“FLAMELESS OXIDATION” APPLIED TO HIGH TEMPERATURE

PROCESS,

OVERVIEW OF GAZ DE FRANCE R&D ACTIVITIES ON THE SUBJECT”

Alain QUINQUENEAU, Frédéric AGUILE, Lynda PORCHERON, Antonin TOUZET,

Frédéric MILCENT GAZ DE FRANCE

Research and Development Division

361, Avenue du Président Wilson

93211 Saint Denis La Plaine

FRANCE

Main contact : alain.quinqueneau@gazdefrance.com

1 CONTEXT Current European industrial furnaces (steel industry, metallurgy, ceramic, glass,

forging, heat treatments, …) massively use natural gas as fuel. In order to save energy, most furnaces include an energy recuperation system on the exhaust gases to preheat the combustion air to temperatures up to 1000 °C (in case of regenerative burners). Enriching combustion air with oxygen also leads to not only a reduction of combustion products volume but also to energy savings. Unfortunately, this is generally achieved while NOx emissions are raised due to a higher flame temperature, and sometimes hot spots appear in furnaces, which can be harmful to the load. Conventional regenerative burners have the same pros and cons as the oxygen enriching air technique.

For more than ten years now, important studies have been realized in Japan, Germany, and USA to develop new types of burners operating with high temperature combustion air (over 1000 °C) while not only reducing NOx emissions, but also increasing furnaces temperature uniformity (by suppressing hot spots).

Today, several manufacturers have this type of burners in catalog, and industrial demonstrations have been validated mainly in Asia (slab, billet or reheating furnaces, …). This new type of burners operate in the « flameless oxidation » mode.

In Europe, the first demonstration appeared 3 years ago, but remain small scale demonstrations. Massive utilization of these techniques on bigger scale installations will only be possible if reliable prediction tools are available to study the different options to modify existing or develop new furnaces.

After recalling some context elements (main purposes of R&D studies, interest of heat

recuperation, and rough description of flameless oxidation), the main current and oncoming R&D actions of Gaz de France will be presented. The main goal of these studies is to promote the diffusion of these new technologies in France and Europe, and to participate in the reduction of CO2 and NOx emissions, while improving the heating quality of products.

2 MAIN GOALS OF R&D STUDIES In terms of developing new industrial equipments operating with natural gas, the

current and future issues mainly concern : • Energy savings,

o Improving energy efficiency, o Optimization of processes,

• Reduction of pollution, o CO2, o NOx, o Noise.

• Product quality, • Process reliability.

With the concern of being able to guarantee with the maximum of certainty the whole of these performances.

2.1 HEAT RECUPERATION : INTEREST AND TECHNIQUES USED

In a general way, improving energy efficiency on installations has been realized in the 80s, by adding preheating of combustion air with the exhaust gases. Figure 1 shows the savings achieved related to the exhaust gases temperature and to the combustion air temperature. This diagram was established for a Lacq type of natural gas, an air/fuel ratio of 1.1, and air at 25 °C.

Figure 1 : Savings induced by preheating the combustion air

The different methods used and constantly improved since the 80s can be classified in 3 groups : • Burners with separated recuperation : in this case, the heat exchanger is generally centralized,

and installed on the furnace. The combustion air is then brought to the corresponding burners by adapted piping. With this type of technique, air preheating is limited to 600 °C.

• Self recuperative burners : the recuperator is integrated around the burner, combustion products are sucked through the heat exchanger, enabling the preheating of the air. This type of burners can achieve preheating temperatures of up to 700 °C. Thus, the unit inlet power is generally limited to 200 kW.

• Regenerative burners : they generally operate in pairs of burners, each one including its thermal capacity. While operating, each burner is alternatively operating either in burner mode or in a “chimney” mode. Combustion products run through the thermal capacity where their temperature is decreased. During the new cycle (from 20s to 2 to 3 minutes) combustion air is preheated while running through the thermal capacity, and can reach temperature as high as 1200 °C. New similar types of burners, so called self-regenerative burners, appeared recently on the market ; in this case, there is only one burner which thermal capacity is divided in two, and the inversion system is integrated in the burner.

