The Reduction Air Pollution Improved Combustion

Energy Convers.Mgmt Vol. 38, No. 10-13, pp. 1335-1341, 1997 Pergamon PH: S0196-8904(96)00163-X

THE REDUCTION

OF AIR POLLUTION COMBUSTION

© 1997 Elsevier Science LRI All rights reserved. Printed in Great Britain 0196-8904/97 $17.00 + 0.00

BY IMPROVED

S. W. CHURCHILL The University of Pennsylvania, Department of Chemical Engineering, 31 I A Towne Building, 220 South 33rd Street, Philadelphia, PA 19104, U.S.A.

A~tract--The contributions of combustion to air pollution and possible remedies are discussed. Control and reduction of air pollution from combustion is more feasible than from other sources because of its discrete localization. The gaseous products of combustion inevitably include H20 and CO2, NO and/or NO2, and may include N20, SO2, SO3 and unburned and partially burned hydrocarbons. Soot, ash and other dispersed solids may also be present, but are not considered herein. Unburned and partially burned hydrocarbons are primafacie evidence of poor mechanics of combustion and should not be tolerated. On the other hand, NOx, SO2 and SO3 are unavoidable if the fuel contains nitrogen and sulfur. The best remedy in this latter case is to remove these species from the fuel. Otherwise their products of combustion must be removed by absorption, adsorption or reaction. NOx from the fixation of N, in the air and CO may be minimized by advanced techniques of combustion. One such methOd is described in some detail. If CO2 must be removed this can be accomplished by absorption, adsorption or reaction, but precooling is necessary and the quantity is an order of magnitude greater than that of any of the other pollutants. © 1997 Elsevier Science Ltd. Thermally stabilized combustion

Air pollution

Incineration

INTRODUCTION

Combustion contributes to air pollution in a number of different ways. It results in the removal of 02 from the air and in the introduction of H20, CO, CO2, oxides of nitrogen, oxides of sulfur and unburned and partially burned hydrocarbons and their products of pyrolysis, as well as soot and other dispersed solids, including metal oxides. Some of these compounds further react in the atmosphere. In addition, combustion is a source of heating of the atmosphere both directly and by virtue of greenhouse effects due to the products of combustion. Incineration, which is essentially a process of pyrolysis and combustion, produces residual concentrations of halogenated hydrocarbons as well as HC1, HBr, HF and all of the products mentioned in conjunction with combustion itself. Industrial and automative combustion along with the incinerative of municipal and industrial wastes is perceived by the public as the major source of air pollution and is often falsely blamed for other sources such as agriculture and the controlled or uncontrolled burning of biomass. The invisibility of many of the pollutants confuses, but does not eliminate this public perception. As public awareness of the real consequences of air pollution grows increased pressure for improved combustion and incineration may be anticipated and perhaps even proposals for their curtailment or elimination. Two manifestations of public anxiety and pressure in the U.S.A. that should serve as a warning may be mentioned. Dr Terry Hoffman, then of McMaster University in Ontario, used to be fond of referring to the U.S.A. as "the great pilot plant to south", implying that Canadian Universities could observe and learn from our successes and failures. Also, one of my colleagues used to say, "every man is of some use, even if only to serve as a bad example." The examples herein are proffered in that sense. This view may be overly optimistic. Many Mexican chemical engineers asserted in the 1960s that they would learn from and avoid the experiences of the U,S.A. with smog. My last visit to Mexico City indicates that they did not succeed. 1335

