JP2024515017A - Method and apparatus for reducing combustible gas pollutants - Patents.com - Google Patents

Abstract

A method (100) for reducing pollutants of a combustible gas (G), comprising: generating (S101) a lean mixture (GM) comprising at least air (A) and a combustible gas (G); fully oxidizing the lean mixture (GM) by a catalytic process (CP) during a first operating phase (P1) to obtain an exhaust gas (EG) (S102); generating (S103) a rich mixture (GM) comprising at least air (A) and a combustible gas (G); and partially oxidizing (S104) the rich mixture (GM) by a catalytic process (CP) during a second operating phase (P2) to obtain a clean combustible gas (CG) preferably for the production of heat and power.

Description

The present invention relates to a method for reducing contaminants in a gas mixture, particularly one containing a combustible gas. The present invention also relates to a corresponding apparatus for using said method, and to a system including said apparatus.

Biomass gasification technology is based on a thermochemical process that converts dry solid biomass, such as wood or agricultural waste, into combustible gas. This combustible gas, also known as wood gas or syngas, can be used as fuel in internal combustion engines (ICEs) and its chemical energy is converted into mechanical, electrical and thermal energy.

Syngas is usually defined as a low heating value fuel since its lower heating value (LHV) can range from 4 to 6 MJ/Nm3. Furthermore, the gas composition can change during operation according to the thermochemical conditions of the gasification reactor and the quality of the biomass fuel itself.

An important criticality regarding the synthesis gas produced from biomass gasification is that it is inherently contaminated with polycondensable organic compounds called tars. The average tar concentration can vary from 0.1 to 50 g/Nm3 depending on the reactor technology employed. The tar concentration is highly dependent on the two phases in which the gasification system operates: start-up/shutdown phase and steady-state phase.

During the start-up and shutdown phases, the high tar content of wood gas makes the gas unsuitable for combustion in an ICE. The gas is therefore diverted to a secondary combustion facility, usually a flare stack, with the aim of completely combusting the gas to comply with legal emission levels.

During the steady-state phase, the tar concentration of wood gas is low (but not zero), so the gas is transferred to the ICE and used as fuel to generate electrical and thermal power. As the tar concentration is a critical issue for the durable and reliable operation of the ICE, the contaminants are usually removed by specific filtration equipment.

Many of the prior art solutions for reducing contaminants in combustible gases are expensive (especially in terms of operating costs) or complex using systems consisting of several parts and components.

The object of the present invention is to provide a method and apparatus for reducing contaminants in combustible gases that is easy to implement and that efficiently produces clean combustible gases for use in heat and power production and other purposes.

This object is solved by a method for reducing contaminants in a combustible gas, comprising generating a lean mixture comprising at least air and a combustible gas, and fully oxidizing the lean mixture by a catalytic process during a first operating phase to obtain exhaust gas. The method also comprises generating a rich mixture comprising at least air and a combustible gas, and partially oxidizing the rich mixture by a catalytic process during a second operating phase to obtain clean combustible gas, preferably for heat and power production.

This allows the catalytic process with a single corresponding catalytic device to be used as a pollutant removal approach in two different operating phases. In particular, in the first operating phase, the mixture is completely combusted by catalytic oxidation, and in the second operating phase, the mixture is purified, i.e., pollutants are reduced, by the catalytic process. The catalytic devices used for the oxidation process in the two operating phases are essentially the same, but the process conditions are different. Indeed, in the first operating phase, the catalytic process is performed on a lean mixture, while in the second operating phase, the catalytic process is performed on a rich mixture. Advantageously, by using the same catalytic device for completely oxidizing the mixture in the first operating phase and for removing pollutants in the second operating phase, this method reduces both the operating costs and the capital expenditures of systems that require the purification of combustible gases by pollutants.

Lean combustion or lean burn refers to the gas combustion process in an internal combustion engine or general-purpose combustion chamber with an excess of air (when considered in terms of the ratio of air to combustible gas). On the other hand, rich combustion or rich burn refers to the gas combustion process in an internal combustion engine or general-purpose combustion chamber with a shortage of air (when considered in terms of the ratio of air to combustible gas).

