WO2004112447A2 | 2004-12-23 | |||
WO2004048851A1 | 2004-06-10 | |||
WO2003066779A1 | 2003-08-14 |
US20080222956A1 | 2008-09-18 | |||
EP1375628A2 | 2004-01-02 | |||
KR20050102958A | 2005-10-27 |
M.A. KOROBITSYN ET AL.: "Possibilities for gas turbine and waste incinerator integration", ENERGY, vol. 24, 1999, pages 783 - 793, XP027472007, DOI: doi:10.1016/S0360-5442(99)00034-1
CLAIMS 1 . A method for the generation of electric power from a chlorine-containing combustible stream, comprising the steps of (a) plasma gasification of the chlorine-containing combustible stream to generate a hot chlorine-containing synthesis gas, (b) generation of high pressure steam using heat from the hot chlorine-containing synthesis gas from step (a), (c) removing a significant portion of the chlorine from the chlorine- containing synthesis gas from step (b) in order to produce a low- chlorine synthesis gas, (d) using the low-chlorine synthesis gas from step (c) as fuel for a gas turbine of which at least part of the mechanical energy is converted to electric power in a first power generator, (e) using heat from the hot exhaust gasses from the gas turbine in step (d) to further increase the temperature of the high pressure steam from step (b) in a superheating step, and (f) feeding the superheated steam from step (e) to a steam turbine of which at least part of the mechanical energy is converted to electric power in a second power generator. 2. The method according to claim 1 wherein the chlorine-containing combustible stream is a waste stream. 3. The method according to claim 1 or 2 wherein the chlorine-containing combustible stream is a solid waste stream, preferably a municipal solid waste stream (MSW) or an industrial waste stream, more preferably a refused derived fuel (RDF) stream which is derived from a solid waste stream, such as a municipal solid waste stream. 4. The method according to any one of the preceding claims wherein the high pressure steam from step (b) is saturated. 5. The method according to any one of the preceding claims wherein the temperature of the high pressure steam from step (b) is at most 300°C and optionally at least 200°C. 6. The method according to any one of the preceding claims wherein the plasma gasification step (a) is a single stage or a two stage gasification step. 7. The method according to any one of the preceding claims wherein the chlorine-containing combustible stream contains at least 5 ppm by volume of chlorine, expressed as hydrogen chloride (HCI). 8. The method according to any one of the preceding claims further comprising a step (g) wherein heat from the hot exhaust gasses from the gas turbine in step (d) is used to generate a second stream of high pressure steam. 9. The method according to claim 8 wherein the high pressure steam from step (g) is fed to a steam turbine of which at least part of the mechanical energy is converted to electric power. 10. The method according to claim 9 wherein the temperature of the high pressure steam from step (g) is further increased by superheating before it is fed to the steam turbine. 1 1 . The method according to claim 10 wherein the high pressure steam from step (g) is combined with the high pressure steam from step (b) before being fed to the same steam turbine. 12. The method according to claim 1 1 wherein the high pressure steam from step (g) is superheated together with the high pressure steam from step (b) in the same superheater. 13. The method according to any one of the preceding claims wherein the metal skin temperatures in the high pressure steam generator of step (b) and/or of step (g), if present, are at most 450°C and optionally at least 350°C. 14. The method according to any one of the preceding claims wherein the superheated steam from step (e) has a temperature of at least 290°C. 15. The method according to any one of the preceding claims wherein the metal skin temperatures in the steam superheater of step (e) are at least 400°C, and optionally not more than 655°C. 16. The method according to any one of the preceding claims wherein the gas turbine of step (d) and the steam turbine of step (f) are driving the same common power generator. 17. The method according to any one of claims 1 -15 wherein the gas turbine of step (d) and the steam turbine of step (f) are driving a different power generator. 18. The method according to any one of the preceding claims wherein the synthesis gas is combusted in the gas turbine using air as the oxygen carrier and/or with an oxygen-containing gas which is richer in oxygen than regular air. 19. The method according to any one of the preceding claims wherein the exhaust gasses from the gas turbine in step (d) have a temperature in the range of 400-600°C. 20. The method according to any one of the preceding claims wherein the hot exhaust gasses from the gas turbine in step (d) are heated, preferably with supplementary firing, more preferably with the combustion of low-chlorine synthesis gas from step (c) and/or by using the excess oxygen which is present in the hot exhaust gasses from the gas turbine, before heat from the exhaust gasses is being used in step (e). 21 . The method according to any one of the preceding claims wherein the steam turbine of step (f) has at least two stages and at least part of the exhaust steam of the higher pressure turbine stage is heated further before it is being used to drive the lower pressure turbine stage. 22. The method according to any one of the preceding claims wherein the steam turbine lowest pressure exhaust is at a pressure of at most atmospheric pressure, preferably at most 0.7 bar absolute (bara). 23. The method according to any one of the preceding claims wherein the exhaust steam of the steam turbine is condensed to form a condensate which is at least partly recycled to the high pressure steam generation in step (b) and/or the high pressure steam generation in step (e). 24. The method according to any one of the preceding claims wherein the chlorine-containing synthesis gas from step (b) are cooled before the chlorine removing step (c). 25. The method according to any one of the preceding claims wherein the chlorine is removed in step (c) by passing the synthesis gas through a system selected from a wet scrubbing system, a semi-wet scrubbing system, a flash dry system, a dry adsorption system, and combinations thereof. 26. The method according to any one of the preceding claims wherein the low-chlorine synthesis gas used in step (d) contains less than 5 ppm by volume of chlorine, expressed as hydrogen chloride (HCI). 27. The method according to any one of the preceding claims for the generation of electric power during a period of increased power consumption, preferably a period of peak power demand. |
FIELD OF THE INVENTION
The present invention relates to the recycling of waste streams into energy. More particularly, the invention relates to the conversion of chlorine containing waste streams into electric power.
