Characterization and Reduction of NO during the Combustion of

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Characterization and Reduction of NO during the Combustion of Biodiesel in a Semi-Industrial Boiler Bahamin Bazooyar, Ahmad Shariati, and Seyed Hassan Hashemabadi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01529 • Publication Date (Web): 11 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015

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Graphical Abstract Flame zone

Staged-Air

Post-flame zone

Thermal NO N=N

T

C≡O

Heat

N=O O

O

Swirl-Air

N=O Prompt NO

CH.

N

N=N N=O

N=O

N=O

Penetration depth

T

N=O O N T O N=O

N=O N=N

O

O

Thermal NO T N=O

N=O

Thermal NO

Heat Staged-Air

Soot

N

C≡O

ACS Paragon Plus Environment

Soot

N

O

O=O

N

N=O

T N=O

N

O

T

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Characterization and Reduction of NO during the Combustion of Biodiesel in a Semi-Industrial Boiler

Bahamin Bazooyara,b, Ahmad Shariatia, Seyed Hassan Hashemabadib

a

Ahvaz Faculty of Petroleum Engineering, Petroleum University of Technology (PUT), Ahvaz, P.O. Box 6198144471, Iran b

School of Chemical Engineering, Iran University of Science and Technology (IUST), 16846-13114 Tehran, Iran

*Corresponding author:

Ahmad Shariati Email: [email protected], Tel: +989163013199, Fax: +9861 35551754

Seyed Hassan Hashemabadi Email: [email protected], Tel: +9821 77240376, Fax: +9821 77240495

1 ACS Paragon Plus Environment

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ABSTRACT: This paper aims to characterize and reduce the level of nitrogen monoxide (NO) during the combustion of rapeseed oil methyl ester (ROME) in a semi industrial boiler. First, the formation of NO is characterized during transitional and steady state operation of boiler-the influence of combustion pressure, excess air, exhaust gas temperature, spray cone angle, and combustion air swirl angle on the level of NO is evaluated, suitable burner operating points for control of NO are recognized, and contributions of thermal and prompt NO to the total level of NO are obtained. In the next level, the potential of air-staging technique (injection of extra air) in the reduction of NO is studied. Results reveal that the level of NO rises significantly in post-flame zone 10 cm after the tail of the flame where only the formation of thermal NO is probable in the chamber. Staged-air is able to reduce the level of NO up to 10% without any negative impact on the operation of the boiler. Results also reveal that fuel spray pattern and air dynamic are able to reduce the level of NO during the combustion. The level of NO at swirl angle 45o is on average 13, 10, 7, and 16% lower than that at swirl angles 30, 37.5, 52.5 and 60o, respectively. Pulverization of biodiesel at 60o pattern leads to the formation of 6, 3, and 10% lower NO compared with that at 30, 45, and 90o patterns, respectively. Thermal NO accounts for about 80% of total NO during the steady-state efficient combustion (combustion pressure  19 bars, and equivalence ratio  0.8 (excess air 25%)) and prompt NO contributes more than thermal NO to the total level of NO during the burner start-up. Key words: NO emission, boiler, biodiesel, combustion, NO reduction. 1.Introduction The utility power plant boilers consume around 40% of energy worldwide. They annually emit huge amounts of NOX as well as other pollutants into the atmosphere 1. Power plants boilers account for about 14% of NOX in the atmosphere 2. Due to such a high contribution of boilers to


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atmosphere total NOX and negative impacts of NOX on the ecosystem (global warming, acid rains, stratospheric ozone depletion, and tropospheric ozone formation), environmental protection agencies throughout the world stipulated that the level of NOX from utility power plant boilers should not reach or go beyond a predefined level (e.g., in Iran it is 350 mg/m3 3, in China it is 100 mg/m3 4). Control of NOX during the combustion thus should be taken into the account of consideration as one of the important factors in the management of boilers. There are two different strategies for reduction of NOX in utility power plants boilers: 1. combustion control: implementation of low NOX burners, air staged technology, exhaust gas recirculation, use of renewable clean alternative fuels and 2. Post combustion control: addition of reagent or antioxidants, steam injection and so forth. However, none of these methods alone are quite perfect and may even lead to malfunction of burners and significant operational problems for boilers. For instance, air staging may increase the unburnt carbon during combustion, elevate the exhaust gas temperature 5, and consequently reduce thermal efficiency of boilers. Likewise, the use of reagents and antioxidants may deteriorate the combustion and elevate the CO concentration at the expense of lower degree of NO 6. The use of clean alternative fuels, i.e., biofuels (alcohol, dimethyl ether, biodiesel, and vegetable oils) is one of the most significant strategies to reduce NOX as well as other pollutants 7. However, triglyceride based fuels (biodiesel 8 and vegetable oils 9

) do not fully satisfy the requirements and expectations of combustion community regarding NOX

reduction objectives. Despite the comparatively low level of biodiesel both regular (CO 10, CO2 11, SO2

12

) and non-regular (particulate matter (PM), polycyclic aromatic hydrocarbons (PAH),

volatile organic compounds (VOC) and aldehydes

13

biodiesel have been usually reported equal with or

11, 13

petrodiesel)

12-18

) emissions in boilers, the level of NOX for higher than that for its counterpart (i.e.,

. Hence, control and reduction of NOX during the combustion of biodiesel is


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essential so that boilers meet requirements of environmental protection agencies regarding their NOX emission standards. NO is the main NOX constituent during combustion in boilers because it accounts for around 95% of NOX 19. It is also the building block of other six nitrogen oxides (NO2, N2O, N2O2, N2O3, N2O4, and N2O5) that constitute NOX 2 (For instance, NO2 forms from oxidation of NO with HO2 ion in cold combustion areas (near the rear walls and recirculation zone) 20). Management of NO thus is a big step toward control and reduction of other nitrogen oxides, i.e., NOX. NO is a multivariable function of different combustion parameters in boilers. Indeed, the level of NO depends on the physicochemical characteristics of biodiesel (flame temperature, density, viscosity, surface tension, length of chains, cetane number, and nature of the methyl ester’s fatty acids. . .) 15

