GAS COMPRESSION IN HYDROGEN-BASED DIRECT REDUCTION

Description

field of technology

[0001] The application relates to a direct reduction plant comprising a catalytic reformer and / or a gas furnace and a gas compression plant with one or more compressors, wherein the gas compression plant comprises at least one compression stage and wherein at least one gas cooler for compressed gas is present. It also relates to a method for operating a direct reduction plant comprising a catalytic reformer and / or a gas furnace and a gas compression plant with one or more compressors, wherein the gas compression plant comprises at least one compression stage and wherein at least one gas cooler for compressed gas is present, wherein after gas compression, reduction gas is introduced into a reduction unit. State of the art

[0002] It is known to reduce metal oxide-containing ore, for example iron oxide-containing ore, by means of direct reduction in a reduction unit, such as a reduction shaft, using reducing gas. In conventional processes currently used on a large industrial scale, the reducing gas is predominantly based on natural gas. This produces a large amount of carbon dioxide CO2, which is undesirable for environmental reasons, among other things. To reduce CO2 emissions during direct reduction, it is known to use hydrogen as a reducing gas. Hydrogen can be used as the sole reducing gas or in combination with other gases, such as natural gas-based reducing gases. The greater the proportion of CO2-neutral hydrogen H2 in the reducing gas, the less CO2 is emitted. The documents CA 3 129 493 A1, US 3 748 120 and CN 107 058 664 A refer to gas compression in hydrogen-based direct reduction.

[0003] However, the currently economically available amount of hydrogen is small, making continuous operation with hydrogen H2 alone as the reducing gas almost impossible. Accordingly, the main focus is on using hydrogen H2, at least temporarily, in conjunction with other gases, such as natural gas-based reducing gases. In principle, it is therefore desirable

[0004] Direct reduction plants should be designed and operated so that they can operate with both natural gas-based reducing gas and hydrogen, as well as mixtures of natural gas-based reducing gas and hydrogen. Production can then be carried out in different ways depending on the availability of the various reducing gases.

[0005] When operating direct reduction plants, the reducing gas or its precursors must be compressed before it enters the reduction unit to overcome the plant's pressure drop. During compression, energy is introduced into the gas to be compressed, which also increases its temperature – consequently placing thermal stress on compressors. Compressors are typically cooled by injecting water into the gas to be compressed or into the compressor.

[0006] Compared to other reducing gases, hydrogen has a significantly lower density, a low molecular weight, and a high speed of sound. This means that compressors designed to economically compress hydrogen have different requirements than compressors designed to process denser gases with higher molecular weights and lower speeds of sound—for example, natural gas-based reducing gases, where the molecular weight also lies within a relatively narrow range. This must be taken into account when operating direct reduction plants in such a way that they can work with both natural gas-based reducing gas and hydrogen, as well as mixtures of natural gas-based reducing gas and hydrogen.If the compressors suitable for the economical compression of hydrogen are also to be able to compress other gases economically and therefore have to cover a wide molecular weight range - this is the case, for example, when gas mixtures of hydrogen with other gases are used. -, These different requirements must be taken into account. Accordingly, the gas compression system of existing direct reduction plants needs to be adapted as the proportion of hydrogen in gas mixtures with other gases, such as natural gas-based reducing gases, increases.

[0007] Furthermore, when using hydrogen, the direct reduction of oxide ores / pellets is endothermic. To cover the thermodynamically necessary heat demand for economical direct reduction with high productivity and to ensure temperature conditions in the reduction shaft, a higher specific gas quantity of reducing gas than required for reduction should be introduced into the reduction unit as a heat energy carrier during hydrogen operation to compensate for cooling due to the endothermic reactions.

[0008] This higher amount of reducing gas, which may consist largely or entirely of hydrogen, can be recycled after passing through the reduction unit, possibly with heating.

[0009] Therefore, when using hydrogen, larger volumes of reducing gas must be compressed compared to conventional operation with exothermic direct reduction using other gases, such as natural gas-based reducing gases. Accordingly, the gas compression system of existing direct reduction plants needs to be adapted as the proportion of hydrogen in gas mixtures with other gases, such as natural gas-based reducing gases, increases.

