CN107250327A - Method for producing methane and electric power - Google Patents

Abstract

A process for the production of methane and electricity comprising the steps of producing synthesis gas from a carbonaceous feedstock and methanating the synthesis gas in two or more methanation reactors and producing in one or more superheaters Superheated steam, a process in which superheated steam is fed to a back pressure turbine to drive a recycle compressor and all or part of the steam used in the turbine is fed into the methanation process to reduce the potential for carbon formation And save recirculation compressor energy. In this way electricity can be produced at a lower price.

Description

Method for producing methane and electricity

The present invention relates to methods of producing methane and electricity. More specifically, the invention relates to feeding superheated steam to a turbine, and using the used steam for process addition or heating, whereby electricity can be produced at a lower than normal production price.

A steam turbine is a device that extracts heat energy from pressurized steam and uses it to perform mechanical work on a rotating output shaft, ie, a turbine uses steam to drive something, such as a pump, compressor or generator. Because turbines generate rotational motion, they are particularly well suited for driving electrical generators. A steam turbine is a form of heat engine that achieves most of the improvements in thermodynamic efficiency by using multiple stages in the expansion of steam, which results in a reversible expansion process that is closer to ideal.

Noncondensing or backpressure turbines are most widely used in process steam applications. The exhaust pressure is controlled by the regulating valve to meet the needs of the process steam pressure. Such turbines are often found in refineries where large volumes of low pressure process steam are required.

A power plant installation with a gas turbine, a steam turbine and a mixed gas/steam turbine is known from US 6.047.549A. The method of isothermal heat supply and heat removal is realized with optimal waste heat utilization by using appropriate networking of the three turbine installations. In particular, the waste heat of the consecutively arranged turbo-sets is used to generate a high-pressure flow, which is used in a backpressure turbine in an efficiency-promoting manner.

US 2010/0263607 A1 describes a system for generating steam for a turbine of an electrical generator. The system includes a superheater that receives steam from the boiler and superheats the steam. The superheated steam is then passed through a heat exchanger to transfer some of the thermal energy of the superheated steam to the water stream. This reduces the temperature of the superheated steam to a temperature suitable for the turbine. The water heated in the heat exchanger may be condensed water that has passed through the turbine, and the water heated in the heat exchanger may be directed to a boiler where it is recycled into steam.

In US 2011/0120127 A1 a method for generating supercritical steam in a low energy ammonia or methanol plant using synthesis gas heat is disclosed. The process involves a reforming or partial oxidation stage, and equipment suitable for operating the process includes at least one supercritical steam generator, at least one superheater, at least one backpressure turbine, at least one extraction and condensation turbine, and at least one boiler feed water pump. By means of this method, energy savings and overall cost advantages, ie better process economy, can be achieved.

Finally, a combined multicomponent gasification, methanation and power island steam turbine system is disclosed in US 2010/0170247 A1. The gasification section includes the heat recovery design and associated controls for obtaining the desired steam to dry gas ratio, and the methanation section includes the first, second, and third methanation reactors with high, medium, and low pressure turbines and High pressure economizer combined with associated heat recovery. Power island steam turbines include high, intermediate and low pressure turbines with an input coupled to the output of a superheater in the methanation process. The subject matter of this reference differs from the subject matter of the present invention in that the high pressure turbine and recycle compressor are not connected and no steam is added to the feed stream of any methanation step.

The present invention differs from the prior art described above in that it is used in the applicant's SNG process, where steam is consumed internally to heat the gas, or is added to the process gas to avoid carbon formation, more specifically Avoid the formation of "whisker carbon". In this process, SNG (Substitute Natural Gas) is produced from cheap carbonaceous raw materials such as coal, petroleum coke, biomass or waste. SNG is rich in methane, and it is used interchangeably with natural gas and is partitioned in the same way.

The conversion of carbonaceous feedstock to SNG is carried out in the following process steps:

- Gasification of the feedstock to produce a gas rich in hydrogen and carbon monoxide;

- transformation, to adjust the ratio between hydrogen and carbon monoxide;

- Acid gas removal, where carbon dioxide and hydrogen sulfide are removed during scrubbing;

- Methanation to convert carbon oxides and hydrogen to methane (SNG), followed by drying and possibly compressing the product SNG to pipeline conditions;

- production of oxygen for the gasification process in the air separation unit, and

- Recovery of sulfur from the acid gas removal unit, which is most commonly accomplished by conversion of hydrogen sulfide to sulfur in a Claus unit according to the following equation:

If desired, the sulfur can then be converted to concentrated sulfuric acid in a wet sulfuric acid (WSA) unit.

Methanation is a process in which carbon oxides and hydrogen are converted to methane according to the following reactions:

with

These reactions are coupled to an equilibrium between carbon monoxide and carbon dioxide as follows:

Both reactions (2) and (3) are highly exothermic, releasing a large amount of reaction heat. Efficient recovery of the heat of reaction is essential for any industrial methanation technology.