According to an embodiment, the combustible gas may be a synthesis gas produced by a gasification process in a gasification system or gasifier. For example, the synthesis gas may be wood gas. The first operating phase may correspond to a start-up phase and/or a shut-down phase of the gasification system, and the second operating phase may be a steady-state phase of the gasification system. In particular, a lean mixture has an air to combustible gas ratio higher than 1, and a rich mixture has an air to combustible gas ratio lower than 1. This results in a complete oxidation by catalytic processes during the start-up phase and/or shut-down phase of the gasification system, and in a partial oxidation decontamination by catalytic processes during the steady-state phase. As a result, a clean gas is produced and can be used to feed the internal combustion engine.

In particular, the air excess factor λ can be defined as the ratio of the actual air-to-flammable gas ratio of the mixture to the stoichiometric air-to-flammable gas ratio. An air excess factor λ greater than 1 means there is too much air. On the other hand, an air excess factor λ less than 1 means there is not enough air for all the flammable gases to burn.

The pollutants treated according to the method may be tars in the combustible gas. The high temperature of the rich gas mixture due to the catalytic process during the second operating stage may determine the decomposition of the pollutants, i.e., tars. Following the decomposition of the pollutants, i.e., tars, the long chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons.

The method of the present invention can also be advantageously used in other industrial processes not necessarily related to biomass gasification, as well as in oil and natural gas processes. The method can be used in all systems that produce a waste gas stream with a time-variable composition according to process operating parameters.

The method includes controlling the oxidation of the lean and/or rich gas mixture over the operation period. In other words, unlike the prior art solutions where the oxidation is varied radially in the reactor to obtain uniform spatial oxidation along the catalyst surface of the reactor, in the present method the oxidation varies over time. This allows for transient operation periods (start-up and shutdown) where the gas quality is low, during which complete oxidation is considered to be beneficial, whereas in other periods (e.g. steady state phase) complete oxidation is considered to result in energy losses. The expression "complete oxidation" here intends that all the required oxygen (no more, no less) is available for combustion. If the oxygen is only distributed spatially (as is customary in the prior art) and not distributed spatially uniformly in time (as in the present method), combustion can occur even with less (e.g. half) or more (e.g. double) the amount of oxygen required.

This method is a dual purpose solution process where control of the catalytic process, such as catalytic oxidation during the first stage, or control of the pollutant removal during the second stage, is obtained by adaptive adjustment of the air flow.

Thus, in an embodiment, the method includes a step of actively adjusting the airflow during a first operational phase and/or a second operational phase, where the airflow during the first operational phase is different from the airflow during the second operational phase. In particular, the method may include a step of automatically switching between a first airflow mode during the first operational phase and a second airflow mode during the second operational phase.

As will be explained in more detail below, the air flow control essentially serves to maintain the temperature at a predetermined range of values during the first and second operating phases. In the first operating phase, the gas is fully oxidized using a catalytic process on a catalytic substrate in order to oxidize the combustible elements of the combustible gas and reduce the pollutant content. This oxidation takes place in a lean combustion mixture (λ>1) with more than stoichiometric amounts of injected air. The air injection control is optimized to keep the mixture lean and the optimum oxidation temperature in a predetermined range, for example 450-550°C. The best result of catalytic oxidation is a fully oxidized gas with a minimum concentration of polluting compounds and thus a low environmental impact. The gases resulting from the catalytic process in the first operating phase are discharged outside the system and are not used for the production of heat and power, but still comply with exhaust gas emission regulations. In the second operating phase, the purpose of the catalytic process is to reduce the proportion of pollutants (e.g. tar). In order to reduce the concentration of pollutants, the injected air is set lower than the stoichiometric ratio (λ<1). Also at this stage, the air injection is controlled to optimize the partial oxidation of the mixture and to keep the optimum temperature within a predetermined range, e.g., 450-550°C. The best outcome of catalytic conversion is a gas with low pollutant concentration and high residual heating value, suitable for the production of heat and power, e.g., by an internal combustion engine or other device.

In one example, the method further comprises actively adjusting the airflow between a minimum and a maximum value, where the minimum value corresponds to zero air insertion. In particular, the method may further comprise switching between a reducing airflow mode without air insertion and an oxidizing airflow mode with air insertion during the first and/or second operation phases. The flexibility to switch from a reducing airflow mode without air insertion (i.e. reducing) to an oxidizing airflow mode with air insertion (i.e. oxidizing) is considered to be important in gasification. Indeed, gas parameters (i.e. temperature, low heating value, etc.) change during the operation time. This gives the method added value compared to other methods of the prior art.

According to an embodiment, the method includes adjusting the airflow during the first operational phase to maintain an air to combustible gas ratio greater than 1, preferably between 3 and 5, and most preferably equal to 4.