BACKGROUND OF THE INVENTION
Most of the waste streams in society in general, as well as many waste streams in industry, contain chlorine. Conversion of such waste streams into gasses at high temperatures above 450°C leads to chlorine being present in the gasses produced, usually in the form of hydrogen chloride (HCI).
This causes problems in the recovery of the heat from those gasses, because HCI causes corrosion when in contact with most metals when the metal surface is at a temperature in the range of 450-500°C.
These corrosion problems are discussed in detail by W.F.M. Hesseling and P.L.F. Rademakers in TNO-Report MEP R-2003, "Efficiency Increase of Waste-to-Energy Plants, Evaluation of Experience with Boiler Corrosion and Corrosion Reduction", March 2003.
Waste streams are preferably recycled as much as possible in the same, or in a lower grade application, as raw materials. This becomes more difficult, if not impossible, if the waste stream is of poor quality. Separation techniques may possibly upgrade particular parts of a mixed waste stream, but their possibilities remain limited. Organic wastes, or organic parts of waste streams, may then be digested, i.e. converted into combustible gasses using bacteria. These gasses may be used for energy recovery, as in gas-driven power plants. What usually remains from mixed waste streams, such as municipal solid waste (MSW) is a non-digestible stream of solids which is still at least partially combustible. Such waste streams may be addressed as refused derived fuel (RDF). They contain a significant amount of energy, and the recovery thereof as useful energy, such as in electric power, is considered the most appropriate way to dispose of these waste streams.
The energy recovery from waste streams is typically based on the combustion of the waste stream and the generation of pressurized steam using the combustion heat. The steam is then used to drive a turbine, which drives an electric power generator.
From the combustion zone, heat is preferably withdrawn by vaporizing pressurized boiler feed water in steel pipes. By only partially vaporizing the water stream, the temperature of the fluid inside the steel pipes remains at the level of the boiling temperature of water at the particular pressure, and remains substantially constant. The temperature of the steel of the pipes, in particular the skin temperatures of the steel piping, will be somewhat higher than the boiling temperature of the water, but may in this way indirectly be maintained below a particular critical level.
The mixture of water and saturated steam inside the pipes is routed to a steam drum, where the steam vapour is separated from the liquid water and under pressure control is withdrawn from the steam drum.
With the steam drum located above the combustion zone, the hot water may be circulated through loops of piping, and the difference in density between the water in the down flowing piping, also called "the downcomers" and the steam/water mixture in the upward flowing piping, also called "the risers", these being exposed to the heat of the combustion zone, also called radiant section of the furnace, or firebox, is usually able to drive a fast circulation through the loops without any need for pumping, such that the vaporization remains partially at the outlet of the risers and the temperature inside the piping remains under control.
The radiant section piping may comprise what is called membrane walls, usually vertical piping arranged as a web enclosing the firebox. The downcomers and risers may be used to form the membrane wall, and typically the spacing between the piping is closed off with metal strips welded to the piping. The temperature limitations of the construction materials determine the maximum skin temperatures which are allowed for the risers, or for the metal strips between the piping of the membrane wall, consequently also the temperature which is allowed inside the risers or piping, and thus indirectly the maximum pressure which is allowed in the steam drum.
A higher steam pressure means a more efficient steam turbine, and thus a higher overall energy recovery. The design pressure of the steam system will thus typically be set as high as possible, as constrained by the governing technical and economical limitations.
Higher steam pressures also require higher purity boiler feed water, in order to control the build up of scale inside the boiling water system because of salt deposits. The preparation of the boiler feed water thus becomes increasingly costly, both in investment cost as in operating cost, with higher steam pressures.
Because the quality of the fuel in solid waste combustion is neither well controlled nor constant, also the supply of combustion air to the combustion zone cannot be adjusted tightly to match the energy contained in the fuel supply, and hence the air-to-fuel ratio cannot be controlled tightly. The combustion is therefore also typically operated with a significant average air excess, which means in addition that the combustion temperatures are significantly lower as compared to burning conventional fuels. Consequently the temperature in the combustion zone is limited, but also bound to vary both in time and location. The same thus applies to the skin temperatures of the steel piping within which the steam is generated.
Because of these variations, a significant safety margin has to be taken into account in setting the design pressure of the steam system.
Waste combustion plants, in particular those intended for the conversion of municipal solid waste, are for these reasons typically designed for generating steam at a pressure of at most 70 bar gauge (70 barg). This steam pressure avoids the need for super high quality boiler feed water, which may require additional water cleanup steps over and above the steps which are typically provided. This pressure also means that the steam is generated inside the risers at its saturation temperature of about 287°C, which is thus also the maximum temperature of the saturated steam leaving the steam drum. This pressure level assures that the skin temperatures of the risers are kept limited to a maximum of about 330°C, such that the corrosion effects described above are avoided or at least controlled at an acceptable level.
