EP2169035B1 - Method for setting the calorific value in fuel gases containing methane - Google Patents

Description

The invention relates to a method for adjusting the calorific value in methane-containing fuel gases.

Methane-containing fuel gases such as biogas, digester gases, mine gases, thermally produced fuel gases and combustion gases produced by degassing can often be fed into the public gas network only by suitable measures. The reason for the limitation of the feed into the public gas network is justified by the German calibration law, since this allows only a deviation of 2% of the average settlement value within a billing period with supply of gas from different sources or directions. In Germany are z.Zt. two fundamentally different natural gases distributed in public gas networks. First, the natural gas H with a calorific value between 9.5 - 13.1 kWh / m ³ and secondly, the natural gas L with a calorific value between 8.4 - 9.5 kWh / m ³ . After processing the raw biogas, biogas has a calorific value of 10.4 - 10.8 kWh / m ³ . Usually this means that the limit of 2% compared to the billing period is exceeded with a standard combustion value of 11.1-11.5 kWh / m ³ .

To achieve a calorific value for the biogas to be fed into the public gas network, which is adjusted to the natural gas calorific value to 2%, technically complex and cost-intensive measures are taken.

Liquefied petroleum gas (LPG) from a propane / butane mixture is now used to increase the calorific value. This mixture comes from fossil sources and is as finite as natural gas and subject to variable market prices. To reduce the calorific value, gas (biogas or natural gas H) is mixed with air via elaborate filtering and drying systems.

It is from the pamphlets WO 00/21911 A1 and WO 03/055795 A1 Methods are known in which methane-containing fuel gases to form a plasma are excited, with predominantly hydrogen and carbon are formed. A similar process for the enrichment of hydrogen in a fuel gas by means of microwaves and a catalyst is described in US Pat US 2008/1735323 A1 described. In the WO 93/12205 A1 discloses a method in which the calorific value of a gas is changed by a pyrolytic process.

It is therefore an object of the invention to provide a method by which the calorific value can be adjusted in methane-containing fuel gases simply and inexpensively.

This object is achieved by a method having the features of claim 1.

Advantageous embodiments are shown in the subclaims.

In the inventive method for adjusting the calorific value in methane-containing fuel gases, the fuel gas is excited by energy input to form a plasma, whereby the fuel gas or product gas then has a modified calorific value. The plasma is preferably generated in a reactor, through which the fuel gas is passed continuously.

The energy input is preferably discontinuous, so that short-term plasmas are generated in the reactor. The plasmas are generated by electric sparks, electric arcs, lasers, microwaves or the like. The electrodes used in this case are preferably made of metal or carbon, with platinum or stainless steel being preferred. Platinum is the most preferred.

It produces a variety of saturated and unsaturated organic compounds and molecular hydrogen. By changing the composition of the fuel gas, the calorific value of the fuel gas is changed, wherein the formation can be controlled so that a predetermined calorific value can be achieved, which can be set by a suitable means, which will be described below, to a defined value.

In this case, the basic organic component methane acetylene / ethyne is formed as the main product. Other products include ethene, ethane, propene, propane, various hydrocarbons with a four carbon carbon skeleton, and hydrogen. There will also be connections with up to six Carbon atoms in the product gas obtained. In addition, carbon black or coal is obtained in good yields.

With the hydrogen formed, the unsaturated compounds are catalytically hydrogenated and the remaining portion of hydrogen can be separated. As the hydrogenation catalyst, known catalysts such as metal asbestos, metals and metal oxides are used. Further suitable catalysts are known to the person skilled in the art.

By separating the hydrogen, the calorific value of the fuel gas is of course also influenced.

The separation is carried out by known methods. These are, for example, the diffusion through clay or ceramic components or hydrogen-permeable membranes of palladium alloys. Further suitable methods are known to the person skilled in the art.

The entire process can advantageously be carried out continuously in all steps.

An apparatus for carrying out the method essentially comprises a reactor, preferably a tubular reactor for the continuous passage of the methane-containing fuel gas. The tubular reactor is provided with means for introducing energy into the fuel gas to form a plasma. This device preferably generates pulsed short-time plasmas.

