Patent Application
20120304539
The present subject matter relates, in general, to natural gas fuel and, in particular, to production of a mixture of hydrogen and natural gas.
Petroleum fuels have been predominantly used in automobiles so far. However, these fuels have a limited resource, and they also produce high levels of emissions. To overcome these challenges, alternative sources of fuel are being explored and tested. Natural gas is one such popular alternative. Although natural gas is regarded as a clean fuel, nitrous oxide (NOx) emissions are still high in this type of fuel. NOx emissions from the combustion of natural gas can be brought down by mixing certain quantity of hydrogen, preferably about 10 to 20% by volume, into the natural gas. Conventionally, hydrogen is produced separately and is later blended with the natural gas to produce an optimum mixture. Hydrogen is produced by employing any of the known processes in the art such as water electrolysis and steam methane reforming (SMR). In the SMR process, natural gas, also referred to as methane, is reformed to produce syngas, which is then converted into carbon dioxide and hydrogen. The process is carried out under extreme operating conditions, i.e., about 20 to 30 bars of pressure and about 850° C. to 950° C. of temperature. Electrolysis involves producing hydrogen from water through electric current However; said methods of producing hydrogen are costly and increase the overall cost of production of the hydrogen-natural gas mixture. Further, storage and transportation of hydrogen is hazardous, in addition to being expensive, considering the inflammable nature of the gas.
In another conventional method, natural gas is directly converted into a mixture of hydrogen and natural gas. In particular, controlled reforming of methane with steam is carried out at a temperature in the range of about 400° C. to 500° C. and pressure of about 1 to 5 atm. Moreover, the reforming takes place in the presence of a catalyst, of which palladium, which is an expansive metal of the platinum group, is an essential component. In addition, the product mixture produced from this method contains carbon monoxide, which is an undesired by-product of the process, and which has to be removed before the product mixture can be used as fuel.
The subject matter described herein is directed to a process for the production of a mixture of hydrogen and natural gas. The process, according to an embodiment of the present subject matter, is carried out at relatively mild operating conditions, i.e. at a lower temperature than the conventional processes. These conditions can be achieved by a relatively less complicated process in a chemical plant. In addition, the product gas mixture obtained from the process described herein is substantially free from carbon monoxide and, therefore, can be used as a fuel in automobiles, directly, after compression. The present subject matter further relates to a plant in which the process to produce the mixture of hydrogen and natural gas can be carried out. The process comprises reforming a stream of natural gas or methane in the presence of steam. The reforming takes place at substantially low temperatures, i.e. around 350° C. to 390° C., and in the presence of a nickel-based catalyst. The temperature range is lower than what is employed in the conventional processes, and can be achieved easily in a chemical plant. In one implementation, the nickel-based catalyst includes nickel, nickel oxide, magnesium oxide, and silica. The nickel-based catalyst is industrially available and inexpensive.
In one embodiment of the present subject matter, the reforming of the stream of natural gas is carried out inside a reformer, which can be, for example, a fixed-bed down-flow vapour phase reformer. In operation, a natural gas stream is first preheated and purified to remove impurities like hydrogen sulphide. The natural gas stream can be preheated through heat exchange with the product-gas mixture obtained after reforming.
The product-gas mixture produced by the process can be adjusted to a predetermined level by varying the quantity of the natural gas intake in the reformer. Based on an outflow demand of the product gas mixture, a portion of the product gas mixture can be recycled. In addition, an inlet water flow rate can also be controlled, based on the outflow demand. When there is no demand of the product gas mixture, the natural gas is entirely recycled and water injection is completely stopped.
These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
The above and other features, aspects, and advantages of the subject matter will be better understood with regard to the following description, appended claims, and accompanying drawings, where:
A process to produce a mixture of hydrogen and natural gas that can be used as a fuel in automobiles is described herein. The automobiles using this mixture of hydrogen and natural gas show low levels of emissions, including NOx emissions. Moreover, the process is substantially a single step process, thereby reducing both cost and complexity involved.
The process involves controlled reforming of a natural gas stream using steam and in the presence of a nickel-based catalyst. The reforming takes place at a controlled temperature in the range of about 350° C. to about 390° C. and at a pressure in the range of about 5 to 7 bars. The nickel-based catalyst is a readily available industrial catalyst, and thus reduces the operating cost. In one implementation, the nickel-based catalyst includes nickel, nickel oxide, magnesium oxide, and silica.
The product mixture obtained after reforming is substantially free from carbon monoxide. Hence, the mixture needs no further treatment and, therefore, can be used directly as fuel, after being compressed to a desired pressure level, in automobiles. Moreover, during the process, the natural gas stream is preheated by exchanging heat with the product gas mixture obtained after the reforming of the natural gas stream. Hence, energy within the system is well utilized, thereby improving the process efficiency and cost effectiveness.
