WO2001037984A1

WO2001037984A1 - Ceramic insulation in reactor for gasification of residual products obtained from pulp production

Info

Publication number: WO2001037984A1
Application number: PCT/SE2000/002302
Authority: WO
Inventors: Bengt Nilsson
Original assignee: Kvaerner Chemrec Ab
Priority date: 1999-11-26
Filing date: 2000-11-23
Publication date: 2001-05-31
Also published as: SE9904284D0; SE9904284L

Abstract

The present invention relates to an improved lining for a reactor for the sub-stoichiometric gasification of waste liquor or black liquor formed in the manufacture of chemical pulp. The lining is divided into at least one ceramic wear layer (4) having a low porosity, a high density and a high thermal conductivity, which is mounted internally to the reaction chamber, and an outer ceramic backing layer (3) that has a lower density and a lower thermal conductivity. The first and second ceramic layers (4 and 3) have suitable thermal conductivities and thicknesses to ensure that the condensation or solidification point (SP) for the resulting gasification products always lies inside the second layer, irrespective of the effective degree of wear and tear of the first layer. The invention also relates to a process for maintaining the desired temperature profile in the lining.

Description

CERAMIC INSULATION IN REACTOR FOR

GASIFICATION OF RESIDUAL

PRODUCTS OBTAINED FROM PULP PRODUCTION

The present invention relates to the lining of a gasifying reactor according to Claim 1. The invention also relates to a process for maintaining the required temperature profile in the lining.

Prior art

Black liquor and other residual chemicals formed in the manufacture of paper pulp are gasified in reactors with a ceramic lining that must be resistant both to the gasification products that are formed and to the high temperatures that prevail in the gasification process, which are typically 900-1200°C. This ceramic lining is formed in the conventional way, using one or more layers of refractory ceramic bricks. It is then closed off with an insulating layer inside the outer steel wall of the reactor. This ceramic lining gradually wears off during use, and, when its wear and tear has reached a predetermined degree, the ceramic lining must be replaced together with the insulating layer. The time for the replacement of the ceramic lining is normally determined on the basis of previous experience. Alternatively, devices that indicate the wear and tear and show the temperature can be installed on the outside of the steel wall, possibly integrated with the ceramic part, this wear-indicating device preferably recording the temperature.

A problem here is that the ceramic lining is subject to the penetration and diffusion of the gasification products formed in the reactor, and these alkaline compounds condense out and/or solidify when they reach the corresponding condensation and/or solidification temperature. The ceramic lining is normally dimensioned without taking into account the area where these substances condense or solidify. Operating experience shows that, when these diffusing and penetrating gasification products condense out or solidify inside the insulating layer that is fitted externally to the ceramics, the said layer loses its insulating effect, and a bursting action is exerted towards both the steel wall and the ceramic material. At this point, the whole ceramic lining, including the insulating layer, must be replaced, and in the worse case even the surrounding pressure vessel has to be replaced. It is therefore desirable according to the present invention to control the position of the condensation or solidification area in such a way that these processes do not take place in the insulating layer.

Another problem is mainly connected with the fact that the reactor is located in the open air. In such a case, it may be difficult to ensure that the condensation and solidification take place in the required area, since this area varies with the temperature and - owing to the outside location of the reactor - the seasons and wind conditions exert an influence, as also does the nature of the insulating layer. It has also been found that, even when the reactor is not in operation, the hydration of the hygroscopic alkaline compounds that takes place in the lining may cause a problem, depending on the moisture, humidity and temperature. These hygroscopic alkaline compounds, which are formed during the gasification process and which diffuse or penetrate into the ceramic lining, can bind water as water of crystallization, which in turn causes a large increase in the volume, leading to the bursting of the material.

European Patent No. 260,867 discloses a furnace for the heat treatment of materials and objects, i.e. it is not really intended for gasification purposes. This means that the patent is not relevant to the present task connected with the condensation and solidification of gasification products.

Instead, the patent mainly relates to the problem of mounting the lining material. It mentions that the outer layer or layers need not be of the same refractory material as the inside layer that faces the hearth of the furnace, since it is this inside layer that experiences the highest temperatures.

