Project Combustion

7/28/2019 Project Combustion

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PRELIMINARY DESIGN OF WASTE FUME INCINERATOR

TERM PROJECT

FUNDAMENTALS OF COMBUSTION

(CHEN 6153)

PREPARED BY: VIJAYARAGAVAN KRISHNAMOORTHY


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INTRODUCTION:

Waste fume incinerator is one of the most important parts of plants using hazardous chemicals. The

hazardous chemicals that are produced from the plant cannot be emitted directly into environment due

to regulatory standards set by pollution control agency. Therefore, it is very much important to

neutralize these hazardous wastes.

There are many ways by which the toxicity emitted from plants can be neutralized. Some of them are

adsorption, absorption, combustion in an incinerator. In this report, we discussed about the design of an

incinerator that destroys such toxic organic compounds.

To destroy organic compounds, high temperature high residence time is required. However, high

temperature process results in excess consumption of fuel as well as emission of NOx. So, two most

important things need to keep in mind are NOx emissions from the incinerator and concentration of

toxic compounds emitted to the atmosphere. The target concentration of NOx and toxic compounds in

the flue gas stream are 15 ppm and 99% destruction of organic toxic species, respectively.

The objective of the report is to find the design a waste plume incinerator that can best destroy organic

species, higher residence time, appropriate injection point, effect of moisture on temperature profile, .


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TECHNICAL APPROACH:

To solve this problem, following procedure is followed:

Tools required: Chemkin 4.1.1 for calculation of residence time, NO emission

Adiabatic temperature for the furnace was calculated using CEA-NASA code

Calculate Tad from CEA code

Determine NO, residence time,T

across the furnace without recycle

Is NO0.8 sCalculate NO,t ,T>1200

for different recycle

No

Yes

Fix the % recycle for further tasks


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Determining the injection point of toxic chemicals in the furnace:

Tools required:

CEA program

Chemkin 4.1.1.

3. Once that injection location and % recycle is determined, the temperature and NO emission should be

given a closer look. If the temperature at 10 ft is not sufficient enough or if the residence time is not

Determine Tad at the burner and Tat

10 ft for 15 % recycle using CEA code

Determine NO emission, residence time,

T emperature at various location of the

furnace using Chemkin for 15 % FGR and

dirty stream injected at 10ft.

Is T >1200 K for residence

time> 1 s

Repeat the above for

injection of dirty

stream at 10 and 15

ft.

yes

Compare the two

process, i.e., injection a

10 ft and splitting the

dirty stream at 10ft and

15ft

Fix the 10 ft injection for remaining tasks


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sufficient enough for 99% destruction of organic species, then a small amount of natural gas can be

mixed with dirty stream to determine the NO emission and temperature in the furnace.

4. Also, the other option to maintain high temperatures in the furnace is to remove the moisture from

the dirty stream entering into the furnace. CEA code is enough to find out the temperature across the

furnace from 10 ft to 40 ft.

RESULTS AND DISCUSSION

The adiabatic temperature for operating CH4-air mixture was determined using CEA code. The adiabatic

temperature thus obtained is used in Chemkin assuming the simplifications as given. The results for

combustion of CH4-air mixture without recycle were obtained. NO emissions, residence time and

temperature across the furnace are shown in Table-1. However, the pressure in system is increases to

10 atm to increase the residence time of dirty species in the reactor.

TABLE-1:Important parameters obtained for combustion of CH4-air mixture

Tad Residence Time NO emission Mole fraction O2 CO

3022.5 F 1.97 s 86.78 ppm (3% O2 wet) 4.43 % 10 atm

Although the temperature is very high, the NO emission is also very high. Very high NO emission seen

here is attributed to high temperature prevailing in the furnace. The higher emission can be

substantially reduced by allowing recycle of flue gas. Following results were obtained when 10% recycle

was computed.


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Table 2: Results of combustion of CH4-air with 10 % recycle

recycle Residence Time NO emission (3% O2 wet) Mole fraction O2 Pressure

2853.2 F 1.93 s 22.12 ppm 4.38% 10 atm

Although 10% recycle reduced NO emissions, the temperature above 1200 K was maintained only for

0.8 s. This is on a lower side to destroy toxic compounds. However, the NO emission decreased

substantially with 10% recycle, which is expected. Higher flue gas recycle can be employed to reduce

the NO emission. However, that has to be compensated by decrease in temperature. The decline in

temperature was determined by allowing 15 % recycle. Moreover, high flue gas recycle decreases the

residence time of the toxic compounds due to increased volumetric flow rate through the furnace. The

results for 15 % recycle are given in Table-3.

Table3: Results of combustion of CH4-air with 15% recycle

recycle Residence Time NO emission (3% O2 wet) Mole fraction O2 Pressure (atm)

2764.8 F 1.91 s 12.69 ppm 4.36 10

The results shows that the temperature is slightly lower for 15 % recycle. However, the NOx emission

dropped substantially. Therefore, all the calculations from here on were performed with 15 % recycle.

DETERMINING INJECTION LOCATION

After the determination of % recycle, it is important to find out the location that best destroys toxic

organic species. As given in the problem statement, two locations were explored, that is at 10 ft and 15


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ft height. Injection at 10 ft will give us an indication whether to go for split at two locations. The

calculations were performed for 10 ft injection and results are tabulated in Table-4.

Table3: Results of combustion of CH4-air/dirty stream with 15% recycle

T Residence time of

dirty stream

NO emission (3%

O2 wet)

Mole fraction of

O2

Pressure

2764.8 F at 0 ft

2259.8 F at 10 ft

1.43 s (10-40 ft) 13.77 5.1 10

It was found, as well as shown in figure-1, that injection at 10 ft would not yield the desired result as the

residence time for greater 1000 K was little more than 0.6 s. This is not sufficient enough to destroy

toxic species. Also, injection into two locations may not yield desired result due to limited residence

time and temperature. Therefore, in this report injection in two locations were not determined.