, the characteristics of the spray (angle, droplet size) 21, the ambient conditions (air temperature

in the chamber, turbulence and heat exchange at the boiler walls), amount and dynamic of the combustion air (equivalence ratio, and air swirling) 22, combustor configuration (spray pattern and, shape of the combustion chamber), and the flame intensity and integrity. Manipulation of these parameters can change the level of NO, thereby provides effective pathways for the reduction of this deleterious emission during the combustion. The formation of NO in boilers is highly qualitative 15. Pereira et al. 21 have shown the amount of nitrogen oxides and PM during biodiesel combustion in boilers is highly sensitive to spray characteristics as far as any improvement in quality of sprays elevates the level of NOX. Ng and Gan

18

have thoroughly researched the

influence of amount of combustion air on the level of biodiesel normalized NO (the level of NO per unit of energy input) over a wide range of equivalence ratios at different injection pressures. They demonstrated that even the level of NO for biodiesel per unit of energy it produced in boilers is more than that for petrodiesel. Bazooyar et al.

15

have identified four physical characteristics


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(adiabatic flame temperature, cetane number, density, and fuel oxygen content) that mostly influence and vary the level of NO in industrial boilers. Tashtoush et al.

11

have experimentally

examined the pre- and post-stoichiometric combustion of biodiesel and petrodiesel at two same energy levels. They have shown that when the combustion occurs at stoichiometric ratio of fuel to air, flue gas of the boiler contains the maximum amounts of NOX. Ghorbani et al. 10 have evaluated the NO formation of biodiesel at different air mass flows to the combustion chamber of a semiindustrial boiler. They postulated that the trend of biodiesel NO in boilers at different operating conditions largely matches the trend of exhaust gas temperature. However, they revealed no relationship (equation) between the level of NO and exhaust gas temperature. Win Lee et al.

14

have shown that the level of biodiesel NOX does not vary significantly when a residential scale hot water boiler combusts B20 instead of petrodiesel. Macor and Pavanello

13

have studied the

biodiesel NOX emission during 24 days continuous operation of a fire-tube boiler. A slight increase in the level of NOX was observed when the boiler fired biodiesel instead of petrodiesel. Bazooyar et al. 12 have compared the level of NOX of six different methyl esters with that of petrodiesel over different air to fuel ratios and combustion pressures in a semi-industrial boiler. They claimed that regardless of the boiler operating condition, all six types of methyl esters emit more amount of NOX than petrodiesel. Within studies above, the formation of NO was only evaluated over the amount of the combustion air and biodiesel characteristics. NO however, can be a strong function of other combustion variables such as: combustion air swirl angle, combustion pressure, and fuel pulverization pattern. To perfectly control NOX, the influence of these yet unknown parameters, which is in this study called “characterization”, on the level of NO should be fully identified. Among various NOX reduction techniques, combustion control can emerge as one of the most promising technologies for biodiesel-fired boilers since it does neither impose any additional


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operational cost on economy nor need any modification to equipment of boilers. For solid-fired boilers, combustion control strategies, especially air staging, fully meet the requirements of NOX reduction objectives 22. Li et al. 23, Wang et al. 24, and Xue et al.

25

have all shown that level of

NOX during coal combustion is susceptible to over –fire air (air staging) in a 1 MW tangentially furnace. Fan et al.

26

saw the two level-air staging during bituminous combustion in a one-

dimensional furnace to be more effective than one-level air staging on reduction of NOX in a furnace. Ribeirete & Costa

27

, Huang et al.

28

, and Jing et al.

29

have reported significant NOX

reduction during pulverized-coal combustion when the ratio of staged air increases. In combustion control techniques, dynamic (swirl, speed) of the combustion air plays a significant role in the reduction of NOX. For instance, swirl of the combustion air has a pronounced effect on the level of NOX during solid-fuel combustion in boilers. Li et al. 30 have shown that the level of NOX during coal combustion can be sensitive even to angle arch-supplied staged air. Dynamic of the combustion air should be wisely manipulated so that it has a desirable effect on level of NO. Improper dynamics of the combustion air may increase the unburned carbon content in fly ash 31, and the exhaust gas temperature 32, that results in the reduction of boiler thermal efficiency. Due to high similarity in operation of biodiesel and solid-fired boilers (both include atomization, vaporization, and combustion), dynamic of the combustion air is expected to play a key role in biodiesel combustion and level of NOX in boilers. Combustion control techniques (air staging, exhaust gas recirculation) are very different in their strategies of NO control with post combustion techniques (antioxidants, reagents, and steam injection). They usually deal with reduction of thermal NO during the combustion, while post combustion techniques decrease the level of prompt NO. Due to different conditions that govern the combustion during the transitional and steady state of operation of boilers (different local


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temperature, level of pollutants, combustion and thermal efficiencies), the dominant NO formation mechanism may be different, thereby different techniques should be used to control NOX during these two modes of operation. Hence, the contribution of thermal and prompt NO to the total level of NO should be determined so that the most efficient NO reduction technique can be identified. The dominant NO formation mechanism is not recognized during transitional operation of boilers. Even that during steady operation of boilers was identified based on speculation: Bazooyar et al. 15, 33

, and Ng and Gan 18 have implicitly shown that during the steady-state combustion of biodiesel,

the thermal NO is the dominant NO constituent. However, the potential of NOX reduction techniques is not evident unless the exact proportion of thermal to prompt NO is obtained during both transitional and steady operation of boilers. This is another facet of characterization of NO that this study aims for. According to discussion above, biodiesel increases the level of NOX compared to petrodiesel during steady and transitional operation of the boilers, thereby control of NOX during combustion of biodiesel is necessary. To find the most suitable NOX reduction strategy and its potential for reduction of NOX, which is the final goal of this paper, NO should be first characterized (recognition of the NO susceptibility to combustion variables, finding of dominant mechanism of NO formation during transitional and continuous operation, and most suitable burner operating points for control of NO as much as possible). Hence, the first objective of this paper is to characterize and analyze the NO formation of biodiesel over different burner operating conditions and fully determine the most influential variables during the transitional and continuous operation of boilers. Suitable burner operating points for reduction and minimizing of the NO are also identified. The second objective of this paper is to assess the potential of air staging technique for the reduction of NO during the combustion of biodiesel in boilers. To these two ends, rape seed