2.2 « FLAMELESS OXIDATION »

To avoid the increase of NOx emissions due to preheated air, many R&D studies have been conducted during the past few years. This is where an entire new combustion system, with very low local levels of oxygen and very high preheated temperature of oxidant was developed. Even if it can be called in a different way (depending on the origin of the R&D studies), this concept is generally called flameless oxidation.

Flameless oxidation is a combustion mode where techniques of reducing NOx emissions (by staging combustion and internal recirculation within the furnace) are pushed to an extreme. Injecting fuel and oxidant at high velocity induces strong internal recirculations of combustion products, and therefore an important dilution of the combustion air appears. The local volumetric concentration of O2 can reach values between 3 and 15 % (see Figure 2). The high temperature of the combustion air (> 1000 °C) preheated by the regenerative system, enables initiation and sustaining of this operating mode. Consequently, there is no longer a structured flame front, because the entire volume of the furnace participates in the heat production.

Figure 2 : High temperature air combustion principles

As shown in Figure 2, the temperature profile generated by this type of flame appears to be more flat than the one achieved with conventional combustion. NOx emissions, mainly influenced by local flame temperature, are now massively reduced, while temperature uniformity in the furnace is strongly improved.

Thanks to the diminution of flame temperature peaks, the average temperature level in the furnace zone can be raised, without its leading to a local overheating around the burners. The heat transfer to the load can therefore be considerably increased. Moreover, combustion noise is reduced.

Flameless oxidation, considered as a future solution to reduce NOx emissions issues, was first developed by Germans and Japanese (see Roman Weber’s article). This technique, now implemented on high temperature processes could have important opportunities in other applications (within the next 10 years in Europe).

From a phenomenological point of view (see Figure 3) high temperature air preheated by combustion products (>1000°C) is supplied to the furnace. In conventional systems, such preheating would definitely lead to high local flame temperatures and therefore to high NOx emissions levels, not complying with current regulations. In flameless oxidation systems, combustion air and fuel injections are separately realized with high velocities. Burner and combustion chamber geometry, associated with high velocities of the flows, create combustion products recirculation toward the burner. The flame can hardly be seen, and it has been shown that combustion is now distributed in the entire furnace volume. The relative temperature and composition uniformity of the combustion chamber is a remarkable characteristic of this process.

AIR

GAZ

Recirculation

Recirculation

Figure 3 : Flameless oxidation principle

The different steps of flameless oxidation are ideally presented (see Figure 4). During step I, preheated combustion air is mixed with recirculated combustion products. After a complete mixing, fuel is added at step II. In the third region, heat energy can be withdrawn by a given application.

Chaleur process

I II

III

Air

Gaz naturel

Produits de combustion

Figure 4 : Different steps of flameless oxidation (Wünning et Wünning 1995)

This combustion mode appeared as the logical evolution of very low NOx regenerative burners. Nevertheless, this technique shows great performances, in terms of thermal flux density, pollutants emissions, and combustion noise, which are not totally explained. This combustion technique is at odds with conventional types of flame such as non-premixed turbulent flame. All the actors working on the subject claim that many studies have to be conducted to totally understand this new type of combustion. The main subject to study are :

• Theory of high temperature jets, • Influence of the different types of positioning of air and gas injectors, • Detailed kinetics chemistry, • Radiated thermal transfer in industrial furnaces, • Procedure of control, command and safety, • Demonstrations in the industry.

Gaz de France is involved in addressing the main issues listed above, and in particular in the integration of these new heat equipments with very high efficiency and very low NOx in the industry field.

