Field of the invention

The invention relates to the field of domestic heating systems. The invention discloses improvements to heat recovery in a domestic heating system, for example a heating system for a building comprising a main heater, such as a boiler, and a heat recovery module.

Prior art

A relevant part of the today's energy consumption is for domestic heating. Moreover, domestic heating systems are known to produce a major contribution to air pollution, expecially in densely populated areas. A domestic heating system releases significant quantities of pollutants such as carbon monoxide (CO), non-combusted hydrocarbons (HC), nitrogen oxides (NOx) and particulate. Many domestic heating systems are obsolete and/or subject to poor maintenance and hence have a low efficiency; their replacement with new systems however is expensive, especially if civil works are needed. Hence, there is a strong incentive to increase the overall efficiency and to lower the emissions of domestic heating systems.

WO 2006/067820 discloses a system where return water is pre-heated in a condensation module coupled to the boiler. Said condensation module is able to cool the combustion fumes down to around 35 °C, by means of a water/fume heat exchanger and a heat pump, thus recovering the latent heat of the fumes and achieving a good efficiency. The pollutants are correspondingly reduced.

With regard to the treatment of fumes, the prior art generally teaches to install fumes treatment devices such as mechanical or catalyst filters in a by-pass duct in parallel with the exhaust duct, see e.g. EP 1 606 554. Recovering the heat of the fumes discharged by the boiler is attractive because the fumes have a relatively high temperature of about 150 °C, suitable for heat recovery at attractive cost, and recovering the heat of fumes allows to enhance efficiency and to reduce pollution.

The prior art heat recovery system are substantially based on heat exchange elements, such as tubes or plates, externally heated by the fumes. A drawback of these systems is that the surface of said heat exchange elements is exposed to fouling caused by soot and particles of the fumes . Said foul ing sign ificantly decreases the heat exchance coefficient, and the design and size of the heat exchange elements must take this aspect into account, i.e. the heat exchange elements must be larger in order to guarantee the nominal performance even when fouling is present. For example, the particles of the fumes may be responsible for cost around 20/30% h igher and size about 40% greater of the heat exchange elements depending on the type of fuel burnt in the boiler; cheaper fuels generates greater fouls. Moreover, also the pressure drops are higher due to increased contact surface and thus friction forces on the heat exchangers walls.

Summary of the invention The aim of the invention is to solve the above drawbacks, and to improve the known technique of heat recovery in a domestic heating system.

This aim is reached with a domestic heating system comprising at least a main heater adapted to heat a thermal fluid by combustion of a fuel, and a heat recovery module arranged to preheat said thermal fluid by recovering heat from combustion fumes discharged by said main heater, said module comprising at least one heat exchanger between said fumes and said thermal fluid, the system being charaterized by comprising a particulate removal system, adapted to remove particulate matter from the fumes, said particulate removal system being located upstream said at least one heat exchanger of the heat recovery module.

The term of particulate, in this description, is used to mean any particulate matter (PM), and in particular the organic matter such as soot particles released by the combustion process.

Said particulate removal system may involve, according to preferred embodiments of the invention, at least a particulate filter and/or a catalytic converter adapted to provide further oxidation of the particulate contained in the fumes, upon the action of a suitable catalyst.

Any suitable filter and/or catalyst may be used in the invention. According to preferred embodiments, the particulate filter is a ceramic fiber or metal fiber filter. The catalytic converter, when provided, comprises a suitable catalyst for oxidizing the particulate, on a suitable carrier. In a preferred embodiment, the catalyst is supported by a high-surface alumina layer of the carrier, to enhance the contact surface between the catalyst and the fumes. Suitable catalysts are known, for example, from the applications to diesel engines and are also applicable to the invention. For example, a catalyst comprising noble metal oxides can be used.

Said main heater may be a conventional boiler, for example a smoke tube boiler. The thermal fluid is preferably water. The heat recovery module preferably comprises a heat exchanger between the fumes and the thermal fluid, and a heat pump disposed to transfer further heat from the fumes to said thermal fluid. Said heat pump is preferably adapted to cool the fumes to their dew temperature, thus recovering the latent heat of vaporization.