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oil methyl ester (ROME) was prepared and tested in an experimental combustion laboratory unit (a semi industrial boiler) at optimum combustion conditions. Rape seed oil is still the first worldwide potential for biodiesel production

34

. Although there are many neat resources (algae,

waste, and non-edible oils), biodiesel production in large scale from such these resources is only theoretically fascinating but economically infeasible. 2. Materials and method Experimentation were performed in the Ahvaz Faculty of petroleum engineering in Ahvaz (Iran), with the collaboration of the Petroleum University of Technology (PUT) and the Iran University of Science and Technology (IUST). 2.1 Semi industrial boiler Fig. 1 shows the schematic of the semi industrial boiler. It includes an oil burner, a combustion chamber, and an air capsule. The burner is a U.K. sterling 90 pressure jet type. Its main technical characteristics are indicated in Table 1. The burner is able to propagate the air in two stages (1. primary and, 2. secondary or over-fire air). The primary air passes through a diffuser with several swirl vanes (30, 37.5, 45, 52.5 and 60o). The secondary air or over-fire air is spread around the flame boundaries and has no swirl. Fuel is pumped from two separate tanks at a ground level and forced at high pressure out of a nozzle. Fuel along the nozzle breaks up into small droplets ‘‘spray’’ and then diffuses into 1 m long and 0.45 m wide horizontal stainless steel chamber with specific patterns (cone angle: 30, 40, 60, and 90o). An air capsule is located near the boiler. The capsule is used for injection of the air into the post flame zone or precise setting the equivalence ratio during the combustion. The equivalence ratio is by definition:

(A )Stoichiometry 1 Equivalence ratio  F   (A )real (1  ) F 100

(1)


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where A denotes the amount of the combustion air and F represents the amount the fuel.  denotes the excess air. Stream of the air from capsule can be entered to the chamber from different sampling points (20 locations at distance of 5 cm) embedded in the body of the chamber. These points can also be used for analyzing the fumes throughout the combustion chamber. The combustion chamber has a water jacket and is installed on a stainless steel frame. Instrumentation attached to the different parts of the boiler allowed direct read-out of inlet and outlet cooling water temperatures, water mass flow rates, inlet air temperature, air mass flow rate, exhaust gas temperature, and fuel flow rate. 2.2 Gas analyzer A Testo 350Z gas analyzer calibrated and retrofitted to analyze flue gas of a wide range of fuels (solid, gaseous, and liquids) is used to measure the NO. The analyzer is equipped with NOlow (0.1 ppm resolution) sensors. This type of sensor has a very much low resolution, even lower than the resolution of standard NO sensors. Thus, record of NO was taken place precisely during the experiments. The analyzer is also equipped with a rarefication system that permits the measurement of hot and concentrated pollutants. The analyzer was calibrated by one of the prestigious manufacturer delegations in Iran, Tehran before the experiments for precisely measuring the level of NO. A PC is connected to the gas analyzer to acquire the level of NO. 2.3 Fuel Rape seed oil methyl ester (ROME) was prepared via alkali-based transesterification of 100% pure ROME, methanol (purity>99.9%), and KOH (purity, 97%). The reaction took place during 1 h in a simple CSTR reactor at optimum conditions: 60 °C, 1:6 vegetable oil to methanol ratio, 1 wt % KOH that were found and widely proven by other researchers 35-37. Neat ROME were washed


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with equal volume of water in several batches for removal of unreacted materials (methanol + KOH). Washed ROME was placed in an oven at 120 °C during 1 h for more dehydration and removal of water form fuel. ROME was then tested for determination of its physical characteristics and fatty acid compositions. Tables 2 and 3 give the physical characteristics and chemical structure of ROME. 2.4 Test methodology Experimentation were performed in two stages. First, characterization of NO was taken place. To this end, NO formation during transient and steady combustion of ROME in the boiler was studied. In this stage, it is shown that the level of NO is a function of which factors during the combustion, and how it varies over effective factors. According to this finding, suitable burner operating conditions for having the minimum level of NO is introduced. The relative level of thermal NO and prompt NO is also determined at optimum combustion conditions (with appropriate amount of combustion air at optimum combustion pressure) from the experiments. Second, the potential of air-staging in the reduction of NO was fully examined when biodiesel combusts at optimum conditions during the steady state operation of the boiler. All tests were performed during three distinct days with the same relative humidity and temperature in winter when there is very much need for heating. Tests were replicated several times. Measurements were highly repeatable during each series of tests. The propagation method of analysis was used to quantify uncertainties of NO measurements 15. Maximum uncertainty was observed during the experimental evaluation of the boiler at transient state that is around 6%. In other cases, uncertainty of NO measurements is below the 4%. To calculate the uncertainty, the following equation was applied. Uncertainty (UT) has two sources. 1. UR, deviation from arrhythmic average value of NO measured after repetition of


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experiments, and 2. UA, apparatus sensitivity which is minimum level of NO that the sensor of the analyzer can measure.

UT   UR2  UA2

(2)

3. Results and discussion 3.1 Characterization of NO 3.1.1. NO during burner start-up Biodiesel was tested at over a range fuel pressures (8-21 bars). When combustion pressure goes below 8 bars, ignition of the fuel in the burner becomes very difficult under standard operating conditions (equivalence ratios (0.75 0.85), equivalent to excess air (0.25 0.33)). Even at high pressures, combustion started with significant production of particulate matters (PM: soot, ashes and other solid aerosols) after which flame became stable and particulate matter disappeared. However, the longevity of this transition period is different at each combustion pressures (5 min at 8 bars to 30 s at 22 bars). Compared to when the flame was sustained in the boiler, combustion start-up emitted significantly lower amounts of NO. Fig. 2 shows the trend of NO in relation to smoke number. Smoke number is an index that qualitatively represents degree of completion of the combustion and magnitude of PM production. When the burner starts operating, the temperature inside the chamber is not sufficient for proper vaporization of the biodiesel, and the complete combustion. As a result, combustion starts with the significant production of black particles (high smoke number) and less NO. As time goes forward, local temperatures in the chamber increase, and this results into more complete combustion of fuel, rise in the level of NO, and disappearance of black particles. Macor and Pavanello