[0010] Furthermore, even when hydrogen is planned as the sole reducing gas, there are operating situations, such as the start-up and warm-up of a direct reduction plant, in which not hydrogen but another gas—for example, nitrogen or other inert gases—with a significantly higher molecular weight than hydrogen is to be pumped. Therefore, compressors designed to be suitable for the economical compression of such gases must meet different requirements than compressors designed to process hydrogen.

[0011] Overall, the problem arises that, due to the different gases to be compressed, gas compression should be economically possible over a wide range of gas densities, molecular weights and speeds of sound, whereby, where appropriate, existing gas compression plants previously used for natural gas-based reducing gas should continue to be usable at least in part.

[0012] With the increasing importance of reduction with hydrogen, the presence of water vapor in the reducing gas has an increasingly detrimental effect, as it reduces the reduction potential of the reducing gas. This problem is exacerbated when a portion of the top gas taken from the reduction unit is recirculated. This problem is exacerbated by the increasing water vapor content of the top gas with increasing hydrogen content – ​​and thus lower natural gas usage – in the reducing gas. It is also exacerbated by the fact that water is injected into the gas to be compressed to cool the compressors. The required cooling capacity of the top gas scrubber increases due to the higher gas volume and higher water vapor content. And the ability to adjust the reducing gas quality, i.e. the ratio of reductants to oxidants (CO+H2) / (CO2+H2O) and H2 / H2O, must be adjusted to control the reforming of natural gas with steam (CH4+H2O → CO+3H2), the reduction, and the product quality. Summary of the invention Technical task

[0013] It is the object of the present invention to provide a solution to at least some of the aforementioned problems. Technical solution

[0014] The object is achieved by a direct reduction plant according to the appended claims 1-6

[0015] One or more gas coolers are present, for example if there are several compression stages there is a gas cooler behind each compression stage.

[0016] The gas cooler condenses water vapor from the compressed gas. Accordingly, the gas cooler allows the water vapor content of the reducing gas to be reduced and / or the reducing gas quality to be adjusted.

[0017] At least one of the compressors is provided with a bypass for recirculating at least a portion of the gas compressed by the compressor into a suction-side gas inlet of the respective compressor. The bypass branches off from the gas outlet downstream of the compressor, as seen in the gas flow direction, and opens into the gas inlet upstream of the compressor, i.e., on the suction side. The compressor downstream of the branch and upstream of the outlet is the "relevant compressor."

[0018] The direct reduction plant also includes at least one reduction unit. It is used for the direct reduction of metal oxide-containing ore, for example, iron oxide-containing ore. The reduction unit can, for example, comprise a reduction shaft, for example, for fixed-bed operation, or a fluidized-bed reactor. The term "ore" also includes metal oxide-containing feedstocks obtained by ore processing, such as pellets, lump ore, fine ore, sinter, oxide briquettes, and remet.

[0019] The flow direction of the gas - also called gas flow direction - is in the direction of the reduction unit, since ultimately the reduction unit of the direct reduction plant is to be supplied with reducing gas.

[0020] According to one variant, the direct reduction plant comprises a catalytic reformer for the production of reducing gas or reducing gas precursor gas, from which reducing gas is produced by carrying out further measures such as admixing additional gases or heating. -; The reducing gas is introduced into the reduction unit to carry out the direct reduction reactions for the production of sponge iron.

[0021] According to another variant, the direct reduction plant includes a gas furnace; in this case, the gas heated in the gas furnace is reducing gas or reducing gas precursor gas. A gas furnace is a device for heating gas, for example, process gas, using hot flue gas from the combustion of top gas or natural gas, or using electric heating.

[0022] The direct reduction plant may also include a catalytic reformer and a gas furnace. The gas furnace is preferably located upstream of the catalytic reformer in the gas flow direction.

[0023] According to the invention, gas emerging from the gas compression plant or from the gas cooler for compressed gas—i.e., compressed gas—is fed directly into the catalytic reformer for reforming to produce reducing gas or reducing gas precursor gas, and / or into the gas furnace. Direct introduction means that the gas is introduced without reducing the amount of CO2 by CO2 removal; thus, the introduction occurs while maintaining the amount of CO2. Direct introduction therefore dispenses with measures to reduce the amount of CO2 in the compressed gas. Direct introduction is carried out without devices for removing CO2 from the compressed gas. If necessary, direct introduction also involves a temperature increase, for example, in a gas-gas heat exchanger; in this case, direct introduction is carried out with devices for temperature increase.In the case of direct discharge, there is no CO2 removal - for example by means of chemical scrubbing - such as MEA Monoethanolamine, KM CDR Kansai Mitsubishi carbon dioxide recovery. -, VPSA vacuum pressure swing adsorption or PSA pressure swing adsorption, so it is initiated without CO2 removal.

