Patent Application
20240018435
Subject matter of the present invention is a method for the conversion of solid waste into syngas using a CO shift unit and a respective plant.
Municipal Solid Waste (MSW) is generated worldwide and has to be dealt with. In the western hemisphere municipal solid waste is either landfilled or incinerated. Incineration is focused on maximizing the use of the energy content of the municipal solid waste in contrast to recycling or reusing the contents of the municipal solid waste on a molecular scale as e.g. in chemical recycling. Incineration generates issues regarding the emission of pollutants into the atmosphere which results—depending on national or even local legislation—in significant technological efforts necessary to meet the limitations provided by national/local legislation. Further, the products generated by the incineration, e.g. fly ash, bottom ash, gypsum and heavy metal and/or dioxin laden active coal, create further challenges regarding their further use and/or processing. Furthermore, the waste to power efficiency, i.e. the amount of caloric value transferred to heat energy is low and is typically in a range of 20 to 25%.
When chemical recycling is desired often the yield, i.e. the final outcome of usable chemical substances needs improvement and/or the energy efficiency is inappropriate. Frequently, a gas comprising carbon monoxide and hydrogen is generated, e.g. by pyrolysis or the like. To increase the amount of hydrogen in the product stream a CO shift reaction might be used as is known e.g. from US 2009/0077892 A1. To optimize the hydrogen output a large excess supply of water steam is necessary resulting in large amounts of water in the product gas of the CO shift reaction. When this product gas is cooled this leads to a considerable amount of condensing and, thus, large amounts of low level heat in the cooling train, while the product gas is cooled to ambient temperature. The majority of this energy is usually discarded which results in a low energy efficiency. It is, thus, an object of the present invention to overcome the drawbacks of the prior art in this regard.
This object is solved by the features of the independent claims. Dependent claims are directed to preferred embodiments of the invention.
According to the method for increasing the hydrogen content in the product gas stream of the torrefaction of solid recovered fuel (SRF) pellets according to the present invention the solid recovered fuel pellets are torrefied to charred pellets while generating torrefaction gas which is converted to torrefaction syngas comprising hydrogen and carbon monoxide, wherein a syngas stream comprising the torrefaction syngas is provided to a CO shift unit for reacting at least a part of the carbon monoxide with water steam to carbon dioxide and hydrogen generating shifted syngas, wherein the shifted syngas is provided downstream of the CO shift unit to a low temperature heat recovery unit in which the shifted syngas is guided through at least two heat exchangers in which the shifted syngas is in thermal exchange with at least two water streams that are used downstream of the respective heat exchangers as process water during the production of the product gas stream for heating said at least two water streams.
The term syngas is understood throughout this document as a gas comprising hydrogen, carbon monoxide and/or carbon dioxide. Solid recovered fuel pellets are generated from solid waste comprising municipal solid waste. The term torrefaction is understood as a substoichiometric oxidization of the solid recovered fuel pellets to generate a raw syngas comprising hydrogen and carbon monoxide. The resulting charred pellets are preferably gasified in a gasification unit creating syngas. The torrefaction syngas comprising hydrogen and carbon monoxide which is generated e.g. by thermal cracking from the torrefaction gas generated during the torrefaction of the solid recovered fuel pellets is provided to a CO shift unit, preferably in combination with the syngas generated during the gasification process. The CO shift unit, which is preferably a two stage CO shift unit comprising a high temperature CO shift unit and a low temperature CO shift unit, is used to support the carbon monoxide (CO) shift reaction in which carbon monoxide and water steam react to carbon dioxide and hydrogen.
Downstream of the CO shift unit the resulting shifted syngas has a temperature above ambient temperature as required by adjusting the chemical equilibrium in the CO shift unit. E.g. in a two stage CO shift unit the syngas enters the high temperature CO shift reactor at a temperature of about 270° C. and leaves the low temperature CO shift reactor at a temperature of about 240° C. Therefore, the shifted syngas has to be cooled down before being able to be stored and/or processed i.e. used as raw material and/or as energy storage medium or provided to a gas cleaning unit in which hydrogen is separated from the remainder of the shifted syngas which is purged as a purge gas. During cooling of the shifted syngas the water steam still being present in the shifted syngas will condense as soon as the temperature is undercutting the dew point. Then, a significant amount of heat energy is released in the form of the condensing energy.