The present subject matter also describes a set up of a plant in which the above said process can be carried out. In such a plant, the mixture of natural gas and hydrogen can be produced on a small scale, for example, for distribution at a retail outlet, as well as on a large scale for distribution to such smaller retail outlets.
Before getting terminated in the impurity removal unit 103, the inlet natural gas line 151 passes through a heat exchanger 102, where the natural gas stream in the inlet natural gas line 151 is preheated. In an implementation, preheating of the natural gas stream in the heat exchanger 102 involves heating of the natural gas stream by a stream of a product gas mixture obtained after reforming. The stream of the product gas mixture is carried to the heat exchanger 102 via a product gas mixture line 152.
The natural gas stream may contain impurities, such as hydrogen sulphide, which are removed in the impurity removal unit 103. The impurity removal unit 103 can contain a substance that adsorbs hydrogen sulphide. In one embodiment, the impurity removal unit 103 is a sulphur guard reactor. In this way, a preheated and purified natural gas stream, referred to as a treated natural gas stream.
Steam that is to be used in the reforming is produced by heating an amount of water that is fed to the plant 100 from a water inlet 104. The water is carried to a heating unit 105 via a water line 153. In one embodiment, the heating unit 105 is a firebox, which is operated by burning a slip stream of the treated natural gas stream. A slip control valve 106 controls a flow rate of the slip stream. The slip stream is carried to the heating unit 105, via a slip stream line 154. At the heating unit 105, the treated gas stream is burnt to heat the water present in the heating unit 105 and produce steam.
Another portion of the treated natural gas stream obtained from the impurity removal unit 103 is reformed to produce the product gas mixture. This portion of the treated natural gas is carried by a main natural gas line 155. In an implementation, a flow rate of the treated natural gas in the main natural gas line 155 can be regulated by a main control valve 108. Further, a portion of the treated natural gas from the main natural gas line 155 is sent to the heating unit 105 through a line 156. This portion of the treated natural gas is heated in the heating unit 105. The flow of this portion of the treated natural gas can be regulated by a control valve 110. Further, the steam generated in the heating unit 105 is mixed with the treated natural gas in the main natural gas line 155, and the mixture is carried to a reformer 107 via a combined natural gas and steam line 157. The portion of the natural gas heated in the heating unit 105 is also mixed with the treated natural gas and steam in the combined natural gas and main natural gas line 157.
In one embodiment, the reformer 107 can be a fixed-bed down-flow vapour phase reformer. The temperature inside the reformer 107 can be in the range of about 350° C. to 390° C. and the pressure in the range of about 5 to 7 bars. Under these reaction conditions, a part of the treated natural gas is reformed to produce the product gas mixture. In an implementation, a catalyst, for example, a nickel-based catalyst, can be used to reform the natural gas. The product gas mixture can have about 10 to 20% of hydrogen by volume and about 70 to 85% of natural gas by volume.
In another embodiment, the slip control valve 106, which controls the flow rate of the slip stream in the slip stream line 154, is regulated by a temperature indicator controller 117. The temperature indicator controller 117 detects a temperature inside the reformer 107 and, based on the detection, directs the slip control valve 106 to control the flow rate of the slip stream.
The product gas mixture is carried via the product gas mixture line 152 to a condensing unit 110. In between, the product gas mixture line 152 passes through the heat exchanger 102 to heat up the natural gas inside the inlet natural gas line 151, as described earlier. The condensing unit 110 separates excess steam from the product gas mixture and condenses the steam to produce water and a dry product gas. The water obtained from the condensing unit 110 is fed to the water line 153 via a condensate line 158.
The dry product gas from the condensing unit 110 is carried via a dry product gas line 159 to a gas analyzer 111. The gas analyzer 111 analyzes the composition of the dry product gas. Based on the analysis of the composition of the dry product gas, if required, an additional quantity of the treated natural gas from the impurity removal unit 103 can be added to the dry product gas, via an adjusting natural gas line 160. The additional quantity of the treated natural gas can be regulated by an adjusting control valve 112. The adjusting control valve is directed by the gas analyzer 111.
In one embodiment, the inflow of natural gas and steam into the reformer 107 is regulated so that the dry product gas from the condensing unit 110 contains less than a pre-determined level of natural gas. This ensures that the product gas composition, after mixing the additional quantity of natural gas, is an optimum composition having desired levels of hydrogen and natural gas, based on a requirement of the fuel. The composition of the final product gas mixture can vary between 15 to 20% of hydrogen by volume and 75 to 90% of natural gas by volume.