13

have tested NO formation of

biodiesel and reported dramatic NO increase after the burner start-up. They showed that the level of NO increases dramatically during transitional operation of the burner, then becomes constant 11 ACS Paragon Plus Environment

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when the operation of the boiler was at steady-state condition. They have shown that about 20 minutes is required before the formation of NO becomes stable and fluctuations in the level of NO of fumes disappear. However, they do not reveal any relation between the level of NO and combustion variables during transitional operation of boiler. During the transitional operation of the boiler, the temperature inside the chamber is not high enough for thermal NO formation, and formation of NO from N2O pathway mechanism and Fenimore mechanisms is only probable during the combustion. At this mode of operation, post combustion techniques are mostly effective in reduction of NOX at the transitional period: by introducing a new reagent into the fuel stream, NOX emission can be perfectly controlled. The formation of NO during steady operation of boilers is different. The in-chamber temperatures at this mode is much higher (T>1000), thereby NO from N2O pathway mechanism becomes insignificant and thermal NO will be the main NO formation mechanism during the steady state mode of operation. These assessment however, are largely speculative and require further experiment and analysis. The paper in the following (section 3.1.3) addresses some of the issues above and measure the relative contribution of thermal and prompt NO. Beforehand, the relative level of NO and CO emissions should be recognized. This can help identify burner set points over which the formation of prompt NO is localized to only to the flame boundaries. 3.1.2. NO vs CO emissions Figs 3 and 4 represent the trend of NO and CO in relation to the combustion pressure and excess air. It can be readily seen from these two graphs that NO emission is maximum when the CO emission is minimum. These two graphs along with fig. 2 attest to qualitative nature of NO formation during the combustion of biodiesel in boilers. It is quite evident in these three figures that the more quality the combustion has in the chamber, the more the level of NO is in fumes.


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Fig. 3 reveals the overall trend of NO in relation to the combustion pressure at equivalence ratio 0.8 (excess air 25%). CO emission becomes almost zero at 19 bars. Below 19 bars, combustion pressure is not sufficient for proper atomization and vaporization of the fuel sprays. As a result, incomplete combustion of ROME occurs even at equivalence ratio 0.8. Indeed, increase in fuel pressure improves the spray characteristics (small droplets with high penetrations) and leads to better and easier vaporization of fuel. This is linked to an increase in average temperature of the chamber and reduction of fuel local rich zones. Therefore, the level of NO at high pressures, when complete combustion takes place is high. The influence of combustion pressure upon NO formation disappeared at 19 bars where the combustion pressure is high enough for complete ignition of fuel. NO emission reaches an almost constant level (71 ppm) above 19 bars. Fig 4. gives the trend of NO and CO emissions in relation to excess air. At excess air 0.8, CO emission is minimum while the level of NO is maximum. At this specific excess air, the amount of the combustion air is very much sufficient for complete combustion and does not also interfere with uniform distribution of fuel in the chamber. The maximum level of NO (71 ppm) is also observed at excess air 0.8 showing that NO shows great susceptibility to combustion quality. The trend of NO over excess air provides a strong authentication for ascendency (the more contribution) of thermal NO over the prompt NO. Maximum NO level (71 ppm) is observed at excess air 25% (equivalence ratio 0.8) where there is a trade-off between maximum possible flame temperature (occurred at lowest excess air) and most oxygen concentration (occurred at highest excess air). Only the formation of thermal NO is influenced by both flame temperature and oxygen concentrations. In contrast, prompt NO is strongly influenced by concentrations of hydrocarbon free radicals during combustion and as consequence, the chemistry of fuels. Neither fatty acid composition

38

nor iodine value, i.e., the degree of unsaturation 15, influence the NOX formation


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during combustion of biodiesel in boilers. Hence, thermal NO forms more than prompt NO during biodiesel combustion. Formation of prompt NO is highly probable in fuel rich flames where fuel plays a significant role in oxidizing the nitrogen. To identify an influential NOX reduction technique and it’s potential for the reduction of NO, the level of prompt and thermal NO should be exclusively measured because, NO X reduction techniques are only able to reduce either the thermal or prompt NO. In the next section, the level of prompt and thermal NO during the combustion of biodiesel at optimum combustion condition is measured. 3.1.3. Thermal vs prompt NO When operation of the boiler is at steady-state condition, thermal and prompt mechanisms are only pathways of NO formation during the combustion of biodiesel. Fig. 5a shows variations of NO along the post flame zone. The likelihood of formation of prompt NO exists only in areas of the chamber that are rich with the fuel and formation of cyanogen (CN) and hydrogen cyanide (HCN) from the reaction of hydrocarbon free radicals (CHX, C2) and nitrogen (N2) is probable 39.

N 2  CH x  HCN  N  ...

(3)

N 2  C 2  2CN

(4)

N  OH  NO  H

(5)

Since the combustion is complete at 19 bars and above (Fig. 3), no significant hydrocarbon free radicals exist in the post flame zone, thereby prompt NO does not form in this region of the chamber. Despite the absence of prompt NO and decreasing trend of temperature along the chamber, there is significant NO formation in the post-flame zone. Note that the timescale of thermal NO is much more than that of oxidation of hydrocarbon radicals. This is due to the time consuming process through which nitrogen dissociates (reaction 4) and reacts with oxygen free radicals: 14 ACS Paragon Plus Environment

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N 2  O  NO  N

(6)

The thermal NO can also form by the two following reactions:

N  O 2  NO  O

(7)

N  OH  NO  H

(8)