3 GAZ DE FRANCE’S ACTIVITIES ON THE SUBJECT Gaz de France has a long lasting experience in the industrial utilizations of natural

gas, and has been working for many years on improving energy and environmental efficiencies of heat equipments. On the flameless oxidation theme, Gaz de France has been particularly active for a few years, and its different actions are presented hereafter.

3.1 TESTING OF NEW BURNERS OPERATING WITH THIS PRINCIPLE

In the aim to better know this type of technique, three commercial regenerative burners operating in the “flameless oxidation” mode were tested at Gaz de France R&D Division facility : Regemat burner 350 Flox® manufactured by WS GmbH, Twinbed FFR burner from North American Manufacturing Co. (under Tokyo Gas Co., Ltd license), and HRS-DL burner manufactured by Nippon Furnace Kyogo. The influence of the main operating conditions (input power, furnace temperature, Air/Fuel ratio and switching time when possible) on NOx emissions and on combustion efficiency was tested. These studies were initiated within European and national projects to which Gaz de France was associated and that partly dealed with these issues.

3.1.1 Equipment tested

3.1.1.1 Regemat 350 Flox® burner from WS

REGEMAT 350 Flox® burner (see Figure 5), has a nominal output of 200 kW. This burner has two operating modes. The first one is for a furnace temperature lower than 850 °C : combustion is done by mixing the air and gas at the burner nozzle and flame is detected by ionization. When the furnace temperature exceeds 850 °C, the burner switches to Flox® mode (Flameless oxidation) and in the self-regenerative mode (the air is thus not preheated when the furnace temperature is below 850 °C).

Figure 5 : REGEMAT 350 Flox® burner from WS

The burner has 6 regenerative blocks intrinsically : while three are pre-heating the combustion air, the three others are extracting the exhaust gases from the furnace. A set of inversion valves, assembled on the burner, allows the swing of the operation of the regenerative blocks.

3.1.1.2 Twinbed FFR burner from North American Manufacturing Co.

The Twinbed FFR 350 burner (see Figure 6), made by the North American Manufacturing Co., has a nominal power of 350 kW. Combustion takes place by mixing at the nozzle with UV cell flame detection. A high voltage electrode provides ignition.

Figure 6 : Twinbed FFR 350 burner from North American Manufacturing Co

The burner has a single regenerative block intrinsically, filled with alumina balls. For the purpose of this testing a full pair of burners and an inversion control system were used. A set of inversion valves allows the swing of operation. The cycle time of inversion was adjustable. The extraction system used is a suction fan, allowing the evacuation of the combustion products through the burner operating in regenerative mode.

3.1.1.3 HRS-DL burner from NFK

HRS-DL burner, has a nominal power of 200 kW. This regenerative burner, developed by NFK, has a thermal capacity of honeycomb type. Burner’s nozzle are closed to the previous Twinbed burner principle (see Figure 6).

Figure 7 : HRS-DL burner from NFK in combustion mode.

The burner has an integrated thermal capacity made of assembled refractory blocks

(see Figure 8) with honeycomb type structure.

Figure 8 : Refractory block with honeycomb type structure.

Each burner is equipped with a reversal valve, installed between the air supply and

the exhaust duct. Switching time can be adapted to suit the process. Below 800 °C, fuel is totally injected in the central air flow using lateral guns. Above this temperature, fuel is injected via two guns located on the periphery of the central jet. The combustion products are evacuated through the regenerative unit using an extractor fan.

3.1.2 Description of the tests

During these tests, the influence of the burner's main operating parameters on the NOx emissions and on the energy efficiency was tested as follows :

• Influence of nominal and semi-nominal power to the burner, • Influence of furnace temperature : 1100 °C, 1200 °C, 1300 °C. • Influence of cycling time (when possible like for FFR and HRS-DL burners), • Influence of the recirculation rate (when possible like for FFR and HRS-DL burners)

from 60 to 85 %. All these tests were performed with a slight pressurization of the furnace of about 1 to

2 mmWC and with LACQ type natural gas. The different measuring points are represented in Figure 9.