According to the present invention the heat energy content of the shifted syngas before entering the low temperature heat recovery unit is not discarded to ambient by e.g. an air or water cooler but is used to heat at least two process water streams used for the production of the product gas. The at least two product water streams are preferably in the liquid and/or gaseous state i.e. liquid water and/or water steam.
The respective product water stream is, for example, a process water stream used for increasing the steam content of the torrefaction or another syngas before entering the CO shift unit. Alternatively, or cumulatively the process water stream is used during the gasification of the charred pellets in a gasification unit, which performs preferably an entrained flow gasification of the charred pellets. Alternatively, or cumulatively boiler feed water for the generation of high-pressure steam is heated. Preferably, the generation of high-pressure steam is maximized at the expense of the generation of low pressure water steam. The term high-pressure steam is understood throughout this document as comprising pressures in the range of 80 bar to 140 bar, whereas low-pressure steam is understood throughout this document as comprising pressures of about 3 bar to 12 bar. The term medium pressure throughout this document is understood as comprising pressures of about 18 bar to 43 bar.
According to a preferred embodiment the shifted syngas is provided to a gas cleaning unit for generating a product gas stream reach of hydrogen, preferably with a hydrogen content of 99.5 vol.-% and more, while generating a purge gas,
This allows to adjust the water content in the syngas provided to the CO shift unit easily. By mixing the purge gas with the torrefaction syngas it is ensured to not loose carbon monoxide and/or hydrogen being possibly present in the purge gas.
According to an embodiment a first syngas stream generated by the gasification of the charred pellets is provided to a high-pressure scrubbing unit to raise the water content in a cleaned syngas stream.
The use of the high-pressure scrubbing unit allows on the one hand to reduce contaminants like long-chain carbon hydrogen compounds in the first syngas stream and on the other hand allows to introduce moisture into the cleaned syngas stream allowing to adjust the water content of the gas stream entering the CO shift unit precisely.
According to an embodiment the shifted syngas is cooled in the low temperature heat recovery unit by bringing into thermal contact with at least one of the following water streams:
In particular, if step c) and at least one of steps a) and b) is realized as well the process is controlled such that the generation of high-pressure steam is prioritized to increase the energy yield of the whole process of producing a product gas stream comprising hydrogen from solid recovered fuel pellets. Preferably, each water stream is passing through at least one heat exchanger which are arranged such that the most efficient use is made of the heat energy content of the shifted syngas.
According to an embodiment the heat exchangers of the low temperature heat recovery unit the pressure of the process water is higher than the pressure of the shifted syngas. This ensures that no syngas penetrates into the respective water stream in case of a leak in the respective heat exchanger.
According to a further aspect of the present invention a plant for the conversion of solid waste provided as solid recovered fuel pellets to a product gas stream rich in hydrogen is proposed, comprising
This allows the efficient use of the low temperature heat in the shifted syngas for heating process water streams used in the production of the product gas stream from solid recovered fuel pellets. The components of the plant according to the present invention are preferably suited and intended to perform the method according to the present invention.
According to an embodiment the low temperature heat recovery unit includes at least two of the following heat exchangers:
The two or more of the second to sixth heat exchangers are arranged in the order of their denotation with a decreasing temperature level of the shifted syngas. E.g. the temperature of the shifted syngas entering the first heat exchanger is higher than the temperature of the shifted syngas entering the third heat exchanger etc. This allows an efficient use of the heat energy of the shifted syngas while cooling the shifted syngas.
According to an embodiment the plant further comprises a saturation column in which a water content at least of the torrefaction syngas is adjustable. The saturation column is preferably a usual wet scrubbing unit in which water is provided on a head of the column and is extracted from a sump of the column while the respective gas stream is flowing against the flow direction of the water. Preferably, the plant comprises a gas cleaning unit downstream of the low heat recovery unit in which hydrogen is separated by a pressure switch adsorption unit, generating a purge gas, whereas the purge gas is provided to the saturation column in combination with the torrefaction syngas.