Although these two reactions (7 and 8) take place faster than reaction 6, thermal NO formation is largely controlled by dissociation of nitrogen (reaction 6) which is a piecemeal process. Hence, the formation of thermal NO initiates just after the flame in post flame zone. This gives rise to the dramatic increase of NO at the beginning of post flame zone. This elevation in the level of NO is however, quenched by the progressive heat transfer and heat loss of fumes along the chamber, thereby variations of NO level show a mild reduction along the chamber near rear walls. Fig 5b. gives the level of NO in flue gas in relation to flame length. As the flame elongates in the chamber, the level of NO in fumes, at first, mildly rises. When the flame length is longer than 85 cm, NO in fumes has a constant level without any significant variation. This is likely due to insufficient residence time of oxygen and nitrogen in the chamber for thermal NO formation. The concentration of NO at the beginning of the post-flame zone (Fig. 5a at chamber length 55 cm) equals to that in fumes of long flames (Fig. 5b at length > 85 cm). These two same level obtained from two different ways likely represent the level of prompt NO (alleged) during biodiesel combustion in the burner. Prompt NO is a mild function of oxygen concentration in the chamber. Fig. 6 depicts the trend of alleged prompt NO in relation to excess air. It can be readily seen that the susceptibility of alleged prompt NO is higher in low excess air (low oxygen content) than in high excess air (high oxygen content). This wholly matches the De Soete formulation of prompt NO

40

. The alleged

prompt NO is also an absolute increasing function of excess air meaning, unlike thermal NO, level of alleged prompt NO is not sensitive to in-cylinder temperature. Habib et al. 15 ACS Paragon Plus Environment

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have both

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Energy & Fuels

experimentally and theoretically shown that the level of prompt NO is not susceptible to combustion temperature in industrial boilers. Hence, majority of alleged prompt NO in fig. 6 is comprised of actual prompt NO. The prompt to thermal NO ratio during biodiesel combustion in boilers is around 0.24 (for combustion pressure 19 and 22 bars are exactly 0.23 and 0.24, respectively.). This ratio in boiler during methane combustion is around 0.3 41. According to discussion above, thermal NO constitutes around 75% of total NO during biodiesel combustion. Hence, an efficient NOX reduction strategy is to limit the formation of thermal NO. Fig 5a shows that majority of NO forms 10 cm after the tail of the flame. Thermal NO is strong function of temperature. Hence, reduction of average temperature in this area of the combustion chamber perfectly leads to less formation of NO during the combustion. There are many different viewpoints regarding the increase in the level of NO when biodiesel ignites in the burners instead of petrodiesel. Many of the authors believe that biodiesel increases the level of both thermal and prompt NO during the combustion compared to petrodiesel 6, 18. This postulation however, needs further analysis and experimentation. Exact comparison of the level of NOX between biodiesel and petrodiesel is made when the level of thermal and prompt NO for these two fuels are obtained from experimentation. The level of NO2 and the relative level of NO and NO2 is also required to draw a better analogy between the level of NOX of these two fuel in burners. 3.2 Minimizing the NO Topology of flames play a pivotal role in the level of NO during the combustion, since they vary the local temperatures 15. Pattern of sprayed fuel to chamber determines the flame thickness and length while, the swirl air leads to stability of flame, more uniform combustion, and integrity of the flame, thereby both may change the level of NO during the combustion. The paper below shows the way and extent these two influence the NO formation.


Energy & Fuels

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Page 18 of 35

3.2.1. Spray cone angle Spray cone angle determines the pattern of sprayed fuel in the chamber. Fig. 7 shows that it plays a pivotal role in level of NO during the biodiesel combustion. The most adapted nozzle to minimize the NO emission in boiler is with cone angles 60, 45, 30, and 90o respectively. When the flame is narrow (cone angle 30o), the continuous flow of fuel is pulverized around a limited area around the chamber centerline, thereby cannot mix with the combustion air as complete as in other spray patterns (45, 60, and 90o). As a result, local fuel rich zones appear more frequently in the chamber. This contributes to higher level of prompt NO in the chamber. When the flame has the most thickness (90o), the combustion takes place uniformly in shortest length of chamber, therefore fumes has the most residence time in the chamber. This leads to formation of more thermal NO in the chamber. Thus, the level of NO is comparatively lower for middle spray cone angles: 45, and 60o. 3.2.2 Swirl angle Swirl of the combustion air plays a key role in dynamic and stability of flames. It increases the penetrating depth of the air jet stream, reduce the exhaust gas temperature and as consequence, the level of thermal NO during the combustion. Meanwhile, swirl of the combustion air increases the mixing rate of fuel and air. This contributes to reduction of fuel local rich zone and decrease in level of prompt NO. On the other hand, swirl air may lead to a more uniform combustion and increase of average in-chamber temperature in flame zone (flame stability). Thus, air swirling leads to increase in level of thermal NO while, it results in reduction of total NO. Fig. 8 gives the level of NO for different swirl angles. Increase of the swirl angle from 30 to 45o results in 15, 6, and 7% reduction of NO at combustion pressures 8, 15, and 22 bars, respectively. NO at combustion pressures 8, 15, and 22 bars is almost 19, 9, and 10% more for swirl angle 60o than that for swirl


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Energy & Fuels

angle 45o. The influence of swirl angle on NO is very decisive at low combustion pressures. Indeed, increase in combustion pressure leads to high combustion quality and more uniform distribution of fuel in the chamber (reduction of the local fuel rich zones), which has exactly the same effect as increase of swirl angle. This results in insusceptibility of NO on swirl angle at high combustion pressures. Bazooyar et al. 15 have also shown that combustion pressure can nullify the influence of some specific fuel properties (density, and viscosity) on NO. Variables such as: swirl angle, biodiesel viscosity, and density is classified as extrinsic parameters that influence NO formation. 4.2 Reduction of NO with Air-staging technique Air-staging is a workable technique for reduction of thermal NO during coal combustion 42. It has also shown great potential for NO reduction during the combustion of biodiesel. Fig. 9 demonstrates that injection of even a small amount of air into the post-flame zone of the chamber has a significant influence on the level of NO. Indeed, injection of the air in post-flame zone leads to reduction of temperature in that area of the chamber. As a consequence, formation of thermal NO becomes limited and the level of NO in the flue gas declines. However, local reduction in temperature increases the probability of soot formation. This can be qualitatively authenticated by the rise in smoke number when the air is injected into the post flame zone. Furthermore, boiler thermal efficiency is highly influenced by the boiler local temperatures. Any reduction in temperature thus reduces the total level of NO at the expense of malfunction and reduction of thermal efficiency of the boiler. Note that injection of up to 10% of combustion air has no significant negative impact on the operation of the boiler, neither significant soot formation nor reduction of thermal efficiency. When this amount of the air enters into the post-flame zone, the local temperatures after the tail of the flame are still high enough for efficient heat transfer along