According to one embodiment of the device, this has after the reactor to a device for the separation of the hydrogen, as described above in the method.

According to a further embodiment of the device, this has between the reactor and the means for separating the hydrogen, a means for the catalytic hydrogenation of the unsaturated compounds in the fuel gas, which flows out of the reactor after the plasma treatment. Suitable catalysts are described above.

Further equipment for process parameters, which are listed below, has the device as needed, in particular, these are devices for controlling the flow rate of the fuel gas in the reactor and the gas pressure.

The following statements relate both to the method according to the invention and to the device for carrying out the method.

The yield of the various products depends on the electrical power, the electrode geometry of the energy input device, its polarity, the flow rate of the fuel gas in the reactor, the gas pressure and the electrode material.

The selectivity is controlled according to the invention by varying these parameters, so that an increase in the calorific value of a methane-containing fuel gas by formation of higher-energy hydrocarbons or a reduction of the calorific value by formation of soot, which is removed from the gas stream occurs. Also by means of hydrogenation and / or separation of the hydrogen, the calorific value is influenced.

The most important and most easily varied parameters in the control of the process are the power of the plasma and the flow rate of the methane-containing fuel gas.

The main products can be classified according to the number of carbon atoms. Thus, in the following, the C2 group (two C atoms), the C3 group (three C atoms) and the C4 group (four C atoms) are considered in detail.

As stated, the concentration of the products increases with the increase of the power of the plasma. The chromatograms according to Fig. 1 and Fig. 2 compare each of the products obtained in the different plasma performances. Shown are the C2 in Fig. 1 and C3-in Fig. 2 ,

Table 1 represents the percent yields of the carbon compounds and the calorific value produced as a function of the voltage and current of the reactor. Tab.1 Dependence of the reaction products on the reaction voltage Flow (l / h) 30 Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 unit tension kV 4.7 6.0 9.0 12.0 15.0 electricity mA 2.4 4.1 7.4 10.6 14.0 hydrogen % 1.51 2.60 3.81 5.10 4.77 methane % 96.81 94.51 90.90 82.52 77,70 Ethan % 0.55 0.48 0.97 1.57 1.39 ethene % 0.17 0.37 0.66 1.57 1.90 acetylene % 0.85 1.73 3.24 7.43 11.34 propane % 0.00 0.00 0.00 0.05 0.04 propene % 0.00 0.00 0.04 0.11 0.14 1,2- % 0.00 0.04 0.05 0.17 0.26 propyne % 0.04 0.10 0.14 0.47 0.77 1,3-butadiene % 0.00 0.00 0.00 0.07 0.11 1.3 Butadiyne % 0.07 0.14 0.13 0.63 1.11 1-butyn % 0.00 0.00 0.00 0.04 0.05 n-pentane % 0.00 0.04 0.05 0.22 0.36 0.00 0.00 0.00 0.06 0.07 condensing kWh ³ 11.11 11.15 11.27 11.92 12.51

Fig. 1 shows superimposed chromatograms and represents the increase in peak height (area) for ethane, ethene, and acetylene. The corresponding information regarding the concentration can be found in Table 1. Sample 1 corresponds to the lowest plasma power (4.7 kV) and Sample 5 to the highest plasma power (15 kV). Particularly noteworthy here is the enormous increase in acetylene. With a voltage of approx. 15 kV, acetylene reaches a peak area of more than ten percent, making it the main product. The sum of the C2 connections is about 15 kV voltage with over 15 percent higher than all other connections.

Fig. 2 shows propene and propadiene as a function of increasing plasma power. There is also a significant increase in peak height with increasing voltage. The proportional peak area of propene and propadiene is about ten times smaller than the sum of the peak areas of the C2 group at about 15 kV. Propane is detectable only at voltages above 9 kV.

An increase of the product concentration by the factor "2" can also be found in Fig. 3 observe where the influence of the change of polarity is examined. This means, that the C3 group can be obtained from negative to positive when the voltage changes from negative to positive with significantly higher yields.

The dependence of the methane conversion on the voltage of the high voltage source, the polarity as well as the gas flow rate is clearly given.