French Patent No. 2,389,826 discloses a lining for use at temperatures of over 1000°C, the purpose of which is to protect the suspension attachments of the lining from the heat. The solution is a modular structure in which an inside layer with a high density protects the suspension attachments, which lie in an outer layer having a lower density. This patent does not mention the condensation and solidification problem in connection with a gasification process and is therefore not relevant to the task set for the present invention.

European Patent No. 434,421 discloses a lining for furnaces intended for metallurgical purposes. This lining is fitted with a protective layer that reacts with molten metals, so that the surface of the protective layer that faces the molten metal becomes hard and heat-resistant, while its surface that faces the lining becomes brittle. As a result, the protective layer can be easily chiselled off when it is ready to be replaced. This patent does not mention the condensation and solidification problem in connection with a gasification process and is therefore not relevant to the task set for the present invention. Furthermore, Fig. 2 in that patent shows that the layer facing the furnace [interior] has a thermal conductivity that is lower than that of the material that forms a layer that is external to it.

German Patent 3,908,206 discloses a lining for use in an installation operated at the very high temperatures of over 1700°C or even over 1900°C. However, this patent does not mention anything about the condensation and solidification problem in connection with a gasification process, so it is not relevant to the task set for the present invention. Instead, its purpose is to prevent the formation of folds or bumps in the inside layer at such high temperatures. The solution, which can be seen from the temperature profile shown in Fig. 1 in the said patent, is that the layer facing the furnace [interior] has a lower thermal conductivity than the material that forms a layer that is external to it.

Brief description of the invention

One of the aims of the present invention is to provide an optimum reactor lining for the sub-stoichiometric gasification of residual products formed in the manufacture of chemical pulp, where the refractory ceramic lining consists of at least two layers, namely an inside "wear lining" or "wear layer", and an outer "backing lining" or "backing layer", as seen from the centre of the reactor.

Another aim of the invention is to reduce the cost of replacing the ceramic lining, since normally only the inside or wear lining needs to be replaced here after a predetermined part of it has worn off.

Another aim of the invention is to choose the material and the dimensions of the refractory ceramic lining according to the gasification process in question, so that the condensation and solidification of the substances present take place in a predetermined area inside the backing layer, while the wear lining is gradually being worn away during the operation of the reactor, reaching a level of wear and tear of about 60%, calculated on the original thickness of the wear lining.

Another aim of the present invention is to ensure that the ceramic lining can be adapted in such a way that the inside or wear lining facing the reaction chamber is made of a denser ceramic material into which the substances in question are less able to diffuse and penetrate, and which preferably has a greater resistance to high temperatures, while the outer or backing layer is made of a material that has a lower density and a lower thermal conductivity. Another aim of the present invention in the case of a preferred embodiment is to ensure that the construction of the lining permits the maintenance of a constant temperature in the outer, backing layer, so that the condensation and solidification of the alkaline compounds take place in the right area, even if the reactor is located out of doors or is subject to fluctuations in the outside temperature for another reason. At the same time, its construction should also make it possible to maintain a constant temperature in the outer, backing layer even when the reactor is not in operation, in order to prevent the hydration of the hygroscopic alkaline compounds in the lining.

Brief description of the illustrations

The invention is described below with reference to the illustrations, where:

Fig. 1 is a cross section showing the first embodiment, which is a ceramic lining according to the invention, placed in a reactor for the sub- stoichiometric gasification of the residual products formed in the manufacture of chemical pulp,

Fig. 2 shows the temperature profile across the ceramic lining illustrated in Fig. 1, and

Fig. 3 shows another embodiment of the invention in the form of a ceramic lining in a reactor for the sub- stoichiometric gasification of the residual products formed in the manufacture of chemical pulp, and it also shows the temperature profile across the lining.

Detailed description

Fig. 1 shows a preferred embodiment of the invention in the form of a ceramic lining for a reactor for the sub- stoichiometric gasification of the residual products formed in the manufacture of chemical pulp. A first, inside refractory ceramic layer 4 faces the reaction chamber 5 which is the site of the gasification of the residual products containing sodium compounds and/or sulphur compounds, preferably in the form of black liquor. At a reaction temperature in the range of 700-1400°C and preferably about 1000°C, the gasification in the reactor gives rise both to a molten material mainly composed of NaA, Na_:C0₃ and NaOH, and to combustion gases mainly consisting of CO, H₂, HS, CH₄ and CO_: .

