The invention relates to a method for reducing contaminants, in particular in a gas mixture containing a burnable gas. Also, the invention relates to a corresponding apparatus for employing said method and to a system comprising the apparatus.

Biomass gasification technology is based on a thermochemical process that converts solid dry biomass, such as wood or agricultural wastes, into a burnable gas. The burnable gas, also known as wood gas or syngas, can be used as a fuel for Internal Combustion Engines (ICE) where its chemical energy is converted into mechanical, electrical or thermal energy.

The syngas is usually defined as a low calorific value fuel, since its Low Heating Value (LHV) can be in the range of 4 to 6 MJ/Nm ³ . Furthermore, the composition of the gas can change during operations according with the thermochemical conditions of the gasification reactor and the quality of the biomass fuel itself.

The major criticality connected with syngas produced from biomass gasification is that it is intrinsically contaminated by heavy condensable organic compounds, called tar. The average tar concentration can vary in the range of 0.1 to 50 g/Nm ³ according to the reactor technology adopted. Tar concentration is highly dependent on the two phases at which a gasification system can be operated, that is Start-up and Shut-down phase and Steady-State phase.

During the Start-up and Shut-down phase, the tar concentration of the wood gas is higher and the gas is therefore unsuitable for ICE combustion. Consequently, the gas is diverted into a secondary combustion equipment, normally represented by a flare stack, whose purpose is to completely burn the gas in order to be compliant with emission levels by legislation.

During the Steady-State phase, the tar concentration of the wood gas is lower (but not zero) and thus the gas is diverted to the ICE where it is used as a fuel to produce electrical and thermal power. Since the concentration of tar represents the main criticality for a durable and reliable ICE operation, the contaminants are usually removed by specific filtering equipment. Prior art solutions to reduce contaminants in burnable gases are often expensive (especially for the operating costs) and/or complicate since use systems made of several parts and components.

An object of the invention is therefore to provide a method and an apparatus for reducing contaminants in a burnable gas that is easy to implement and that can be efficient in producing a cleaned burnable gas used for heat and power production or other purposes.

The object is solved by a method for reducing contaminants in a burnable gas comprising producing a lean gas mixture comprising at least air and the burnable gas and completely oxidizing the lean gas mixture through a catalytic process during a first operating phase to obtain an exhaust gas. Also, the method comprises producing a rich gas mixture comprising at least air and the burnable gas and partially oxidizing the rich gas mixture through a catalytic process during a second operating phase to obtain a cleaned burnable gas, preferably for heat and power production.

In this way, a catalytic process using a single corresponding catalytic device can be used as contaminant abatement approach during two different operating phases. In particular, in the first operating phase the gas mixture is completely combusted through a catalytic oxidation and in the second operating phase the gas mixture is cleaned, i.e. the contaminants are reduced, through the catalytic process. It is noted that whereas the catalytic device used for the oxidation processes during the two operating phases is essentially the same, the process conditions are different. In fact, during the first operating phase the catalytic process is carried out on a lean gas mixture whereas in the second operating phase the catalytic process is carried out on a rich gas mixture. Advantageously, by using the same catalytic device for completely oxidizing the gas mixture in the first phase and for removing contaminants in the second phase, this method can reduce both operating and capital expenditures of systems requiring the production of burnable gases cleaned by contaminants.

Lean combustion or lean-burn refers to a burning process of a gas with an excess of air (considered in an air to burnable gas ratio) in an internal combustion engine or in a generic combustion chamber. Rich combustion or rich-burn refers, on the other hand, to a burning process of a gas with a deficiency of air (considered in an air to burnable gas ratio) in an internal combustion engine or in a generic combustion chamber. According to an embodiment, the burnable gas can be a syngas produced by a gasification process in a gasification system or gasifier. For example, the syngas can be a wood gas. The first operating phase can correspond to a Start-up and/or a Shut-down phase of the gasification system and the second operating phase can be a Steady-State phase of said gasification system. In particular, in the lean gas mixture, the airto burnable gas ratio is higher than 1 , and in the rich gas mixture, the air to burnable gas ratio is lower than 1. In this way, the complete oxidation is carried out through the catalytic process during the Start-up and/or Shut-down phase of the gasification system and the contaminant removal with a partial oxidation is carried out through the catalytic process during the Steady-State phase. Therefore, a clean gas is produced and can be used to feed an internal combustion engine.