According to an embodiment, the method includes adjusting the airflow during the second operating phase to maintain an air to combustible gas ratio lower than 1, preferably between 0 and 0.5, and most preferably equal to 0.3.

To control the temperature change during the catalytic process, in an embodiment, the method includes a step of measuring the temperature value of the mixed gas after it has been oxidized by the catalytic process. In particular, the temperature value during the catalytic process needs to be maintained within a predetermined range, namely between a minimum temperature value Tmin and a maximum temperature value Tmax. The minimum temperature value Tmin is the temperature at which the pollutants start to decrease, for example the temperature at which long-chain hydrocarbons start to break down into simpler molecules such as lighter hydrocarbons. At temperatures below Tmin, the pollutant concentration does not decrease and it is clear that the catalytic process has no effect on the purification of the gas. This temperature value may depend on the type of pollutant and the catalyst material used in the catalytic process.

The maximum temperature value Tmax is the melting temperature of the catalytic substrate used in the catalytic process. At temperatures higher than Tmax, the catalytic device used in the catalytic process is no longer usable and it is clear that no oxidation process takes place. Before reaching the melting temperature, the device can reach a so-called material stress temperature Tstress. The material stress temperature Tstress is the temperature at which the catalytic material starts to wear out in such a way that the performance of the catalytic device used in the catalytic process is reduced. Therefore, the temperature values during the catalytic process must be maintained within the range Tmin and Tmax, preferably within the range Tmin and Tstress, where Tmin<Tstress<Tmax. According to an embodiment, the temperature values during the catalytic process must be maintained within the range 400°C and 550°C, where Tmin is about 400°C, Tmax is about 550°C and Tstress is about 500°C.

If the temperature value is greater than a maximum temperature value (Tmax), the method includes managing the airflow. As previously described, the maximum temperature value (Tmax) is the melting temperature of the catalytic substrate used in the catalytic process. If the temperature value is less than a minimum temperature value (Tmin), the method includes running the heating process and setting the control hysteresis to a predetermined temperature range (Trange). As previously described, the minimum temperature value (Tmin) is the temperature at which the contaminants begin to decrease. According to an embodiment, Tmin is about 400°C, Tmax is about 550°C, and Trange is between 400°C and 450°C.

In one aspect of the present invention, there is provided an apparatus for reducing contaminants in a combustible gas, the apparatus being advantageously used in a method according to one of the previously described embodiments.

The apparatus includes a housing having an inlet region for receiving a gas mixture including at least air and a combustible gas, an outlet region for discharging a contaminant-reduced gas, and a catalyst region downstream of the inlet region and upstream of the outlet region for partially or fully oxidizing the gas mixture by a catalytic process, the housing being removably insertable into the gasification system. In particular, in a first operating phase, the catalyst region is configured to fully oxidize the gas mixture, and in a second operating phase, the catalyst region is configured to partially oxidize the gas mixture. Also, in the first operating phase, the gas mixture is a lean gas mixture, and the ratio of air to combustible gas is greater than 1, and in the second operating phase, the gas mixture is a rich gas mixture, and the ratio of air to combustible gas is less than 1.

Advantageously, the device is a catalytic device, in particular a catalytic converter. Compared to prior art systems that perform catalytic procedures in complex reactors, the method of the present invention uses a device that is simple to use and more compact. In fact, the device is comparable to a commercial (automotive) catalytic device, with a housing that allows easy installation and/or removal from the gasification system. As a result, the device is compact, inexpensive (both OPEX and CAPEX), adaptable and easy to maintain.

In an embodiment, the catalytic region comprises a catalytic substrate, which is preferably a multi-stage assembly. The catalytic substrate materials are typically platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, platinum), nickel, and rhenium. The catalytic region may be constructed in any configuration that maximizes contact between the catalytic material and the contaminants, but other design factors such as thermal expansion and mechanical durability must also be considered.

The apparatus further comprises an air inlet disposed in the inlet region and a gas inlet disposed in the inlet region spaced apart from the air inlet, the air inlet being coupled to the fan element.

The apparatus may further comprise a control device for switching between a first operating phase in which the gas discharged from the outlet region is exhausted to the outside and a second operating phase in which the gas discharged from the outlet region is preferably used for the production of heat and electricity. The first and second operating phases correspond to the above-mentioned first and second operating phases, respectively, in which the mixed gas is fully or partially oxidized.