It is known from the art of steam reforming that, depending on operating conditions and actual catalyst composition, catalysts can form carbon. Carbon can be formed on the catalyst from methane, carbon monoxide or higher hydrocarbons. Carbon formation from methane and carbon monoxide can be represented by the following reaction:

The carbon formed depends on operating conditions and catalysts. Typically, the carbon on the Ni catalyst is in the form of whisker carbon. Said whisker carbons are described in the literature, see eg "Concepts in Syngas Manufacture" by J. Rostrup-Nielsen and Lars J. Christiansen, Catalytic Science Series vol. 10, 2011, pages 233-235. As noted above, the choice of catalyst and operating conditions will determine whether carbon is formed. According to the so-called equilibrium gas principle, carbon is formed if thermodynamics predicts the formation of carbon from one or more reactions (5)-(7) after equilibration of reactions (2)-(4). See, eg, pages 247-252 of the above reference. In this case, means of avoiding carbon formation include lowering the temperature and increasing the steam content of the feed gas to the reactor.

The heat released by the above methanation reaction is most efficiently recovered as high pressure superheated steam.

The basic idea of the invention is to feed hot steam feed to a back pressure turbine and to use the spent steam for process addition or heating. In this way, electricity can be generated at a relatively low price, since in the case of heating, 100% of the condensation energy is utilized. In the case of steam addition to the process, it can be seen that the latter results in lower energy utilization compared to the addition of saturated steam and the use of condensing turbines.

Accordingly, the present invention relates to a process for the production of methane and electricity comprising the steps of producing synthesis gas from a carbonaceous feedstock in a manner known per se, and passing the feed synthesis gas over sulfur for desulphurisation. Preservation is followed by methanation in two or more methanation reactors, where

- generating saturated steam in one or more boilers and feeding it to one or more superheaters for conversion into superheated steam;

- feeding at least a portion of the superheated steam to a back pressure turbine to drive a recycle compressor which compresses a portion of the effluent from the last methanation reactor; and

- Feed all or part of the steam used in the turbine to the methanation process to reduce the potential for carbon formation and save energy in the recycle compressor.

The utilization of superheated steam is crucial in the present invention. Superheated steam is steam at an absolute pressure (at which the temperature is measured) at a temperature above its evaporation point (boiling point).

In the boiler, process waste energy is transferred to liquid water to generate steam. In the boiler used in the method of the invention, the water is always at boiling point. Once the boiling point is reached, the temperature of the water stops rising and remains constant until all the water has evaporated. Water passes from a liquid state to a vapor state and receives energy in the form of latent heat of vaporization. As long as some liquid water is present, then the temperature of the steam is the same as that of the liquid water. Then, steam is called saturated steam.

When all the water has been evaporated, any subsequent addition of heat will raise the temperature of the steam. When steam is heated beyond the saturated steam level, it is called superheated steam.

Industry typically uses saturated steam for heating, drying or other operations. Superheated steam is used almost exclusively in turbines to drive generators, compressors, pumps, etc.

A back pressure turbine is used to drive the recirculation compressor. Typically, compressor drives are electric in SNG plants due to their relatively low power consumption. Even so, the power consumption of the present invention is lower, practically zero.

The feed gas is preferably a gas in which the combined concentration of hydrogen and carbon oxides is at least 60%.

The process of the invention is shown in the figures. The synthesis gas feed (A) preferably comprises at least 7 mol % methane on a dry weight basis, more preferably at least 11 mol % methane on a dry weight basis, most preferably at least 15 mol % methane on a dry weight basis, to be mixed with small amounts The water is passed through a sulfur guard (SG) followed by a gas conditioning reactor (GC). The water can be (but need not be) high pressure steam HP. It is also possible to use small amounts of liquid water, which are then evaporated in the piping upstream of the reactor.

In a gas-regulated reactor (GC), the shift reaction takes place

The aim is to raise the temperature to about 320°C. At lower inlet temperatures there is a risk of gum formation in the first methanation reactor (M1). Since gel formation may also occur due to high CO concentrations, the use of a gas-regulated reactor (GC) has a "double" advantage, since the shift reaction lowers the CO concentration. The amount of steam added upstream of the sulfur guard (SG) is so low that when entering the SG the concentration is 0.01 to 1.00%, typically 0.3%.

The conditioned gas is then passed through two or more methanation reactors. The embodiment shown in the figures includes two methanation reactors (M1 and M2). The heat of reaction from the methanation reactor is used in boilers (B1 and B2) and steam superheaters (S1 and S2), then the superheated steam, or at least a portion thereof, is fed to a turbine (T) to drive a recycle compressor (RC ).

In the process of the invention, the steam pressure is preferably 30 bar higher than the feed gas pressure.

Spent steam is fed from the turbine to the methanation reactor to reduce the potential for carbon formation. After a possible split, steam is added to the system at a point immediately upstream of the first methanation reactor M1 (where a shift-active catalyst may also be present). In this way, the added steam will not go directly to the GC or M2. Adding steam would be most effective at a location between reactors GC and M1. When both a shift catalyst and a methanation catalyst are present, then these two catalysts are advantageously placed in separate reactors, ie in the GC and M1 respectively. The separate reactor was operated as an adiabatically operated composite reactor together with the GC reactor placed immediately before M1. This composite reactor was patented by the applicant, see GB 2 018 818B. If no shift active catalyst is present, steam can be added to the second methanation reactor M2 after the split. This option will be more energy efficient. A portion of the used steam from the backpressure turbine can be added to the process stream upstream of the desulfurization step.