Steam turbines are preferably not operated on saturated steam. By the expansion of the steam in the turbine, also the temperature drops, and the steam partially condenses. This condensation occurs in the form of small water droplets, which at the high fluid velocities impinge on the turbine blades and may cause erosion. More water droplets are forming inside the turbine as the steam expands further towards the steam outlet of the turbine. The condensation of steam inside the turbine has to be kept limited because of this erosion, which jeopardizes the integrity of the very expensive turbine blades and hence the useful lifetime of the turbine. A turbine may tolerate some condensation, but this is typically kept limited to about 4% by weight at the point where the steam leaves the turbine at the low pressure end.
When only saturated steam is available to drive the turbine, this limitation on the amount of condensing water imposes a severe limitation on the fluid conditions inside the turbine, by limiting the pressure drop that may be allowed over the turbine from steam inlet to steam outlet. It is however the inlet-over-outlet pressure ratio which sets the amount of power which the turbine is able to withdraw from the steam. The rest of the energy remains in the steam leaving the turbine, and is usually lost downstream where the steam is condensed for recovery as liquid condensate. With a tight limit on the pressure drop allowed over the turbine, the power from the turbine is limited and the overall recovery of energy of the power plant is significantly impaired.
Power plants, based on solid waste incineration and working on the basis of saturated steam generation only, have been built and put in operation. The skin temperatures of the tubes in these plants are kept below the critical range of 400-450°C to avoid the high corrosion rates caused by HCI, and these plants run at relatively low energy efficiencies, with a yield of electrical power relative to the available combustion energy in the range of only about 20-22%. Their power yield is limited because of several reasons. In addition to the skin temperature limitation already mentioned above, firstly, the temperatures of the flue gases must be kept above their condensation temperature, in order to avoid condensation. The condensate of the flue gasses of a municipal-waste-to-power (MWP) plant is known to be very aggressive. Secondly, a lower pressure at the turbine outlet is not allowed because of the limit imposed on the amount of condensation in the steam outlet of the turbine. Operating outside of these boundaries quickly reduces the lifetime of the construction materials. It would severely reduce the availability of the power plant and increase its maintenance costs beyond what is economically viable in view of the current value obtained for the disposal of the waste (the "waste gate fees") and the value obtained for the generated electrical power (energy prices, kWh tariffs).
The second limitation, related to the turbine outlet pressure, may possibly be moved by superheating the steam which is fed to the turbine. With superheated steam at the turbine inlet, the steam which expands as it travels through the turbine remains dry for a significant part of its trajectory, and may only dive into the two-phase zone towards the end of the turbine, even when the pressure, and thus also the temperature, at the steam outlet are brought significantly lower as compared to with saturated steam at the inlet. With superheated steam, the inlet-over-outlet pressure ratio of the turbine may thus be significantly increased without having to increase the inlet pressure, which means that a significantly higher part of the energy in the high pressure steam may be converted into turbine power and hence into electrical power. Superheating the steam may thus significantly increase the overall efficiency of the power plant.
Superheating the steam in waste incineration is however not easy to accomplish. The steam leaving the steam drum has a typical temperature of at most 287°C and a pressure of 70 barg. This steam should be superheated, at the high pressure, which will increase its temperature, preferably as much as possible. This means that the skin temperatures of the high pressure piping of such a steam superheater will also increase.
Providing a steam superheater in the combustion zone is less preferred as compared to in the downstream convection zone, because the combustion temperatures in the combustion zone are higher and not well controlled. The flue gas temperature when combusting municipal solid waste or RDF, i.e. the temperature of the combustion gasses leaving the combustion zone, is still typically as high as 800°C, significantly lower however as compared to a typical 1000°C with conventional fuels burned with a good and well-controlled air-to-fuel ratio. The reasons for this lower temperature have been explained above in detail and are primarily due to the poor fuel quality of the solid waste streams.
Even with a co-current heat exchanger, giving lower metal temperatures as compared to a counter-current heat exchanger, which would be designed for taking the saturated steam at 287°C and heating it against flue gasses which are cooling down from 800°C, the critical skin temperatures of 450-500°C are readily reached as soon as the steam itself reaches temperatures in the range of 400-450°C.
The temperature of the superheated steam therefore remains significantly limited because of the chlorine present in the fuel and the corrosion which this may cause on the superheater piping. This temperature limit imposes a significant limit on the energy yield of a power plant based on chlorine containing wastes as fuels. This limit is not present in power plants based on fuels containing no or hardly any chlorine or other halogens. Such power plants typically achieve a much higher efficiency, which may be as high as 55%.
With chlorine containing fuels however, if the superheater(s) in the convection section of the furnace, and the membrane walls which may be present in the radiant section of the furnace are not protected, and these temperature limits are not respected, their lifetime is very short, ranging from only a few weeks to at most a few months. The only industrially acceptable protection so far is an overlay of inconel steel, in which case the equipment lifetime may extend to one or two years. This however represents a significant investment cost increase, which is rarely affordable in most current economic climates in which these power plants have to operate.
There therefore remains a need for a method for the energy recycling of chlorine containing fuels, such as refused derived fuel obtained from the separation of municipal solid waste, with higher energy efficiencies or energy yields, as compared to what is known and obtainable in the state of the art.
