Conductive Heat Transfer

Conductive Heat Transfer Heat transfer takes place as conduction if there is a temperature gradient in a solid or fluid

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Conduction will take place if there exist a temperature gradient in a solid (or stationary fluid) medium.

Energy is transferred from more energetic to less energetic molecules when neighboring molecules collide. Conductive heat flow occur in direction of the decreasing temperature since higher temperature are associated with higher molecular energy.

Fourier's Law express conductive heat transfer as

q = k A dT / s         (1)

where

A = heat transfer area (m2, ft2)

k = thermal conductivity of the material (W/m.K or W/m oC, Btu/(hr oF ft2/ft))

dT = temperature difference across the material (K or oC, oF)

s = material thickness (m, ft)

Example - Heat Transfer by Conduction

A plane wall constructed of solid iron with thermal conductivity 70 W/moC, thickness 50 mm and with surface area 1 m by 1 m, temperature 150 oC on one side and 80oC on the other.

Conductive heat transfer can be calculated as:

q = (70 W/moC) (1 m) (1 m) ((150 oC) - (80 oC)) / (0.05 m)

    = 98,000 W

    = 98 kW

Heat Transfer through Plane Walls In Series

Heat conducted through several walls in good thermal contact can be expressed as

q =( T1 - Tn) /((s1/k1A) + (s2/k2A) + ...  (sn/knA))     (2)

where 

T1 = temperature inside surface (K or oC, oF)

TN = emperature inside surface (K or oC, oF)

Example - Heat Transfer through a Furnace Wall 

A furnace wall of 1 m2 consist of a 1.2 cm thick stainless steel inner layer covered with a 5 cm this outside insulation layer of asbestos board insulation. The inside surface temperature of the steel is 800 K and the outside surface temperature of the asbestos is 350 K. The thermal conductivity for stainless steel is 19 W/mK and for asbestos board 0.7 W/mK. 

The conductive heat transport through the wall can be calculated as

q =((800 K) - (350 K)) /(((0.012 m)/(19 W/mK)(1 m2)) + ((0.05 m)/(0.7 W/mK)(1 m2)))

          = 6245 W/m2

Thermal Conductivity and Common Units Btu/(h ft2 oF/ft) Btu/(h ft2 oF/in) Btu/(s ft2 oF/ft) MW/(m2 K/m) kW/(m2 K/m) W/(m2 K/m) W/(m2 K/cm) W/(cm2 oC/cm) W/(in2 oF/in) kJ/(h m2 K/m) J/(s m2 oC/m) kcal/(h m2 oC/m) cal/(s cm2 oC/cm)

1. Real fluids

The flow of real fluids exhibits viscous effect, that is they tend to "stick" to solid surfaces and have stresses within their body.

You might remember from earlier in the course Newtons law of viscosity:

This tells us that the shear stress, , in a fluid is proportional to the velocity gradient - the rate of change of velocity across the fluid path. For a "Newtonian" fluid we can write:

where the constant of proportionality, is known as the coefficient of viscosity (or simply viscosity). We saw that for some fluids - sometimes known as exotic fluids - the value of changes with stress or velocity gradient. We shall only deal with Newtonian fluids.

In his lecture we shall look at how the forces due to momentum changes on the fluid and viscous forces compare and what changes take place.

2. Laminar and turbulent flow

If we were to take a pipe of free flowing water and inject a dye into the middle of the stream, what would we expect to happen?

This

this

or this

Actually both would happen - but for different flow rates. The top occurs when the fluid is flowing fast and the lower when it is flowing slowly.

The top situation is known as turbulent flow and the lower as laminar flow.

In laminar flow the motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the pipe walls.

But what is fast or slow? And at what speed does the flow pattern change? And why might we want to know this?

The phenomenon was first investigated in the 1880s by Osbourne Reynolds in an experiment which has become a classic in fluid mechanics.

He used a tank arranged as above with a pipe taking water from the centre into which he injected a dye through a needle. After many experiments he saw that this expression

where = density, u = mean velocity, d = diameter and = viscosity

would help predict the change in flow type. If the value is less than about 2000 then flow is laminar, if greater than 4000 then turbulent and in between these then in the transition zone.