This inside ceramic layer 4 is a "wear layer" that is gradually consumed during the operation of the reactor, its main function being to withstand the high reaction temperatures and the chemical effects exerted e.g. by the diffusion and penetration of the substances present. This inside ceramic wear layer 4 should therefore have the highest possible density, i.e. a high density δ_α and so a relatively high thermal conductivity ki . For example, the wear layer may consist of a ceramic material that is obtained by fusion casting and has a density of δi = 3540 kg/m³ and a thermal conductivity of k_x = 4.9 W/m°K.

A second ceramic layer, the backing lining 3, is fitted externally to the inside, wear layer 4. This layer 3 is instead optimized with a lower thermal conductivity k₂, which can be achieved with the aid of a preferably refractory ceramic material having a lower density δ₂ than the first or wear layer 4. It is a characteristic of the invention that the second or backing layer 3 consequently has a lower density than the wear layer 4, i . e . δ₂ < δi, and the thermal conductivity of the second or backing layer 3 is lower than that of the wear layer 4, i.e. k₂ < ki . For example, the backing layer may consist of a ceramic material that is obtained by fusion casting and has a density of δ₂ = 3260 kg/irr and a thermal conductivity of A = 1.7 W/m°K.

An insulating layer 2, preferably consisting of mineral wool, is fitted externally to the backing layer, after which the construction is closed off with a steel wall 1 and possibly with an insulating material that is external to the steel wall and which is not shown.

The sub-stoichiometric gasification process in question is used for the gasification of the residual products that are formed in the manufacture of chemical pulp and which comprise alkaline compounds and sulphur-containing compounds, preferably in the form of waste liquor or black liquor. At a reaction temperature of about 1000°C, the gasification in the reactor gives rise to the formation of a molten product mostly consisting of Na₂S, Na₂C0₅ and NaOH, and to the formation of combustion gases mainly consisting of CO, H₂, H₂S, CH and C0₂, in accordance with the following equilibrium reactions given here as examples:

CH₄ + H₂0 CO + 3H₂

Na₂C0₃ + H₂S <=> Na₂S + C0₂ + H₂0

2NaOH + C0₂ Na₂C0₃ + H₂0.

The temperature region for the condensation and solidification of the alkaline compounds is normally between 300 and 650°C in the present case. This solidification or condensation temperature depends to a large extent on the composition of the combustion gases and on the ratio between Na₂S, Na₂C0₃ and NaOH in the molten material in question.

In the gasification of black liquor, the reaction equilibria are displaced, e.g. because of the high temperatures and the high partial vapour pressures, and the formation of for example NaOH is favoured. The vapour phase of the alkaline compounds is maintained at a temperature of well over 700°C, and increases noticeably as the temperature rises further. This vapour phase of the alkaline compounds mainly consists of sodium hydroxide (NaOH) and potassium hydroxide (KOH) , together with elementary sodium (Na) and potassium (K) . The water in the black liquor evaporates in the upper part of the gasification reactor, where the burners are situated. This evaporation shifts the equilibrium to the left, with an increased formation of NaOH, according to the following equilibrium reaction:

2NaOH + C0₂ <=> Na₂C0₃ + H₂0.

As a result, the condensation or solidification temperature is further lowered in the reactor, at least locally. The combined effect is that the solidification or condensation temperature is usually comprised in the range 300-650°C.