M.A. Korobitsyn et al, "Possibilities for gas turbine and waste incinerator integration", Energy 24 (1999) 783-793, proposes to combine a solid waste incineration plant with a conventional gas- fired "steam-and-gas" ("STAG" or in its Dutch version "STEG") power plant, even more widely known as a "combined cycle gas turbine" (CCGT) plant, which is the currently most preferred way to generate electrical power from natural gas or from synthesis gas obtained from coal. The gas is fired in a gas turbine, of which the mechanical power is converted to electrical power. The exhaust gasses from the turbine are hot, and may be used for the generation of steam, preferably by some additional burning of gas in the steam generation zone. The steam may be superheated using the heat from the flue gasses, which are low in chlorine. The superheated steam is then used to drive a steam turbine, which may for instance be a two-stage turbine with intermediate superheating in order to further improve the energy yield. The mechanical power of the steam turbine is then also converted to electrical power. The proposed combination is characterised in that the solid waste incineration plant only produces saturated steam, but which is superheated against the hot flue gasses from the CCGT plant, which are low in chlorine, and which do not cause the same corrosion problems. It is claimed that this combination of an incineration process for chlorine- containing solid waste with a conventional CCGT process achieves energy yields which are significantly higher as compared to the two processes operating in separation. The advantage of this combination is that the chlorine-based corrosion problem is alleviated. The drawback is that the CCGT process still needs a significant external supply of fuel, such as natural gas or synthesis gas, and which needs to be low in chlorine. Plasma gasification is a process, already applied on commercial scale, for converting all kinds of organic matter into synthesis gas (syngas) by using plasma processing. The technology uses an electric arc gasifier, also called a plasma torch, to turn a gas such as steam, air or nitrogen into a high temperature ionized gas at 2200-13900°C, and which is used to break organic matter primarily into syngas and solid waste (slag) in a controlled vessel, called the plasma converter. Its main commercial use is as a waste treatment technology as it allows full decomposition and disintegration of organic components. The slag is inert and may be granulated and used in construction. The lack or shortage of oxygen and the high temperatures in the plasma reactor prevent the main elements of gas from forming toxic compounds, such as furans, dioxins, NO x , or sulphur dioxide. Extensive filtration removes inorganic residue (ash) from the gas, and gaseous pollutants (NO, HCI, H 2 S, etc.) and allows the production of ecologically clean synthesis gas. The gaseous compounds do not contain any phenols or complex and heavy hydrocarbons. The dry purified syngas may thus be used as a clean fuel. The water circulating through the filtering systems has removed the hazardous substances and must be cleaned. The process is intended to be a net generator of electricity, depending upon the composition of input wastes, and to reduce the volumes of waste being sent to landfill sites.
KR 10-2005-0102958 discloses such an integrated system combining waste plasma gasification with power generation. City type solid waste having a caloric value of 1500 kcal/kg is fed to the plasma gasification step. The high temperature plasma torch (2000-
7000°C) converts the organics in the waste to a synthesis gas having a calorific value of 2510 kcal/m 3 . The incombustible inorganic material is converted into an inert slag which is claimed to be useful as a construction material. The synthesis gas is cooled and purified before it is used as fuel to a combined cycle power generation, i.e. a STEG power plant, where the synthesis gas is burned in a gas turbine driving a first power generator, and the waste heat from the gas turbine is used to generate steam which drives a steam turbine driving a second power generator. The process disclosed in KR 10-2005-0102958 does not utilize the heat in the synthesis gas from the plasma gasification step.
WO 03/066779 is concerned with the greenhouse effect and discloses the plasma gasification of waste, whereby carbon dioxide recovered from stack effluents from cement works and/or coal fired power stations is used as the inert gas to the plasma torch as well as to further control the atomic carbon/oxygen ratio and the temperature in the gasification reactor. The syngas leaving the plasma gasification reactor is cooled down to 1200°C (1473°K) by injection of extra carbon dioxide, before its heat is used to generate steam. A second injection of carbon dioxide may be needed to further reduce the temperature to a lower level, more compatible with the presence of HCI. The syngas leaving the steam generator, at a temperature of 500°K (227°C), is contacted with sodium bicarbonate powder to form a solid mixture of sodium chloride, sodium sulphate and sodium carbonate, which is recovered and supposedly used as raw material in the chemical industry. The steam from the steam generator is driving a steam turbine, which is driving a power generator. Exhaust steam from the steam turbine is used to dry the waste feed to the plasma gasifier down to 10% water, before it is condensed and recycled as condensate to the steam generator. The purified syngas produced may have many uses, one being as a fuel in a CCGT plant ("installation Turbine-Gaz-Vapeur"), to be burned in the presence of pure 0 2 which is obtained by air permeation. The only link of the plasma gasification process with the CCGT plant is the supply of purified synthesis gas. The drawback of the process of WO 03/066779 is that the hot synthesis gas, against which the steam for the turbine is generated, contains chlorine. Because of this chlorine, the temperatures in the steam generator must remain limited, resulting in a limited efficiency of the turbine driven by this steam.
There thus remains a need for the conversion of chlorine-containing solid waste into power with a higher efficiency or yield, while not needing a large external source of low-chlorine-containing fuel. The present invention aims to obviate or at least mitigate the above described problem and/or to provide improvements generally.
SUMMARY OF THE INVENTION
According to the invention, there is provided a method of power generation as defined in any of the accompanying claims.
The present invention provides for a method or process for the generation of electric power from a chlorine-containing combustible stream, comprising the steps of
(a) plasma gasification of the chlorine-containing combustible stream to generate a hot chlorine-containing synthesis gas,
(b) generation of high pressure steam using heat from the hot chlorine-containing synthesis gas from step (a),
(c) removing a significant portion of the chlorine from the chlorine- containing synthesis gas from step (b) in order to produce a low- chlorine synthesis gas,
(d) using the low-chlorine synthesis gas from step (c) as fuel for a gas turbine of which at least part of the mechanical energy is converted to electric power in a first power generator,
(e) using heat from the hot exhaust gasses from the gas turbine in step (d) to further increase the temperature of the high pressure steam from step (b), and
(f) feeding the superheated steam from step (e) to a steam turbine of which at least part of the mechanical energy is converted to electric power in a second power generator.