1336

CHURCHILL: REDUCTIONIN AIR POLLUTION

The first example of a pervasive public reaction in the U.S.A. is that regarding the incineration of municipal wastes to generate steam, which is epitomized by the acronym NIMBY meaning not in my back yard. The history of the nuclear power industry in the U.S.A. serves as a second example. The future of this industry has been virtually ruined, at least for the current generation, by poor management, poor technology and a failure to deal openly with the public. In working with Admiral Rickover on thermal aspects of the design of the Nautilus submarine, the first nuclear-powered one, I interacted with a number of representatives of the utility industry who were then first considering the generation of electricity by means of nuclear reactors. It was painfully apparent that these representatives were not technically competent to deal with the problems of this new technology because their experience had all been gained in a mature industry. The original posture of the automobile manufacturers to the production of pollutants by their products is a classical one from which we might learn much in another sense. When smog began to increase rapidly in the Los Angeles basin the manufacturers first denied the existence of the problem and then when it became undeniable, blamed other sources, in particular decaying vegetation and agriculture. When automobiles were established beyond denial as a major contributor, the manufacturers asserted that the problem was soluble with present technology, primarily by better-tuned engines. When this failed they begged for more time to redesign the engines and fuels. Finally, they turned to catalytic afterburners, but even then asserted that the public would not accept the greater costs for the automobiles. A curious after effect was the elimination of tetraethyl-lead as an anti-knock compound because it poisoned the catalysts. This entire scenario has since been reenacted with diesel engines. The leadership of the current U.S. Congress started out bravely with a plan to repeal all existing laws on environmental protection, including those related to air pollution, on the basis that: (1) the regulations were a nuisance and a burden to industry; and (2) that the public did not really care. Our Congress was quickly disabused of the latter notion and they have gradually retreated in the face of a broadly based public outcry. They are currently exploring less visible means of achieving the same goal by: (1) reducing funding to the agencies with environmental oversight; (2) eliminating or handicapping enforcement of the existing laws by various means; and (3) transferring responsibility from the federal government to the individual 50 states. This latter step is a transparent subterfuge since the state governments have less funds, less technical capability and are more susceptible to bribes and blackmail, including the threat by companies to move their operations to a more tolerant state. Furthermore, pollutants introduced into the air or streams do not respect state boundaries. As leaders in the development of new processes for energy conversion, the attendees at this conference should be more responsible and far-sighted. Air pollution is a world problem; there are no barricades for the atmosphere at national and continental boundaries, just as there are no non-smoking seats on an airplane. Scientists and engineers working in energy conversion should be proactive and not simply passive observers. It would be wise to straighten-up our house instead of waiting for the public or governmental agencies to pressure us to do so, or fighting a rear guard action that we are certain to lose in the long run. Buying time by denial, obfuscation and political chicanery will only soil our professional reputations. With respect to combustion, the hour is already late. Unless we act rapidly and effectively we may in another decade find ourselves in the same situation as the nuclear-power industry. We must devise and implement processes of combustion and incineration that minimize the production of pollutants and/or develop associated devices and processes that will remove or convert the pollutants after they are formed. On grounds of simplicity, the former choice is obviously to be preferred insofar as possible. POLLUTANTS FROM COMBUSTION Let us now examine, briefly, the individual pollutants formed by combustion and incineration. In a recent paper, Prather and Logan [1] have estimated that the total rate of combustion of fossil fuels increased from 5.4 gigatonnes per year in 1980 to 6.26 in 1991. The uncontrolled burning of oil in Kuwait, as initiated by Iraq, temporarily contributed about 3% to the total. Prather and

CHURCHILL:

REDUCTION IN AIR POLLUTION

1337

Logan further estimate the quantity and primary sources of the major pollutants released to the atmosphere, as listed in Table 1. In this listing, GNG designates greenhouse gases, ODG oxygen-depleting gases and OXG oxygen-generating gases. The first item, CO2, is by far the largest and the most controversial pollutant. If control of CO2 becomes necessary, the most obvious remedy is to reduce the use of carbonaceous fuels to below the limit of biological regeneration of the CO2. All other means are short-term since the absorption of CO2 in the oceans or its absorption in rocks have finite limits. There exists in the U.S.A. a small, but very vocal, group that advocates solution of this problem by the replacement of carbonaceous fuels with H2. If pressed as to the source of the hydrogen they generally refer to electrolysis of water by solar radiation. They ignore the possible production of thermal NOx due to the combustion of H2.

