolers and the like.
CO2 and N2 separators can be cryogenic distillation devices that separate CO2 and N2 from gas mixtures so that pure liquid methane is purified.
One of the main features of Embodiment 5 is the use of a cryogenic separation process to purify LFG, and the feeding of the purified methane to Embodiment 2 (
Secondly, in a cryogenic process, individual components can be separated and can serve as co-products to create an extra value for the process, such as high purity nitrogen and carbon dioxide. Energy balance and efficiency calculations are included in Table 1.
Below is a discussion of process efficiencies. Energy steams and efficiency calculations for the five embodiments are summarized in Table 1. Further evaluation of the LH2 production system of the present invention requires a discussion of two different efficiencies. The first law efficiency is defined in Equation 10 and the second law efficiency is defined in Equation 11 below.
Methane combustion ΔHCombustion=ΔHo298 K=802.6 kJ/mole and Hydrogen high heating value ΔHCombustion=ΔHof 298 K=285.9 kJ/mole. ηMethane and ηHydrogen are methane input moles and moles of hydrogen produced, respectively; ΔGof(=228.5 kJ/mole) is the Gibbs free energy change with the formation of one mole of water under conditions where hydrogen and oxygen are subsequently used. While hydrogen cooling and liquefaction energy (ΔHCooling+ΔHLiquefaction)Hydrogen can be calculated from
The work produced from the engine can be calculated as: ΔHLH2 *ηcarnot. Considering heat losses and leakage in a cryogenic system, the cryogenic process efficiency, ηC, is estimated to be 30% from input energy to cooling energies required for separation and hydrogen liquefaction. In addition to the energy required for the entire process, heat leakage and heat recovery losses exist throughout the process. If approximately 10% of total heat is lost in the process, the final second law efficiency is calculated as 90% less than its original value (Equation (11)).
The SMR process requires a large portion of energy inputs while on the other hand, the methane autothermal reaction is an energy saving process, reducing the total input energy requirement. A close look at the results of Embodiments 1 and 2 reveals that combining SMR and WGSR into one reactor reduces energy input requirements, thereby increasing first law efficiency.
With regard to the second law efficiency, although the total energy input for Embodiment 4 is the lowest of five embodiments (35947.5 kJ/hr), its second law efficiency is also the lowest (44.83%) because of the low amount of LH2 produced, 0.9506 mole compared to the 1.1852 mole produced in the other four embodiments. The second law efficiencies of Embodiments 1 and 2 are similar, but Embodiment 2 combines SMR and WGSR into one reactor, therefore, resulting in a lower capital cost.
The second law efficiency is determined by total energy inputs and LH2 produced based on the same moles of methane input. Table 2 shows that the cooling energy share of the total energy input is the more important part in the second law efficiency calculation. Improvements of the second law efficiency requires an increase in the activities of the SMR and WGSR reactors.
Total mass balance and methane conversion use Embodiment 2 as an example to calculate component balances. Table 2 shows the total flow rates of input and outlet components.
The balance calculation is shown below.
The above calculations indicate satisfactory mass balances on input and outlet components. The results also show that more than ¾ of total hydrogen is produced from methane while less than ¼ is from water splitting. With an increase of hydrogen production through methane pyrolysis, the hydrogen to carbon dioxide ratio will be reduced to 4.21 in comparison to SMR, CH4+2H2O=4H2+CO2, from which one half hydrogen is produced from methane and one half from water. Since there is no carbon monoxide or methane discharged during the process, both methane conversions and hydrogen yield achieve 100%. These results further indicate the great advantage of this process.