The maximum recoverable heat of vaporization is the difference between lower heating value (LHV) and higher heating value (HHV); depending on the fuel and the combustion conditions it is possible to know in advance the maximum recoverable energy from the water phase change. In the following table gives the gross energy percentage of recovery achievable trough condensation.

FUEL Gross Gross Theoretical

Condensing Cooling Maximum

Energy Margin Energy Recoverabli

Margin

ΔΤ = 100°C

Coal 2.3% 3.52% 5.8%

Diesel 6.30% 3.41% 9.7%

Natural gas 10.4% 3.62% 14.0%

Methane 10% 3.65% 13.3%

Hydrogen 15.4% 3.34% 18.7%

Catalytic conversion usually require a high temperature to operate, for example around 350 °C. If the particulate removal system includes a catalytic converter, the heating system or the heat recovery module preferably comprises a heater adapted to heat the fumes to a suitable temperature for the catalyst. According to a preferred embodiment, the main heater is fed with a rich fuel mixture containing some excess of fuel compared to stoichiometric value, to promote the further combustion that takes place in the catalytic converter.

The catalytic conversion has the advantage that an additional heat is del ivered and the temperature of the fumes is increased; hence the downstream heat exchange elements of the recovery module have the benefit of a greater difference of temperature between the fumes and the thermal fluid. The advantage can in standard conditions around 0.3-0.4%; by means of a mod ified rich bu rn generated for the purpose, the advantage can be greater and reach around 1 % of recovery with minimal or null exchanger performance reduction.

Accord ing to another aspect of the invention , at least one catalytic converter is positioned on the path of the fumes, in a suitable position so that the temperature of the fumes flowing through said at least one catalytic converter is in the range required for operation of the catalyst. In this way, no additional heat input is necessary to bring the fumes to a temperature suitable for the catalytic reaction . For example, suitable catalyst carriers may be inserted inside the tubes of a smoke tube boiler. More in detail , a smoke tube boiler may comprise tubes arranged in multiple passages, usually three passages. The temperature of the fumes gradually decreases along the tubes, while heat is exchanged with the hot water outside the tube bundles. The catalyst carriers are provided at an appropriate position where, under normal operation, the temperature of the fumes is suitable for the catalytic reaction. Said temperature is usually in a range which spans from 300 to 450 °C with a preferred value around the place were 350 °C according to the standard local temperature profile.

According to a further and preferred aspect of the invention, the tubes comprise swirlers (also called turbulators) that act as catalyst carriers. In a preferred embodiment, a smoke-tube boiler comprises at least some tube swirlers that directly support a catalyst adapted to oxidize particulate. For example the swirlers are coated with a h ig h-surface alumina layer supporting a suitable catalyst.

Still another aspect of the invention is that conventional swirlers may be treated to directly support a catalyst: for example, metal swirlers of a tube smoke boiler are coated with a high-surface alumina layer supporting a suitable catalyst.

An inventive feature is the following. A boiler comprising heat exchange tubes and swirlers inside said tubes can be revamped by coating at least partly the tube swirlers with a suitable catalyst, or by replacing at least some of the swirlers with catalyst-coated swirlers. Some or all of the tube swirlers may be coated with catalyst, or replaced with catalyst-coated swirlers, and each of said swirlers may be coated partly or fully with the catalyst. Preferably said catalyst is supported by a high-surface washcoat such as alumina coating. Preferably the catalyst is adapted to oxidize the particu late of the fumes; an appropriate catalyst may be selected depending on the fuel and composition of fumes; more preferably the catalyst includes a percentage of Palladium in the mixture, especially for diesel fuels.