[0024] Direct introduction takes place via the direct introduction from the gas compression plant or the gas cooler to introduce gas directly into the reformer and / or the gas furnace.

[0025] If the gas cooler is located downstream of the last compression stage in the direction of gas flow, the direct discharge originates from the gas cooler; if the gas cooler is not located downstream of the last compression stage in the direction of gas flow, the direct discharge originates from the last compression stage in the direction of gas flow. If the gas cooler is located downstream of the last compression stage in the direction of gas flow, it is not considered part of the gas compression system; if the gas cooler is not located downstream of the last compression stage in the direction of gas flow, it is considered part of the gas compression system.

[0026] In any case, compressed gas is introduced directly into the reformer and / or the gas furnace by means of direct injection. Advantageous effects of the invention

[0027] The gas compression system comprises one or more compressors. A compressor has a gas inlet – for gas to be compressed – and a gas outlet – for compressed gas. A bypass allows a gas flow to be redirected past the compressor to the suction side. According to the invention, at least one of the compressors is provided with a bypass connecting its gas inlet and gas outlet for recirculating at least a portion of the gas compressed by the compressor. The bypass branches off from the gas outlet downstream of the compressor, viewed in the gas flow direction, and opens into the gas inlet upstream of the compressor, i.e., on the suction side.

[0028] This bypass allows for precise control of the reduction gas flow rate if the desired reduction gas flow rate does not match the compressor flow rate. In the case of speed control of one or more compressors, it may occur that the minimum speed for the compressor cannot be undercut—for example, 30% of the nominal speed—for reasons of compressor and / or engine lubrication or compressor and / or engine cooling.If a compressor is fundamentally designed to compress the gas volumes required for pure hydrogen operation, operation at minimum speed may still deliver excessive volumes when mixing hydrogen with, for example, natural gas. To avoid delivering too much gas while maintaining the minimum speed, a portion of the compressed gas can be recirculated via a bypass and fed into the gas inlet on the suction side, thus varying the delivery rate. The portion of compressed gas that is recirculated is returned unchanged; the recirculated gas then corresponds to the compressed gas branched off from the gas outlet when fed into the gas inlet on the suction side.

[0029] Preferably, at least one gas cooler has a bypass. A gas cooler has a gas inlet and a gas outlet. Using a bypass, a gas flow can be directed past the gas cooler without passing through it. The bypass line branches off from the gas inlet upstream of the gas cooler, as seen in the gas flow direction, and opens into the gas outlet downstream of the gas cooler. A gas cooler cools the gas, with the purpose of condensing water vapor as a result of the cooling; it could also be referred to as a gas cooling condenser or condenser.

[0030] In this way, the water vapor content of the reducing gas can be easily varied. The gas flowing through this bypass line is not cooled in the gas cooler. Depending on how a gas stream is divided with respect to its passage through the gas cooler or this bypass line, the water vapor content after reunification will vary. For example, if a gas stream is divided into 90% into a first portion passing through the gas cooler and 10% into a second portion passing through the bypass line, the water vapor content after the two partial streams are reunited downstream of the gas cooler will be lower than if the proportions were reversed – because in the first case, more water will condense out in the gas cooler and be removed from the gas stream than in the second case.

[0031] In pure hydrogen H2 operation, a portion or the entire amount is passed through the gas cooler to adjust the water vapor content in the reducing gas within a target range, preferably in the range of 0.5 vol% or higher, more preferably 3 vol% or higher, up to 10 vol%, more preferably down to less than 8 vol%, most preferably down to less than 6 vol%. If necessary, a portion is also passed through the gas cooler bypass.

[0032] Even when operating with mixtures of natural gas and hydrogen, a portion or the entire amount is passed through the gas cooler to adjust the water vapor content in the reducing gas within a target range, preferably in the range of 0.5 vol% or higher, particularly preferably 3 vol% or higher, up to 10 vol%, particularly preferably down to less than 8 vol%, and most particularly preferably down to less than 6 vol%. If necessary, a portion is also passed through the gas cooler bypass.