We have found that the process according to the present invention brings the advantage of higher overall energy efficiency, i.e. a higher yield of electric power output from the same amount of combustible energy input. While the overall yield of the process may not necessarily reach the levels achieved with a stand alone CCGT plant running on a pristine and low-chlorine fuel, it is able to reach a higher level than the processes currently known in the art for converting a chlorine-containing combustible stream into power. The applicants believe that this is thanks to the extra superheating occurring in step (e) and thus the higher temperatures of the high pressure steam which is generated from the heat in the chlorine- containing synthesis gas.
The process according to the present invention brings the further advantage of being less dependent, preferably not dependent at all, on an external supply of a low-chlorine fuel as feed to the gas turbine.
The process according to the present invention brings the further advantage that it is readily able to respond to fluctuations in the electrical load, i.e. in fluctuations in the demand for electric power. The chlorine-containing combustible stream is typically a stream which may be kept in storage or inventory. The feed rate of the chlorine-containing combustible stream may thus readily be changed in order to increase or decrease the power output of the overall process, such that it may more closely match the demand for electric power or respond to changes in the power demand. This is a major advantage, because on the one hand any excess power which necessarily needs to be generated for process reasons may need to be sold at a relatively low value or even dissipated at zero value, while on the other hand extra power which occasionally may need to be imported from an external source may be scarce and demand a relatively high value. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a process flow chart of one embodiment of the process according to the present invention.
DETAILED DESCRIPTION
The present invention will be described in the following with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein can operate in other orientations than described or illustrated herein.
The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
In the context of the present invention, high pressure steam is defined as steam at a pressure in the range of 48-130 bar gauge (barg), preferably at least 50 barg, more preferably at least 60 barg, even more preferably at least 65 barg and yet more preferably at least 68 barg. A higher pressure of the steam brings the advantage of a higher inlet pressure available to the turbine, which is beneficial to the amount of mechanical power which the turbine is able to extract from the high pressure steam. The high pressure steam may have a pressure of at most 120 barg, preferably at most 1 10 barg, more preferably at most 100 barg, even more preferably at most 90 barg, yet more preferably at most 80 barg, preferably at most 70 barg. A lower pressure brings the advantage that the quality requirements on the boiler feed water which is used for the steam generation are reduced. A lower pressure also brings the advantage that the boiling temperature during steam generation is lower, such that the metal temperatures, in particular the skin temperatures, of the equipment used in the high pressure steam generation, may be kept limited, which is beneficial for the equipment lifetime.
In the context of the present invention, synthesis gas, or syngas in short, is defined as a gas which contains significant amounts of hydrogen and carbon monoxide, and which is characterised by a significant heat of combustion. The synthesis gas may further contain other gaseous components such as carbon dioxide and/or water vapour. The synthesis gas may contain further combustible components, such as hydrocarbons.
In the context of the present invention, the chlorine-containing synthesis gas contains at least 15 ppm by volume of chlorine, expressed as hydrogen chloride (HCI), preferably at least 20 ppm by weight, more preferably at least 30 ppm, even more preferably at least 40 ppm and yet more preferably at least 50 ppm by volume. In the context of the present invention, the chlorine content of a low-chlorine containing synthesis gas is less than 15 ppm by volume, on the same basis, preferably at most 10 ppm by volume, more preferably at most 7 ppm by volume, even more preferably at most 4 ppm by volume, and yet more preferably at most
2.0 ppm by volume. Most preferably the chlorine content of low-chlorine containing synthesis gas is less than 1 .0 ppm by volume.
The chlorine content of a synthesis gas may be determined using techniques known in the art. Preferably the chlorine content of a synthesis gas is determined by a method based on the principle of laser spectroscopy, which is typically much simpler than the older extraction methods and brings the advantage that the measurement may even be performed in-situ or in-line. Suitable apparatus are for instance available from the company Ankersmid M&C (The Netherlands). This method also has a very low detection limit, typically of about 0.05 ppm volume HCI. A further advantage is that this same principle may be used to measure for sulphur, expressed as H 2 S, with the same advantages and a detection limit of about 3 ppm volume H 2 S. Another suitable method to analyse for sulphur as H 2 S is by UV absorbance, such as with the Hydrogen Sulfide Analyzer OMA- 300-H2S obtainable from the company Applied Analytics, which may also be arranged in situ or in-line.
In an embodiment of the present invention, the chlorine-containing combustible stream is a waste stream.
In another embodiment of the present invention, the chlorine-containing combustible stream is a solid waste stream, preferably a municipal solid waste stream (MSW) or an industrial waste stream. Suitable industrial waste streams are contaminated wood, tyres, car fluff or the like. A particularly suitable waste stream is a refused derived fuel (RDF) stream which is derived from a solid waste stream, such as a municipal solid waste stream.
The chlorine-containing synthesis gas, or syngas in short, which is generated by the plasma gasification step may have a temperature of at least 700°C, preferably at least 800°C, more preferably at least 900°C, even more preferably at least 950°C, yet more preferably at least 1000°C. Typically this syngas temperature is not more than 1800°C, preferably not more than 1500°C, more preferably not more than 1200°C. The lower temperatures bring the advantage that more common refractory material may be used in the construction of the plasma chamber and/or of the connection to the downstream convection section where the high pressure steam is generated according to step b) of the process according to the present invention.