A further inventive feature is the following. Use of swirlers inside heat exchange tubes is known, to improve the overall heat exchange coefficient of a fluid current flowing inside the tubes. Known swirlers have continuous or roughly segmented surfaces or flanges to direct the fluid current, thus driving said current in a fluid pattern having substantially a "corkscrew" shape, following said flanges. Said pattern is however relatively regular and only a thin fluid layer is actually in contact with the surface of the flange. In an inventive arrangement, a swirler for a heat exchange tube has a plurality of flanges having passing-through apertures, such as cuts or holes, allowing passage of the fluid current from one side to the other side of the flange. In other words, the fluid current is allowed to cross the surfaces of the swirler; this produces more deviation of particle patterns that are led to impact more easily in the flange surface. A first advantage is that the heat exchange coefficient is further increased; in preferred embodiments the swirler is at least partly coated with a catalyst layer, to remove pollutants from the fluid current, e.g. to remove particulate from an exhaust gas current. In this case, a further advantage is that a closer contact is achieved between the fluid current and the catalyst layer thus increasing dramatically the amount of removed particle. For example, the swirler is formed as a twisted ribbon made of metal and covered by alumina or similar supporting substrate, and coated by catalytic coating of pure or noble metal mixtures.

In preferred embodiments, said swirler(s) is/are inserted in specific location of the heat exchange tubes, where under normal condition the temperature of the fluid is suitable to promote the effect of the catalyst. For example swirlers coated with a catalyst adapted to oxidize particulate matter are placed in smoke tubes, preferably where the temperature of the fumes is 300 to 450 °C.

Other embodiments of the invention provide a combination of a catalytic converter and a particulate trap. For example, a relatively coarse catalytic converter may be provided to remove pollutants, and a filter such as a centrifugal filter may be adopted downstream the catalytic converter, to remove the particulate.

The above disclosed features may be combined to provide several embodiments of the invention.

The advantages of the invention are now discussed.

The removal of particulate makes any downstream heat exchange more efficient. By reducing the main source of fouling, the heat exchange coefficient is made more constant during time, and then smaller and less expensive heat surfaces are sufficient. Hence, the heat recovery is more effective and more attractive from economical point of view.

The invention has an important advantage also from the environmental point of view. The particulate is now one of the major concerns with regard to air pollution, particularly in natural gas-fired systems which uses a "clean" fuel" and then cause no or little concerns about other pollutants such as SOx or HC. The health effects of particulate are known to include damage to lungs and arteries, causing risk of heart disease and other problems; hence, the reduction of particulate emissions from domestic heating systems, which is made possible by the invention , is a great advantage.

Simple embodiments of the invention with a mechanical particulate filter have the advantage of a low cost, and requ ire maintenance (filter cleaning) about every 6 months or once a year, which is fully acceptable in a domestic heating system. Embodiments using catalytic converters have the advantage of a h igh efficiency in removing the particulate and , moreover, the further advantage that the catalytic reaction increases the temperature of fumes and then gives more heat to be recovered by the downstream heat exchangers. Embodiments with a catalytic converter and a particulate filter combine the advantage of a lower cost for the catalyst, and very efficient removal of pollutants from the fumes.

A further advantage of the invention is the ability to recover the latent heat of the fumes, thus providing a high thermal efficiency. A related and remarkable advantage is that by condensing the steam contained in the fumes, the volumetric flow rate is greatly reduced. This is quite important because in some cases, especially in old buildings, the exhaust stack does no longer comply with safety regulations, e.g. in terms of possible leakage of CO, HC, etc... and need to be internally l ined by adding a jacket of a suitable material. Adding the jacket however may be incompatible with the minimum required internal diameter. Thanks to the invention, the volumetric rate of fumes is smaller and then also the minimum applicable internal diameter of the stack is smaller; an existing stack can be l ined with stainless steel tubes, or a new and smaller stainless steel stack can be inserted in an existing old stack. In this last case, another advantage is that the free space around the new stack will act as a very cheap heat insulator allowing an optimal discharge of exhaust.

A further advantage of the invention is the drying of fumes, allowing a better stability in the combustion according to the following explanation. Fumes with water vapours has a greater mass than dry fumes, on the other hand vapour density is less than the average density of the gas contained in the fumes allowing a good evacuation in standard conditions. When the pressure is low and/or the temperature is low or in general when the chimney insulation is not good, the vapours contracts its volume lowering the temperature faster than the gases due to local condensation phenomena and in the worse conditions th is could lead to sensible counterpressure in the chimney, heat pipes and thus the combustion chamber leading to a bad combustion producing high quantity of unburnt and particles in the fumes.