[0033] In the direct reduction plant according to the invention, one or more devices for injecting water into a gas stream to be compressed or into the compressor are preferably also present; these devices can be used to cool the compressors and / or adjust the water vapor content.

[0034] Preferably, the gas compression plant comprises a device for controlling and / or regulating the water vapor content in the gas stream exiting the gas compression plant.

[0035] Such a device can, for example, act on the distribution of a gas flow between the gas cooler and its bypass, or on a device for injecting water into a gas flow to be compressed or into the compressor, or on the addition of steam into a gas flow exiting a compressor, or on the cooling medium temperature of the gas cooler - in gas coolers operated with a cooling medium, for example cooling water, the cooling medium temperature influences the water vapor content -, or it can receive, process, and / or output relevant control and / or regulation signals – for example, via appropriate sensors, data processing devices, actuators, valves, etc. Preferably, a device is also provided for controlling and / or regulating the gas flow exiting the gas compression system – which is introduced, for example, into a catalytic reformer or the gas furnace – by means of flow measurements of this gas flow and influencing one or more compressors, preferably frequency-converter positive displacement compressors.

[0036] The gas compression system comprises one or more compressors. Preferably, all compressors in the gas compression system are positive displacement compressors. Rotary lobe compressors are preferred. -, but they can also be other types such as reciprocating piston compressors or screw compressors, cell wheel compressors or Wankel compressors. Positive displacement compressors adapt easily to changes in operating conditions, such as gas composition, inlet and outlet temperature, etc., with appropriate changes until the operating limits are reached, whereby the outlet pressure is not significantly dependent on the gas composition and thus the speed of sound, as is the case with radial compressors. Positive displacement compressors usually have silencers. Preferably, at least one positive displacement compressor has a silencer. It is preferred that at least one of the gas coolers is integrated into a silencer of a positive displacement compressor.This reduces space requirements and leads to lower costs as only one pressure vessel is required.

[0037] Preferably, the gas compression plant has one or more frequency converter displacement compressors in at least one compression stage.

[0038] A variable frequency drive positive displacement compressor has speed control via VFD control -The delivered gas flow rate is essentially proportional to the compressor speed, which is controlled by the frequency of the alternating voltage. A variable-frequency positive displacement compressor can be operated at various speeds, which can be easily changed by adjusting the frequency using a frequency converter. Positive displacement compressors are typically operated at a fixed speed or within a narrow range of speeds; thus, they can be operated economically within a narrow range of gas densities, molecular weights, sound velocities, and gas flow rates.A frequency converter positive displacement compressor, on the other hand, allows operation over a comparatively wider range of speeds due to the frequency converter and can therefore be operated economically for a wider range of gas volume flows, gas densities, molecular weights and speeds of sound.

[0039] The presence of a frequency converter-driven positive displacement compressor thus allows for a response to increasing hydrogen content in the reducing gas, as its operation can be easily adapted to changes in gas density, molecular weight, sound speed, and gas flow rate. A continuous increase in the hydrogen content or gas flow rate is possible, as adaptation only requires an increase in the compression frequency by controlling and / or regulating the frequency converter.

[0040] Existing compressors in the compression stages can be retained and supplemented with frequency converter compression compressors. This makes adapting existing direct reduction plants operating with natural gas-based reducing gas for hydrogen operation simple, cost-effective, and resource-efficient.

[0041] The frequency converter displacement compressor(s) can be connected in parallel or in series to each other or to other types of compressors in the compression stages.

[0042] Varying the flow rate via frequency conversion is more energy efficient than varying the flow rate using a bypass.

[0043] However, a variation by means of a bypass and a variation via frequency conversion can complement each other well, for example in transition areas where the frequency control cannot reduce further due to minimum speed restrictions, or when starting up volume conveyors.

[0044] A further subject of the present application is a method for operating a direct reduction plant according to the appended claims 7-10.

[0045] Such a process can, for example, be used to operate a direct reduction plant as described above.

[0046] A portion of the gas compressed by a compressor is returned to the suction side of the compressor via a bypass.

[0047] According to the process, gas is compressed in the gas compression plant, producing compressed gas. The gas compression plant serves to provide compressed gas through gas compression. The compressed gas is cooled in the gas cooler. The compressed gas is reducing gas or reducing gas precursor gas.