H20 is in even greater concentrations in the atmosphere than CO2, but is not generally considered to be a pollutant and its total concentration is not greatly affected by combustion. However, the proposed fleet of 500 civil transport supersonic aircraft cruising at 20 km of altitude may have a significant effect in the lower stratosphere, producing a 10-20% increase in H20 vapor in the northern midlatitudes.

Table 1. Radiatively and chemically active gases Concentration (10 -9 vol/vol) Current

Residence time (year)

% Change due to combustion

120~

> 90

Compound

Preindustrial

carbon dioxide CO2 methane CH4 nitrous oxide N20 chlorofluorocarbon- 11 CFCh chlorofluorocarhon- 12 CF2C12 hydrochlorofluorocarbon-22 CHF2CI methyl chloroform CH3CCI3 methyl chloride CH3CI methyl bromide CH3Br tetrafluoromethane CF~ carbon monoxide (northern hemisphere) CO nitric oxides (free troposphere) NOx nonmethane hydrocarbons NMHC tropospheric water

278,000

356,000

Impact

700

1714

14

10

GHG, OXG, ODP

270

310

120

20

ODP, G H G

GHG

0

0.268

50

0

ODP, G H G

0

0.503

102

0

ODP, G H G

0

0.105

13

0

ODP, G H G

0

0.160

5.4

0

ODP, G H G

<0.600

0.600

1.3

>0

ODP

0.006

0.012

i.4

50

ODP

0

0.070

0

GHG

> 999

60

120

0.2

>90

OXG

?

10-1000

<0.03

>50

OXG

>0

OXG

?

?

10,000,000

same

0-0.24 •

0

GHG, OXG

~2

10

GHG, ODG

> 50

GHG, OXG

(t)H20 stratospheric water (s)H20 tropospheric ozone (t)O, stratospheric zone (s)O3 carbonyl sulfide OCS

3500

5500

25

50

4000

3800

<0.500

0.500

< 0.10 ~2

<5

GHG, ODG, OXG

30

>0

anti-GHG, ODP

sulfates SO~/SO~"

?

>?

a

> 90

anti-GHG

black carbon aerosols soot C

?

>?

"

> 90

GHG

a--Variable.

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CHURCHILL:

REDUCTION IN AIR POLLUTION

The concentration of the trace gases (those other than 02, N2, H20, COL and A) are generally controlled by chemical and photochemical processes as follows: (1) N20 appears to be increasing at a rate of four megatons of N per year, of which one megaton may be due to combustion. However, these values are very uncertain and variable with respect to location. (2) The concentration of halocarbons is already being reduced as a result of the Montreal protocols. Combustion of wastes may be a remaining significant source. (3) CO, which is entirely a product of combustion, controls the level of OH. Oxidation of CO in the presence of NOx produces 03. (4) Combustion has produced a four-fold increase in NOx but the atmospheric concentrations are quite nonuniform. (5) The existing concentration of SO2 is primarily due to combustion. (6) Soot is wholly due to combustion.

METHODS OF ALLEVIATION Table 1 represents our heritage. What can be done to alleviate the present concentrations and prevent future increases? The most widely proposed method is to manipulate the temperature and the concentration of 02 in burners by staged combustion. Lower temperatures result in less NOx and CO. Excess fuel results in higher temperatures, more CO and some residual hydrocarbons. The addition of NH3 has been used to reduce NOx. Other methods have been discussed at this symposium. Removal of nitrogen and sulfur from fuels is generally more effective than removal of their products of combustion. An example is shale oil, which is often cited as a fuel of the future. The high concentration of nitrogen (as much as 1%) must almost certainly be removed before combustion. Thermally stabilized combustion, a process we have been studying in my laboratory for over 30 years, will now be discussed as one means of drastically reducing the production of NOx, CO and unburned hydrocarbons. Most conventional processes of combustion recycle the energy needed for preheating and igniting the fuel by backmixing some of the hot burned gas with the unburned fuel and air. For example, in Bunsen-type burners the backmixing takes place by molecular diffusion, in bluff-body-stabilized flames the backmixing is accomplished by the recirculating zones, while in jet-mixed flames the backmixing occurs by large-scale turbulent eddies. These processes have the advantage of a wide range of stable operating conditions, but the back-mixed, burned gas dilutes the unburned mixture and some bypassing of the flame occurs due to the lack of confinement. With thermally stabilized flames the backmixing of energy is not accompanied by a proportionate backmixing of burned gas. The purely thermal backmixing may be accomplished by conduction, convection or radiation, or by a combination of these mechanisms. Cellier and Galant [2], Hanamura and Echigo [3], Hardesty and Weinberg [4] and Churchill and Tepper [5] are among those that have investigated the use of convective feedback, while Takeno et al. [6], Hsu et al. [7] and Tong and Sathe [8] are representative of those that have utilized conduction and radiation in a porous solid. The simplest means is probably that of Churchill and coworkers, who have utilized wall-to-wall radiation and in-wall conduction in a cylindrical tube or channel, as illustrated in Fig. 1. The optional characteristics of these burners may be summarized as follows.