The correlation of gas flow, voltage and the concentration of educt and products is in the Fig. 4-6 played. The course of the methane concentration ( Fig. 4 ), the acetylene concentration ( Fig. 5 ) of the hydrogen concentration ( Fig. 6 ).

Since methane is converted in plasma, its concentration decreases with increasing plasma power. Looking at the course in its entirety, one sees the largest sales at very high voltages and low gas flows.

The formation of higher hydrocarbons, in addition to the plasma power, also depends strongly on the gas flow rate. Basically, sales increase with increasing plasma output. In the considered experimental interval, the highest sales are obtained at high flows.

Table 2 shows that higher concentrations of higher hydrocarbons are obtained at 60 l / h than at 30 l / h.

The concentration of hydrogen in the product gas mixture correlates with low gas flows and high voltages. At low gas flows carbon is formed, which is recognizable as carbon fouling. The hydrogen yield then increases, the concentration of higher hydrocarbons decreases as carbon is formed. Table 2: Dependence of the concentration as a function of the flow rate Flow (l / h) 60 30 unit tension kV 9.1 9.0 electricity mA 7.2 7.4 hydrogen 3,542 2.43 3.81 methane % 11,064 89.56 90.90 Ethan % 19.537 1.11 0.97 ethene % 19.537 0.96 0.66 acetylene % 19.537 5.04 3.24 propane % 25 0.00 0.00 propene % 25 0.07 0.04 1,2-propadiene % 25 0.10 0.05 propyne % 25 0.00 0.14 1,3-butadiene % 35 0.26 0.00 1.3 Butadiyne % 35 0.03 0.13 1-butyn % 35 0.33 0.00 n-pentane % 40 0.00 0.05 0.11 0.00 condensing KWh / m ³ 11.65 11.27

Fig. 5 shows that acetylene concentrations of over one percent at 30 l / h and about 15 kV are obtained.

The concentration of hydrogen in the product gas mixture correlates with low gas flows and high voltages. Fig. 6 shows that more than 5% by volume hydrogen was measurable at reaction conditions of 30 l / h and 15 kV in the product gas.

Likewise, a dependence on the material of the electrodes used and their geometry in the reaction space is given, in particular, the selective increase in the yield of ethane, ethene and acetylene is particularly noteworthy.

The preceding statements concern the use of stainless steel electrodes.

Table 3 shows the results of methane conversion at 60 l / h with a platinum and a stainless steel electrode

The volume percentages are shown. Repeatedly, the tests were carried out with positive and negative high voltage. Only the results of the positive voltage are shown. The trend is identical for negative voltage

It can be seen that there is a higher yield when using the platinum electrode. Especially at very high voltages (15 kV) an increase of acetylene of up to 37% can be observed. When ethane is observed one Increase in yield by 85% and ethene still a yield increase of 63%. With increasing carbon number and number of multiple bonds decreases the yield increase. electrode material platinum stainless steel platinum stainless steel platinum stainless steel platinum stainless steel tension 6 6.93 9.1 9.08 12.1 12 15.1 15.05 electricity 2.95 3.6 6.4 7.24 9.7 10.4 13.2 E 3.7 hydrogen 1.15 1.29 2.24 2.43 3.23 3.45 4.34 4.12 methane 93.55 95.31 86.04 89.56 80.51 82.26 71.52 78.78 Ethan 1.13 0.68 1.76 1.11 2.33 1.56 3.11 1.68 ethene 0.62 0.38 1.44 0.96 2.15 1.67 3.24 1.97 acetylene 3.00 2.04 6.98 5.04 9.68 8.62 14,23 10.35 propane 0.00 0.00 0.04 0.00 0.06 0.05 0.09 0.05 propene 0.04 0.00 0.10 0.07 0.15 0.14 0.22 0.16 1,2-Propandlen 0.06 0.04 0.15 0.10 0.21 0.22 0.35 0.29 propyne 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1,3-butadiene 0.16 0.09 0.40 0.28 0.57 0.58 0.96 0.76 1.3 Butadiyne 0.00 0.00 0.06 0.03 0.07 0.09 0.13 0.12 1-butyn 0.23 0.13 0.61 0.33 0.76 1.00 1.33 1.25 n-pentane 0.00 0.00 0.03 0.00 0.04 0.06 0.07 0.07 0.06 0.04 0.17 0.11 0.23 0.31 0.41 0.40 condensing 11.48 11.28 12.04 11.65 12.40 12.25 13.12 12,49

