GB2170898A

GB2170898A - Method and apparatus for recovering and making available process heat

Info

Publication number: GB2170898A
Application number: GB08526053A
Authority: GB
Inventors: Klaus Knizia
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-02-02
Filing date: 1985-10-22
Publication date: 1986-08-13
Also published as: GB8526053D0; NL8502863A; ZA859762B; AU4928185A; FR2577034A1; DE3503610A1; JPS61184301A

Abstract

Process heat is recovered from a furnace 10 and a product gas line by means of heat exchangers 9 and 21 respectively. These heat exchangers are in a circuit 7 in which sodium, which is liquid at low pressures and high temperatures, is circulated. In heating the sodium, process heat is recovered from the hot gas stream from the furnace 10 and from the product gas and is supplied to heaters 6 and 22 for heating the feed gas fed to a coal gasification, gas cracking or hydrocracking plant 2. For this purpose, the process air preheater 6, which heats charge gases, and the heater 22 which forms a process stream generator are traversed by the heated sodium, in order to achieve both a preheating of the process air and of the charge gases and also a superheating of the process steam up to high temperatures, without the heat necessary for this purpose having to be obtained from the combustion of a portion of the product gas. <IMAGE>

Description

SPECIFICATION Method and apparatus for recovering and making available process heat This invention relates to methods and apparatus for recovering and making available process heat, in which the process heat is transferred to a heat exchanger.

For many technical processes, such as coal gasification with air or oxygen with the addition of steam, gas conversion, for example the cracking of methane into a mixture of carbon monoxide and hydrogen, and also hydrocracking by thermochemical processes, or high-temperature electrolysis, it is necessary to preheat the feed substances, such as air, oxygen or other gases and steam, to high temperatures.

Thus, for example, the gasification of coal by an oxidation process, is endothermic and sub-stoichiometric combustion is involved.

This oxidation gasification can take place with air or oxygen and the addition of steam. The high temperatures required in gasification can be achieved either solely by partial combustion of the coal or by partial combustion of the coal and preheating of the air or oxygen necessary for the gasification and the added steam. A high preheat temperature of the air or oxygen and of the steam favour the reacion kinetics.

A high preheat temperature of this kind for air, oxygen, other gases and steam, can be obtained if a portion of the gas produced in the coal gasification is burnt, in order to heat air, oxygen or gas preheaters and steam superheaters. This portion of the gas produced is thus lost for other processes, so that the gasification plant must be designed not only for a specific quantity of product gas but additionally also for the production of the combustion gas necessary for the preheating.

Between the gasification plant and the preheating plant, lines for the preheated air or preheated oxygen respectively and the superheated steam are necessary. In these lines, pressure losses, heat losses and thermal expansions occur, which are dependent upon the flow rates and pressures and involve a high cost of material. Added to this is the space required for these lines.

Similar conditions exist, for example, with the gas conversion of methane into carbon monoxide and hydrogen, whereas in hydrocracking the necessary energy for the reaction must come from outside. A feature common to all three cases is that only the supply of heat at a high temperature makes the process easy or indeed possible.

The object of the present invention therefore is to provide a method and an apparatus for recovering and making available process heat, by which it is possible to convey heat at a high temperature level with low pressure and heat losses and without very high special equipment costs, the efficiency in the recovery and transmission of the process heat and of the method itself being high.

To this end, according to one aspect of this invention, a method as initially described is characterised in that heat is removed from the heat exchanger to make it available by circulating through the heat exchanger a heat carrier which is liquid at low pressures and high temperature.

The heat carrier liquid is preferably sodium which has a high thermal capacity and high thermal conductivity. The sodium can absorb heat in a closed circuit at high temperature from a source and convey large quantities of heat at low pressures in lines of small crosssection and give this heat up in heat exchangers to the feed substances. Since sodium becomes liquid at about 100"C and does not boil at ambient pressure until about 890 C, it is capable of transferring heat within this temperature range in a circuit which is unpressurized except for friction losses, without a phase change taking place. Furthermore, the saturation temperature of the sodium increases steeply with increasing pressure, so that it can also when liquid transmit heat at higher temperatures at comparatively low pressure (e.g. 1000'C at approximately 2.7 bar).Small pipe cross-sections and low pressures make the problems of thermal expansion, insulation and materials capable of solution when sodium is used, at conveying distances such as occur in the aforementioned processes on an industrial scale, for heat at a high temperature level. Sodium is therefore particularly useful at low pressures and high temperatures that is, for example, below 3 bar and above 800'C.