Stability of a process is one of the most important parameters in measuring the capability of the process to maintain steady state operating conditions when reaction condition changes. Because methane and carbon monoxide are trapped in the cryogenic processes and recycled, the entire process can hold a steady state when, for example, the catalyst is gradually deactivated. The best method to simulate the deactivation of the SMR or WGSR is to change the reaction temperatures. As a result, the conversions of carbon monoxide and methane will vary with the temperature change. The variation of the conversions of SMR or WGSR will, in turn, influence the steady state of the process. Table 3 shows that even with a wide range of WGSR temperatures (500 to 900° C.), LH2 and LCO2 outlets remain unchanged. The total energy to LH2 ratio and system efficiencies vary only slightly. The change of conversions of SMR and WGSR, results in total energy inputs are different. The stability feature of the process shown in this invention is better than hydrogen production via other processes. In addition to the stability discussions above, Table 2 shows one other important fact. There is an optimal WGSR temperature of 650° C. corresponding to the maximum system efficiency of 94.56%.
Referring now to the ratio of carbon dioxide to hydrogen RCO2/H2, carbon dioxide produced from SMR or WGSR can be separated within the system in its pure liquid form so that it can be sequestered or used as a co-product. In this way, the environmental impact of the greenhouse gas produced through the new processes of the present invention can be greatly reduced. However, efforts to minimize RCO2/H2 are always important for reducing the greenhouse effect. Table 1 shows the ratios of carbon dioxide to hydrogen for the five embodiments. Except for Embodiment 4, they all have very similar RCO2/H2 ratios. If methane is separated from carbon monoxide and recycled to the methane autothermal reactor for pyrolysis, the ratio is greatly reduced. Thus, it is demonstrated that SMR produces high concentrations of carbon dioxide. If one mole of methane is completely converted into hydrogen and carbon dioxide in an SMR, RCO2/H2=1*44.010/4/2.016=5.46. The other extreme example is methane totally decomposed to produce carbon and hydrogen according to Reaction: CH4=C+2H2 with RCO2/H2=0.00. However, both SMR and pyrolysis require greater energy inputs.
In summary, the present invention shows, for the first time, five embodiments of liquid hydrogen production from methane and landfill gas using cryogenic separation processes. The processes show a highly efficient production of high purity liquid hydrogen for use in fuel cells as well as in propellants for NASA and other hydrogen fuel applications. The development and analyses of flow diagrams illustrate that the proposed processes are unique in combining hydrogen production with hydrogen liquefaction to achieve maximum utilization of cryogenic energy in hydrogen recovery and purification.
Hydrogen recovery from the cryogenic processes is as high as 99.99% with a purity of 99.9999%. Mass balance results indicate that the input methane and landfill gases are up to 99.99% utilized for hydrogen production. Simultaneously with the liquid hydrogen production, high purity carbon dioxide generated from the processes is separated in liquid form and can serve as a co-product or can be sequestered to reduce environmental impact. The intermediate reaction products, carbon monoxide and unreacted methane in the processes can be recycled through Water Gas Shift Reaction (WGSR) and Steam Methane Reforming (SMR) reactors to produce additional hydrogen. The total first law efficiencies of the processes exceed 81% and 79% for methane and landfill gas, respectively. The highest second law efficiency is 57% for methane and 51% for landfill gas under the assumption of a 10% of heat loss and 30% efficiency of cryogenic processes. The weight ratio of carbon dioxide to hydrogen in these processes is 4.21 (g/g). Recycling methane to the autothermal reactor, reduces the carbon dioxide to hydrogen ratio to as low as 3.027 (g/g).
Additionally, the present invention provides a process for zero emission LH2 production directly from methane or landfill gas when the input energy to the system is a renewable energy, such as solar energy. The advantages of the processes disclosed herein, include, but are not limited to, zero emissions, high process efficiency, greater than approximately 85% 1st low thermodynamics and greater than approximately 55% 2nd law thermodynamics, pure LCO2 and LH2 products, a compact design utilizing a cryogenic system for both liquefaction and separation.
While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.
This invention claims priority based on U.S. Provisional Application Ser. No. 60/578,212 filed on Jun. 9, 2004 and the invention was funded in part by the National Aeronautics and Space Administration through Glenn Research Center (NASA-GRC) under Grant NAG32751.
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