References on Improvement of a Device
Reduction of NOx emission
of a 125 t/h capacity gas-fired boiler
at the BC-Power Plant (BorsodChem)
by means of highly-effective minor modifications (2016)
of a 125 t/h capacity gas-fired boiler
at the BC-Power Plant (BorsodChem)
by means of highly-effective minor modifications (2016)
The beauty of this case is that the final result needed only strikingly small (cheap) modifications.
This case is a shining example of applying smart solution with minimum cost of physical modification.
Client:
This case is a shining example of applying smart solution with minimum cost of physical modification.
Client:
BC-Power Plant Ltd, (BC-Erőmű Kft. Kazincbarcika, Hungary)
Hardware:
1 boiler of fresh steam flow rate of 125 t/h, equipped with two pieces of LowNOx burner of 49.5 MW capacity each (Babcock-Borsig)
Fuels: natural gas and hydrogen-rich gas
Fuels: natural gas and hydrogen-rich gas
The regulations of Kyoto Protocol concerning the harmful product emissions just became stricter lately. For example, in the case of using natural gas as fuel, the limit of emission of nitrogen-oxides has decreased from 150 to 100 mg/Nm3. Notwithstanding that the given boiler has been equipped with Low-NOx burners from the first place, at nominal load with pure natural gas combustion the emission would have been somewhere near to 170 mg/Nm3. So, the load of the boiler had to be confined in order to comply even the former environmental limit. Therefore, the smart reduction of NOx emission underneath the new limit seemed a compelling challenge.
Since it is prohibited to operate the boiler with higher nitrogen-oxides emission than the new environmental limit, certain obvious questions arose:
- What do we do to push back the NOx emission under the new environmental limit?
- Can it be done without the expensive replacement of the given burners?
- If so, what kind of modifications will be necessary on the burners to do that?
- What do we do to push back the NOx emission under the new environmental limit?
- Can it be done without the expensive replacement of the given burners?
- If so, what kind of modifications will be necessary on the burners to do that?
This boiler mainly utilizes natural gas, which is occasionally supplemented by hydrogen, a by-product of the adjacent chemical factory. Therefore, in the beginning, the investigation focussed on the natural gas combustion as a starting point of the observation.
In order to get to know the existing circumstances, a 3-dimensional mathematical model of the actual boiler has been built. The 12-million-cell model contains the exact geometry of the real burners, including their various adjustable parts, with which the structure of the flame can be altered within a range.
The analysis of the spatial parameter distributions revealed constructional weaknesses of the burners. In oder to get rid of these, several different sets of small modifications took shape. Similarly, the effectiveness of these variations has been predicted by means of numerical modeling, so their probable results could be compared to each other. Fortunately, one of them appeared to be very promising considering every aspects. Therefore, this scientifically well-grounded modification set has been suggested for realization and, consequently, it has been done on the real boiler.
In order to get to know the existing circumstances, a 3-dimensional mathematical model of the actual boiler has been built. The 12-million-cell model contains the exact geometry of the real burners, including their various adjustable parts, with which the structure of the flame can be altered within a range.
The analysis of the spatial parameter distributions revealed constructional weaknesses of the burners. In oder to get rid of these, several different sets of small modifications took shape. Similarly, the effectiveness of these variations has been predicted by means of numerical modeling, so their probable results could be compared to each other. Fortunately, one of them appeared to be very promising considering every aspects. Therefore, this scientifically well-grounded modification set has been suggested for realization and, consequently, it has been done on the real boiler.
This modification set contained minor changes of the burners, e.g.: cutting out about 10 kg steel, conscious settings of the variable parts, and re-programming the controlling algorithm of the burners. Inserting new components was not needed at all. The cutting out was done within a couple of hours by two workmen. Apart the re-programming, the cost of the physical modification was, in fact, negligible.
After performing the suggested modifications, a series of measurement followed using various boiler loads and various fuel mixtures. Independent measuring system has proved, that the proposed modification of the burners has successfully reduced the nitrogen-oxide emission. For example, for burning pure natural gas, the measured nitrogen-oxide emission is 95 mg/Nm3. It turned out that the changes in every other parameters are in the acceptable range, actually, some of them are improved. Eventually, the whole process exceeded even the wildest expectations.
For more details, please click "play" of the animation below.
After performing the suggested modifications, a series of measurement followed using various boiler loads and various fuel mixtures. Independent measuring system has proved, that the proposed modification of the burners has successfully reduced the nitrogen-oxide emission. For example, for burning pure natural gas, the measured nitrogen-oxide emission is 95 mg/Nm3. It turned out that the changes in every other parameters are in the acceptable range, actually, some of them are improved. Eventually, the whole process exceeded even the wildest expectations.