Energy & Fuels

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Page 20 of 35

the body of the chamber. Note that the influence of staged-air on the level of NO is more drastic at low combustion pressures. When the in-cylinder pressure and as a result temperature is high, more air is needed to have a same effect than that at low combustion pressures. This is likely due to the more combustion quality at high combustion pressures. 4. Conclusions This study aims to reduce the NO emission during the combustion of biodiesel in a semiindustrial boiler by characterizing the NO formation over different operating conditions. The influence of air-staging is also verified on the reduction of NO of the boiler. The main conclusions are: I. The level of NO is germane to quality of the combustion during both steady and transitional operation of the boiler. The maximum NO level appears when the CO level is minimum. During transitional operation of the boiler, NO is a linear function of smoke number: it rises from a minimum value at burner start-up to a maximum value at steady-state condition. It is postulated that prompt NO forms more during the transitional operation of the boiler. II. Apart from biodiesel characteristics, NO shows significant susceptibility to the operating conditions (combustion pressure, excess air, exhaust gas temperature, spray cone angle, and combustion air dynamic) of the burner. Within these, suitable pulverization pattern, and air swirling angles can reduce the level of NO during the combustion. III. Prompt NO forms more than thermal NO during the burner start-up (transitional period) because the average combustion temperature is not high enough for dissociation of N2 and thermal NO formation. IV. NO level can be reduced in the boiler if biodiesel sprayed to the chamber at 60o pattern and ignites with 45o swirl air. Combustion of biodiesel with 45o swirl air emits on average 13, 10,


Page 21 of 35

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Energy & Fuels

7, and 16% lower NO than that with 30, 37.5, 52.5 and 60o swirl air, respectively. 6, 3, and 10% lower NO were obtained when biodiesel sprayed to the chamber with 60o pattern than when it sprayed with 30, 45, and 90o patterns, respectively. V. The prompt/thermal NO ratio varies from 0.23 to 0.24 at the optimum combustion condition in the boiler- An effective strategy is to reduce thermal NO such as: air-staging, and exhaust gas recirculation. Post-combustion strategies are inappropriate for biodiesel combustion, neither efficient nor economical. VI. Air staging is an efficient technique for NO reduction. With increasing the swirl air angle, the divergent angle decreases and as a consequence, the penetrating depth of the jet stream (flame length) increases. This results in the reduction of exhaust gas temperature and NO. Acknowledgment The financial support of National Iranian Gas Company (NIGC) for this research is greatly appreciated. Literature cited 1. Lior, N., Sustainable energy development: The present (2011) situation and possible paths to the future. Energy 2012, 43, (1), 174-191. 2. EPA, Nitrogen oxides (NOx), why and how they are controlled. US Environmental Protection Agency 1999, Available online at http://www.epa.gov/ttn/catc/dir1/fnoxdoc.pdf. 3. Parlimant, I. R., Standards and limits of outputs from factories and industrial workshops. The fifteenth provision of legislation on control and limit the air pollutants. 1995, Available on line at http://www.nigc-khrz.ir/utils/getFile.aspx?Idn=11510. 4. Ti, S.; Chen, Z.; Li, Z.; Xie, Y.; Shao, Y.; Zong, Q.; Zhang, Q.; Zhang, H.; Zeng, L.; Zhu, Q., Influence of different swirl vane angles of over fire air on flow and combustion characteristics and NOx emissions in a 600 MWe utility boiler. Energy 2014, 74, 775-787. 5. Li, S.; Xu, T.; Hui, S.; Wei, X., NOx emission and thermal efficiency of a 300 MWe utility boiler retrofitted by air staging. Applied Energy 2009, 86, (9), 1797-1803. 6. Gan, S.; Ng, H. K., Effects of antioxidant additives on pollutant formation from the combustion of palm oil methyl ester blends with diesel in a non-pressurised burner. Energy Conversion and Management 2010, 51, (7), 1536-1546. 7. Ghorbani, A.; Bazooyar, B., Optimization of the combustion of SOME (soybean oil methyl ester), B5, B10, B20 and petrodiesel in a semi industrial boiler. Energy 2012, 44, (1), 217-227.