In an embodiment of the method according to the present invention, the high pressure steam from step (b) is saturated.
This brings the advantage that the metal skin temperatures of the equipment in step (b) which contain the high pressure steam may be well controlled and be kept below the critical levels, such as for instance to avoid or alleviate the negative effects which chlorine and/or other halogens may have on the construction materials at too high temperatures, as explained above.
In an embodiment of the method according to the present invention, the temperature of the high pressure steam from step (b) is at most 300°C and optionally at least 200°C. By keeping this steam at a temperature of at most 300°C, the skin temperatures of the equipment containing the high pressure steam of step (b) may be well controlled and kept below the critical levels, in order to avoid or control the negative effects of chlorine and/or other halogens at an acceptable level.
In an embodiment of the method according to the present invention, the plasma gasification step (a) is a single stage or a two stage gasification step. In a single stage plasma gasification step, the fuel is directly exposed to the plasma torch, and gasification occurs inside the plasma reactor. With two stage plasma gasification, the organic starting material is fed to a gasification chamber where it is thermally decomposed with the heat generated from combusting recirculated syngas and insufficient primary air to support full combustion. The gas from the gasification chamber is then fed to the plasma reactor. The gasification chamber itself may be preceded by a drying chamber, wherein water from the starting material may be evaporated and recovered as steam. The single stage plasma gasification is more limited in the quality of waste fuels it may process, but produces a better quality syngas, i.e. with a higher caloric value. The two stage process is more versatile in terms of fuel quality, but requires a higher throughput to be economical. The two stage process is preferred for municipal solid waste, organic waste, or streams derived therefrom.
In an embodiment, the method according to the present invention further comprises a step (g) wherein heat from the hot exhaust gasses from the gas turbine in step (d) is used to generate a second stream of high pressure steam. There may be more heat available in the exhaust gasses from the gas turbine than what may be used in step (e), and/or the exhaust gasses may be cooled further down than what is desirable or possible with step (e). It is therefore preferred to also make use of this extra available heat for increasing the useful energy output provided by the method of the present invention.
In the embodiment with a step (g), the high pressure steam from step (g) is preferably fed to a steam turbine of which at least part of the mechanical energy is converted to electric power. This brings the advantage that the power output provided by the method of the present invention is further increased.
In an embodiment of the method according to the present invention, the temperature of the high pressure steam from step (g) is further increased by superheating before it is fed to the steam turbine. This brings the advantage that more of the energy in the high pressure steam from step (g) may be converted into mechanical energy output of the steam turbine while at the same time the possible erosion of condensing steam particles in the steam turbine remains controlled at an acceptable level.
In an embodiment of the method according to the present invention, the high pressure steam from step (g) is combined with the high pressure steam from step (b) before being fed to the same steam turbine. This brings the advantage that only one steam turbine needs to be provided, which reduces complexity of the plant operation as well as the investment cost for the equipment.
In an embodiment of the method according to the present invention, the high pressure steam from step (g) is superheated together with the high pressure steam from step (b) in the same superheater. This brings the advantage that only one steam superheater needs to be provided, which also reduces complexity of the plant operation as well as the investment cost for the equipment.
In an embodiment of the method according to the present invention, the metal skin temperatures in the high pressure steam generator of step (b) and/or of step (g), if present, are at most 450°C, preferably at most 430°C, even more preferably at most 425°C, yet more preferably at most 420°C, preferably at most 415°C, more preferably at most
410°C, yet more preferably at most 405°C, most preferably at most 400°C, and optionally at least 350°C, preferably at least 375°C, more preferably at least 390°C, even more preferably at least 400°C, yet more preferably at least 410°C, and preferably at least 420°C. Metal skin temperatures of piping in furnaces may readily be measured by techniques known in the art. We prefer to use contact thermocouples based on the Seebeck-effect, of which certain types are commercially available able to measure temperatures up to 800°C and even up to 1 100°C. Suitable thermocouples are obtainable from the company Fabritius (Belgium)
In an embodiment of the method according to the present invention, the superheated steam from step (e) has a temperature of at least 290°C, preferably at least 300°C, more preferably at least 350°C, even more preferably at least 400°C, yet more preferably at least 450°C, preferably at least 500°C, more preferably at least 550°C and even more preferably at least 570°C. These higher steam temperatures bring the advantage that a higher portion of the energy contained in the steam may be extracted by the steam turbine of step (f), while at the same time the possible erosion of condensing steam particles in the steam turbine remains controlled at an acceptable level.
In an embodiment of the method according to the present invention, the metal skin temperatures in the steam superheater of step (e) are at least 400°C, preferably at least 420°C, more preferably at least 440°C, even more preferably at least 460°C, yet more preferably at least 480°C, preferably at least 500°C, more preferably at least 520°C, even more preferably at least 540°C, yet more preferably at least 560°C, and optionally not more than 650°C, preferably at most 630°C, more preferably at most 610°C, even more preferably at most 580°C, yet more preferably at most 560°C. It is a major benefit of the present invention that the steam superheater of the process according to the present invention is exposed to flue gasses originating from combusting fuel which is sufficiently low in chlorine, or does not contain any chlorine or other halogens at levels which raise a concern for chlorine corrosion of metals at high temperatures.
In an embodiment of the method according to the present invention, the gas turbine of step (d) and the steam turbine of step (f) are driving the same common power generator. Such a so-called "single shaft" operation brings the advantage of a lower overall investment cost, reduces operating complexity, reduces the footprint required for the equipment, and reduces the start up costs for the method.
