A Waste-to-Energy plant comprising: an incineration chamber in which waste is combusted generating flue gas; an economizer heating feedwater using heat from the flue gas; an evaporator producing steam from the heated feedwater using heat from the flue gas; a steam drum receiving heated feedwater from the economizer and supplying heated feedwater, the steam drum receiving steam from the evaporator and supplying steam; and a superheater receiving and heating steam from the steam drum to a superheated steam using heat from the flue gas; the incineration chamber comprising a first PCM-wall and a second PCM-wall each comprising a plurality of pipes and a layer of PCM provided between the pipes and the incineration chamber, the pipes in the first PCM-wall receiving heated feedwater from the steam drum and producing additional steam therein and the pipes of the second PCM-wall additionally heating steam therein using radiant heat from the incineration chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a filing under 35 U.S.C. 371 as the National Stage of International Application No. PCT/SG2017/050314, filed Jun. 23, 2017, entitled “WASTE-TO-ENERGY PLANT,” which claims priority to Singapore Application No. SG 10201605197Q filed with the Intellectual Property Office of Singapore on Jun. 23, 2016 and entitled “WASTE-TO-ENERGY PLANT,” both of which are incorporated herein by reference in their entirety for all purposes.

FIELD

This invention relates to a waste-to-energy plant.

BACKGROUND

In Waste-to-Energy (WtE) plants, hot flue gas generated during combustion of waste is used to produce steam by heating feedwater and thereby regain energy by using the steam to drive turbines or for other energy processes such as cogeneration. Heat recovery of thermal power in the gas generated during the combustion process is currently completely carried out by traditional steam generation boilers which usually comprise an incineration chamber having water-walls (i.e. radiant evaporators), as well as evaporators, economizers and superheaters that use heat from the hot flue gas as their energy source. In the economizer, feedwater is heated prior to being fed to the evaporator which produces steam by converting the heated feedwater into wet steam. The wet steam is then converted into dry steam and raised to useful temperatures by the superheater. Dry heated steam from the superheater is channelled to produce useful work as electricity and/or heat. The water-walls of the incineration chamber where combustion of waste takes place also absorb heat released during combustion and this heat is also used to evaporate feedwater into steam that is channelled into the superheater.

However, steam generation boilers suffer thermal power fluctuation due to the waste combustion being characterized by a high variability in net calorific value. This leads to a few technical issues which limit the maximum net electric efficiency achievable in existing WtE plants, namely:i. the maximum steam temperature achievable by the superheater is limited due to corrosion of the superheater occurring at high metal surface temperature; andii. steam production by the evaporator fluctuates due to non-homogeneous composition of waste resulting in inconstant temperature of the hot flue gas that heats the feedwater into steam.

SUMMARY

Disclosed is a WtE energy plant developed around a technology based on Phase Change Material (PCM) capable of controlling steam temperature and steam production fluctuation by means of thermal energy storage. In the present system, the typical water-wall technology of existing boilers is replaced with a PCM-based technology capable of storing a variable heat flux coming from a high temperature heat source (i.e. the incineration chamber) and to release it on demand as a steady heat flux. This technique for heat storage allows designing thermal energy storage systems with a high energy density capable of storing heat at high temperature (>300° C.). By introducing the PCM-based technology, an extra degree of freedom is introduced in the heat recovery/management which did not exist before in combustion processes. The steady heat flux in the present system is used to avoid steam production fluctuation and to increase temperature of superheated steam over current corrosion limits (450° C.) without requiring the use of expensive coated superheaters. The PCM-based technology used in the present system has the potential to increase net electric efficiency above 30%, while still maintaining low maintenance costs and high plant availability.

According to a first aspect, there is provided a Waste-to-Energy plant comprising: an incineration chamber in which waste is combusted generating hot flue gas; at least one economizer to heat feedwater using heat from the hot flue gas; at least one evaporator to produce steam from the heated feedwater using heat from the hot flue gas; at least one steam drum to receive the heated feedwater from the at least one economizer and serve as a supply for the heated feedwater, the at least one steam drum further to receive the steam from the at least one evaporator and serve as a supply for the steam; and at least one superheater to receive the steam from the at least one steam drum and to further heat the steam to a superheated steam using heat from the hot flue gas; wherein the incineration chamber comprises a first PCM-wall and a second PCM-wall each comprising a plurality of pipes and a layer of PCM provided between the pipes and the incineration chamber, the plurality of pipes in the first PCM-wall receiving the heated feedwater from the steam drum and producing additional steam in the plurality of pipes in the first PCM-wall using radiant heat from the incineration chamber, and the second PCM-wall additionally heating steam in the plurality of pipes of the second PCM-wall using radiant heat from the incineration chamber to.