Fig. 7 shows a schematic structure of a device. This device 10 has a fuel gas source 11 with a discharge valve 12, from which the fuel gas is passed via a gas line to the flow reactor 13, in which a device for energy input 14 is arranged. Subsequently, the fuel gas / product gas changed in the calorific value is first conducted to a device for hydrogenation 15 and then to a device for the separation of hydrogen 16. In addition, the last device provided is a gas chromatograph 17 with which the composition of the product gas can be analyzed.

The gas analysis after carrying out the process with the device prototype described below shows, in addition to the main component methane, exclusively saturated higher hydrocarbons and no hydrogen. Thus, the partial conversion of methane into higher hydrocarbons and the complete hydrogenation of unsaturated hydrocarbons such as acetylene and the complete separation of hydrogen has succeeded. The increase in calorific value was over 11%. The control of the ignition energy of the plasma allows adjustment of the calorific value. This means that almost every calorific value that is greater than 11.064 kWh / m3 can be set by varying the process parameters. Table 4 Results of gas analysis based on methane gas composition in front of the reactor after the reactor components % % methane 100 98.765 Ethan 1,024 propane 0,128 butane 0,069 pentane 0,004 neopentane 0.005 hexane 0.005 hydrogen 0 0 total 100 100 kWh / m ³ kWh / m ³ condensing 11,064 11.193

The energy used is listed in tabular form below: Table 5 fuels reactants Products kWh kWh Methane 100 l 1.1064 Heating furnace hydrogenation and 0,175 High voltage for methane reactor 0.09 Product gas 98.8% 1.1059 Hydrogen 1.2% 0.0036 total 1.3714 1.1095 efficiency 80.9%

As a prototype of the device for increasing the calorific value in fuel gases, a 2-inch gas pipe was equipped with spark plugs. The electrical discharge in the gas stream converts the methane in the manner described above. The design consists of commercially available components and can be used in continuous operation.

The methane reactor consists of a square tube (edge length 10 cm) in which at a distance of 5 cm twelve spark plugs are screwed. By a transformer, the electrical mains voltage (230V, 50Hz) is transformed into 12 volts, then rectified and distributed on twelve coils. The required high voltage is achieved by a relay circuit with a very high clock frequency. In this case, semiconductor relays are controlled by means of high-frequency pulsed DC voltage and thus clock the supply voltage of the coils. The control of the ignition energy via a frequency controller and the amount of supply voltage for the coils.

It has been shown that it is possible to increase the calorific value of methane-containing gases by means of electrical energy. The unsaturated hydrocarbons thereby formed were catalytically hydrogenated. The remaining hydrogen was completely separated and removed from the system. It can be obtained separately and very fine and used energetically (fuel cell, etc.). The not yet optimized efficiency is 81%. The generated gas can be directly into a public gas network as a substitute gas for natural gas H. Any conditioning with LPG can be saved.

Claims (4)

1. Method for setting the calorific value in fuel gases containing methane, characterized in that the fuel gas is continuously conveyed through a reactor, wherein an excitation of the fuel gas for the purpose of plasma formation is performed, by means of energy input, in such a manner that ethyne, as a main product, is formed from methane and a fuel gas having an increased calorific value, which is indicated in kWh/m³, is discharged from the reactor, wherein the plasma is generated by means of electric sparks, arcs, lasers and microwaves and wherein the fuel gas having a changed calorific value is hydrogenated.
2. Method according to claim 1, characterized in that hydrogenation is performed catalytically.
3. Method according to claim 1 or 2, characterized in that the hydrogen still present by changing the calorific value in the fuel gas and after hydrogenation is separated from the fuel gas.
4. Method according to any one of claims 1 to 3, characterized in that the calorific value is set by variation of at least one of the following parameters:

   electrical power of the plasma,

   flow velocity of the fuel gas,

   electrode geometry,

   polarity of the electrodes,

   pressure of the fuel gas, and

   electrode material.

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