Figure-1: Temperature Vs Plug flow residence time (from 10 ft to 40 ft)


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DETERMINING THE MIXING WITH NATURAL GAS

From the earlier calculations, we found that temperature and residence time is not sufficient enough to

completely destroy organic species. Therefore, mixing with natural gas may help increase the

temperature slightly. However, it is expected that the addition of natural gas may increase NO emission

and reduces residence time. To evaluate the effects of each parameter on burning dirty stream with

natural gas, chemkin and CEA calculations were performed. The results of the calculations are tabulated

in Table5.

Table 5: Results for injection of methane with dirty stream at 10 ft height of the furnace

T Residence time of

dirty stream

NO emission Mole fraction of O2 Pressure (atm)

2764.8 F at 0 ft

2264.5 F at 10 ft

1.41 13.36 ppm 4.46 10

The NO emission decreased on addition of natural which is surprisingly an interesting result. This is

attributed to competition between methane combustion and Nitrogen oxidation. Moreover, addition of

natural gas did not increase the temperature measurably and compensates the decline in temperature

of addition of dirty stream. Therefore, it is concluded that addition of methane does not have any effect

of temperature on the furnace..

EFFECT OF MOISTURE IN THE DIRTY STREAM

After all the calculations that were performed, it was found that NO is no longer a constraint. However,

the major constraint here is temperature and the residence time. Removing moisture could come at a

cost of reducing the dirty stream temperature that would affect the temperature in the furnace.


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However, from CEA code it was found that removing moisture does not help much in terms of

temperature at 10 ft level. So, to understand the effect of moisture, CEA calculation assuming the dirty

stream with no moisture was performed and following results were obtained.

Temperature at 10 ft with Natural gas injection

and wet dirty stream at 505.37 K

2264.5 F

Temperature at 10 ft with natural gas injection and

dry dirty stream at 505.37 K

2271.38 F

This clearly indicates that removal of moisture does not have a major effect in increasing the

temperature of the furnace and therefore it is concluded that this is not a right option.

CONCLUSION

Analysis of waste fume incinerator for a given conditions were performed. It was found that 15 %

recycle could limit NO emission to the regulatory standards. However, the major setback is the

temperature profile across the furnace. Even with 10% recycle, temperature profile is not sufficient

enough to destroy the organic species. However, the other constraint with 10 % flue gas recirculation is

total NO emission, which is above the regulatory limits. Even with 10ft injection of flue gas, a

satisfactory result was not obtained due to limitations in residence time and temperature in the furnace.

Addition of natural gas does not have any effect on temperature profile in the furnace. However, NOx

emissions were reduced on addition of natural gas. The same can be said for moisture. Moisture has

slight effect on temperature profile. However, it is not sufficient enough to destroy all the organic

species with sufficient residence time.

The best case scenario for the given problem is to operate the furnace at 15 % recycle with natural gas

mixed injected at 10 ft level.


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RECOMMENDATIONS

1. The flue gas can be injected at height below 10 ft, so that the higher temperature and increasedresidence time can be utilized for 99% destruction of toxic species.

2. Equivalence ratio of CH4-air mixture can be increased for better temperature profile in thefurnace.

3. 50 F/ft assumption seems to be too high a heat loss. Sufficient measures like covering thefurnace with insulation may help reducing heat loss.

4. Volume of the reactor can be increased to increase the residence time of toxic compounds inthe reactor.

APPENDIX

Figure A: NO emission Vs Distance for combustion of CH4-air without recycle and dirty stream


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Figure B: O2 concentration Vs plug flow residence for combustion of CH4-air without recycle and dirty

stream

Figure C: Temperature Vs plug flow residence for combustion of CH4-air without recycle and dirty

stream


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Figure D: Mole fraction of CO emitted by combustion of CH4-air without recycle and dirty stream

II Recycle with 10% flue gas with no dirty stream

Figure A1:Mole fraction of O2 Vs Distance for combustion of CH4-air with 10% recycle and no dirty

stream


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Figure B1:Temperature Vs Residence time for combustion of CH4-air with 10% recycle and no dirty

stream

Figure C1:CO emission Vs distance for combustion of CH4-air with 10% recycle and no dirty stream

15 % Recycle of flue gas with no dirty stream


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Figure A2:NO emission Vs Distance for combustion of CH4-air with 15% recycle and no dirty stream

15% Recycle _ Dirty stream_10Ft

Figure A3:Temperature Vs Distance for combustion of CH4-air with 15% recycle and dirty stream

injected at 10 ft height in the furnace


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Figure B3:Temperature Vs residence time for combustion of CH4-air with 15% recycle and dirty

stream injected at 10 ft height in the furnace

Figure C3:Mole fraction NO Vs distance for combustion of CH4-air with 15% recycle and dirty stream



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Figure D3:Mole fraction O2 Vs distance for combustion of CH4-air with 15% recycle and dirty stream


15% Recycle_10ft_Natural Gas

Figure A4:Temperature Vs residence time for combustion of CH4-air with 15% recycle and dirty

stream-CH4 injected at 10 ft height in the furnace


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Figure B4:Mole fraction NO Vs Distance for combustion of CH4-air with 15% recycle and dirty stream-CH4 injected at 10 ft height in the furnace

Figure C4:Mole fraction O2 Vs Distance for combustion of CH4-air with 15% recycle and dirty stream-

CH4 injected at 10 ft height in the furnace

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