The PCM may comprise one of: aluminium and an inorganic eutectic aluminium alloy.

The steam in the plurality of pipes of the second PCM-wall may be superheated steam from the at least one superheater.

The superheater may be a low-pressure superheater, the steam in the plurality of pipes of the second PCM-wall may be the steam supplied from the steam drum, superheated steam from the second PCM-wall may be passed through a high pressure turbine, the superheater may receive and reheat steam from the high pressure turbine, and reheated steam from the superheater may be passed through a low pressure turbine.

The plant may further comprise a pump to control mass flow rate of steam between the first PCM-wall and the steam drum.

For at least one of the first PCM-wall and the second PCM-wall, the layer of PCM may be enveloped within a PCM-container, and the PCM-container may be made of at least one of: carbon steel having an Al2O3coating, carbon steel having a WC—Co coating, a ceramic inert to molten aluminium and molten aluminium alloys, and microencapsulation.

The PCM-container facing the plurality of pipes may be shaped to conform to a shape of the plurality of pipes.

For at least one of the first PCM-wall and the second PCM-wall, an air gap may be provided between the layer of PCM and the plurality of pipes.

DETAILED DESCRIPTION

Exemplary embodiments of a WtE plant100will be described below with reference toFIGS. 1 to 4. The same reference numerals are used in the different figures to denote the same or similar parts.

The WtE plant100is a PCM-based technology that exploits the working principle of thermal energy storage based on latent heat. This kind of heat storage system stores or releases latent heat when a PCM undergoes a phase transition from solid to liquid, or vice versa. The storage and release of heat occurs at the phase transition temperature of the PCM14in the WtE plant100which can be considered to be constant. This technique for heat storage allows designing thermal energy storage systems with a high energy density capable to store heat at high temperature (>300° C.). The PCM14used in the present WtE plant100is based on aluminium, its eutectic and near-eutectic alloys because they offer good thermal properties amongst high temperature PCM (Kenisarin 2010):High thermal conductivity (solid state>200 W/mK to water state>90 W/mK);High latent heat of fusion (280 to 560 KJ/kg)High melting temperature (470 to 660° C.)

FIG. 1shows a first exemplary WtE plant100integrating PCM-based technology with traditional heat recovery components in order to avoid fluctuation of steam production and to increase steam parameters. The traditional components include the incineration chamber20in which waste is combusted generating hot flue gas21, at least one economizer30using heat from the hot flue gas21to heat feedwater31, at least one evaporator40using heat from the hot flue gas21to produce wet steam41from the heated feedwater31, and at least one superheater50using heat from the hot flue gas21to further heat dry steam91to a superheated steam51. A steam drum90receives heated feedwater31from the economizer30and wet steam41from the evaporator40, and supplies the dry steam91to the superheater50and heated feedwater31to the evaporator40.

This integration is obtained by providing the incineration chamber20with PCM-walls11,12as shown inFIG. 2. Each PCM-wall11,12comprises a plurality of pipes13that are closely placed together for maximum heat absorption of the radiation heat from the incineration chamber20. A layer of PCM14is provided between the pipes13and the incineration chamber20. The PCM14is enveloped within a PCM-container15. For the material of the PCM-container15(which can be considered as the PCM-heat exchanger), several solutions are contemplated:Carbon steel container with Al2O3coating, applied through an aluminizing and oxidation treatment, or WC—Co coating applied through high velocity oxygen-fuel technique (López & Rams 2015);Ceramics characterized as inert to molten aluminium and its alloys (such as graphite, alumina-silicate refractories, AlN, Al2O3, Si3N4, and sialons) (Yan & Fan 2001);Microencapsulation of Metal-based PCM (Nomura et al. 2015).