For more details, please click "play" of the animation below.
SRF & rubber co-firing within conventional pulverized coal burning heavy duty boiler – Development of suitable combustion system for the usage of alternative fuels (2013.*)
Troubleshooting of the evaporating system of a stationary fluidized bed boiler – CFD modeling of the main parts of the evaporating system and the fluidized bed combustion system (2012.*)
Development of the cooling system of the motors of water intake pumps of the Paks Nuclear Power Plant (2011.*)
Development of an algorithm for integration a Pumping Storage Plant (PSP) into the national grid – CFD modeling of pressure loss of a given PSP (2008.*)
Troubleshooting of a gas burner of an inline-heater in the oil-refinery of MOL (2007.**)
Modeling different angle of orientation of gas lances of the boiler #10 and #12 in Dunamenti Power Plant (2005.**)
Troubleshooting of problem of overheating in the boiler #3 (VÉRT) – Termination of Overheating of plates near the liquid seal (2003.*)
Reduction of NOx emission Far Below the Environmental Limit within a coal-fired heavy duty boiler, without using any additional equipment or a single welded seam (2002.*)
Client:
VÉRT Vértesi Power Plant Share Company,(Hungary)
Hardware:
4 units of 60 MWe capacity each
with one OB-230-type
pulverized-coal-fired boiler
(Fresh steam flow rate: 230 t/h/boiler)
with one OB-230-type
pulverized-coal-fired boiler
(Fresh steam flow rate: 230 t/h/boiler)
By accession to the Kyoto Protocol the regulations concerning the harmful product emissions have become stricter in Hungary, too. Power plants have to pay a considerable penalty due to their total mass of emitted harmful products. To cap it all, it had been prohibited to operate with higher nitrogen oxides emission than the environmental limit, which is 650 mg/Nm3 for the given case. Both of these restrictions are individually sufficient to inspire the owner, the Vértesi Power Plant Share Company (Hungary), to do something in order to decrease its original value of about 1,000 mg/Nm3.
The Client invited tenders for retrofit of all the four units and specified a stricter contractual limit of 400 mg/Nm3 for NOx. The scheduled modifications were accomplished with outstanding speed by the winning contractor (SES, Slovakia). The burners of the boilers were replaced by modern LowNOx burners. During the commission, it turned out that the measured NOx emission (~500 mg/Nm3) was found under the environmental limit but it broke the contracting limit. At this stage ERBE Power Engineering and Consulting Ltd. (the former employer of L. L. Varga) offered its collaboration in order to relieve the retrofit program against undesirable consequences. The time has come to use the FLUENT-aided development.
The Client's main questions are:
- Can the NOx emission of the given boilers be beaten down below the contractual limit of 400 mg/Nm3 ?
- If so, what kind of modifications will be necessary on the boilers to do that?
- Can the NOx emission of the given boilers be beaten down below the contractual limit of 400 mg/Nm3 ?
- If so, what kind of modifications will be necessary on the boilers to do that?
You need to perform numerical modeling in order to compare quantitatively the effects of the NO-reducing procedures on the given boiler. For this reason, first and foremost, a three-dimensional numerical model for the reference case has been built up based on the accurate geometry and the applied boundary conditions of the given boiler. After the verification this virtual model - a state of the art development tool - was ready for finding the most appropriate solution.
In accordance with the results of FLUENT reference case it has been found that the main NOx formation mechanisms of the given boiler are thermal NO and fuel NO (they shared approximately equally). The prompt NO mechanism can be neglected under these circumstances.
Accordingly, you will produce considerably NO emission if you combine the high local temperature with the oxygen-rich zone. If you decrease at least one of these parameters (temperature, oxygen content) the emission will be moderated.
Regarding to the characteristics of different NOx formation mechanisms, the quantity of NOx can be reduced by controlled mixing of fuel and oxidizer. This practically means that, as far as possible, there must be exactly the same amount of air within every zone of the furnace than that of the fuel molecules inside the current zone are needed to produce low NOx emission. In the reality this can be approached by means of air-staging or flue-gas-recirculation.
Several calculations have been performed with different sets of boundary conditions (staged air combustion with different primary/secondary/tertiary air ratios; flue gas recirculation with different recirculating ratios). The nitrogen oxides emission calculations have been performed for every case, thus they could be compared to each other.