Fig. 2 shows, as an example, the temperature profile across a reactor lining illustrated in Fig. 1 according to the invention, both in the case of an intact wear layer and in the case when the thickness of the wear layer has been reduced by 50%. This illustrates the displacement obtained here in the solidification point SP. This is given in the figure as the distance (depth) from the inside surface of the backing layer that lies against the wear layer to the position of the effective solidification temperature ST. The lower, dashed curve B shows the temperature profile during the operation of the reactor with an intact wear layer 4 having a thickness of ti . The inside or wear layer 4, which has a thickness of A and a higher density δi, reduces the temperature from the reaction temperature of 1000°C to a level around 850°C at the outer surface of the wear layer 4, i.e. the surface facing the second ceramic layer 3. The temperature then falls further as the heat traverses this second ceramic layer 3, which has a thickness of t₂, and specifically from 850°C to about 425°C. Finally, the temperature drops as the heat traverses the insulating layer 2, and specifically from about 425°C to about 150°C, which is basically the outer temperature of the steel wall 1. The solidification temperature ST lies here at about 625°C, depending on the conditions prevailing in the reactor. As a result, the solidification point SP lies in the middle of the second ceramic layer 3 when the wear layer 4 is intact . After operating the reactor for a certain time, the thickness of the wear layer is reduced to 50% owing to wear and tear, i.e. to 0.5 ti, which gives a temperature profile across the reactor lining that is indicated by the dotted line (curve A) . In this case, the temperature in the inside or wear layer 4 is reduced from the reaction temperature of 1000°C to about 925°C at the outer surface of the wear layer 4, i.e. the surface facing the second ceramic layer 3 (transition between A and t₂) . The temperature then drops further from 925°C to about 500°C as the heat traverses the second ceramic layer 3, which retains its thickness of t₂. The solidification point SP is shifted by the distance dSP as the wear layer 4 is being consumed. As a result, the solidification point SP comes to lie at a distance (depth) corresponding to 25-30% of the original thickness of the second ceramic layer 3, taken from the outer surface of this layer facing the insulating layer. Finally, the temperature falls during the passage of the heat through the insulating layer 2, and specifically from 500°C to about 220°C, which basically corresponds to the outside temperature of the steel wall 1 as well.

The thicknesses t₁ and t₂ of these layers and their materials are so chosen that the area where the alkaline compounds condense and solidify is always inside the second ceramic layer 3 if the wear layer 4 is replaced at regular intervals well before 60% and preferably 50% of its original thickness A is worn away, provided that the black liquor is gasified under the conditions that correspond to the dimensioning parameters.

In the construction illustrated, the second ceramic layer 3 has the same thickness as the wear layer 4, i.e. A = t₂, with a value suitably about 150 mm, while the insulating layer 2, consisting of mineral wool, has a thickness that is about one-tenth of that of the second ceramic layer or that of the wear layer. The thickness of the insulating layer can be between one-fifth and one-fiftieth, preferably between one- tenth and one-fortieth and more especially about one-twenty- fifth of the combined thickness of the ceramic layers. Instead of mineral wool, it is also possible to use insulating bricks, in which case a greater thickness is needed, because of the lower insulating ability of this material.

The ceramic lining of the reactor is therefore adapted to suit the prevailing operating conditions. Since both molten materials and combustion gases are formed in the gasification of black liquor, the ceramic insulation is subject to both penetration and diffusion processes. It is an important design feature of the ceramic lining that the wear layer has a very low porosity and a high stability to the gasification conditions, including thermal shock. The second ceramic layer has a lower thermal conductivity and can withstand the resulting substances as they condense or solidify. As the wear layer is being consumed, the area where condensation or solidification takes place is gradually displaced towards the colder, outer surface of the second ceramic layer, but it never ends up outside this ceramic layer, i.e. in the insulating layer.

The whole ceramic lining must be so constructed that the solidification and condensation area lies inside the second ceramic layer, for the mineral-wool insulating layer should be prevented from absorbing the condensable or solidifiable alkaline compounds that reduce its insulating ability, affect the steel wall, and can also lead to an inward-directed bursting effect as more material builds up in it.

Fig. 3 shows a second preferred embodiment of the ceramic lining according to the invention for a reactor for the sub- stoichiometric gasification of the residual products formed in the manufacture of chemical pulp. Fig. 3 also shows the resulting temperature profile across this lining.

The reactor lining shown in Fig. 3 is provided with a space 7 between the ceramic lining (which consists of two ceramic layers 3 and 4) and the insulating layer 2. In this space or gap 7 a basically constant temperature is ensured by a nest of pipes consisting of a pipe coil 8 mounted in such a way that it runs around the ceramic lining. This pipe coil is fed with feed water or condensate used for steam production. The nest of pipes can be lagged with a fibrous insulating material or it may be embedded in a graphitic compound or the like, used as a filler.

This pipe coil 8 suitably extends along the whole length/height of the reactor, with the necessary number of turns. For example, when the reactor is located outdoors, it is possible to generate in this pipe coil 8 high-pressure steam e.g. at a pressure of 64 bar and a temperature of 281°C. The pipe coil 8 is fed with feed water or condensate, suitably from the base upwards, with the aid of a feeding device (pump), which is not shown. Owing to the "heat leakage" from the gasification reaction, the condensate traverses the vaporization (steam-formation) temperature at the prevailing pressure as it flows in the pipe around the reactor lining. As a result, steam flows out of the pipe coil 8 and it can be used in the pulp-making process or in a similar operation. Since the vaporization (steam-formation) temperature is traversed in the pipe coil, it is possible to keep the temperature in the space 7 basically constant. This in turn makes it possible to ensure the required temperature profile across the inside or wear layer 4 and across the second ceramic layer 3. The condensation or solidification point SP can therefore be localized in the required position inside the second ceramic layer 3. An advantage here is that steam is produced for another purpose at the same time.