In particular, an air excess factor l can be defined as the ratio of the actual airto burnable gas ratio of the gas mixture relative to the stoichiometric air to burnable gas ratio. An air excess factor l higher than 1 means that there is too much air. On the other hand, an air excess factor l lower than 1 means that there is not enough air for all of the burnable gas to combust.

It is noted that the contaminants treated according to the present method can be tars in the burnable gas. The high temperature of the rich gas mixture through the catalytic process during the second operating phase can determine a cracking of the contaminants, i.e. of said tars, according to which long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons.

The inventive method can be advantageously used in other industrial or Oil&Gas processes not necessarily related to biomass gasification. This method can be used for all the systems producing waste gas streams which have a variable composition during time, according to process operational parameters.

It is noted that the present method comprises the step of controlling the oxidation of the lean gas mixture and/or the rich gas mixture over operational time periods. In other words, differently from prior art solutions where oxidation is changed along the radius of the reactor in order to obtain a uniform spatial oxidation along the reactor catalytic surface, in the present method the oxidation changes along time. In this way, it is possible to deal with transient operational times (start-up and shut down) when gas quality is low. In such periods, completely oxidizing is considered a positive effect, whereas in other periods (e.g. the steady state phase) the complete oxidation would bring to energy losses. It is pointed out that the expression “completely oxidizing” is intended here as the possibility of using all the necessary oxygen (not more nor less) for the combustion. In case the oxygen is only distributed over space (as usually provided in prior art) and not overtime (as in the present method), although spatially uniform, the combustion can also occur with a lower amount (e.g., the half) or higher amount (e.g., the double) of the necessary oxygen.

The present method is a dual-purpose solution process, wherein the control of the catalytic process such as a catalytic oxidation during the first phase or the control as a contaminant removal during the second phase is obtained through an adaptive modulation of the air flow.

Therefore, in an embodiment the method comprises the step of active modulating the air flow during the first operating phase and/or the second operating phase, wherein the air flow during the first operating phase is different from the air flow during the second operating phase. In particular, the method can comprise the step of automatically switching between a first airflow mode during the first operating phase and a second air flow mode during the second operating phase.

As explained in more detail below, the air flow control serves essentially to maintain the temperature within a predefined range of values during the first and the second operating phases. In the first operating phase, in order to oxidize the combustible elements of the burnable gas and reduce the pollutant content, the gas is fully oxidized using a catalytic process on a catalytic substrate. The oxidation is performed in a lean combustion mixture (l> 1 ) , wherein the injected air is above the stoichiometric quantity. The air injection control is optimized in order to dilute the gas mixture and keep the optimal oxidation temperature in a predefined range, for example 450-550 °C. The optimal result of the catalytic oxidation is a fully oxidized gas with a minimal polluting compounds concentration and, as a consequence, a minor environmental impact. It is noted that the gas resulted from the catalytic process in the first operating phase is discharged outside the system and is not eventually used for heat or power production but is compliant with emission limits for an exhaust gas. In the second operating phase, the purpose of the catalytic process is to abate the contaminants (for example tars) fraction. In order to reduce the concentration of the contaminants, the injected air is set below the stoichiometric ratio (l<1). Also, in this phase the air injection is controlled in order to optimize the partial oxidation of the mixture and keep the optimal temperature in a predefined range, for example 450-550 °C. The optimal result of the catalytic conversion is a gas with a low contaminant concentration and a high residual heating value, suitable for example for heat and power production with an internal combustion engine or other devices.

In one example, the method further comprises active modulating the air flow between a minimum value and a maximum value, wherein the minimum value corresponds to zero air insertion. In particular, the method can further comprise the step of switching between a reduction air flow mode without air insertion and an oxidation air flow mode with air insertion during the first operating phase and/or the second operating phase. It is noted that the flexibility of switching from a flow mode with no air insertion (i.e., reduction) to a flow mode with air insertion (i.e., oxidation) can be considered crucial in gasification. As a matter of fact, gas parameters (i.e., temperature, low heating value, etc.) are variable during operation time. This gives an added value to the present method compared to other methods of prior art.