In another embodiment of the method according to the present invention, the gas turbine of step (d) and the steam turbine of step (f) are driving a different power generator. Such a so-called "multishaft" operation brings the advantage of higher operating flexibility, higher reliability. In a preferred embodiment, two gas turbines are provided, of which the second gas turbine may be kept as a spare for when the first gas turbine requires an intervention, such as maintenance.
In an embodiment of the method according to the present invention, the synthesis gas is combusted in the gas turbine using air as the oxygen carrier and/or with an oxygen-containing gas which is richer in oxygen than regular air.
In an embodiment of the method according to the present invention, the exhaust gasses from the gas turbine in step (d) have a temperature in the range of 400-600°C, preferably at least 420°C, more preferably at least 440°C, and optionally at most 560°C, preferably at most 540°C and more preferably at most 520°C.
In an embodiment of the method according to the present invention, the hot exhaust gasses from the gas turbine in step (d) are heated, preferably with supplementary firing, more preferably with the combustion of low-chlorine synthesis gas from step (c) and/or by using the excess oxygen which is present in the hot exhaust gasses from the gas turbine, before the exhaust gasses are being used in step (e). The extra heating of the exhaust gasses from the gas turbine brings the advantage that the overall energy efficiency of the method is increased, in particular when the equipment is operated at partial load. The extra heating step also provides for a very convenient extra control possibility to respond to power demand fluctuations. The extra heating may bring the temperature of the hot exhaust gasses from the gas turbine up to a level in the range of 600-900°C, preferably at least 650°C, more preferably at least 700°C, even more preferably at least 750°C.
In an embodiment of the method according to the present invention, the steam turbine of step (f) has at least two stages and at least part of the exhaust steam of the higher pressure turbine stage is heated further before it is being used to drive the lower pressure turbine stage.
In an embodiment of the method according to the present invention, the steam turbine low pressure exhaust is at a pressure of at most atmospheric pressure, preferably at most 0.7 bar absolute (bara), more preferably at most 0.5 bara, even more preferably at most 0.4 bara, yet more preferably at most 0.3 bara. The lower exhaust pressure allows the turbine to extract a higher proportion of the energy contained in the high pressure steam as mechanical energy, which improves the overall power output for the same energy input.
In an embodiment of the method according to the present invention, the exhaust steam of the steam turbine is condensed to form a condensate which is at least partly recycled to the high pressure steam generation in step (b) and/or the high pressure steam generation in step (g), if present, the condensate preferably being degassed or deaerated before it is recycled to the high pressure steam generation, the degassing or deaeration preferably being performed by injection of low pressure steam, the condensate preferably being preheated in an economiser before being recycled to the high pressure steam generation in step (e) or to the upstream deaeration step, if present. The condensing of the low pressure turbine exhaust allows the exhaust pressure of the turbine to be controlled by a vacuum pump, which is much more environmentally friendly than by steam jets. The condensing step also allows for an at least partial recovery of the condensate and recycle thereof as boiler feed water to the steam generation steps of the process or method, such that less fresh boiler feed water needs to be prepared, which requires chemicals and produces chemical waste. The heat from the condensing of the steam turbine low pressure exhaust is preferably utilized for drying the chlorine-containing combustible stream, or put to other use, such as for heating an aquaculture, or for city heating. In an embodiment this heat is released to the environment, preferably by condensing the water vapour in an air cooler. In an embodiment of the method according to the present invention, the chlorine-containing synthesis gas from step (b) is cooled before the chlorine removal step (c). The preferred temperature for the chlorine removal step depends on the selected system. The person skilled in the art will readily find the synthesis gas temperature which is preferred as entry temperature for the selected chlorine removal step.
In an embodiment of the method according to the present invention, the chlorine is removed in step (c) by passing the synthesis gas through a system selected from a wet scrubbing system, a semi-wet scrubbing system, a flash dry system, a dry adsorption system, and combinations thereof.ln a wet scrubbing system, the gas is fed into a water, hydrogen peroxide or/and a washing solution containing a reagent, such as for instance a sodium hydroxide solution. The reaction product is aqueous. A wet scrubbing system typically consists of a first wet scrubber at low pH to remove mainly HCI, and also HF, if present. If desired, it may be followed by a second wet scrubber at hig pH of 6-8 primarily for the removal of S02. Three or more stages may also be used, whereby the first low pH stage is being sub-divided into additional stages for specific purposes. The wet scrubbing system typically provides the benefit of the highest removal efficiency of soluble acid gases such as HCI, but also HF and S0 2 , when compared to its possible alternatives. They provide the possibility to treat these gases separate from particulates, which are often and preferably removed upstream, if present. A further advantage is that the plasma step produces hardly any dioxins. The typical risk for buildup of dioxins in the wet scrubbers is thus advantageously low.
In a semi-wet scrubbing system, also called semi-dry, a sorption agent is added to the gas in an aqueous solution (e.g lime milk) or suspension (e.g. as a slurry). The water solution evaporates and the reaction products are dry. The residue may be recirculated to improve reagent utilisation. A subset of this technique are the so-called flash-dry systems which consist of the injection of water, thereby giving fast gas cooling, and reagent at a filter inlet.
In a dry adsorption system a dry sorption agent, such as lime or sodium bicarbonate, is added to the gas flow. The reaction product is also dry.