According to an embodiment, the control device can be configured to control air flow and/or temperature values during the catalytic process.

To maximize the mixing of air and combustible gas in the gas mixture, the device may further comprise a swirl-inducing element disposed in the inlet region. In particular, the swirl-inducing element may be disposed in a space located between the inlet region and the catalyst region.

To measure the temperature value of the mixed gas, the device comprises at least one temperature transducer. The temperature transducer can be connected to the control device and can be located upstream and/or downstream of the catalytic region.

To measure the pressure value of the mixed gas, the device includes at least one pressure transducer. The pressure transducer can be connected to the control device and can be located upstream and/or downstream of the catalyst region.

To measure the composition of the gas mixture, the device includes at least one gas composition sensor that can be connected to the control device and can be located upstream and/or downstream of the catalyst region.

To measure the ratio of air to combustible gas, the device comprises a lambda sensor connected to the control device. The lambda sensor is advantageously positioned downstream of the catalyst area.

To vary the temperature, the device comprises a heating element connected to the control device. In particular, the heating element is arranged in the catalyst area, for example in contact with a catalytic substrate used in a catalytic process.

According to embodiments, the components of the device can be made from stainless steel AISI 409 (not limited to) for the piping and 1.4767 (CrAI6) (not limited to) for the catalytic substrate used in the catalytic process.

In another aspect of the present invention, there is provided a system, in particular a gasification system, comprising an apparatus according to any one of the previously described embodiments.

The method and apparatus of the present invention is a contaminant removal technique or system that has a dual purpose at different stages of operation.

During the first operation phase, e.g. the start-up and/or shutdown phase of the gasification system, the main objective is to avoid the presence of a free flame by lowering the temperature at which oxidation occurs through the use of a catalytic device. In this operation phase, a high degree of control of the combustion is established by a control system capable of producing stable reactions during the LHV oscillations of the wood gas. This control is obtained by actively adjusting the air injection flow to the range λ>1 (lean burning condition). This reduces the volume of the heat generating (high temperature) zone and keeps the combustion in a closed volume suitable for the indoor environment.

During the second operational phase, e.g. the steady state phase in a gasification system, the main objective is to convert pollutants (e.g. tar) to lighter combustible gases. In this operational phase, a high degree of control of the pollutant conversion is established by an active control system that can adjust the air injection in the range λ<1 (rich combustion conditions). This increases the overall reliability of the system, e.g. the gasification system, and reduces the maintenance needs.

During the first operating phase (i.e. start-up and/or shutdown phase), the control of the oxidation process is established by a combination of control of the optimum conversion temperature by an external heater and direct control of the air injection flow. When the second operating phase (i.e. steady-state phase for heat and power supply) begins, the same device changes its target to partial combustion in order to reduce pollutants. Whereas the target in the first operating phase is to release the heating value of the combustible mixture by complete combustion of the gas, the target in the second operating phase is to keep the heating value of the gas as constant as possible while at the same time removing the pollutants (tar) to make the gas suitable for e.g. ICE.

In the figures, the subject matter of the invention is shown diagrammatically, and elements of identical or similar function are usually provided with the same reference numbers.

1 is a flowchart of a method according to an embodiment. FIG. 2 is a schematic diagram illustrating the functionality of two operational stages of a method according to an embodiment. FIG. 1 is a schematic diagram of an apparatus according to an embodiment, shown in a perspective view. FIG. 2 is a schematic diagram of an apparatus according to an embodiment, shown in side view. 1 is a schematic diagram of an apparatus according to an embodiment, shown in cross-section. FIG. 2 is a schematic diagram of an apparatus according to an embodiment. FIG. 2 is a schematic diagram of a swirl-inducing element, shown in a perspective view. FIG. 2 is a schematic diagram of a turning-inducing element, shown in a front view. FIG. 2 is a schematic diagram of a turning-inducing element, shown in rear view. 1 is a graph showing the change in temperature and λ as a function of time in two stages of a method according to an embodiment. FIG. 2 is a block diagram of the control elements of the device. 4 is a flow chart of the corresponding control process.

With reference to FIG. 1, a method 100 for reducing contaminants of a combustible gas G is shown in a schematic flow chart. The method 100 provides different steps based on the operational phase in which the method 100 is performed. By operational phase, we mean the mode of operation of a system used to treat the combustible gas G, such as a gasification system.