It should be noted that the individual features specified in the claims may be combined with one another in any desired technologically reasonable manner and form further embodiments of the invention. The specification, in particular taken together with the figures, explains the invention further and specifies particularly preferred embodiments of the invention. Particularly preferred variants of the invention and the technical field will now be explained in more detail with reference to the enclosed figures. It should be noted that the exemplary embodiment shown in the figures is not intended to restrict the invention. The figures are schematic and may not be to scale. The figures display:
CO+H2O↔CO2+H2
The reaction is in chemical equilibrium which can be influenced either in direction of the educts or the products in a usual way e.g. by use of the respective temperature and the use and concentration of a specific catalyst. As the reaction is endothermic the water is usually provided as water steam. To shift the chemical equilibrium to the product side to increase the production of hydrogen a molar steam to carbon monoxide ration of about 2.3 to 2.7, in particular about 2.5, has been found to be advantageous. The steam is provided preferably at a pressure which is above the pressure at which the shift reaction is taking place which is preferably about 40 bar. If the molar steam to carbon monoxide ratio of 2.5 is used a surplus of 1.5 moles steam for every mole carbon dioxide remains in the reactor. This results in large amounts of water in the product gas of the CO shift reaction. When this product gas is cooled this leads to a considerable amount of condensing and, thus, so large amounts of low level heat in the cooling train, while the product gas is cooled to ambient temperature. The majority of this energy is usually discarded which results n a low energy efficiency.
The CO shift unit 500 is part of a plant 1 for the conversion of solid waste into a gas comprising hydrogen, in particular into syngas comprising hydrogen and hydrogen. After preparing solid recovered fuel pellets from solid waste like municipal solid waste 103, preferably with additional added biomass, in a in pelletizing facility 100 the respective solid recovered fuel pellets 117 are transported to the plant 1 and are provided to a torrefaction unit 200 in which the pellets are torrefied, i.e. oxidized sub stoichiometrically at temperatures of 250° C. to 300° C. The term torrefaction is understood as a thermochemical treatment of the solid recovered fuel pellets at temperatures of 250° C. to 320° C. It is carried out under atmospheric pressure and without adding further oxygen, e.g. without providing air. During the torrefaction process water contained in the solid recovered fuel pellets evaporates as do volatiles included in the solid recovered fuel pellets. Biopolymers included in the solid recovered fuel pellets partly decompose under release of volatiles. The product of the torrefaction process are charred pellets and torrefaction gas. The torrefaction of the pellets results in charred pellets 201, which are gasified in a gasification unit 300. Another product of the torrefaction is torrefection gas 202 which is provided to a torrefaction gas processing unit 400. The product of the torrefaction gas processing unit 400 is a torrefaction syngas 401, whereas the product of the gasification unit 300 is a syngas 301. Both the syngas 301 and the torrefaction syngas 401 comprising water steam, carbon monoxide and hydrogen. Both syngases 301, 401 are introduced into the CO shift unit 500 which is described in detail with respect to
The syngas 401 being generated in the torrefaction gas processing unit 400 is conveyed to a saturation column 510 via a conveying means 511. By the conveying means 511 the syngas 401 is compressed, preferably to a pressure of 40 bar. The conveying means 511 can comprise several compressors with intercoolers which provide for a multistage intercooled compression of the syngas 401. Likewise, the purge gas 602 from the gas cleaning unit 600 is conveyed via a conveying means 512 to the saturation column 510 as well. By the conveying means 512 the purge gas 602 is compressed, preferably to a pressure of 40 bar. The conveying means 512 can comprise several compressors with intercoolers which provide for a multistage intercooled compression of the purge gas 602. The saturation column 510 is a conventional wet scrubber which is provided with process water 513 from a low temperature heat recovery unit 524 being described later on via conveying means 514. The water 515 collected in a sump 516 of the saturation column 510 is used as process water in a low temperature heat recovery unit 524 described below. By the saturation column 510 the syngas 401 and the purge gas 602 are mixed and, simultaneously, moisture is added to the resulting saturation column product gas 517. Depending on the parameters of the operation of the saturation column, i.e. in particular on the water flow and the water temperature, the moisture of the saturation column product gas 517 can be controlled.
The saturation column product gas 517 is provided together with the cleaned syngas 509 to a first heat exchanger 518. In the first heat exchanger 518 energy is transferred from a high temperature CO shift reactor offgas 519 to the cleaned syngas 509 and the saturation column product gas 517 which are combined downstream of the first heat exchanger 519 to a combined syngas stream 520. By the heat exchange in the first heat exchanger 518 the respective high temperature CO shift reactor offgas 519 is cooled while the cleaned syngas 509 and the purified gas stream 517 are heated. Preferably, the saturation column 510 is driven in such a way that the combined syngas stream 520 has a molar ratio of water steam to carbon monoxide of between 2.0 and 3.0, preferably between 2.4 and 2.6, in particular of about 2.5.