The features and the advantages of the invention shall become clearer from the following detailed description and with the help of the attached figures.

Brief description of the figures

Fig. 1 is a scheme of a domestic heating system according to a first embodiment of the invention.

Fig. 2 is a scheme of a domestic heating system according to a second embodiment of the invention

Fig. 3 is a temperature - heat diagram of the fumes in a heating system according to Fig. 1 .

Fig. 4 is a temperature - heat diagram of the fumes in a heating system according to Fig. 2.

F ig . 5 is a cut-out view of a heat exchange tube showing a novel arrangement for a swirler according to a further inventive feature.

Detailed description

Fig . 1 shows a first embodiment of the invention . A domestic heating system comprises: a boiler 1 with a combustion chamber C, a heat recovery condensation module 2, radiators 3, hot water delivery and return pipes 4, 5, a duct 6 for directing the hot fumes from the combustion chamber C to the modu le 2, and a flue pipe 7. The heat recovery condensation module 2 has a condensate discharge 8.

Hot water for the radiators 3 is delivered for example at around 90 °C to pipe 4, and returns for example at around 60 °C through pipe 5. Before entering the boiler 1 , the return water is pre-heated in the condensation module 2, where the fumes are cooled down to dew point in order to recover also the latent heat.

Said condensation module 2 comprises a fume/water heat exchanger 10 and a heat pump 1 1 , composed basically of an evaporator 12, compressor 13, condenser 14 and lamination valve 15. The working fluid of said heat pump 1 1 is a suitable refrigerant, such as R134a or any other suitable fluid or mixture. The boundary of the module 2 is indicated by the dotted line in Fig. 1 .

The hot fumes F in the d uct 6 are for exam ple at arou nd 1 50 °C . Circulation of the fumes through the exchanger 10 and evaporator 12 is allowed by a fan 16.

The return water in pipe 5 is first pre-heated in the condenser 14, receiving heat from the condensing refrigerant of the heat pump 1 1 . The water may be heated for example from 60 to around 64 °C. The pre-heated water 5a then is further heated by the combustion fumes F in the gas/water heat exchanger 10, exiting as preheated water stream 5b for example around 67 °C. It can be noted that the fumes F, after passing through the gas/water heat exchanger 10, act as the heat source of the evaporator 12. Evaporation of the working fluid of the heat pump 1 1 takes place at a low temperature, for example less than 20 °C; hence the fumes can be cooled down to 35 - 40 °C , thus exploiting also the latent heat of vaporization of the water vapour contained in the fumes F. Using special mixtures with boiling boint lower than R134a or similar, the undercooling wall could be maintained at very low temperature below the dew point reaching the almost complete drying of fumes the sole limitation being in the mixing phenomena of the fume patterns.

The advantages of the module 2 are that a further amount of heat is recovered from the fumes, and that dry fumes F' are discharged to the flue pipe 7. The dry fumes F' have a much smaller volume than fumes F.

According to the embodiment of Fig . 1 , a catalytic converter 20 is integrated in the heat recovery condensation module 2, upstream the heat exchanger 10 and evaporator 12.

When a catalytic converter is used, the fumes needs to be at a sufficient temperature for the catalytic reaction, typically around 350 °C. To this purpose, the current of fumes F is heated by a gas heater 21 upstream the catalytic converter 20. The gas heater 21 may be realized for example with electric resistances immersed in the gaseous current of fumes, or by induction.

The benefit of the converter 20 is that the fouling of the exchanger 10 and evaporator 1 2 is su bstantially reduced and then the nom inal heat exchange coefficient is greater, and smaller and less expensive units can be adopted. It should be noted that the heat input of the gas heater 21 is partially recovered by the downstream heat exchanger 10 and evaporator 12; thanks to this synergistic effect of converter 20 and condensation module 2, the need of a further heat input in order to catalytically remove the particulate, does not compromise the overall efficiency. Moreover, the catalytic reaction of the particulate in the converter 20 gives a further temperature increase (around 10 °C, for example) of the fumes, to the benefit of the efficiency of the module 2.