In step S101, a lean mixture gas GM is generated. This mixture gas GM comprises at least a combination of air A and a combustible gas G. The lean mixture gas GM is generated by acting on the air flow, i.e. by increasing the amount of air in the mixture gas GM. In step S102, during a first operating phase P1, the lean mixture gas GM is completely oxidized by a catalytic process CP. In particular, the mixture gas GM is oxidized on a catalytic substrate and the generated gases are discharged to the outside as exhaust gas EG. During these steps, the air flow is optimized to dilute the mixture gas GM and to maintain an optimal oxidation temperature. If the method 100 is used to treat a combustible gas G in a gasification system, the first operating phase P1 corresponds to a start-up and/or shutdown phase.

In step S103, a rich mixture gas GM is generated. This mixture gas GM comprises at least a combination of air A and a combustible gas G. The rich mixture gas GM is generated by acting on the air flow, i.e. by reducing the amount of air in the mixture gas GM. In step S104, during a second operating phase P2, the rich mixture gas GM is partially oxidized by a catalytic process CP. In particular, the mixture gas GM is oxidized on a catalytic substrate and the generated gas is a clean gas CG with reduced pollutants. During these steps, the air flow is controlled to optimize the partial oxidation of the mixture gas GM and to maintain an optimal oxidation temperature. This means that the concentration of pollutants is reduced to a minimum and the combustible gas G can maintain a high residual heating value. If the method 100 is used to process a combustible gas G in a gasification system, the second operating phase P2 corresponds to a steady-state phase.

Figure 2 shows the application of method 100 in two operational phases P1 and P2.

In a first operating phase P1, the combustible gas G is combined with air A to form a lean mixture GM, i.e. a mixture GM with an air to combustible gas ratio higher than 1. This mixture GM is fully oxidized by a catalytic process CP and the resulting gas EG is exhausted to the atmosphere. The exhaust gas EG has no calorific value and cannot be used, for example, for heat production. For example, if the combustible gas G is wood gas with a lower heating value (LHV) of about 5 MJ/Nm3, the exhaust gas EG may have an almost zero calorific value, i.e. LHV = 0 MJ/Nm3.

In the second operating phase P2, the combustible gas G is combined with air A to form a rich mixture GM, i.e. a mixture GM with an air to combustible gas ratio less than 1. This mixture GM is partially oxidized by a catalytic process CP, and the resulting clean gas CG is a gas with reduced pollutant concentration. The clean gas CG still has a calorific value and can be used, for example, for heat production, in an internal combustion engine (ICE). For example, if the combustible gas G is wood gas with an LHV of about 5 MJ/Nm3, the clean gas CG may have a slightly reduced calorific value, i.e. LHV=4 MJ/Nm3.

3A-3C show details of the device 10 according to an embodiment. The device 10 is configured for use in the method 100 for reducing contaminants of a combustible gas G, as previously described. The device 10 basically comprises a housing 30 having an inlet area 12 for receiving the mixed gas GM, an outlet area 14 for discharging the gas, and a catalytic area 16 located between the inlet area 12 and the outlet area 14. The device 10, and in particular the housing 30, are configured to be mounted downstream of the reactor area of a gasification system and to be removed downstream from the reactor area. The catalytic process can be carried out inside this compact device 10 without the need to build an additional reactor in the gasification system. The mounting of the device 10 in a gasification system occurs by simply connecting the inlet area 12 to the combustible gas G supply line and the air A supply line of the gasification system, and connecting the outlet area 14 to the gas outlet line of the gasification system.

In the inlet region 12, combustible gas G is premixed with air A. The structure and length of the inlet region 12 are designed to maximize the homogeneity of the mixture and to avoid, for example, backpuff and backfire phenomena from the combustion section (not shown).

The catalytic substrate of the catalytic device is arranged in a catalytic area 16. The diameter and length of the catalytic area 16 are optimized to ensure complete conversion according to the residence time of the mixed gas GM in the catalytic device. In case of weak chemical reactions, the temperature of the catalytic substrate can be controlled by a heating element 27 that keeps the optimal surface temperature within a predetermined range of values (see especially Figures 6A and 6B).

In the outlet region 14, the oxidized gas is discharged from the device 10 and transferred to an exhaust system or an engine, e.g., an ICE, depending on the operating phase of the implemented system. In particular, based on whether the device 10 is used during the first operating phase P1 or the second operating phase P2, the discharged gas is either an exhaust gas EG to be discharged to the outside or a clean gas CG, which can potentially be used for the production of heat or electricity.