ChimneyT (°C)O2 (%)CO2 (%)CO (%)NOx (%)

FurnaceTf (°C)Pf (mmCE)O2 (%)CO2 (%)CO (%)NOx (%)Total Flux (kW/m²)

AirGasAirTair (°C)Pair (mbar)Qair (m3(n)/h)

Tgaz (°C)Pgaz (mbar)Qgaz (m3(n)/h)

Burner 2Burner 1

onoff

ExtractorT (°C)O2 (%)CO2 (%)CO (%)NOx (%)Qf (m3(n)/h)Psuc (mbar)

+ furnace informationQwater (m3(n)/h)Twater inlet (°C)Twater outlet (°C)Twalls (°C)

Figure 9 : Location of measuring points

3.1.3 Main results

Generally, no problem of operation on the burners was noted. The lighting and the detection of flame did not raise any particular problem. The combustion hygiene is also very satisfactory since the burners does not produce CO, and the levels of NOx always remained below 350 mg/m3(n) at 3% excess O2, regardless of the operating conditions tested.

Figure 10 shows the influence of the furnace temperature on NOx emissions. A 2 to 3 ratio was observed between 1100 °C and 1300 °C. NOX emissions in this case appear to be closely linked to the furnace temperature.

50

100

150

200

250

300

350

400

1050 1100 1150 1200 1250 1300 1350

Furnace temperature (°C)

NOx

(mg/

m3(

n) @

3%

exc

ess

O2)

Figure 10 : Influence of furnace temperature on NOx emissions

Figure 11 shows the influence of furnace temperature on combustion efficiency. The increase of furnace temperature leads to a slight diminution of combustion efficiency. Between 1100 and 1300 °C, up to 8 points of combustion efficiency can be lost. This can be explained because the losses through the chimney are higher when furnace temperature increases.

60

65

70

75

80

85

1050 1100 1150 1200 1250 1300 1350

Furnace temperature (°C)

Com

bust

ion

effic

ienc

y (%

of L

HV)

Figure 11 : Influence of furnace temperature on combustion efficiency

3.1.4 Conclusions

Measurements show that very low NOx emissions can be achieved with this new burner generation. Thanks to the flameless oxidation technique, implemented on these burners, tests carried out always showed NOx emissions below 350 mg/m3(n) @ 3% excess O2, regardless of the operating conditions (particularly with high temperature preheated air or furnaces), without affecting combustion efficiencies. Very low NOx burners are now commercially available, and industrial demonstrations in Europe, in various industry fields, should be launched within the next few years.

Moreover, these tests in non-stationary mode, isolating every operating parameter independently one from another, were carried out thanks to the great flexibility of our testing furnaces (particularly good control of the operating conditions thanks to an adjustable water-cooled load).

Consequently, the influence of the main operating parameters of regenerative burners

has been shown :

• The incrrease of NOx emissions and the decrease of combustion efficiency when furnace temperature increases,

• The effect of the Air/Fuel ratio and of the furnace temperature on NOx emissions, • Finally, the small influence of the input power on NOx emission or the combustion

efficiency.

3.2 FORCAST’S DEMONSTRATION

With the same objective we participated to a demonstration project in the field of metallurgy. This operation concerns a forging furnace built and brought to service by Ermat in 1999 for FORCAST International’s factory in Thionville (France). FORCAST International is part of the Swedish group AKERS, world leader in manufacturing either forged or moulded rolling mill cylinders.

3.2.1 The forging furnace

The furnace (8 m long, 3,3 m wide and 1,95 m high) has a maximal net load of 120 tons, for a maximum operating temperature of 1 250 °C (See Figure 12).