The staged air combustion method uses two main combustion zones. In the first zone the NO production is limited by the reduced amount of oxidizer, then, in the second zone, the low temperature is the restricting factor. However, we have to take into consideration that insufficient amount of air in a certain volume yields high CO production and there is an emission limit for this molecule too.
It has been found that the staged air combustion method provides more efficient possibility for NOx reduction, therewith it has smaller disadvantages. The total heat transfer to the evaporating surfaces slightly decreases thus the outgoing saturated steam mass flow rate of the drum decreases as well. Certain heat drop is allowable because the heat transfer to the second passage and the injected feed water mass flow rate will increase simultaneously with this effect. Thus the produced total fresh steam mass flow of the boiler remains at its nominal value.
In accordance with the numerical calculation the staged air combustion have been proved the most appropriate procedure for reducing NOx emission of the given case. Beyond that, it was worth to examine a special circumstance of the operation of the given boiler: the three-burner-operation.
The original burners were replaced by new LowNOx burners which were designed for boilers of full-burner-operation. In Oroszlány, however, the given boiler operates generally with three (out of four) burners only. In order to keep up the helical flue gas stream within the furnace the secondary air nozzles of burners deliver the same amount of combustion air, regardless whether the burner operates or not. Thus, the secondary air of the burner which is under repair modifies the local fuel/air ratio near to this burner. To avoid this undesirable effect you have to decrease the secondary air mass flow rate of the non-working burner to some degree. The proposed value of this moderation has been determined by numerical modeling.
A practicable set of boundary conditions has been specified for the boiler by means of FLUENT-aided development. These values have been adjusted manually on the real boiler. The resulting NOx emission value has been verified by measuring sequence. Based on the numerical calculations and the subsequent empirical experiences a list of concrete adjustments has been determined for the given boiler. This list gives you definite instructions on how to operate the boiler with NOx emission below the contracting limit of 400 mg/Nm3. You just have to take this list into your hands, go to the control room of the given boiler and follow the instructions step by step (e.g., set the secondary air control valves of the working burners to 40% closed; set the secondary air control valve of the non-working burner to 32% closed; open the tertiary air control valves to 80%; and so on). By the time you complete the execution of the last step (and wait for a while to reach the thermal equilibrium), the boiler will operate below the contracting limit.
Later, according to the list of adjustments the algorithm of the boiler controlling system in the whole operating range has been modified by the manufacturer (SES). From this time onward, the given boiler has been operating with NOx emission below the specified contracting limit.
In accordance with the results of FLUENT reference case it has been found that the main NOx formation mechanisms of the given boiler are thermal NO and fuel NO (they shared approximately equally). The prompt NO mechanism can be neglected under these circumstances.
Accordingly, you will produce considerably NO emission if you combine the high local temperature with the oxygen-rich zone. If you decrease at least one of these parameters (temperature, oxygen content) the emission will be moderated.
Regarding to the characteristics of different NOx formation mechanisms, the quantity of NOx can be reduced by controlled mixing of fuel and oxidizer. This practically means that, as far as possible, there must be exactly the same amount of air within every zone of the furnace than that of the fuel molecules inside the current zone are needed to produce low NOx emission. In the reality this can be approached by means of air-staging or flue-gas-recirculation.
Several calculations have been performed with different sets of boundary conditions (staged air combustion with different primary/secondary/tertiary air ratios; flue gas recirculation with different recirculating ratios). The nitrogen oxides emission calculations have been performed for every case, thus they could be compared to each other.
The staged air combustion method uses two main combustion zones. In the first zone the NO production is limited by the reduced amount of oxidizer, then, in the second zone, the low temperature is the restricting factor. However, we have to take into consideration that insufficient amount of air in a certain volume yields high CO production and there is an emission limit for this molecule too.
It has been found that the staged air combustion method provides more efficient possibility for NOx reduction, therewith it has smaller disadvantages. The total heat transfer to the evaporating surfaces slightly decreases thus the outgoing saturated steam mass flow rate of the drum decreases as well. Certain heat drop is allowable because the heat transfer to the second passage and the injected feed water mass flow rate will increase simultaneously with this effect. Thus the produced total fresh steam mass flow of the boiler remains at its nominal value.
In accordance with the numerical calculation the staged air combustion have been proved the most appropriate procedure for reducing NOx emission of the given case. Beyond that, it was worth to examine a special circumstance of the operation of the given boiler: the three-burner-operation.