Figure 3B is a side view of device 10, and Figure 3C shows a longitudinal cross-section of device 10 of Figure 3B along line A-A.

The combustible gas G is injected through the gas inlet 13 and the air A is injected through the air inlet 11 via the fan element 15 (not shown here). The fan element 15 helps to ensure a flow rate that can cover a wide range of air to combustible gas ratios (λ) in both the sub-stoichiometric range (λ<1) and the over-stoichiometric range (λ>1). The apparatus 10 includes sensor ports 17 located both upstream and downstream of the catalyst area 16. The sensor ports 17 are configured to accommodate measuring instruments such as temperature transducers, pressure transducers, lambda sensors and/or gas composition analyzers.

The apparatus 10 further includes a heating element 27, e.g., an electrical resistor, disposed on the outer surface of the catalyst substrate of the catalyst area 16. The heating element 27 is for ensuring a minimum activation temperature during mild heating conditions. The heating element 27 can be controlled by a local controller, such as a thermostat, or a general system controller, such as a programmable logic controller (PLC).

As is evident from FIG. 3C, the catalyst substrate can be a multi-stage assembly, specifically having two stages 18, 19 separated by a gap 20. The multi-stage design is employed to ensure appropriate combustion and residence times for variable combustible gas and air flow rates. To ensure higher conversion efficiency at lower cost, more than two stages can be employed with the goal of increasing the residence time of the gas in the catalyst device. The number of stages and type of catalyst material can be selected in relation to the degree of system efficiency required.

Advantageously, the device 10 also comprises a connection 21 located between the inlet area 12 and the catalyst area 16 and between the catalyst area 16 and the outlet area 14. The connection 21 is realized by two annular surfaces connected to each other by a connecting means such as a number of screws, between which a sealing element is arranged. The connection 21 essentially serves to remove the catalyst area 16 from the inlet area 12 and/or the outlet area 14. This is extremely useful for maintenance operations of the device 10, for example when a catalytic device used in a catalytic process needs to be regenerated or replaced.

Figure 4A shows that the device 10 can additionally comprise a swirl-inducing element 22, which is preferably arranged in the inlet region 12 on the inlet side of the catalyst region 16. This allows for maximum mixing of the air and the combustible gas to ensure homogeneous combustion of the mixed gas GM entering the catalyst region 16.

As shown in Figures 4B-4D, the swirl-inducing element 22 is composed of a ring structure 29 and a central helical structure 24. The ring structure 29 is provided with a number of through holes 23 distributed on the edge of the ring structure 29. The swirl-inducing element 22 has a protruding rim 25 on its rear face (Figure 4D) that accommodates the helical structure 24. The through holes 23 serve to fix the swirl-inducing element 22 to the device 10 at the joint 21 between the inlet area 12 and the catalyst area 16. The protruding rim 25 is then located in the inlet area 12.

Figure 5 shows the variation of two parameters, namely temperature and air to combustible gas ratio, during the first and second operating phases. As mentioned before, these two parameters are controlled according to the air flow.

In the first operation phase P1, e.g. the start-up and/or shutdown phase, the airflow control is based on adding fresh air. As shown in FIG. 5, the minimum airflow is adjusted to maintain the air to combustible gas ratio λmin equal to 4. If the control temperature exceeds a value of 500° C., the control device 26 is configured to increase the airflow to the device 10, thereby increasing the rarefaction of the combustible gas and then decreasing the oxidation temperature.

In the second operating phase P2, e.g. the steady state phase, the control is based on partial combustion. To establish partial oxidation of the mixed gas GM, the air flow rate is reduced below the stoichiometric value (λ=0.3). A condition of 500° C. is sufficient for the conversion of pollutants. If the control temperature exceeds a value of 500° C., the control device 26 is configured to reduce the air flow rate to the device 10, thereby reducing the oxygen for the exothermic reaction and subsequently reducing the temperature.

By employing this hybrid, dual-purpose control strategy (e.g., a proportional-integral-derivative PID controller as the control device 26 can control both elements), it is possible to control the temperature change below or above the target value. In fact, the objective of this control strategy is to maintain an optimum temperature (500° C. in the example) for the following reasons:
- To ensure optimal working conditions of the catalytic substrate of the catalytic device used in the catalytic process CP. The surface of the catalytic substrate is strongly affected by high temperatures, which can cause premature wear and tear, resulting in a decrease in both conversion efficiency and system life.
- Ensuring minimum pollutant concentrations during all operational phases (start-up, shutdown, steady state, part load, etc.). Different pollutant species may be present at different operating conditions, allowing the temperature to be adjusted accordingly to match the highest efficiency of the catalytic system to each condition.
- Ensuring combustion safety by keeping lambda control away from air/combustible gas ratios outside the safe range.