Each PCM-wall11,12can adopt a rear-ventilated solution by maintaining an air gap16between the PCM-container15and the pipes13. Sealing air in the air gap16ensures a non-corrosive atmosphere at the pipes13and consequently a long lifetime of the pipes13. Alternatively, if heat transmission is preferred to or prioritized over corrosion protection, the PCM-walls11,12can be installed in direct contact with the pipes13and the air gap16can be avoided or eliminated. Preferably, a surface15-1of the PCM-container15that faces the pipes13is shaped to conform to the shape of the pipes13in order to maximize heat absorption by feedwater31in the pipes13. On another side of the pipes13away from the PCM-container15and PCM14is a layer of insulation17to minimize heat loss from the pipes13.

The PCM-walls11,12serve as two heat exchange components respectively: a PCM-Evaporator64and a PCM-Superheater65. The PCM-walls11,12store a part of the fluctuating thermal power generated by the waste combustion process. The stored thermal energy is then steadily transferred to the PCM-Superheater65for steam temperature increase and to the PCM-evaporator64for steam production control. Specifically, the PCM-Evaporator64receives heated feedwater31from the steam drum90via pump66to produce additional wet steam641that is fed back to the steam drum90. The PCM-Superheater65receives superheated steam from the superheater50and further increases temperature of the superheated steam51.

In particular, aluminium is the most suitable PCM in the PCM-Superheater65for steam superheating because it has the highest melting temperature (660° C.). The use of aluminium as high temperature PCM allows heating the superheated steam up to 550-600° C., thus leading to very high efficiency of the WtE plant. For steam production control, the eutectic alloy Al-12Si is more suitable as the PCM in the PCM-Evaporator64because of its higher latent heat of fusion (560 KJ/kg) and lower melting point (576° C.). Steam generation into the PCM-evaporator64can be easily managed by varying the mass flow rate of a pump66which connects the steam drum90to the PCM-evaporator64; generated wet steam641from the PCM-evaporator64is then used to completely avoid fluctuation in steam production.

FIG. 3shows a second exemplary embodiment of the WtE plant100configured for obtaining high steam parameters. In the description that follows, reference numerals shown in brackets refer to reference numerals shown in rectangular boxes inFIG. 3indicating sequential flow of the heat exchange fluid (water/steam) in the plant100. In this embodiment, feedwater31is pumped (1) into an economizer30that heats the feedwater31using heat from the flue gas21arising from combustion in the incineration chamber20. The heated feedwater31is then channelled (2) through a steam drum90and fed (3),(4) into two evaporators40-1,40-2. A mixture of water and steam41generated by the two evaporators40-1,40-2using heat from the flue gas21is channelled (5), (6) back to the steam drum90. Heated feedwater31in the steam drum90is pumped (7) through a pump66into a first PCM-wall11of the incineration chamber20, the first PCM-wall11serving as a PCM-Evaporator64. Additional steam641produced by the PCM-Evaporator is fed (8) into the steam drum90. Dry steam91in the steam drum90is passed (9) into a superheater50for further heating using heat from the flue gas21. Superheated steam51from the superheater50is fed (10) into a second PCM-wall12of the incineration chamber20, the second PCM-wall12serving as a PCM-Superheater65. The PCM-Superheater65additional heats the superheated steam51from the superheater50to produce superheated steam651at high temperature that is passed to (11) and used to drive a turbine80. Exhaust steam801from the turbine80is passed (12) through a cooling system85where it is condensed and then recovered in a feedwater tank33. Feedwater31from the feedwater tank33is channelled (13) into a feedwater pump32that pumps (1) the feedwater31into the economizer30.