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8. Hoekman, S. K.; Robbins, C., Review of the effects of biodiesel on NOx emissions. Fuel Processing Technology 2012, 96, (0), 237-249. 9. Daho, T.; Vaitilingom, G.; Sanogo, O.; Ouiminga, S. K.; Zongo, A. S.; Piriou, B.; Koulidiati, J., Combustion of vegetable oils under optimized conditions of atomization and granulometry in a modified fuel oil burner. Fuel 2014, 118, (0), 329-334. 10. Ghorbani, A.; Bazooyar, B.; Shariati, A.; Jokar, S. M.; Ajami, H.; Naderi, A., A comparative study of combustion performance and emission of biodiesel blends and diesel in an experimental boiler. Applied Energy 2011, 88, (12), 4725-4732. 11. Tashtoush, G.; Al-Widyan, M. I.; Al-Shyoukh, A. O., Combustion performance and emissions of ethyl ester of a waste vegetable oil in a water-cooled furnace. Applied Thermal Engineering 2003, 23, (3), 285-293. 12. Bazooyar, B.; Ghorbani, A.; Shariati, A., Combustion performance and emissions of petrodiesel and biodiesels based on various vegetable oils in a semi industrial boiler. Fuel 2011, 90, (10), 3078-3092. 13. Macor, A.; Pavanello, P., Performance and emissions of biodiesel in a boiler for residential heating. Energy 2009, 34, (12), 2025-2032. 14. Lee, S. W.; Herage, T.; Young, B., Emission reduction potential from the combustion of soy methyl ester fuel blended with petroleum distillate fuel. Fuel 2004, 83, (11–12), 1607-1613. 15. Bazooyar, B.; Ebrahimzadeh, E.; Jomekian, A.; Shariati, A., NOx Formation of Biodiesel in Utility Power Plant Boilers. Part A: Influence of Fuel Characteristics. Energy & Fuels 2014, 28, (6), 3778-3792. 16. Bazooyar, B.; Hallajbashi, N.; Shariati, A.; Ghorbani, A., An Investigation of the Effect of Input Air Upon Combustion Performance and Emissions of Biodiesel and Diesel Fuel in an Experimental Boiler. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2013, 36, (4), 383-392. 17. Bazooyar, B.; Shariati, A., A Comparison of the Emission and Thermal Capacity of Methyl Ester of Corn Oil with Diesel in an Experimental Boiler. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2013, 35, (17), 1618-1628. 18. Ng, H. K.; Gan, S., Combustion performance and exhaust emissions from the non-pressurised combustion of palm oil biodiesel blends. Applied Thermal Engineering 2010, 30, (16), 2476-2484. 19. Daho, T.; Vaitilingom, G.; Sanogo, O., Optimization of the combustion of blends of domestic fuel oil and cottonseed oil in a non-modified domestic boiler. Fuel 2009, 88, (7), 1261-1268. 20. Turns, S. R., An Introduction to Combustion. 2nd Edition ed.; McGraw-Hill: New York, 2000. 21. Pereira, C.; Wang, G.; Costa, M., Combustion of biodiesel in a large-scale laboratory furnace. Energy 2014, 74, (0), 950-955. 22. Liu, H.; Chaney, J.; Li, J.; Sun, C., Control of NOx emissions of a domestic/small-scale biomass pellet boiler by air staging. Fuel 2013, 103, 792-798. 23. Li, S.; Xu, T.; Sun, P.; Zhou, Q.; Tan, H.; Hui, S., NOx and SOx emissions of a high sulfur self-retention coal during air-staged combustion. Fuel 2008, 87, (6), 723-731. 24. Wang, J.; Fan, W.; Li, Y.; Xiao, M.; Wang, K.; Ren, P., The effect of air staged combustion on NOx emissions in dried lignite combustion. Energy 2012, 37, (1), 725-736. 25. Xue, S.; Hui, S. e.; Liu, T.; Zhou, Q.; Xu, T.; Hu, H., Experimental investigation on NOx emission and carbon burnout from a radially biased pulverized coal whirl burner. Fuel Processing Technology 2009, 90, (9), 1142-1147. 26. Fan, W.; Lin, Z.; Kuang, J.; Li, Y., Impact of air staging along furnace height on NOx emissions from pulverized coal combustion. Fuel Processing Technology 2010, 91, (6), 625-634. 21 ACS Paragon Plus Environment

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Energy & Fuels

27. Ribeirete, A.; Costa, M., Detailed measurements in a pulverized-coal-fired large-scale laboratory furnace with air staging. Fuel 2009, 88, (1), 40-45. 28. Huang, L.; Li, Z.; Sun, R.; Zhou, J., Numerical study on the effect of the Over-Fire-Air to the air flow and coal combustion in a 670 t/h wall-fired boiler. Fuel Processing Technology 2006, 87, (4), 363-371. 29. Jing, J.; Li, Z.; Liu, G.; Chen, Z.; Liu, C., Measurement of Gas Species, Temperatures, Coal Burnout, and Wall Heat Fluxes in a 200 MWe Lignite-Fired Boiler with Different Overfire Air Damper Openings. Energy & Fuels 2009, 23, (7), 3573-3585. 30. Li, Z.; Liu, G.; Chen, Z.; Zeng, L.; Zhu, Q., Effect of angle of arch-supplied overfire air on flow, combustion characteristics and NOx emissions of a down-fired utility boiler. Energy 2013, 59, (0), 377-386. 31. Costa, M.; Azevedo, J. L. T., Experimental characterization of an industrial pulverized coalfired furnace under deep staging conditions. Combustion Science and Technology 2007, 179, (9), 1923-1935. 32. Li, Z.; Fan, S.; Liu, G.; Yang, X.; Chen, Z.; Su, W.; Wang, L., Influence of Staged-Air on Combustion Characteristics and NOx Emissions of a 300 MWe Down-Fired Boiler with Swirl Burners. Energy & Fuels 2010, 24, (1), 38-45. 33. Bazooyar, B.; Shariati, A.; Hashemabadi, S. H., Economy of a utility boiler power plant fueled with vegetable oil, biodiesel, petrodiesel and their prevalent blends. Sustainable Production and Consumption 2015, 3, (0), 1-7. 34. Gui, M. M.; Lee, K. T.; Bhatia, S., Feasibility of edible oil vs. non-edible oil vs. waste edible oil as biodiesel feedstock. Energy 2008, 33, (11), 1646-1653. 35. Leung, D. Y. C.; Wu, X.; Leung, M. K. H., A review on biodiesel production using catalyzed transesterification. Applied Energy 2010, 87, (4), 1083-1095. 36. Bazooyar, B.; Ghorbani, A.; Shariati, A., Physical Properties of Methyl Esters Made from Alkali-based Transesterification and Conventional Diesel Fuel. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2015, 37, (5), 468-476. 37. Qiu, F.; Li, Y.; Yang, D.; Li, X.; Sun, P., Biodiesel production from mixed soybean oil and rapeseed oil. Applied Energy 2011, 88, (6), 2050-2055. 38. San José, J.; Sanz-Tejedor, M. A.; Arroyo, Y., Effect of fatty acid composition in vegetable oils on combustion processes in an emulsion burner. Fuel Processing Technology 2015, 130, (0), 20-30. 39. Saravanan, S.; Nagarajan, G.; Anand, S.; Sampath, S., Correlation for thermal NOx formation in compression ignition (CI) engine fuelled with diesel and biodiesel. Energy 2012, 42, (1), 401410. 40. De Soete, G. G., Overall reaction rates of NO and N2 formation from fuel nitrogen. Symposium (International) on Combustion 1975, 15, (1), 1093-1102. 41. Habib, M. A.; Elshafei, M.; Dajani, M., Influence of combustion parameters on NOx production in an industrial boiler. Computers & Fluids 2008, 37, (1), 12-23. 42. Kuang, M.; Li, Z.; Liu, C.; Zhu, Q., Experimental study on combustion and NOx emissions for a down-fired supercritical boiler with multiple-injection multiple-staging technology without overfire air. Applied Energy 2013, 106, 254-261.