The final product gas mixture can be directly stored in a low pressure storage 113 and can also be compressed further through a compression system for directly using as an automobile fuel. In an implementation, the compression of the final product gas mixture can be carried out through two compression stages, 114A and 114B, with a compression ratio of about 5 to 6 in both the stages. Following the compression, the final product gas mixture can be stored in a high pressure storage 115. This compressed final product gas mixture can be directly dispensed info the automobiles through a dispenser 116.
Based on an outflow demand, a portion of the final product gas mixture can be recycled to the reformer 107 via a recycling line 161. In an implementation, the quantity of the final product gas mixture in the recycling line can be controlled by a recycling control valve 118.
At block 201, the inlet natural gas is preheated. Preheating can be carried out using a product gas mixture obtained after reforming the natural gas. The product mixture is at a higher temperature than the inlet natural gas, and the heat of the product mixture is utilized in preheating the inlet natural gas.
After preheating, at block 202, the natural gas is purified by removing impurities like hydrogen sulphide. In one implementation, the purification involves adsorbing hydrogen sulphide by an appropriate substance.
The reforming of the natural gas, as described later, is carried out by steam. The steam required for the reforming of the natural gas is generated in a steam generating process, at block 203. A part of the natural gas obtained after the purifying step, at block 202, can be used as fuel to heat water. Along with the steam generation, a part of the natural gas to be reformed can a so be heated.
At block 204, the natural gas obtained after purifying at block 202, is reformed by the steam generated at the block 203, in the presence of a nickel-based catalyst. The reforming of the natural gas is carried out at within a controlled temperature range. In an implementation, the controlled temperature range is about 350° C. to 390° C. The pressure to be maintained during the reforming can be maintained in a range of about 5 to 7 bars. In one implementation, the nickel-based catalyst comprises nickel, nickel oxide, magnesium oxide, and silica. Reaction at these conditions leads to the formation of a product gas mixture that is substantially free from carbon monoxide. Further, a gas hourly space velocity on dry basis in a range of about 1500 hr-1 to 2250 hr-1 can be maintained during the reforming.
The reforming of the natural gas takes place through following reaction steps:
CH4+H2O→CO+3H2 (1)
CO+H2O→CO2+H2 (2)
The natural gas, that is, methane, is reformed by steam to produce carbon monoxide in reaction (1). However, carbon monoxide gets converted to carbon dioxide during reaction (2). The reaction (2) is also termed as water-gas shift reaction. This reaction proceeds predominantly in a forward direction under the reaction conditions as disclosed earlier. Hence, almost all of the carbon monoxide produced as an intermediate product is converted into carbon dioxide, and thereby the product gas mixture gets substantially free from carbon monoxide.
At block 205, the product gas mixture obtained after reforming the natural gas is condensed to separate excess steam. The water obtained as a condensate, along with the inlet water, is again heated in the steam generating process at block 203 to generate steam. After separating the excess steam, a dry product gas mixture is obtained whose composition is adjusted at block 206.
At block 206, the composition of the dry product gas mixture is analyzed and, based on the analysis, an additional quantity of the treated natural gas obtained after the purifying, at the block 202, is added to the dry product gas mixture to obtain a final product gas mixture having a desired composition of hydrogen and natural gas.
A portion of the final product gas mixture, obtained after the composition is adjusted, is recycled, at block 207. The quantity of the natural gas that is recycled is based on an outflow demand of the product gas. A quantify of the inlet water that is used for steam generation in the steam generating process is also based on the outflow demand of the product gas.
When the outflow demand is low, the quantity of the inlet water is proportionately low. When there is no outflow demand, supply of the inlet water is completely stopped and the product gas mixture is entirely recycled. Hence, the process can be started and stopped easily, or its rate can be varied depending on the demand of the product gas.
The product gas having an appropriate composition can be compressed, at block 208. The compressed product gas hence obtained can be directly used as a fuel in automobiles.
The table 300 has two sets of rows: 310 and 320. The set of rows 310 represents operating conditions under which the experiments are carried out, while the set of rows 320 represents compositions of different gases in the product gas mixture obtained as a result of the experiments. The table 300 includes data from three experiments, represented by column 301, column 302, and column 303.
All the three experiments were carried out in a fixed-bed down-flow vapour phase reformer. Experiment 1 and experiment 2 were carried out in the presence of 100 cc of a nickel-based catalyst, as described in the description of
Column 303-1 represents data for an initial run of the experiment 3, whereas column 303-2 represents data after a number of start-up and shut down cycles of the plant. Further, row 311 represents temperature values whereas row 312 represents pressure values in the reformer at which the experiments are carried out. Also, row 313 represents steam to carbon molar ratio and row 314 represents a gas hourly space velocity. As it can be seen, in all the three experiments, the product gas mixture is substantially free of carbon monoxide.
Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2656/DEL/2009 | Dec 2009 | IN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IN10/00787 | 12/3/2010 | WO | 00 | 6/14/2012 |