According to an embodiment, the method comprises the step of calibrating the air flow during the first operating phase to maintain the air to burnable gas ratio higher than 1 , preferably between 3 and 5 and most preferably equal to 4.

According to an embodiment, the method comprises the step of calibrating the air flow during the second operating phase to maintain the air to burnable gas ratio lower than 1 , preferably between 0 and 0.5 and most preferably equal to 0.3.

In order to control the temperature variation during the catalytic process, in an embodiment the method comprises the step of measuring the temperature value of the gas mixture, after the gas mixture is oxidized through the catalytic process. In particular, the temperature value during the catalytic process needs to be maintained within a predefined range, i.e. within a minimum temperature value Tmin and a maximum temperature value Tma _X . The minimum temperature value Tmin is the temperature at which the contaminants start to be reduced, for example, the temperature at which the long- chain hydrocarbons start to break down into simpler molecules such as light hydrocarbons. It is clear that for temperatures lower than Tmin, the contaminants concentration is not reduced and the catalytic process does not provide any effect in cleaning the gas. This temperature value can depend on the type of contaminants as well as on the catalytic material used for the catalytic process.

The maximum temperature value Tm _ax is the melting temperature of a catalytic substrate used in the catalytic process. It is clear that for temperatures higher than T _ma x, the catalytic device used for the catalytic process is no more usable and the oxidation process does not occur anymore. It is pointed out that before reaching the melting temperature, the apparatus can reach a so called material stress temperature T _st r _ess that is the temperature at which the catalytic material starts to be worn in such a way to compromise the performance of the catalytic device used for the catalytic process. Accordingly, the temperature value during the catalytic process needs to be maintained Within Tmin and Tmax, preferably within Tmin and Tstress, wherein Tmin Tstress Tmax- According to an embodiment, the temperature value during the catalytic process needs to be maintained within 400°C and 550°C, wherein Tmin is about 400°C, T _ma x is about 550°C and Tstress is about 500°C.

If the temperature value is higher than the maximum temperature value (T _ma x), the method comprises the step of regulating the air flow. As defined before, the maximum temperature value (Tmax) is the melting temperature of a catalytic substrate used in the catalytic process. If the temperature value is lower than the minimum temperature value (Tmin), the method comprises the step of activating a heating process and setting a control hysteresis in a predefined temperature range (T _ra nge). As defined before, the minimum temperature value (Tmin) is the temperature at which the contaminants start to be reduced. According to an embodiment, Tmin is about 400°C, T _ma x is about 550°C and Tr _a ng _e is between 400°C and 450°C.

In an aspect of the invention, an apparatus is provided to reduce contaminants in a burnable gas. The apparatus is advantageously employed in the method according to one of the preceding embodiments.

The apparatus comprises a housing having an inlet region for receiving a gas mixture comprising at least air and the burnable gas, an outlet region for releasing a gas with reduced contaminants and a catalytic region located downstream the inlet region and upstream the outlet region for partially or completely oxidizing the gas mixture through a catalytic process, wherein the housing is insertable in a removable way in a gasification system. In particular, in a first operating phase the catalytic region is configured to completely oxidize the gas mixture and in a second operating phase the catalytic region is configured to partially oxidize the gas mixture. Also, in the first operating phase the gas mixture is a lean gas mixture, wherein air to burnable gas ratio is higher than 1 , and in the second operating phase the gas mixture is a rich gas mixture, wherein air to burnable gas ratio is lower than 1 .

Advantageously, the apparatus is a catalytic device, in particular a catalytic converter. Compared to prior art system in which the catalytic method is carried out in a complex reactor, the present method uses an apparatus that is simple to use and more compact. As a matter of fact, the apparatus can be equivalent to a commercial (automotive) catalytic device and comprises a housing that can be easily mounted to, and/or removed from, a gasification system. As a consequence, the apparatus is compact, cheap (both OPEX and CAPEX), flexible and maintenance friendly.

In an embodiment, the catalytic region comprises a catalytic substrate, wherein preferably the catalytic substrate is a multi-stage assembly. The materials for the catalytic substrate are generally the Platinum-group metals ( ruthenium, rhodium, palladium, osmium, iridium, and platinum) and Nickel and Rhenium. About the structure of the catalytic region, any kind of structure that maximize the contact between the catalytic material and the contaminant can be used, but other design factor must be taken into account like thermal expansion or mechanical resistance.