Preferably, the heating of air, oxygen or other gas preheaters and a steam superheater can be carried out by a process heat source such as the flue gas stream of a furnace or the helium gas stream in a helium gas circuit of a high-temperature nuclear reactor. If the flue gas stream of a furnace is chosen as the process heat source, then the advantage is obtained that through the heat transferred from this furnace by means of the sodium circuit into the coal gasification, gas conversion and hydrocracking plants, any fuels, even of low quality, can be used. If the heat is obtained from the helium gas stream of a high-temperature nuclear reactor, then nuclear heat is supplied to the processes.

If gasification is carried out with air, the product gas obtained achieves, with the high preheat temperature of the air and the corresponding superheating of the steam, a higher heat value, since the quantity of heat required for preheating the air and superheating the steam is not released by the sub-stoichiometric combustion in the gasification plant and consequently the nitrogen fraction of the air which would be necessary for this part of the combustion is not introduced into the gasification process.A transfer of the heat directly from the flue gas or from the helium to the gases and the steam cannot be carried out directly in gas generation and gas conversion plants, since the large volumetric flows would have to be conducted in large, hot, pressurised ducting systems, of which the thermal expansion, insulation and strength problems could not be solved and for which sufficient space would not be available.

Since, in coal gasification, gas conversion and hydrocracking, high temperatures occur in the product gases, the sensible heat contained in them can furthermore be utilized by product gas-sodium heat exchangers connected to the coal gasification, gas conversion and hydrocracking plants, and by process air preheaters and process steam generators and superheaters through which the heated sodium flows, and also by other gas preheaters.

It is also possible that, where processes are combined with one another, for example coal gasification with oxygen and steam and hydrocracking, process heat may also be exchanged between these processes via the sodium circuit.

Thus the invention also consists, according to another of its aspects, in apparatus comprising a steam generator having a furnace with a flue gas duct, or a nuclear reactor having a helium gas circuit, a process air preheater and/or a process steam superheater and/or other gas heaters, a heat exchanger in the flue gas duct or the helium gas circuit and a circuit for causing sodium to flow through the heat exchanger and through the preheater and/or the superheater and/or the other gas heaters.

It also consists, according to yet another of its aspects, in apparatus comprising a coal gasification plant or a gas conversion plant or a hydrocracking plant having a product gas outlet, a process steam generator and/or a process air preheater and/or other gas heaters, a heat exchanger in the product gas outlet and a circuit for causing sodium to flow through the heat exchanger and through the steam generator and/or the preheater and/or the other gas heaters.

In the furnace chamber of a steam generator, a sufficient quantity of heat at a high temperature level is available for preheating the air or oxygen and superheating the process steam, since here temperatures well above 1000cm exist. The flue gas-sodium heat exchanger can be located in the furnace chamber of the steam generator at a position favourable for the heat exchanger, whereas highly loaded parts of the furnace chamber of the steam generator are cooled with water as is usual and thus have considerably lower wall temperatures.

It is furthermore advantageous that no special quality requirements need to be imposed upon the fuels used in the furnace of the steam generator, so that it is possible to introduce into the coal gasification, via the sodium circuit, heat which originates from a fuel which quite possibly would otherwise be unsuitable or not well suited for coal gasification.

Added to this are the high thermal capacity of the sodium and its low flow resistance in the liquid state, which make sodium particularly suitable for heat transfer at a high temperature level over great distances, such as occur between a steam generator and a coal degasification or gasification plant. To carry out such a heat transfer by means of large gas streams would be prohibited by the difficulties resulting from the thermal expansion and insulation, on account of the pressure loss in the gas streams and on account of the cost of materials for the ducting and piping.

The high thermal conductivity of sodium, by contrast, and its heat transfer circuit operating without pressure or at a small pressure, make possible the construction of heat exchangers which give rise to only small pressure losses in the air, steam or gas streams which are to be heated.

Since high temperatures occur in the gasification of the coal and in gas conversion plant and hydrocracking plant, the sensible heat contained in the product gas from any of these plants can be utilized, according to this invention, by a product gas-sodium heat exchanger connected to the coal gasification plant or gas conversion plant or hydrocracking plant and also by a process steam generator or process air preheater through which the heated sodium flows.

An example of a method and of apparatus in accordance with the invention will now be described with reference to the accompanying drawing which is a circuit diagram of the apparatus.

The example of the apparatus shown is a coal gasification plant coupled to a steam power plant. The apparatus includes a crusher plant 1, in which coal dust is produced and is fed into a gasification reactor 2. The product gas is removed from the reactor 2 via a product gas line 3. The air required for the gasification is compressed by means of a compressor 4 into a process air line 5 and flows into the gasification reactor 2. In the process line 5, a process air preheater 6 is disposed and it is supplied with heat from a sodium circuit 7 having a sodium circulating pump 8. The steam necessary for the gasification is generated in a process steam generator 22, which is supplied with feedwater via a pump 23 and a line 24. If instead the process steam generator 22 is supplied with steam, then it acts as a process steam superheater. The sodium absorbs heat in a sodium heating heat exchanger 9 from the flue gas from a furnace 10 of a steam generator 11.