In a simpler and less expensive embodiment, said catalytic converter 20 may be replaced with a particulate filter; in this case the gas heater 21 is not necessary.

Fig. 2 shows another embodiment, where the boiler 1 is a smoke-tube boiler. The hot fumes (around 600 °C) produced in the flame region B of the combustion chamber, passes through one or more tube passages, in the example three tube passages 30, 31 and 32, exchanging heat with the hot water. The hot fumes cools along said tube passages (for example from 600 to 150 °C), while the hot water is heated from the temperature of the pre-heated flow 5b to the delivery temperature of flow 5a.

According to this embodiment of the invention, suitable catalyst carriers 22 are placed inside the tubes of the boiler 1 , at a suitable location that corresponds to the fume temperature suitable for the catalytic reaction. In other words, in this second embodiment the catalytic converter is "shifted" to intercept the hot current of fumes, where the fumes are at the right temperature for the catalytic reaction, thus avoiding the need of a gas heater.

As shown in Fig . 2, the catalyst carriers 22 may be part of the heat recovery module 2 , i .e . sa id cata lyst carriers may form part of a revamping/boosting of the boiler 1 , by adding said module 2.

In a preferred embodiment, the catalytic elements 22 are internal swirlers of the tubes, coated with a h igh-surface alumina layer supporting a suitable catalyst.

Fig . 3 shows a temperature-heat diagram of hot fumes F in a system according to Fig. 1 , wherein:

- in regions (c), (d) and (e), the fumes are cooled in the boiler 1 , for example in the various tube passages of the boiler,

- in regions (f) and (g), the fumes at around 150 °C are re-heated to about 350 °C by the heat input of the gas heater 21 , and by the further heat input coming from the oxidation of particulate contained in the fumes, taking place in the converter 20,

- in regions (h) and (i), the fumes releases heat to the heat exchange means of the module 2, namely to the gas/water heat exchanger 10 and to the evaporator 12.

It may be appreciated that the module 2 is able to cool the fumes to a very low temperature (lower than that of return water 5) thanks to the heat pump 1 1 . It is further appreciated from Fig. 3 that the heat input of the gas heater 21 is recovered in an efficient way by the heat exchange in regions (h) and (i).

Fig . 4 shows a temperature-heat diagram of hot fumes F in a system according to Fig. 2, wherein:

- in regions (c), (d) and (e), the fumes are cooled in the boiler 1 , for example in the various tube passages of the boiler, and regions (g) indicate the temperature increase in the fumes while passing through the catalytic elements 22 inside the tubes,

- in regions (h) and (i), the fumes releases heat to the gas/water heat exchanger 10 and to the evaporator 12, as above.

Fig . 4 allows to appreciate that the fumes pass through the catalyst elements 22 while they are at the temperature of about 350 °C, namely the temperature of maximum efficiency of the catalytic reaction. Fig. 4 further allows to appreciate that the increase in the temperature (regions (g)) gives a positive effects to all the downstream heat exchange equipments.

Fig . 5 shows an exempl ificative embod iment for a swirl device or turbulator. A heat exchange tube or pipe 100 comprises an internal swirler 101 . The swirler 101 in the shown embodiment consists of twisted metal strips 102, 103, each metal strip having edges lying on a helix. This shape is shown as an example and other shapes, according to the known art of turbulators for boilers, are possible.

The twisted strips 102, 103 are covered by alumina or equivalent high- surface supporting substrate, and said substrate is further coated by a suitable catalyst, preferably pure or noble metal mixtures. The surface of the strips 102, 103 has a plurality of cuts 104 allowing the gaseous flow to "cross" the strips from one side to the other. This cross-flow of the gas allows more intimate contact with the catalyst.

For example, the tube 100 is a smoke tube of a boiler, such as tubes in passages 30, 31 , 32 of the boiler 1 (Fig. 2) and the catalyst is adapted to oxidize the particulate matter contained in the fumes. An arrangement as in Fig. 5 allows a better removal of said particulate and then more clean fumes are obtained.