If necessary, the synthesis gas may also be dedusted, upstream or downstream of the chlorine removal step, or in between different stages of the chlorine removal step, if present. Dedusting may be performed by using equipment selected from cyclones and/or mulit- cyclones, electrostatic precipitators (ESPs), bag filters, and combinations thereof.
Gas polishing steps may be performed, instead or in addition to dedusting steps, by the use of one or more bag filters, wet- ESPs, electrodynamic venture scrubbers, agglo-filtering modules, ionizing wet scrubbers, and combinations thereof. Also double bag filtration may be used, not necessarily immediately adjacent to each other.
For the removal of acid gas components such as HCI, HF and/or SOx, a variety of alkaline reagents may be used, such as sodium of potassium hydroxide, lime, lime stone, sodium bicarbonate, and combinations thereof.
In order to cope with varying concentrations of HCL, but also of HF and/or SOx, a gas polishing system may be added in which remaining traces from the upstream HCI removal step may be removed. Preferably a wet system is selected for such an additional polishing system.
In an embodiment of the method according to the present invention, also most if any sulphur present is removed from the chlorine-containing synthesis gas before the gas is fed to the gas turbine.
Preferably the synthesis gas which is fed as fuel to the gas turbine has a sulphur content, expressed as H 2 S, of at most 1000 ppm volume, preferably at most 500 ppm volume, more preferably at most 200 ppm volume, even more preferably at most 100 ppm volume, and yet more preferably at most 50 ppm volume. The applicants have found that the removal steps preferred for removing chlorine are also contributing to the reduction of the sulphur content of the synthesis gas which is treated, in case sulphur is present.
In an embodiment of the method according to the present invention, also the water concentration of the synthesis gas being fed as fuel to the gas turbine is reduced such that the syngas is "dry" at ambient temperature, i.e. that there is no water condensing when the gas is cooled down to a temperature of 25°C, preferably only 15°C, more preferably only 10°C, and even more preferably only 0°C. A suitable method for removing excess water is to cool down the wet synthesis gas such that water is condensed and may be removed by simple phase separation and be withdrawn. Such condensing may be performed in a stainless steel cooler. In order to remove excessively corrosive components upstream of this cooler, a wet gas washing step may be included before the cooling step.
In an embodiment of the method according to the present invention, the synthesis gas which is fed as fuel to the gas turbine is brought to a temperature of about 55°C. In case the gas needs to be warmed up to 55°C, a suitable method is to recycle part of the syngas compressor from the compressor outlet back to the compressor inlet via a simple bypass.
In an embodiment, the method according to the present invention is for the generation of electric power during a period of increased power consumption, preferably a period of peak power demand. It has been explained above that the method according to the present invention is for several reasons particularly suitable for responding to demand fluctuations, one main reason being that an inventory of the feed stream of the method may readily be kept, of which may be consumed at a variable rate which is adapted to a change in demand, such as during a period of peak power demand.
The plasma gasification in step (a) may be performed using methods known in the art. Suitable methods may for instance be found in WO 03/066779 or KR 10-2005-0102958. The plasma gasification step uses an electric arc gasifier, also known as a plasma torch, to create a high-temperature ionized gas which breaks organic matter in the feed stream up into synthesis gas. Most of the inorganic matter in the feed stream is usually retrieved as solid waste or slag. EXAMPLES
The present invention is now illustrated further using the flow sheet shown in Figure 1. Chlorine-containing combustible stream 1 is fed to plasma gasification step 100, where hot chlorine-containing synthesis gas 2 is generated. The hot chlorine-containing synthesis gas 2 is fed to a first steam boiler 101 , where heat from the gas is used to convert boiler feed water supply 15 into typically saturated high pressure steam 1 1 . The cooled chlorine-containing synthesis gas from steam boiler 101 is fed to a wet syngas cleanup step 102. The partially cleaned synthesis gas 4 from this wet cleanup step is passed through a stainless steel cooler/condenser 103 to remove a major portion of the water from the stream 4. The cooled and partially dewatered chlorine containing synthesis gas 5 from the cooler/condenser 103 is fed to the dry syngas cleanup step 104. In the combination of the wet syngas cleanup step 102, the cooler/condenser 103 and the dry cleanup step 104, a significant portion of the chlorine from the chlorine-containing synthesis gas is removed in order to produce the low- chlorine synthesis gas 6. This low-chlorine synthesis gas 6 is then compressed in syngas compressor 105. The compressed synthesis gas 7 is then used as fuel in gas turbine 106, of which at least part of the mechanical energy is converted to electric power in a first power generator (not shown). The combustion in gas turbine 106 produces hot exhaust gasses 8. The hot exhaust gasses 8 are fed to a steam superheater 107 wherein part of the sensible heat from the hot exhaust gasses is used to superheat the substantially saturated steam produced in first and second steam boilers 101 and 108, thereby producing a partially cooled stream of exhaust gas 9. The partially cooled exhaust gas 9 is fed to the second steam boiler 108, where further heat is removed from the partially cooled exhaust gasses 9 to convert boiler feed water stream 16 into substantially saturated high pressure steam 17. The thereby further cooled exhaust gas 10 may be discarded, such as as flue gas to the atmosphere. The superheated steam 12 from superheater 107 is used to drive steam turbine 200, of which at least part of the mechanical energy is converted to electric power in a second power generator (not shown). The turbine low pressure exhaust steam 13 is fed to condenser 201 , generating condensate 14, which is fed to degasser 202. The degassed water from degasser 202 is then pumped by a high pressure boiler feed water pump (not shown) as stream 15 to the first steam boiler 101 and as stream 16 to the second steam boiler 108.