Figure 12. Picture of Forcast's forging furnace

It’s equipped with 16 REGEMAT selfregenerative burners manufactured by WS, with

a nominal thermal input of 200 kW. These burners are mounted on both sides of the furnace : one row on the upper part of one side, and another row in the lower part of the opposite side. This set-up allows a very good mixing of the furnace atmosphere by the combustion products, and therefore enhances a very good temperature uniformity. The furnace temperature control is divided in 8 zones of two burners and one thermocouple each. Although, pressure control is ensured by four exhaust valves located on the top of the furnace. Temperature raise on this furnace can reach up to 75 °C/h, with a temperature uniformity of +/- 10 °C, at 1250 °C.

3.2.2 Main results

A measuring campaign was conducted on this furnace late in 1999. It aimed at achieving the heat balance over an operation of reheating two stainless steel ingots weighing 56 tons each before forging. The thermal cycle studied was as follows :

• Furnace was preheated @ 900 °C the night before, • Heating cycle was launched after the second ingot was put in the furnace. This

heating consisted of rapidly increasing the furnace temperature (75 °C/h) up to 1220 °C (this lasted for approx. 4h20),

• Maintaining 1220 °C in the furnace to increase the ingot inside temperature, • Opening of the furnace door to start forging the first ingot.

This total cycle lasted for approximately 27 hours, during which several data was

collected such as combustion products analysis (composition and flows), furnace pressure, instantaneous natural gas flow, and furnace temperature.

The heat balance of the furnace was achieved, and is given in Table 1. In this table, we show the three heat balances achieved over the cycle. The first one deals with the balance of the heating phase (including the two steps heating and equalising). The second one was done over the phase of maintaining 1220 °C in the furnace, and the last one is done over the entire cycle.

Heating balance Maintaining balance Overall balance

Time of operation (min) 660 951 1611ACTIVE Fuel 20804 96% 10148 89% 30527 94%

Load oxidation 760 4% 1211 11% 1860 6%Σ 21564 100% 11359 100% 32387 100%

PASSIVE Load 6579 30% 0 0% 6579 21%Supports 1756 8% 0 0% 1756 6%

Stocked energy (refractory) 1988 9% 0 0% 1988 6%Walls 3053 14% 4399 39% 7452 24%Water 112 1% 162 1% 274 1%

Parasite air inlet 0 0% 1100 10% 1102 3%Exhaust gases 8120 38% 5618 50% 12370 39%

Σ 21608 100% 11279 100% 31521 100%RELATIVE DIFFERENCE 0,2% 0,7% 2,7%

Table 1. Heat balance (data in kWh)

Load oxidation was evaluated through the weight of calamine generated. It corresponds to approximately 1% in weight of the total load. This result appears to be a ‘classical’ one, proving that flameless oxidation techniques doesn’t seem to have any different impact on oxidation of the load.

Losses through the exhaust gases account for at least 38 % of the energy consumption. This appears to be higher than conventional regenerative burners. This result is closely linked to the fact that recuperation of the combustion products through these selfregenerative burners accounts for a little less than what could be achieved by improving the operating parameters of the furnace.

Finally, the net efficiency over the heating phase (ratio between energy accumulated by the load over the energy supplied by the fuel) is 32 %.

3.3 COLLABORATION PROJECT WITH ARCELOR/IRSID, STEIN-HEURTEY AND ADEME

The project: «Integration of the new low-NOx and powerful techniques known as «flameless oxidation» in the industrial processes running with natural gas », proposed by Gaz de France together with the Arcelor/IRSID group, the furnaces manufacturer Stein-Heurtey, and financed by Ademe, aims supporting the diffusion of these new burners in France and Europe, and at taking part in a concrete way, with the reduction of CO2 and NOx emissions. The technical program was elaborated jointly by the partners on the base of the end-users requests. This project, which is a three years long, has been started in October 2000. The main steps of the project are briefly described in the following paragraphs: • Industrial technological survey

The objective of this first step is to show to the actors, that new equipments become available and to put together our knowledge on the subject in order to constitute a common base to start the project. • Burner studies