An outside insulating layer 9 is suitably arranged outside the wall 1 of the reactor.

The reactor according to the embodiment shown in Fig. 3 also has the advantage that, when it is not in operation, a high, preferably constant temperature can be maintained in the outer part of the lining, and so one can prevent the problem presented by the hydration of the hygroscopic alkaline compounds in the lining, which may otherwise happen as the temperature falls in the reactor. In this case, it is convenient to reverse the flow in the pipe coil in the sense that the latter is fed with steam, e.g. high-pressure steam at 64 bar and a temperature of 281 °C, from the high-pressure steam circuit of the factory (not shown) . As it flows through the pipe coil 8, this steam fully or partly condenses out, which makes it possible to keep the temperature in the lining basically constant, although the reactor is not in operation.

It will be realized that the pressure of 64 bar mentioned above and the corresponding vaporization (steam-formation) temperature of 281 °C are merely examples, and the best temperature is found in each case by process optimization, which can easily be carried out by the expert in the field. It will also be realized that the pipe coil design can also be used for the reactor with only one layer of refractory ceramic material. The design can be further modified by dividing up the pipe coil 8 into a number of pipe coils arranged around the reactor. This permits the production of steam with different quality ratings, and the cooling of the reactor lining can therefore be varied along the reactor wall. The thickness of the ceramic layers in the lining can also be varied along the reactor wall for the same reason. If for example an extra thick wear layer is used, it can be consumed to a greater extent than the specified 50-60% before it is replaced.

The invention can be modified in different ways without violating the scope of the claims. For example, the second ceramic layer or the wear layer can in turn be divided into two or more ceramic layers. The layers can be rearranged conventionally so that the joints between the stones in the respective layers only intersect the joints in the adjacent layer lying externally or internally to it, in which case the penetration and diffusion of the substances through the joints can be hindered or prevented.

The present invention also covers the mode of operation without the presence of sulphur-containing compounds, where Na_S is missing from the molten material, and HS is missing from the combustion gases. In this mode of operation, the conversion can be either a sub-stoichiometric combustion or a super-stoichiometric combustion. Furthermore, the invention also covers the mode of operation where the alkaline compounds are basically only potassium compounds.

Claims

1. Lining for a reactor for gasification and preferably the sub-stoichiometric gasification of the residual product formed in paper pulp manufacture by delignifying fibrous raw materials, which residual product comprises alkaline and/or sulphur-containing compounds, preferably in the form of waste liquor and/or black liquor, where the gasification in the reactor results in the formation of both a solid and/or molten material and a gaseous phase at a reaction temperature in the range 700-1400°C and preferably 900-1100°C and more especially 950-1000°C, characterized in that the lining comprises the following parts:

- a first, inside ceramic wear layer (4) that faces the reaction chamber (5), has a very low porosity, a density (δ_α) and a thermal conductivity (ki)

- at least one second ceramic layer (3) that has a density (δ₂) and a heat conductivity (k₂) and is arranged externally to the first ceramic wear layer (4)

- an insulating layer (2) that is arranged externally to the ceramic layers (3, 4) and has a thermal conductivity (k₃) that is lower than the thermal conductivities (ki, k₂) of the ceramic layers (3, 4) lying internally to it, and

- an outside protective casing (1), where the thermal conductivity (ki) of the first, wear layer (4) is higher than that of the thermal conductivity (k₂) of the second ceramic layer (3), and where the first ceramic wear layer (4) and the second ceramic layer (3) have thicknesses (ti, t₂) that make it possible to ensure that the condensation or solidification point (SP) of the said prevailing gasification products always lies inside the second ceramic layer (3) during the wearing process of the first, wear layer (4) from the intact state to one in which up to about 60% of it has worn away.

2. Reactor lining according to Claim 1, characterized in that the density (δi) of the first ceramic wear layer (4) is higher than the density (δ ) of the second ceramic layer (3) .