By using steam in this way, there are at least three surprising advantages:

1) By changing the O/C ratio and H/C ratio in the reactor, the carbon balance is directly shifted;

2) More steam to the turbine driving the compressor creates higher recirculation, which in turn leads to lower temperatures, thereby moving process conditions away from the carbon zone; and

3) Lower temperature in the reactor results in higher conversion of synthesis gas to methane due to equilibrium shift towards methane at low temperature.

So by first generating high pressure steam and using some of it in a back pressure turbine (which drives the recycle compressor at reduced pressure) and then feeding all or a portion of the reduced pressure steam to the process, it is much different from adding steam directly Energy consumption can be minimized and the risk of carbon formation can be reduced compared to plants that would otherwise, for example, electrically drive recirculation compressors.

The invention is further illustrated by the following examples.

Example 1

This example compares three different cases, more specifically the conventional SNG process (first case) and two cases of the back pressure turbine based method of the present invention (second and third case). The second and third cases are referred to as "turbine case" and "additional turbine case", respectively.

In the first case of the conventional SNG process, the steam production was about the same as the other two cases, and the outlet temperature from the first methanation reactor was 675 °C. Electricity consumption to drive the recirculation compressor is 1818 kW.

In the second case, feeding additional steam to the gas conditioning reactor increased the distance from the first methanation reactor to carbon formation to 15°C. As in the first case, the outlet temperature from the first methanation reactor is 675°C, but the electricity consumption to drive the recycle compressor is zero.

For the third case (additional turbine case), the outlet temperature from the first methanation reactor was raised from 675 to 690°C while still being 10°C away from carbon formation. Likewise, the electricity consumption to drive the recirculation compressor is zero.

The results are summarized in Table 1 below:

Table 1

normal situation turbine condition Additional Turbine Conditions Steam generation, t/h 216.1 211.8 215.1 Power consumption, kW 1818 0 0 Outlet temperature from the first methanation reactor, °C 675 675 690 Distance from carbon formation, °C 10 15 10

Example 2

This example illustrates the use of a condensing turbine in the method according to the invention. More specifically, a condensing turbine is used as opposed to a back pressure turbine where the effluent steam can be reused in the process. Condensing turbines extract the maximum possible amount of energy from the steam, leaving behind steam condensate.

Two cases using condensing turbines have been tested (condensing turbine case and alternative condensing turbine case). When compared to the backpressure turbine case in Example 1, these two condensing turbine cases show that the overall steam balance is best for the embodiment of the backpressure turbine according to the invention.

The results are summarized in Table 2 below:

Table 2

Condensing Turbine Condition Alternative Condensing Turbine Cases Steam generation,t/h 209.2 205.4 Power consumption, kW 0 0 Outlet temperature from the first methanation reactor, °C 675 675 Distance to carbon formation, °C 10 15

These examples clearly show that it is possible to simultaneously minimize energy consumption and reduce the risk of carbon formation compared to any plant directly adding steam and otherwise driven recycle compressors.

Claims (10)

1. a kind of method for producing methane and electric power, the described method comprises the following steps：In a way known from carbon Matter charging production synthesis gas, and make feed synthesis gas (A) by being used for after the sulphur protection (SG) of desulfurization at two or more Methanation is carried out in multiple methanators (M1, M2), wherein

- saturated vapor is produced in one or more boilers (B1, B2), and the saturated vapor is fed to one or more Superheater (S1, S2) is to be converted into superheated steam；

- at least a portion superheated steam is fed to back pressure turbine (T), to drive recycle compressor (RC), it is described again Recycle compressor compresses a part for the effluent from last methanator；With

- used all or part of steam will be added in the turbine in methanation, to reduce the possibility of carbon formation And save the energy of recycle compressor.

2. according to the method described in claim 1, wherein the reaction heat from the methanator (M1, M2) is used for into institute State in boiler (B1, B2) and the steam superheater (S1, S2), to produce superheated steam.

3. method according to claim 1 or 2, wherein being entered by the turbine (T) to driving recycle compressor (RC) Expect more steaminess to obtain higher recycling.

4. method according to claim 3, wherein the higher recycling causes at the first methanator (M1) In lower outlet temperature, thus process conditions are removed from carbon region.

5. method according to claim 1 or 2, wherein synthesis gas charging (A) is rubbed based on dry weight meter comprising at least 7 Your % methane.

6. method according to claim 5, wherein synthesis gas charging based on dry weight meter comprising at least 11 moles % Methane.

7. method according to claim 6, wherein synthesis gas charging based on dry weight meter comprising at least 15 moles % Methane.

8. high 30 bar of method according to claim 1 or 2, wherein vapour pressure ratio feed gas pressure.

9. high 60 bar of method according to claim 1 or 2, wherein vapour pressure ratio feed gas pressure.

10. method according to claim 1 or 2, wherein a part of from the back pressure turbine used is steamed Vapour is added in the technique stream of desulfurized step upstream.