CHARACTERISTICS OF THERMALLY STABILIZED C O M B U S T I O N

(1) It produces a virtual step in temperature, pressure and composition (to CO and H20). (2) The temperature of the gas exceeds the adiabatic flame temperature (hence the term "excess-enthalpy flame"). (3) The temperature of the wall approaches, but is less than the adiabatic flame temperature. (4) The flame is virtually invisible because of the absence of unburned carbon and hydrocarbons and this process has, therefore, sometimes been called "flameless combustion".

CHURCHILL:

REDUCTION IN AIR POLLUTION

1339

(5) Radiative participation by the gas is negligible. (6) The flame is quiet due to the absence of oscillations. (7) The process is virtually fuel-independent because the rate of combustion is controlled by thermal feedback and the rupture of a C-C bond. (8) The rate of combustion depends critically on the adiabatic flame temperature, which varies only slightly from fuel to fuel. (9) Atomized liquid fuels behave similarly to gaseous fuels if evaporated before ignition. (10) Solid fuels may be burned with thermal stabilization, but behave differently due to the extended flame zone. (11) The range of stability is narrow and hence the turndown ratio is limited. (12) The stability limits differ fundamentally from those of ordinary flashback and blowoff. (13) Multiple stationary states are possible. (14) The flame is highly resistant to perturbations owing to the great thermal inertia of the ceramic wall. (15) The ceramic block must be preheated in order to stabilize a flame in the burner. (16) The formation of "prompt NO" is completely avoided. (17) The production of "thermal NO" may be minimized at the expense of unburned CO. (18) The simplicity of the flow aids in the complete modeling with free-radical kinetics. (19) The integro-differential energy balance for the wall is difficult to solve. (20) A blower is required for the air because of the significant pressure drop in the burner. (21) The confinement of the flow allows the contiguous operation of a heat exchanger and/or scrubber. The temperature profiles of the gas stream and the wall are illustrated schematically in Fig. 2, while the tradeoff between NOx and CO is illustrated in Fig. 3. The contiguous operation of a water boiler is shown in Fig. 4 and the resulting wall and gas temperatures in Fig. 5. The formation of NOx is rapidly quenched in the boiler, but CO continues to oxidize down to its equilibrium value, as illustrated in Fig. 6. Due to the high-velocity confined flow, the heat transfer coefficient is as much as 100 times that for a conventional water-boiler furnace, resulting in a corresponding decrease in overall size for the same duty. Further details on this process are summarized by Churchill [9]. The possible use of this process for the incineration of chlorinated hydrocarbons is currently under investigation. Preliminary results indicate that all such compounds are reduced below one part per million. In this application an absorber or absorber for the HCI, which is the principal product, operates contiguously with the heat exchanger. SUMMARY

AND CONCLUSIONS

Combustion and incineration are viewed by the public as the principal contributors to air pollution and, indeed, these processes are responsible to a significant degree. Improved methods and devices are urgently needed to alleviate this aspect of energy conversion. One novel process of combustion was described in some detail as an example of possible improvements. Adiabatic flame temperature oo---It #B

ffh-~rjj.,~ g ~ ' ~ A r ~

/ Tube wall

~

J hqf~r~r~rj~rj~ rjt~lFl~

~-~ front

Fig. 1. Schematic of mechanisms of heat transfer within a thermally stabilized combustor.