3. Reactor lining according to Claim 1 or 2, characterized in that the said thicknesses (A, t₂) and the said thermal conductivities (ki, k₂) of the wear layer (4) and the second ceramic layer (3) respectively are chosen in such a way that the point (SP) where the gasification products condense or solidify is situated inside - and at about the mid-point of - the second ceramic layer (3) when the first, wear layer (4) is still intact, but it is located in an area that lies further outward from this mid-point when the first or wear layer (4) has worn away to an extent of about 60%.

4. Reactor lining according to Claim 3, characterized in that, when the first layer (4) has been worn away to an extent corresponding to about 60% of its thickness, the condensation or solidification point (SP) for the gasification products lies at a distance from the outer surface of the second ceramic layer (3) which distance corresponds to at least about 20% of the original thickness (t₂) of the second layer.

5. Reactor lining according to any one of the preceding claims, characterized in that the lining also comprises a space (7) between the said second ceramic layer (3) and the said insulating layer (2), which space (7) accommodates a pipe coil (8) that is so arranged that it is connected to a feeding device for preheated feed water and/or steam (and preferably high-pressure steam) , depending on the mode of operation.

6. Reactor lining according to any of the preceding claims, characterized in that only the wear layer (4) is destined to be replaced after experiencing wear and tear, while the second ceramic layer (3) and possibly the pipe coil (8) and the insulating layer (2) are destined to remain in place after the wear layer has been replaced.

7. Reactor lining according to any one of the preceding claims, characterized in that the thickness of the insulating layer (2) is substantially smaller than the combined thickness of the ceramic layers (3, 4), being in the range of 1/5 - 1/50, preferably 1/10 - 1/40 and more especially about 1/25 of that, and in that the insulating layer (2) is made of some form of mineral wool.

8. Process for achieving the desired temperature profile in a lining of a reactor used for gasification, and preferably for the sub-stoichiometric gasification of the residual product formed in the manufacture of paper pulp by delignifying fibrous raw materials, which residual product comprises alkaline and/or sulphur-containing compounds, preferably in the form of waste liquor and/or black liquor, which gasification results in the formation of both solid and/or molten material and a gaseous phase at a reaction temperature in the range 700-1400°C, preferably 900-1100°C and more especially 950-1000°C, characterized in that:

- it has a first, inside wear layer (4) that faces the reaction chamber (5) and is made of a first ceramic material with a very low porosity and with a density (δi) and a thermal conductivity (ki)

- it has a second layer (3) that is arranged externally to the first ceramic wear layer (4) and is made of a second ceramic material with a density (δ₂) and a thermal conductivity (k₂)

- it has an insulating layer (2) that is arranged externally to the ceramic layers (3, 4) and is made of an insulating material that has a thermal conductivity (k₃) which is lower than the thermal conductivities (k , k₂) of the ceramic layers (3, 4) arranged internally to it, and

- it also has an outside protective casing (1), where the said first material used for the wear layer (4) has a thermal conductivity (ki) that is higher than the thermal conductivity (k₂) of the second ceramic layer (3), and where the first ceramic wear layer (4) and the second ceramic layer (3) have thicknesses (A, t₂) that make it possible to ensure that the condensation or solidification point (SP) for the said combustion products always lies inside the second ceramic layer (3) during the wearing process of the first or wear layer (4) from an intact state to one in which up to about 60% of it has been worn away.

9. Process according to Claim 8, characterized in that the lining is also provided with a space (7) between the said second ceramic layer (3) and the said insulating layer (2), which space (7) accommodates a pipe coil (8) that is connected either to a feeding device for preheated feed water and/or to a source of steam (and preferably high-pressure steam), depending on the mode of operation.

10. Process according to Claim 9, characterized in that, when the reactor is in operation, the said pipe coil (8) is connected to the said feeding device for preheated feed water, and the flowing material is allowed to take up heat from the lining and so pass through the vaporizing (steam-formation) temperature as it flows through the pipe coil (8) around the reactor, so that steam is generated, and preferably high- pressure steam, whereas, when the reactor is not in operation, the said pipe coil is connected to a source of steam and preferably high-temperature steam, which steam wholly or partly condenses out, and so gives off its heat to the lining as it flows in the pipe coil (8) around the reactor.