6A and 6B are diagrams of the control elements of the device 10 and the corresponding control process 200 for maintaining the temperature within a predetermined range of values. The control elements may include a heating element 27, a fan element 15, and a temperature transducer 28, all connected to a controller 26. The fan element 15 is located at the air inlet 11 to manage the air flow in the device 10. The heating element 27 is located at the catalyst area 16 of the device 10, for example at the substrate of a catalytic device used in the catalytic process CP. The temperature transducer 28 is located at the outlet area 14, and the temperature is measured after the mixed gas GM has passed through the catalytic process CP. Alternatively, the temperature transducer 28 may be located directly on the catalyst surface at the catalyst area 16.

The heating element 27 is activated when the weak LHV combustible gas G cannot guarantee self-ignition. The control hysteresis can be set in the range of 400-450°C. The air injection flow rate can be controlled by a variable frequency driver (VFD), through a valve-controlled bypass or a mechanical system.

Referring to FIG. 6B, in step S201, the temperature value of the mixed gas GM is measured after the catalytic process CP, for example by the temperature transducer 28 of FIG. 6A. This allows the temperature change during the catalytic process CP to be continuously monitored.

If the temperature value is higher than a maximum temperature value (Tmax) (S202), the method includes a step S203 of managing the air flow. As already mentioned, Tmax is the melting temperature of the catalytic substrate used in the catalytic process CP, e.g. 550°C. The process then returns to step S201, where the temperature of the mixed gas GM is continuously measured. In step S204, it is determined that the temperature value is lower than a minimum temperature value (Tmin). In this case, the method includes a step S205 of operating the heating process and setting the control hysteresis to a predetermined temperature range. As already mentioned, Tmin is the temperature at which the pollutants start to decrease, e.g. 400°C.

The process then returns to step S201, where the temperature of the mixed gas GM is continuously measured. If the temperature is below Tmax and above Tmin, the temperature has reached its optimum value and the method is maintained in step S206.

Explanation of symbols

10 Apparatus 11 Air inlet 12 Inlet area 13 Gas inlet 14 Outlet area 15 Fan element 16 Catalyst area 17 Sensor port 18 Stage 1
19 Stage 2
20 gap 21 joint 22 swirl-inducing element 23 through hole 24 helical structure 25 protruding rim 26 control device 27 heating element 28 temperature transducer 29 ring structure 30 housing 100 method for reducing contaminants 200 controlled process

Claims (14)