J Inlet

~es

/

.-,,, Wall mbt~at ~

_

T. . . .

~ .................

Distance ~

Outlet

Fig. 2. Sketch of longitudinal profiles of the gas and temperatures wall in thermally stabilized combustion in a refractory tube.

1340

CHURCHILL:

REDUCTION IN AIR POLLUTION

0.5 ff Be X

0.4

Be

ef f

x/

/

Bo oe

/

0.3-

eB

B

x

0.2-

/ ,,"

/

ff

ee

/x X

0.1-

/

o~4'

/ o°

/x ff

/

/ I

0

J

I

I

++•

30-

f

•

I

0

X

%0

I0-

I

J

I

I

2 4 6 8 I0 Distance of flame front from tube inlet (inches)

Fig. 3. The tradeoff between NOx and CO for different flame front locations: X--experimental; O--computed. (From M. H. Bernstein and S. W. Churchill, 16th Symposium (International) on Combustion, 1977, p. 1737. The Combustion Institute, Pittsburgh, PA, with permission.) Junction between Flame combustion and front heat exchanger

t

I

I ¢

¢ |

i , g

i s

-.constantRe ,-constant Re: >2100 ,', <2100 , Steam

Boiling water

Increasing Re

- t////////Atr/////////~ Premixed ~ edmme - . and air

~~

......fi'~i'l'[~'~t~.......i Cooled /~ . ~ - ~ combustion products Distance from inlet

Fred water Fig. 4. Contiguous water boiler.

Fig. 5. Reynolds number regimes in a 9.5 mm ID thermally stabilized burner and cooling tube.

CHURCHILL:

REDUCTION IN AIR POLLUTION

1341

80 B i

60

Increasing ~4o 0 Z

.#-

20 O "

__

I i

0

1

I

I

D

•

iO41

.•

Increasing

0 102

to =

0

I

I

I

I

20 4o 60 80 Diameters from inlet of cooled tube

I0O

Fig. 6. Measured concentrations (water-free basis) vs distance from inlet of cooled tube. (From M. R. Strenger and S. W. Churchill, Numer. Heat Transfer A, 1992, 22, 1. Taylor and Francis, Washington, DC. Reproduced with permission. All rights reserved.)

REFERENCES 1. Prather, M. J., and Logan, J. A., Combustion's impact on the global atmosphere. In Proceedings of the 25th Symposium (International) on Combustion, 1994, pp. 1513-1527. The Combustion Institute, Pittsburgh, PA. 2. Cellier, M. and Galant, S., Rev. G~n. Therm., Fr., 1981, 20, 287; English transl., Intern. Chem. Engng, 1982, 22, 234. 3. Hanamura, K. and Echigo, R., Wdrme- und Stofpdbertragung, 1991, 26, 377. 4. Hardesty, D. R. and Weinberg, R. J., Combust. Sci. Technol., 1974,8, 201 5. Churchill, S. W. and Tepper, P., Indust. Engng Chem. Proc. Des. Develop., 1985, 24, 542. 6. Takeno, T., Sato K. and Hase, K., A theoretical study on an excess enthalpy flame. In Proceedings of the 18th Symposium (International) on Combustion, 1981, pp. 465-472. The Combustion Institute, Pittsburgh, PA. 7. Hsu, P:-F., Howell, J. R. and Matthew, R. D., J. Heat Transfer, Trans. ASME, 1993, 115, 744. 8. Tong, T. W. and Sathe, S. B. J. Heat Transfer, Trans..4SME, 1991, 113, 423. 9. Churchill, S. W., Chem. Enging Technol., 1989, 12, 249.