The combined syngas stream 520 is provided downstream of the first heat exchanger 518 to a high temperature CO shift reactor 521 in which the above-described shift reaction is taking place. The high temperature CO shift reactor offgas 519 has a reduced water/steam and carbon monoxide content and an increased hydrogen (H2) content compared to the combined syngas stream 520. The high temperature CO shift reactor offgas 519 is—as will be described in the following—guided through several heat exchangers to reduce its temperature including the first heat exchanger 518 to a low temperature CO shift reactor 522 in which the CO shift reaction as described above is taking place. A low temperature CO shift reactor offgas 523 is increased in its hydrogen (H2) content compared to the high temperature CO shift reactor offgas 519 that is entering the low temperature CO shift reactor 522.
The low temperature CO shift reactor offgas 523 is guided downstream of the low temperature CO shift reactor 522 through a low temperature heat recovery unit 524. In this low temperature heat recovery unit 524 the heat content of the low temperature CO shift reactor offgas 523 is used to increase the temperature of several water streams. After entering the low temperature heat recovery unit 524 the low temperature CO shift reactor offgas 523 is subsequently guided through a second heat exchanger 525, a third heat exchanger 526, a fourth heat exchanger 527, a fifth heat exchanger 528 and a sixth heat exchanger 529. While passing these heat exchangers 524, 525, 526, 527, 528, 529 the temperature of the low temperature CO shift reactor offgas 523 is reduced steadily before leaving the low temperature heat recovery unit 524 as a syngas stream 530 and is passed, optionally and if necessary, through an air cooler 531. Thereafter, the syngas stream 530 is provided to the gas cleaning unit 600 as discussed above.
First, in the second heat exchanger 525 the heat content of the syngas stream 530 is used to heat feed water 508 to be used in the gasification unit 300. E.g. it is thus possible to heat this water 508 from 158° C. to 205° C. Second, in the third heat exchanger 526 water is heated that can e.g. be used to generate high pressure steam. The water is e.g. heated from typically 155 to 200° C. Third, in the fourth heat exchanger 527 process water 513 is heated, typically from 135 to 158° C., which is used in the saturation column 510. Further use of this water as feed water 508 is possible, in particular after passing the second heat exchanger 525. In the fourth heat exchanger 527 the low temperature CO shift reactor offgas 523 is cooled below its dew point, thus, large amounts of condensing heat are released and used for the respective heat transfer. Fourth, in the fifth heat exchanger 528 boiler feed water 536 is heated, typically from to 155° C., which is preferably used to prepare high pressure steam. Fifth, in the sixth heat exchanger 529 water is heated from typically 25° C. to 135° C. for use in the gasification of the charred pellets 201 in the gasification unit 300.
The feed water 508 for the gasification unit 300 is at least a part of the water 515 collected in the sump 519 of the saturation column 510 as described above. The water 515 collected in the sump 516 of the saturation column 510 is entering the fourth heat exchanger 527. Water 521 from a waste water treatment unit (not shown) can be mixed with the water 515 after having passed the sixth heat exchanger 529. Downstream of the second heat exchanger 527 the heated fourth heat exchanger water off-stream 533 is in part used as process water from low temperature heat recovery 513 in the saturation column 510 and in part it is guided through the second heat exchanger 525 to be used downstream of the second heat exchanger 525 partly as condensed water 508 to be used partly as feed water 503 in the high-pressure scrubbing unit 502 and partly as process water in the torrefaction unit 200. A further part of water downstream of the sixth heat exchanger 529 is used as condensate 534 provided e.g. to a water stripper. Other water like e.g. external high pressure boiler feed water 535 generated for applications outside the plant 1 is guided through the fifth heat exchanger 528 to be partly used as boiler feed water 536 e.g. in the gasification unit 300 and/or for the generation of low pressure steam and/or in the torrefaction unit 200. Another part of the boiler feed water 535 is guided downstream the fifth heat exchanger 528 through the second heat exchanger 526. Thereafter, it is passed through a seventh heat exchanger 537.
The CO shift unit 500 as part of the plant 1 for the conversion of solid waste into a product gas stream comprising hydrogen allows an energy efficient use of the low temperature heat energy in the low temperature heat recovery unit 524 to heat process water streams used in the plant 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20204800.5 | Oct 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/079467 | 10/25/2021 | WO |