A method (100) for reducing pollutants of a combustible gas (G), comprising:
Producing a lean mixed gas (GM) containing at least air (A) and the combustible gas (G) (S101);
during a first operating phase (P1), completely oxidizing the lean mixture (GM) by a catalytic process (CP) to obtain an exhaust gas (EG) (S102);
Producing a rich mixed gas (GM) containing at least air (A) and the combustible gas (G) (S103);
Partially oxidizing said rich mixed gas (GM) by a catalytic process (CP) during a second operating phase (P2) to obtain (S104) a clean combustible gas (CG) preferably for the production of heat and power;
The method (100) comprising: 2. The method (100) of claim 1,
a) the combustible gas (G) is synthesis gas produced by a gasification process in a gasification system, the first operation phase (P1) corresponds to a start-up and/or shut-down phase of the gasification system, and the second operation phase (P2) is a steady-state phase of the gasification system, and b) in the lean gas mixture (GM) the air to combustible gas ratio is higher than 1, and c) in the rich gas mixture (GM) the air to combustible gas ratio is lower than 1.
Method. The method (100) of claim 1 or 2, further comprising controlling oxidation of the lean gas mixture (GM) and/or the rich gas mixture (GM) over an operating period. The method (100) of any one of claims 1 to 3, further comprising:
a) actively adjusting the airflow during the first operation phase (P1) and/or the second operation phase (P2), the airflow during the first operation phase (P1) being different from the airflow during the second operation phase (P2), and/or b) automatically switching between a first airflow mode during the first operation phase (P1) and a second airflow mode during the second operation phase (P2);
The method includes: The method (100) of any one of claims 1 to 4, further comprising actively adjusting the air flow between a minimum and a maximum value, the minimum value corresponding to zero air insertion. The method (100) according to any one of claims 1 to 5, further comprising switching between a reducing airflow mode without air injection and an oxidizing airflow mode with air injection during the first operating phase (P1) and/or the second operating phase (P2). The method (100) of any one of claims 1 to 6, further comprising:
a) adjusting the airflow during said first operation phase (P1) to maintain an air to combustible gas ratio higher than 1, preferably equal to 4, and/or b) adjusting the airflow during said second operation phase (P2) to maintain an air to combustible gas ratio lower than 1, preferably equal to 0.3;
The method includes: The method (100) of any one of claims 1 to 7, further comprising:
a) measuring the temperature value of the mixed gas (GM) after said catalytic process (CP) to control the temperature change during said catalytic process (CP), and/or b) maintaining the temperature value during said catalytic process (CP) between a minimum temperature value (Tmin) and a maximum temperature value (Tmax), preferably between 400°C and 550°C, said minimum temperature value (Tmin) being the temperature at which the pollutants start to decrease and said maximum temperature value (Tmax) being the melting temperature of the catalytic substrate used in said catalytic process (CP), and/or c. monitoring (S201) the temperature change during the catalytic process (CP) by measuring the temperature value of the mixed gas (GM) after the catalytic process (CP); if the temperature value is higher than a maximum temperature value (Tmax) (S202), the method comprises a step of managing the air flow (S203); if the maximum temperature value (Tmax) is the melting temperature of the catalytic substrate used in the catalytic process (CP), preferably 550°C, and if the temperature value is lower than a minimum temperature value (Tmin) (S204), the method comprises a step of running a heating process and setting a control hysteresis to a predetermined temperature range (S205), the minimum temperature value (Tmin) is the temperature at which the pollutants start to decrease, preferably 400°C, and the predetermined temperature range is preferably between 400°C and 450°C;
The method includes: A device (10) for reducing pollutants of a combustible gas (G) using the method (100) according to any one of claims 1 to 8, comprising:
an inlet area (12) for receiving a mixed gas (GM) containing at least air (A) and the flammable gas (G);
an outlet area (14) for discharging a pollutant-reduced gas (EG; CG);
a catalytic zone (16) located downstream of said inlet zone (12) and upstream of said outlet zone (14) for partially or completely oxidizing the mixed gas (GM) by a catalytic process (CP);
a housing (30) having a gasification system (10), the housing (30) being removably insertable into the gasification system;
Apparatus (10). The device (10) according to claim 9, which is a catalytic device, in particular a catalytic converter. The device (10) according to claim 9 or 10,
a) the catalytic region (16) comprises a catalytic substrate, the catalytic substrate being preferably a multi-stage assembly, and/or b) the apparatus (10) further comprises an air inlet (11) disposed in the inlet region (12) and a combustible gas inlet (13) disposed in the inlet region (12) spaced apart from the air inlet (11), the air inlet (11) being coupled to a fan element (15);
Device. The device (10) according to any one of claims 9 to 11, further comprising:
a) a control device (26) for switching between a first operating phase (P1) in which the gas (EG) discharged from the outlet region (14) is discharged to the outside and a second operating phase (P2) in which the gas (CG) discharged from the outlet region (14) is used, preferably for the production of heat and electricity; and/or b) a control device (26) for controlling the air flow and/or for controlling the temperature values during the catalytic process (CP); and/or c) a swirl-inducing element (22) arranged in the inlet region (12) for maximizing the mixing of the air (A) and the combustible gas (G) in the mixed gas (GM);
An apparatus comprising: The device (10) according to claim 12, further comprising:
a) at least one temperature transducer (28) connected to said control device (26) and arranged upstream and/or downstream of said catalyst area (16), and/or b) at least one pressure transducer connected to said control device (26) and arranged upstream and/or downstream of said catalyst area (16), and/or c) at least one gas composition sensor connected to said control device (26) and arranged upstream and/or downstream of said catalyst area (16), and/or d) a lambda sensor connected to said control device (26) and arranged upstream of said catalyst area (16), and/or e) a heating element (27) connected to said control device (26) and arranged at said catalyst area (16),
An apparatus comprising: A system, in particular a gasification system, comprising a device (10) according to any one of claims 9 to 13.

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