It is one of the main task of the project. The objective is to characterize in a complete way an equipment working on the flameless oxidation principle and to discuss the way of representing it to carry out industrial furnace simulations. The support burner of our study is burner is the HRS – DL type from NFK company. This regenerative burner has a thermal capacity of honeycomb type (see §2.1 in which the main experimental results of the burner are included). Within the framework of a PHD work with the CORIA of Rouen, detailed measurements in the flame (temperature, velocity, species and radiative flux) in stationary mode are going to be done. These experimental data will be used not only for a better understanding of this new combustion type but also for the numerical simulation validation. They will also contribute to the discussion on the methods to represent this type of burner in a 3D simulation at an acceptable cost (low number of meshes) when representing the whole furnace. • Semi-industrial scale and validation of the tools

Before using the tools at industrial scale and simulating the whole installation, a preliminary work is on the way to validate them at semi-industrial scale. To do that, an experimental set-up (representing a furnace section and including the transfer to the load) has been design and equipped in order to provide input/output measurements and boundaries conditions. It will help us to specify and discuss the validity domain of the tools and the methodology for using them. • Demonstration project

For this demonstration project, a site is going to be identified and a furnace equipped with regenerative burners in the preheated zone (increasing the capacity from 10 to 15 %). A heat balance of the furnace will make it possible to quantify the real profits of energy, and the impact on NOx and CO2 emissions. The experimental results will also be used to validate the predictions of the models developed during the preliminary study and to adapt the tools.

3.4 OTHER ACTIONS

• Calculations of chemical kinetics applied to flameless oxidation

Preliminary simulation of such a combustion was committed using the Passerel tool coupled with the kinetic mechanism GDF-Kin2.0®. Taking into account the relative homogeneity of the temperatures and strong recirculations, simplifying assumptions could be done to describe aerodynamics. The results are encouraging, simulations will be continued with in particular the objectives of evaluating the GDF-Kin2.0® mechanism under extreme conditions for which it was not still validated.

• Exchanges with partners

For a few years, meetings have been organized on the subject within the framework of the GERG (European Gas Research Group). This group is extended with burner manufacturers and European organizations which are active on the subject. These annual meetings make it possible each one to present its work and to exchange on the subject.

4 CONCLUSION Regenerative burners are efficient systems, and their implementing on furnaces can

optimize not only the size of the industrial installation, but also its productivity, while reducing the CO2 emissions. Several manufacturers can supply this type of burners, and industrial demonstrations have been validated, mainly in Asia (Slab, billet, or heat treating furnaces, …). Measurements showed that very low NOx emissions can be achieved with this new generation of burners. Thanks to the flameless oxidation technique implemented on these burners, tests carried out always showed NOx levels below 350 mg/m3(n) @ 3% excess O2 regardless of the operating conditions (particularly with very high preheated air or furnace temperatures), without altering combustion efficiency. Industrial demonstrations in Europe, in all industrial sectors, should be launched within the next few years.

Gaz de France is particularly active on the subject since the past few years. The main goal of its studies is to promote the diffusion of these new burners in France and in Europe, through a better knowledge of the phenomena inherent to this new combustion mode, by developing new designing tools to ensure the installation efficiency, and on the heating quality. Indeed, numerical or physical simulation, associated with detailed in-flame measurements, are tools particularly suited to study the phenomena implemented in these industrial heat equipments. Moreover, the increasing number of parameters to vary during the design of an equipment, can lead to numerous, long lasting and costly tests. Numerical simulation, closely carried out to experimentation (for validation purposes for example), can definitely limit the number of tests, facilitating the analysis and the post processing of the results.

Finally, even if the first demonstrations were mainly carried out in the metallurgy field, this new combustion mode will certainly be fitted in the next future to other industrial sectors such as ceramic, glass, waste treatment, petrochemistry, gas turbine, or even industrial boilers.

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