The apparatus further comprises an air inlet placed in the inlet region and a gas inlet placed in the inlet region separated from the air inlet, wherein the air inlet is coupled to a fan element.

In addition, the apparatus can comprise a control unit to switch between a first operating phase, in which the gas released from the outlet region is discharged to the outside, and a second operating phase, in which the gas released from the outlet region is preferably used for heat and power production. It is noted that the first and second operating phases correspond to the first and second operating phases defined above, wherein the gas mixture is completely or partly oxidized, respectively.

According to an embodiment, the control unit can be configured to control the air flow and/or to control the temperature value during the catalytic process. In order to maximize the mixing of air and burnable gas in the gas mixture, the apparatus can further comprise a swirl inducing element placed in the inlet region. In particular, the swirl inducing element can be placed in a space located between the inlet region and the catalytic region.

In order to measure the temperature value of the gas mixture, the apparatus comprises at least a temperature transducer. The temperature transducer can be connected to the control unit and placed upstream and/or downstream the catalytic region.

To measure the pressure value of the gas mixture, the apparatus comprises at least a pressure transducer. The pressure transducer can be connected to the control unit and placed upstream and/or downstream the catalytic region.

To measure the composition of the gas mixture, the apparatus comprises at least a gas composition sensor. This sensor can be connected to the control unit and placed upstream and/or downstream the catalytic region.

To measure the value of the air to burnable gas ratio, the apparatus comprises a lambda sensorthat is connected to the control unit. The lambda sensoris advantageously placed downstream the catalytic region.

In order to change the temperature, the apparatus comprises a heating element connected to the control unit. In particular, the heating element is placed in the catalytic region, for example in contact with the catalytic substrate used in the catalytic process.

According to an embodiment, the components of the apparatus can be made of stainless steel AISI 409 (not exclusively) for the piping and 1 ,4767 (CrAI6) (not exclusively) for the catalytic substrate used in the catalytic process.

In another aspect of the invention, a system, in particular a gasification system is provided. The system comprises the apparatus according to any one of the preceding embodiments.

The inventive method and apparatus represent a contaminant abatement approach or system with a double purpose during the different phases of operations. During the first operating phase, for example the Start-up and/or Shut-down phase in a gasification system, the main purpose is to decrease the temperature at which the oxidation happens by using a catalytic device and therefore avoiding the presence of a free flame. During this operating phase, a high control of combustion is established through a control system that is capable of creating a stable reaction during LHV oscillation of the wood gas. The control is obtained through an active modulation of the air injection flow in the range l>1 (lean combustion condition). In this way, there is a decrease of the volume of the exothermal (high temperature) area and the combustion is maintained into an enclosed volume, suitable for indoor environments.

During the second operating phase, for example the Steady-State phase in a gasification system, the main purpose is to convert the contaminants (for example tars) into combustible lighter gases. During this operating phase, a high control of contaminants conversion is established through an active control system capable of modulating the air injection in the range l<1 (rich combustion condition). In this way, the reliability of the overall system, for example a gasification system, is increased and the maintenance requirements are decreased.

The control of the oxidation process during the first operating phase (i.e. Start-up and/or Shut-down phase), is established by controlling the optimal conversion temperature trough an external heater in combination with a direct control of the air injection flow. Once the second operating phase (i.e. Steady-State phase for heat and power generation) starts, the same apparatus changes its target to a partial combustion in order to abate the contaminants. The target of the first operating phase is the complete combustion of the gas in order to release the calorific value related with the combustible mixture, whereas the target during the second operating phase is to keep as much as possible unchanged the heating value of the gas and, simultaneously, eliminate the contaminants (tar) to make the gas suitable for example for an ICE.

In the figures, the subject-matter of the invention is schematically shown, wherein identical or similarly acting elements are usually provided with the same reference signs.

Figure 1 shows a flow chart of the method according to an embodiment. Figure 2 shows a schematic representation of the functioning of the two operating phases of the method according to an embodiment.

Figures 3A-C show a schematic representation of the apparatus according to an embodiment in a perspective view (A), in a lateral view (B) and in cross- section (C).