The original burners were replaced by new LowNOx burners which were designed for boilers of full-burner-operation. In Oroszlány, however, the given boiler operates generally with three (out of four) burners only. In order to keep up the helical flue gas stream within the furnace the secondary air nozzles of burners deliver the same amount of combustion air, regardless whether the burner operates or not. Thus, the secondary air of the burner which is under repair modifies the local fuel/air ratio near to this burner. To avoid this undesirable effect you have to decrease the secondary air mass flow rate of the non-working burner to some degree. The proposed value of this moderation has been determined by numerical modeling.
A practicable set of boundary conditions has been specified for the boiler by means of FLUENT-aided development. These values have been adjusted manually on the real boiler. The resulting NOx emission value has been verified by measuring sequence. Based on the numerical calculations and the subsequent empirical experiences a list of concrete adjustments has been determined for the given boiler. This list gives you definite instructions on how to operate the boiler with NOx emission below the contracting limit of 400 mg/Nm3. You just have to take this list into your hands, go to the control room of the given boiler and follow the instructions step by step (e.g., set the secondary air control valves of the working burners to 40% closed; set the secondary air control valve of the non-working burner to 32% closed; open the tertiary air control valves to 80%; and so on). By the time you complete the execution of the last step (and wait for a while to reach the thermal equilibrium), the boiler will operate below the contracting limit.
Later, according to the list of adjustments the algorithm of the boiler controlling system in the whole operating range has been modified by the manufacturer (SES). From this time onward, the given boiler has been operating with NOx emission below the specified contracting limit.
The NOx emission of the modified boiler has been supervised by means of an independent and authenticated measuring system. At nominal load the average value of the measured NOx emission data was 385 mg/Nm3 which is 59 percent of the environmental limit. Moreover, the CO emission has remained below its limit and the boiler efficiency has not decreased.
The controlling algorithm of another three boilers has been modified in the same way after the successful verification. Since then all four boilers have been operating in this Very-LowNOx mode. The presented development saves up 400,000 € per year for the Vértesi Power Plant Share Company, and what is more, everyone is able to breathe less polluted air.
The controlling algorithm of another three boilers has been modified in the same way after the successful verification. Since then all four boilers have been operating in this Very-LowNOx mode. The presented development saves up 400,000 € per year for the Vértesi Power Plant Share Company, and what is more, everyone is able to breathe less polluted air.
For this work, L.L. Varga won the third prize from "The Foundation of Thecnical Development of Industry" in 2002. (Hungarian)
See more: A Magyar Villamos Művek Közleményei, 2003/3. pp 43-47. (Hungarian)
Due to practical reasons, Lajos L. Varga has performed some of these references on behalf of institutes other than CALTECH deposit company. These particular cases have been marked as follows:
- The reports of the case studies marked by an asterisk (*) above have been released on behalf of MVM ERBE Power Engineering & Consulting Private Company Limited.
- The reports of the case studies marked by two asterisks (**) above have been released on behalf of The Technical University of Budapest, Faculty of Mechanical Engineering, Department of Fluid Dynamics.
Despite this circumstance, all of the projects specified above have been worked out completely by Lajos L. Varga alone, except the "Development of suitable combustion system for the usage of SRF and rubber fuels (2013.), in which Mr. György Plaveczky has been an extremely valuable associate researcher, furthermore, Mr. Nándor Udvarhelyi and the complete measurement group of MVM ERBE Ltd. have provided indispensable help, especially regarding the development – and later the usage – of a special measuring device for determination of the drag velocity of the irregular shaped SRF particles, with which the modeling of the trajectory of this material has become numerically possible.
- The reports of the case studies marked by an asterisk (*) above have been released on behalf of MVM ERBE Power Engineering & Consulting Private Company Limited.
- The reports of the case studies marked by two asterisks (**) above have been released on behalf of The Technical University of Budapest, Faculty of Mechanical Engineering, Department of Fluid Dynamics.
Despite this circumstance, all of the projects specified above have been worked out completely by Lajos L. Varga alone, except the "Development of suitable combustion system for the usage of SRF and rubber fuels (2013.), in which Mr. György Plaveczky has been an extremely valuable associate researcher, furthermore, Mr. Nándor Udvarhelyi and the complete measurement group of MVM ERBE Ltd. have provided indispensable help, especially regarding the development – and later the usage – of a special measuring device for determination of the drag velocity of the irregular shaped SRF particles, with which the modeling of the trajectory of this material has become numerically possible.
Do you have questions? Please contact us.
The technical details for some of these case studies are missing, mainly due to the continuous construction of this webpage.
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