[0048] Preferably, at least temporarily, a portion of a compressed gas directed to a gas cooler is bypassed past the gas cooler by means of a bypass.

[0049] For example, out of 100 m3 of compressed gas, 80 m3 is cooled in a gas cooler, while 20 m3 is not cooled; these 20 m3 are, for example, bypassed by the gas cooler that cools the 80 m3.

[0050] The water vapor content of the gas stream obtained during gas compression is preferably controlled and / or regulated, preferably by injecting water into a gas stream to be compressed or into a compressor. The water vapor content can be adjusted by means of devices for injecting water into a gas stream to be compressed or into the compressor; thus, such devices can be used not only for cooling the compressors, but also for controlling and / or regulating the water vapor content.

[0051] Preferably, gas compression is carried out in at least one compression stage by means of a frequency converter displacement compressor. Short description of the drawings

[0052] The present invention is described below by way of example with reference to several schematic figures. Figure 1 shows schematically a direct reduction plant according to the invention. Figure 2shows schematic details of a gas compression plant. Figure 3 shows schematic integration of a gas cooler into a silencer. Description of the embodiments Examples

[0053] Figure 1shows a schematic diagram of a direct reduction plant 10. Material containing metal oxide – in the illustrated case iron oxide – such as ore, pellets – 20 is fed into a reduction unit 30 to be reduced therein using reducing gas. During operation of the direct reduction plant 10, after gas compression, reducing gas is introduced into the reduction unit 30. Shown is a reducing gas line 40 through which reducing gas is introduced into the reduction unit 30 – here a reduction shaft. A gas compression system 50 ensures that the reducing gas or its precursors are compressed in order to have the pressure required for carrying out the direct reduction in the direct reduction plant 10. For reasons of clarity, Figure 1Among other things, the illustration of recirculation lines for top gas has been omitted. According to the invention, the gas compression system 50 comprises at least one compression stage. At least one gas cooler 51 for gas compressed in the gas compression system 50 is present in the direct reduction system 10, here downstream of the last compression stage as seen in the direction of gas flow toward the reduction unit 30. Also schematically shown is an optionally present device 52 for controlling and / or regulating the water vapor content in the gas stream exiting the gas compression system 50.

[0054] Also shown is an existing catalytic reformer 60, into which gas compressed in the gas compression system is directly introduced via direct inlet 70. In principle, the element with reference numeral 60 could also represent a gas furnace.

[0055] The detail according to the invention that a bypass is provided on at least one of the compressors for returning at least a portion of the gas compressed by the compressor into a suction-side gas inlet of the compressor in question is described in Figure 2 shown.

[0056] Figure 2shows a schematic view of a gas compression plant 80 and a gas cooler 90. There are two compression stages, A and B, where A comprises three compressors and B comprises two compressors. A gas cooler 90 is provided downstream of the last compression stage in the gas flow direction—represented by the arrowheads. Optionally, the gas cooler 90 has a bypass 100, shown in dashed lines. By means of the bypass 100, a gas stream can be guided past the gas cooler 90 without passing through it. The line of the bypass 100 branches off from the gas inlet 110 upstream of the gas cooler 90, viewed in the gas flow direction, and flows into the gas outlet 120 downstream of the gas cooler. During operation of the direct reduction plant, after gas compression, at least a portion of the compressed gas is cooled in the gas cooler 90.The gas compression system 80 includes a bypass 130, shown in dashed lines, which branches off from the gas outlet 140 of a compressor of compression stage B downstream of this compressor and flows into its gas inlet 150 upstream of the compressor. The bypass 130 allows at least a portion of the gas compressed in the compressor to be returned, at least temporarily, to the suction side of this compressor.

[0057] As a device for controlling and / or regulating the water vapor content of the gas stream obtained during gas compression, a device 160 for injecting water into a gas stream to be compressed or into a compressor is also shown schematically.

[0058] The compressors shown are one frequency converter displacement compressor 171, 171 per compression stage.

[0059] Figure 3shows schematically how a gas cooler 180 is integrated into a silencer 190 of a positive displacement compressor 200. A silencer 191 is arranged downstream of the compression section 210 of the positive displacement compressor 200 in the gas flow direction - indicated by arrowheads. The parts serving for gas cooling, such as cooling water inlet 220, packing 230, cooling water outlet 240, and condensate drain 250, are arranged in the

[0060] Integrated silencer 190. For sound attenuation, the silencer 190 has a cross-sectional constriction 260.