Figures 4A-D show a schematic representation of the apparatus according to an embodiment (A) and of the swirl inducing element according to a perspective view (B), frontal view (C) and rearview (D).

Figure 5 shows a graphic of the temperature and l variation as a function of time in the two phases of the method according to an embodiment.

Figures 6A-B show a block diagram of a control elements of the apparatus (A) and a flow chart of a corresponding control process (B).

With reference to Figure 1 , a method 100 for reducing contaminants in a burnable gas G is illustrated according to a schematic flow chart. The method 100 provides different steps based on the operating phase during which the method 100 is carried out. It is noted that with operating phase is intended a mode of operation of a system used for handling the burnable gas G, such as a gasification system.

At step S101 , a lean gas mixture GM is produced, the gas mixture GM comprising a combination of at least air A and the burnable gas G. The lean gas mixture GM is produced by acting on the air flow, i.e. by increasing the quantity of air in the gas mixture GM. At step S102, during a first operating phase P1 , the lean gas mixture GM is completely oxidized through a catalytic process CP. Specifically, the gas mixture GM is oxidized on a catalytic substrate and the resulting gas is discharged to the outside as exhaust gas EG. During these steps, the air flow is optimized in order to dilute the gas mixture GM and keep the optimal oxidation temperature. In case the method 100 is used for handling a burnable gas G in a gasification system, the first operating phase P1 would correspond to the Start-up and/or Shut-down phase.

At step S103, a rich gas mixture GM is produced, the gas mixture GM comprising a combination of at least air A and the burnable gas G. The rich gas mixture GM is produced by acting on the air flow, i.e. by decreasing the quantity of air in the gas mixture GM. At step S104, during a second operating phase P2, the rich gas mixture GM is partly oxidized through a catalytic process CP. Specifically, the gas mixture GM is oxidized on a catalytic substrate and the resulting gas is a clean gas CG with reduced contaminants. During these steps, the air flow is controlled to optimize the partial oxidation of the gas mixture GM and keep the optimal oxidation temperature. This means that the concentration of contaminants is reduced to a minimum and the burnable gas G can maintain a high residual heating value. In case the method 100 is used for handling a burnable gas G in a gasification system, the second operating phase P2 would correspond to the Steady-State phase.

Figure 2 illustrates the application of the method 100 during the two operating phases P1 and P2.

In the first operating phase P1 , the burnable gas G is combined with air A to form a lean gas mixture GM, i.e. a gas mixture GM having an air to burnable gas ratio higher than 1. The gas mixture GM undergoes a fully oxidation through the catalytic process CP and the obtained gas EG is discharged into the atmosphere. It is noted that the exhaust gas EG has no heating value and cannot be used for example for heat production. For example, in case the burnable gas G is a wood gas with a low heating value (LHV) of about 5 MJ/Nm ³ , the exhaust gas EG would have almost zero heat value, i.e. LHV=0 MJ/Nm ³ .

In the second operating phase P2, the burnable gas G is combined with air A to form a rich gas mixture GM, i.e. a gas mixture GM having an air to burnable gas ratio lower than 1 . The gas mixture GM undergoes a partial oxidation through the catalytic process CP and the obtained clean gas CG is a gas with a reduced concentration of contaminants. It is noted that the clean gas CG has still a heating value and can be used for example for heat production and can be employed in an internal combustion engine (ICE). For example, in case the burnable gas G is a wood gas with a LHV of about 5 MJ/Nm ³ , the clean gas CG would have a slight reduced heat value, i.e. LHV=4 MJ/Nm ³ .

Figures 3A-3C illustrate details of the apparatus 10 according to an embodiment. The apparatus 10 is configured to be employed in the method 100 for reducing contaminants of a burnable gas G, as described above. The apparatus 10 basically comprises a housing 30 having an inlet region 12 for receiving the gas mixture GM, an outlet region 14 for releasing a gas and a catalytic region 16 located between the inlet region 12 and the outlet region 14. It is noted that the apparatus 10, and in particular the housing 30, is configured to be mounted downstream to, and removed downstream from, a reactor region of a gasification system. The catalytic process can be carried out inside this compact apparatus 10 without the necessity of building additional reactors to the gasification system. The mounting of the apparatus 10 to the gasification system occurs by simply connecting the inlet region 12 to the burnable gas G supply line and the air A supply line of the gasification system and by connecting the outlet region 14 to the gas outlet line of the gasification system.