Energy & Fuels

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Table 1 Technical characteristics of the Burner Characteristic

Value

Manufacturer

Bentone (Sweden)

Power output

30–150 kW

Fuel outlet pressure

8-22 bar

Acceptable fuel viscosity

1.5-7.5@ 40 (mm2/s)

Nozzles

1.35 GPH 30, 45, 60, and 90° (hollow cone)

Flow rate

4-10 kg/h


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Page 25 of 35

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Energy & Fuels

Table 2 Properties of ROME Property

ROME

Fuel chemical formula

C18.02H33.09O2

Stoch. air to fuel (mass basis)

12.28

Specific gravity @ 15 ºC

0.8895

Kinematic viscosity @ 40 ºC (cST)

5.405

Cetane number

43

Flash point (ºC)

179

Cloud point (ºC)

-1

Pour point (ºC)

-2

Gross calorific value (MJ/kg)

41.130

Fuel Quality Acid number (mg KOH/g)

0.21

Iodine number (gI/100 g)

53

Carbon residue (% m/m)

0.051

Cold filter plugging point (ºC)

-2

Corrosion, copper strip rating 3 h @ 50 ºC

1a

Sulphated ash (% m/m)

0.005

Oxidative stability @110 ºC (h)

13

Total contamination (mg/kg)

14

Water content (% w/w)

0.044

Sulphur (ppm)

0.95

Sodium + potassium (mg/kg)

1.89

Calcium + magnesium (mg/kg)

0.11

Phosphorus content (mg/kg)

0.72

Methanol content (% m/m)

0.18

Linolenic acid methyl ester (% m/m)

0.99

Monoglyceride content (% m/m)

0.214

Diglyceride content (% m/m)

0.191

Triglyceride content (% m/m)

0.021

Unsaturated ME (Double bond>4) (% m/m)

0.13

Free glycerol (% m/m)

0.003

Total glycerol (% m/m)

0.112


Energy & Fuels

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Table 3 Fatty acids composition of ROME (wt.%) C14:0

C16:0

C16:1

C17:1

C18:0

C18:1

C18:2T C18:2C – C

C18:3C- C C20:0

C23:0

C24:0

0.09

3.91

0.11

0.49

2.74

60.51

0.59

18.83

0.03

0.25

20.14

0.31

C(x:y): x, number of carbons; y, number of double bonds


Air Flowmeter

Page 27 of 35

Energy & Fuels

Exhaust

Air Flow

1 Staged-air 2 3 Flame Temperature Probe t5 4 5 6 Burner Air t3 control 7 8 9 10 Burner Control Box Oil Pump 11 12 13 t2 Analysis points 14 Diffuser Fan damper 15(secondary air) (primary air) 16 17 18 Pump Pressure Gauge 19 Water Drain Valve Oil Flowmeter Water 20 Drain Valve 21 t1 22 23 24 25 Water Temperature Control Cooling Water Flow Control 26 Oil Isolating Valve 27 Oil Filter 28 29 Self seal coupling 30 Cooling Water Flowmeter Self seal coupling 31 32 33 Water Isolating Valve 34 35 Priming Pump 36 Three-way valve 37 38 39 40 Fuel Tanks 41 42 43 44 45 46 47 48 ACS Paragon Plus Environment 49 50 Figure 1 Boiler schematic 51 +

+

0.50

0.75

0.25

0.75

5

0.50

0.25

_

_

Air

3

Testo 350z Gas Analyzer Water Flow switch Condensate Drain Cooling Water Drain Non-return valve

Energy & Fuels

30 s 60

Y = -2.47 X + 49.23 r † = 0.996 3 min

NO (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

40

20

5 min

0 0

2

4 8 bars

6 15 bars

8

10

12

22 bars

Smoke number

Figure 2. NO in relation to smoke number during transitional operation of the boiler (excess air = 25%).


Page 29 of 35

80

3500 NO

CO 3000

70

2000 50 1500

CO (emission)

2500

60

NO (emission)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40 1000 30

500

20

0 8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

Pressure (bars)

Figure 3. NO and CO in relation to combustion pressure (excess air = 25% (equivalence ratio=0.8)).


Energy & Fuels

100

72 NO

CO

70

80

60 66 40 64 20

62

60

0 0

20

40

60

80

100

Excess air (%)

Figure 4. NO and CO in relation to excess air (combustion pressure= 19 bars).


CO (ppm)

68

NO (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Page 31 of 35

100

120

Post-flame

Flame

Rear walls 100

80

Stock

80

60

NO (ppm)

NO (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Energy & Fuels

Rear walls 40

60

40

22 ppm

22 ppm

20

20

19 ppm 0 50

55

60

65

70

75

80

85

90

95

19 ppm

0

100 Stock

30

40

Chamber lenght (cm)

50

60

70

Flame lenght (cm)

(a)

(b) 19 bars

22 bars

Figure 5. NO (a) along the chamber, (b) in relation to flame lenght (excess air 25%).


80

90

100

Energy & Fuels

19.0

NO (ppm)

18.9

18.8

18.7

18.6

18.5 20

30

40

50

60

70

80

90

100

Excess air (%)

20

Pressure 19 bars 19 18

NO (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

17 16 15 14 13 0

20

40

60

80

100

120

Excess air (%)

Figure 6. Alleged prompt NO in relation to excess air (combustion pressure=19 bars).


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85

80

NO (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

75

70

65

60 20

40

60

80 o

Spray cone angle ( ) 8 bars

15 bars

22 bars

Figure 7. NO in relation to spray cone angle.


100

Energy & Fuels

80

70

60

NO (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

50

40

30

20 25

30

35

40

45

50

55

60

o

Swirl angle ( ) 8 bars

15 bars

22 bars

Figure 8. NO in relation to combustion air swirl angle.


65

Page 35 of 35

12

10

8

NO reduction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6

4

2

0 2

4

6

8

10

Staged-Air (%) 8 bars

15 bars

22 bars

Figure 9. NO reduction potential with staged-air.


Characterization and Reduction of NO during the Combustion of

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