In a third exemplary embodiment of the WtE plant100that includes a steam reheating cycle as shown inFIG. 4, feedwater31is pumped into an economizer30that heats the feedwater31using heat from the flue gas21arising from combustion in the incineration chamber20. The heated feedwater31is then channelled through a steam drum90into two evaporators40-1,40-2. A mixture of water and steam generated by the two evaporators40-1,40-2using heat from the flue gas21is channelled through the steam drum90. Feedwater31is pumped through a pump66into a first PCM-wall11of the incineration chamber20, the first PCM-wall11serving as a PCM-Evaporator64. Additional steam641produced by the PCM-Evaporator64is fed into the steam drum90. Dry steam91from the steam drum90is passed into the second PCM-wall12serving as a PCM-Superheater65to additionally heat the steam. Additionally heated steam651(that may have a pressure of about 90 bar and a temperature of about 600°) from the PCM-Superheater65is passed to (11) and used to drive a high-pressure turbine80. Output steam801(that may have a pressure of about 23 bar and a temperature of about 260° C.) from the high-pressure turbine80is passed (11) into a low-pressure superheater52which reheats the steam using heat from the flue gas21. Output reheated steam521(that may have a pressure of about 23 bar and a temperature of about 420° C.) from the low-pressure superheater52is used to drive a low-pressure turbine81. Exhaust steam811from the low-pressure turbine81passes (13) through passed through a cooling system85where it is condensed and then recovered in a feedwater tank33. Feedwater31from the feedwater tank33is channelled (14) into a feedwater pump32that pumps (1) the feedwater31(that may have a pressure of about 90 bar and a temperature of about 130° C.) into the economizer30.

Table 2 below shows an exemplary WtE plant100configuration using PCM-based technology for steam superheating and steam production control.

The above described WtE plant100integrating PCM-walls11,12not only uses PCM14as a heat storage system but also exploits the PCM14as an interface between the heat source (i.e. waste incineration in the incineration chamber20) and the heat transfer fluid (i.e. water31/steam41,641,51,651). This additional characteristic allows decoupling of the heat exchange between the heat source and the heat transfer fluid, which means that the thermal behaviour of both the heat source and the heat transfer fluid depends only on the melting temperature of the PCM14. Thus, this feature of the PCM-walls11,12, combined with the feature of heat storage, enables controlling superheated steam temperature and mass flow rate in the WtE plant100, while increasing waste combustion control. In particular, it provides the following advantages (in order of importance):I. The presently disclosed WtE plant100allows the installation of additional superheaters (i.e. PCM-Superheaters65) in the incineration chamber20, where the temperature of the heat source (waste combustion) is highest. This particular arrangement enables the generation of superheated steam at a higher temperature than that achievable in current WtE plants. In fact, the closer to the heat source is the superheater, the higher the steam temperature and the overall efficiency of the WtE plant. It is worth noting that in current WtE plants, the installation of superheaters within the incineration chamber is hindered by the thermal power fluctuations, which lead to uncontrollable overheating of the tubes and consequent tube failure, resulting in additional maintenance costs and shutdown of the plant. Currently, only the radiant evaporators (i.e. the water-walls) can operate properly within the incineration chamber since they work at constant temperature (evaporation temperature) exploiting the water vaporisation (characterized by high value of latent heat).II. The presently disclosed WtE plant100provides an additional degree of freedom on control systems, which allow optimization of the waste combustion control without affecting the steam production (as currently occurs in WtE plants). This feature leads to increase of the waste throughput while increasing the overall energy efficiency of the whole plant100. Both the PCM-evaporator64and the PCM-superheater65have this feature.III. The presently disclosed WtE plant100absorbs directly a part of the thermal power fluctuation from the incineration chamber20, thereby allowing the downstream heat recovery components (evaporator40, superheater50and economizer30) to operate with a lower thermal power fluctuation, thus increasing their lifetime.

Table 2 below shows a comparison between a latest prior art configuration to improve WtE plant efficiency and the presently disclosed WtE100adopting the PCM-based technology. The comparison is based on the methodology described in (Main, Armin Maghon 2010) and it considers as a baseline (under the column “Basis”) the typical WtE plant configuration (i.e., 400° C. at 40 bar). It can be clearly seen that the presently disclosed WtE100adopting PCM-based technology can achieve a steam temperature up to 600° C. leading to a dramatic increase in gross electrical efficiency up to 31.4%.

Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations and combination in details of design, construction and/or operation may be made without departing from the present invention. For example, the number of economizers, evaporators, superheaters, and steam drums may be varied as desired from the numbers disclosed for the embodiments described above. For example, further flue gas cooling by means of an external economizer heating boiler feedwater and a heat exchanger preheating primary and secondary air may be provided.