The steam produced in an evaporator 13 of the steam generator is conducted first through a superheater 14 and then passes into a steam turbine 16 having a high-pressure section and a low-pressure section. The steam turbine 16 drives an electric generator 12. Between the high-pressure and low-pressure sections of the steam turbine 13, a reheating superheater 15 is disposed. At the steam turbine 16, there are bleed points which serve for supplying steam for feedwater preheating in a feedheater 9. The steam issuing from the steam turbine 13 is condensed in a condenser 17, is compressed and delivered by a condensate pump 18 into the feedwater preheater 19 and is conducted by means of a feedwater pump 20 into the steam generator 11.

The sodium circuit 7 also absorbs heat contained in the product gas by means of a heat exchanger 21, in order to recover and make available this heat together with heat from the heat exchanger 9 for heating the air preheater 6 and the process steam generator 22. In the circuit diagram, the heat exchanger 9 in the steam generator 11 and the heat exchanger 21 in the product gas stream, are connected in parallel, as also are the air preheater 6 and the process steam generator 22.

Depending upon the quantities of heat produced and the temperatures involved, the heat exchangers may alternatively be connected in series.

Instead of the heat source, which is supplied in the furnace 10 by combustion of coal, a high-temperature nuclear reactor may be used, in which the heat is relesed by nuclear reactions and is transmitted via a helium circuit to the sodium heating heat exchanger 9.

The coal gasification plant 2 is, if gasification is carried out with oxygen instead of air, supplied with oxygen via the compressor 4 and the line 5, this oxygen being preheated in the preheater 6.

Instead of the coal gasification plant 2, a gas conversion or hydrocracking plant may be used. The coal feed from the crusher plant 1 is then omitted. Instead, methane is preheated in the case of the gas cracking plant and steam or other feed stuff in the case of the hydrocracking plant, up to high temperatures by the sodium circuit.

With the method and the apparatus in accordance with this invention it is possible to recover and make available heat at a very high temperature from coal burnt in normal furnaces or from nuclear fission in high-temperature reactors, in an economically and technically practical manner, and transport the heat, over the distances involved in full-size plants, into plants in which endothermic processes at high temperatures are carried out. These include coal gasification, gas conversion and hydrocracking. With the sodium circuit in the example of this invention, these processes can be carried out on a full industrial scale.

Claims

1. A method of recovering and making available process heat in which the process heat is transferred to a heat exchanger characterized in that heat is removed from the heat exchanger to make it available by circulating through the heat exchanger a heat carrier which is liquid at low pressures and high temperatures.

2. A method according to Claim 1, in which the heat carrier liquid is sodium.

3. A method according to Claim 1 or Claim 2, in which the process heat is removed by the heat exchanger from the flue gas stream of a furnace.

4. A method according to Claim 1 or Claim 2, in which the process heat is removed by the heat exchanger from a helium gas circuit of a high-temperature nuclear reactor.

5. A method according to Claim 1 or Claim 2, in which the process heat is removed by the heat exchanger from the product gas stream of a coal gasification plant.

6. A method according to Claim 1 or Claim 2, in which the process heat is removed by the heat exchanger from the product gas stream of a gas conversion plant.

7. A method according to Claim 1 or Claim 2, in which the process heat is removed by the heat exchanger from the product gas stream of a hydrocracking plant.

8. Apparatus for carrying out the method in accordance with Claim 3 or Claim 4, comprising a steam generator having a furnace with a flue gas duct, or a nuclear reactor having a helium gas circuit, a process air preheater and/or a process steam superheater and/or other gas heaters, a heat exchanger in the flue gas duct or the helium gas circuit and a circuit for causing sodium to flow through the heat exchanger and through the preheater and/or the superheater and/or the other gas heaters.

9. Apparatus for carrying out the method in accordance with any one of Claims 5, 6 and 7, comprising a coal gasification plant or a gas conversion plant or a hydrocracking plant having a product gas outlet, a process steam generator and/or a process air preheater and/or other gas heaters, a heat exchanger in the product gas outlet and a circuit for causing sodium to flow through the heat exchanger and through the steam generator and/or the preheater and/or the other gas heaters.

10. A method according to Claim 1, substantially as described with reference to the accompanying drawing.

11. Apparatus according to Claim 8 or Claim 9, substantially as described with reference to the accompanying drawing.