In the inlet region 12, the burnable gas G is pre-mixed with air A. The structure and length of the inlet region 12 is designed in order to maximize the mixture uniformity and avoid for example back-puff or backfire phenomena from a combustion section (not shown in the figure).

A catalytic substrate of a catalytic device is located in the catalytic region 16. The diameter and length of the catalytic region 16 is optimized in order to guarantee a complete conversion accordingly with the residence time of the gas mixture GM into the catalytic device. In case of weak chemical reaction, the temperature of the catalytic substrate can be controlled with an heating element 27 that keeps the surface optimal temperature in a predefined range of values (see in particular figures 6A and 6B).

In the outlet region 14, the oxidized gas is discharged from the apparatus 10 and is diverted to an exhaust system or to an engine, for example ICE, depending on the operational phase of the implementing system. In particular, based on the fact that the apparatus 10 is employed during the first operating phase P1 or the second operating phase P2, the released gas is an exhaust gas EG to be discharged outside or a clean gas CG to be potentially used for heat or power production.

Figure 3B is a lateral view of the apparatus 10 and the figure 3C represents a longitudinal cross section of the apparatus 10 of figure 3B along the line A-A.

The burnable gas G is injected from the gas inlet 13 and air A is injected through a fan element 15 (not shown in these figures) from the air inlet 11 . The fan element 15 serves to guarantee a flowrate that can cover a wide range of air to burnable gas ratios (l), both in the sub stoichiometric range (l<1) and over stoichiometric range (l>1). The apparatus 10 comprises sensor ports 17 located both upstream and downstream the catalytic region 16 and configured to install measurement instruments such as temperature transducers, pressure transducers, lambda sensors and/or gas composition analyzers.

The apparatus 10 furthermore comprises a heating element 27, for example an electrical resistance, that is installed on the external surface of the catalytic substrate in the catalytic region 16 in orderto guarantee the minimum activation temperature during weak exothermal conditions. The heating element 27 can be controlled through a local controller, such as thermostat, or through a general system controller, such as a Programmable Logic Controller (PLC).

From figure 3C, it is notable that the catalytic substrate can be a multi- stage assembly, in particular having two-stages 18, 19 separated by an empty gap 20. The multistage design is adopted to ensure a suitable combustion degree and residence time for variable burnable gas and air flowrates. In orderto ensure a higher conversion efficiency at lower costs, more than one stage can be adopted with the target to increase the residence time of the gas into the catalytic device. The number of stages and type of catalytic material can be selected in relation with the degree of system efficiency needed.

Advantageously, the apparatus 10 also comprises coupling sections 21 located between the inlet region 12 and the catalytic region 16 and between the catalytic region 16 and the outlet region 14. The coupling sections 21 are realized by two ring surfaces connected to each other by connecting means, such as a plurality of screws, with a sealing element placed therebetween. The coupling sections 21 basically serve to detach the catalytic region 16 from the inlet region 12 and/or the outlet region 14. This is extremely useful for a maintenance work of the apparatus 10, for example in case the catalytic device used for the catalytic process needs to be regenerated or replaced.

Figure 4A shows that the apparatus 10 can be additionally provided with a swirl inducing element 22. This element is preferably placed in the inlet region 12 at the entrance of the catalytic region 16. In this way, it is possible to maximize the mixing of the air and the burnable gas, in order to have a homogeneous combustion of the gas mixture GM entering the catalytic region 16.

As shown in figures 4B to 4D, the swirl inducing element 22 comprises a ring structure 29 and a central helical structure 24. The ring structure 29 is provided with a plurality of through-holes 23 distributed on the edge of the ring structure 29. In the rear face (figure 4D), the swirl inducing element 22 comprises a protruding rim 25 for containing the helical structure 24. It is noted that the holes 23 serve to fix the swirl inducing element 22 to the apparatus 10 at the coupling section 21 between the inlet region 12 and the catalytic region 16, wherein the protruding rim 25 is placed in the inlet region 12.

Figure 5 illustrates the variation of two parameters, i.e. the temperature and the air to burnable gas ratio, during the first and second operating phases. As mentioned above, these two parameters are controlled acting on the air flow.