[0061] Of course, devices for the computer-implemented operation of a direct reduction plant according to the invention or a method according to the invention may also be provided; for the sake of clarity, they have not been shown in the figures. List of reference symbols

[0062] 10Direct reduction plant 20Material containing metal oxide 30Reduction unit 40Reduction gas line 50Gas compression plant 51Gas cooler 52Device for controlling and / or regulating the water vapor content 60Catalytic reformer 70Direct inlet 80Gas compression plant 90Gas cooler 100Bypass 110Gas inlet 120Gas outlet 130Bypass 140Gas outlet 150Gas inlet 160Device for injecting water into a gas stream to be compressed or into a compressor 170Frequency converter positive displacement compressor 171Frequency converter positive displacement compressor 180Gas cooler 190Silencer 191Silencing section 200Positive displacement compressor 210Compression section 220Cooling water inlet 230Packing 240Cooling water outlet 250Condensate drain 260Cross-sectional constriction

Claims

1. Direct reduction plant (10) comprising a catalytic reformer (60) and / or a gas furnace and a gas compression plant (50) having one or more compressors, wherein the gas compression plant (50) comprises at least one, preferably at least two compression stages (A, B), and wherein at least one gas cooler (51) for gas compressed in the gas compression plant is present, preferably at least downstream of the last compression stage as viewed in the direction of the gas flow, characterized in that a direct introduction line (70) for introducing compressed gas directly into the reformer (60) and / or into the gas furnace proceeds from the gas compression plant (50) or the gas cooler (51) with preservation of the amount of CO2, and, at least at one of the compressors, a bypass (130) for returning unchanged at least a portion of the gas compressed by the compressor to a suction-side gas introduction line of the relevant compressor is provided.

2. Direct reduction plant according to Claim 1, characterized in that at least one gas cooler (90) has a bypass (100).

3. Direct reduction plant according to Claim 1 or Claim 2, characterized in that the gas compression plant (80) has a device (160) for the open-loop and / or closed-loop control of the water vapour content in the gas stream exiting the gas compression plant (80).

4. Direct reduction plant according to any of Claims 1 to 3, characterized in that all the compressors of the gas compression plant are positive-displacement compressors (200).

5. Direct reduction plant according to any of Claims 1 to 4, wherein at least one positive-displacement compressor has a sound suppressor, characterized in that at least one of the gas coolers is integrated in a sound suppressor (190) of a positive-displacement compressor (200).

6. Direct reduction plant according to any of Claims 1 to 5, characterized in that the gas compression plant (80) has one or more variable-frequency-drive positive-displacement compressors (170, 171) in at least one compression stage.

7. Method for operating a direct reduction plant (10) comprising a reduction unit, a catalytic reformer (60) and / or a gas furnace and a gas compression plant (50), which is intended for providing compressed gas by gas compression and has one or more compressors, wherein the gas compression plant (50) comprises at least one compression stage (A, B), and wherein at least one gas cooler (51) for gas compressed in the gas compression plant is present, wherein reduction gas is introduced into the reduction unit (30) after gas compression, characterized in that at least a portion of the compressed gas is cooled, preferably at least after the last gas compression as viewed in the direction of the reduction unit (30), and compressed gas from the gas compression plant (50) or the gas cooler (51) is introduced directly into the reformer (60) and / or the gas furnace with preservation of the amount of CO2, and, at least intermittently, a portion of a gas compressed by a compressor is returned unchanged to a suction-side gas introduction line of the relevant compressor by means of the bypass (130).

8. Method according to Claim 7, characterized in that, at least intermittently, a portion of a compressed gas conducted to a gas cooler (90) is conveyed past the gas cooler (90) by means of the bypass (100).

9. Method according to Claim 7 or Claim 8, characterized in that the water vapour content in the gas stream obtained during the gas compression is controlled in open-loop and / or closed-loop fashion, preferably by spraying water into a gas stream to be compressed or into a compressor.

10. Method according to any of Claims 7 to 9, characterized in that gas is compressed in at least one compression stage (A, B) by means of a variable-frequency-drive positive-displacement compressor (170, 171).