In the first operating phase P1 , for example the Start-up and/or Shut-down phase, the air flow control is based on the addition of fresh air. As shown in figure 5, the minimum air flowrate is calibrated to maintain an air to burnable gas ratio Amin equal to 4. If the control temperature overtakes the value of 500°C, a control unit 26 is configured to increase the air flowrate into the apparatus 10 in order to enhance the dilution of the burnable gas and subsequently decrease the oxidation temperatures.

In the second operating phase P2, for example the Steady-State phase, the control is based on a partial combustion. The air flowrate is decreased below stoichiometric value (A=0.3) in order to establish a partial oxidation of the gas mixture GM. The 500°C condition is sufficient for the conversion of the contaminants. If the control temperature overtakes the value of 500°C, the control unit 26 is configured to decrease the air flowrate into the apparatus 10 in order to decrease the oxygen for the exothermal reactions and subsequently decrease the temperature.

The adoption of this hybrid dual purpose control methodology (both elements can be controlled by for example a proportional integral derivative PID controller as control unit 26) makes possible to control the temperature variation either under the target value and above the target value. In fact, the purpose of the control strategy is to keep optimal temperature (in the example 500°C) for the following reasons:

- guarantee the optimal working condition of the catalytic substrate of the catalytic device used in the catalytic process CP. The surface of the substrate is strongly sensitive to high temperatures that may cause premature wear and consequently decrease both the conversion efficiency and lifetime of the system; obtain minimum contaminants concentration during all possible operational phases (i.e. start-up, shut down, steady state, partial load etc.). Different contaminants species can be present in different operational conditions, temperature can be adjusted accordingly in order to tune for each condition the best efficiency of the catalytic system; ensure combustion safety by keeping l control farfrom air/burnable gas ratio that are out of the safety range.

Figures 6A and 6B show a diagram of a control elements of the apparatus 10 and a corresponding control process 200 for maintaining the temperature within a predefined range of values. The control elements can comprise a heating element 27, a fan element 15 and a temperature transducer 28, all connected to a control unit 26. The fan element 15 is located at the air inlet 11 to regulate the air flow in the apparatus 10. The heating element 27 is located in the catalytic region 16 of the apparatus 10, for example at the substrate of the catalytic device used in the catalytic process CP. The temperature transducer 28 is located in the outlet region 14 and the temperature is measured after the gas mixture GM has undergone the catalytic process CP. Alternatively, the temperature transducer 28 can be located in the catalytic region 16 directly on the catalytic surface.

The heating element 27 is switched on in case the weak LHV of the burnable gas G cannot guarantee a self-sustaining combustion. The control hysteresis can be set in the range of 400-450 °C. The air injection flowrate can be controlled through a Variable Frequency Driver (VFD), through a bypass or a mechanical system controlled by valves.

With reference to figure 6B, at step S201 , the temperature value of the gas mixture GM is measured after the catalytic process CP, for example by the temperature transducer 28 of figure 6A. In this way, the temperature variation during the catalytic process CP is continuously monitored.

In case the temperature value is higher than a maximum temperature value (T _max ) (S202), the method comprises the step S203 of regulating the air flow. As already mentioned, T _max is the melting temperature of the catalytic substrate used in the catalytic process CP, for example 550°C. The process then goes back to step S201 to continuously measure the temperature of the gas mixture GM. At step S204, the temperature value is determined to be lower than a minimum temperature value (T _m in). In this case, the method comprises the step S205 of activating a heating process and setting a control hysteresis in a predefined temperature range. As already mentioned, Tmin is the temperature at which the contaminants start to be reduced, for example 400°C. The process then goes back to step S201 to continuously measure the temperature of the gas mixture GM. In case the temperature is lower than Tm _ax and higher than T _mi n, the temperature has reached its optimized value and the method is maintained at step S206.

Reference Signs

10 apparatus

11 air inlet 12 inlet region

13 gas inlet

14 outlet region

15 fan element

16 catalytic region 17 sensor ports

18 stage 1

19 stage 2

20 empty gap

21 coupling section 22 swirl inducing element

23 through-hole

24 helical structure

25 protruding rim

26 control unit 27 heating element

28 temperature transceiver

29 ring structure

30 housing

100 method for reducing contaminants 200 control process