This value is known as the Reynolds number, Re:

Laminar flow: Re < 2000

Transitional flow: 2000 < Re < 4000

Turbulent flow: Re > 4000

What are the units of this Reynolds number? We can fill in the equation with SI units:

i.e. it has no units. A quantity that has no units is known as a non-dimensional (or dimensionless) quantity. Thus the Reynolds number, Re, is a non-dimensional number.

We can go through an example to discover at what velocity the flow in a pipe stops being laminar.

If the pipe and the fluid have the following properties:

water density = 1000 kg/m3

pipe diameter d = 0.5m

(dynamic) viscosity, = 0.55x103 Ns/m2

We want to know the maximum velocity when the Re is 2000.

If this were a pipe in a house central heating system, where the pipe diameter is typically 0.015m, the limiting velocity for laminar flow would be, 0.0733 m/s.

Both of these are very slow. In practice it very rarely occurs in a piped water system - the velocities of flow are much greater. Laminar flow does occur in situations with fluids of greater viscosity - e.g. in bearing with oil as the lubricant.

At small values of Re above 2000 the flow exhibits small instabilities. At values of about 4000 we can say that the flow is truly turbulent. Over the past 100 years since this experiment, numerous more experiments have shown this phenomenon of limits of Re for many different Newtonian fluids - including gasses.

What does this abstract number mean?

We can say that the number has a physical meaning, by doing so it helps to understand some of the reasons for the changes from laminar to turbulent flow.

It can be interpreted that when the inertial forces dominate over the viscous forces (when the fluid is flowing faster and Re is larger) then the flow is turbulent. When the viscous forces are dominant (slow flow, low Re) they are sufficient enough to keep all the fluid particles in line, then the flow is laminar.

In summary:

Laminar flow

Re < 2000 'low' velocity Dye does not mix with water Fluid particles move in straight lines Simple mathematical analysis possible Rare in practice in water systems.

Transitional flow

2000 > Re < 4000 'medium' velocity Dye stream wavers in water - mixes slightly.

Turbulent flow

Re > 4000 'high' velocity Dye mixes rapidly and completely Particle paths completely irregular Average motion is in the direction of the flow Cannot be seen by the naked eye Changes/fluctuations are very difficult to detect. Must use laser. Mathematical analysis very difficult - so experimental measures are used Most common type of flow.

3. Pressure loss due to friction in a pipeline.

Up to this point on the course we have considered ideal fluids where there have been no losses due to friction or any other factors. In reality, because fluids are viscous, energy is lost by flowing fluids due to friction which must be taken into account. The effect of the friction shows itself as a pressure (or head) loss.

In a pipe with a real fluid flowing, at the wall there is a shearing stress retarding the flow, as shown below.

If a manometer is attached as the pressure (head) difference due to the energy lost by the fluid overcoming the shear stress can be easily seen.

The pressure at 1 (upstream) is higher than the pressure at 2.

We can do some analysis to express this loss in pressure in terms of the forces acting on the fluid.

Consider a cylindrical element of incompressible fluid flowing in the pipe, as shown

The pressure at the upstream end is p, and at the downstream end the pressure has fallen by p to (p-p).

The driving force due to pressure (F = Pressure x Area) can then be written

driving force = Pressure force at 1 - pressure force at 2

The retarding force is that due to the shear stress by the walls

As the flow is in equilibrium,

driving force = retarding force

Giving an expression for pressure loss in a pipe in terms of the pipe diameter and the shear stress at the wall on the pipe.

The shear stress will vary with velocity of flow and hence with Re. Many experiments have been done with various fluids measuring the pressure loss at various Reynolds numbers. These results plotted to show a graph of the relationship between pressure loss and Re look similar to the figure below:

This graph shows that the relationship between pressure loss and Re can be expressed as

As these are empirical relationships, they help in determining the pressure loss but not in finding the magnitude of the shear stress at the wall w on a particular fluid. If we knew w

we could then use it to give a general equation to predict the pressure loss.

4. Pressure loss during laminar flow in a pipe

In general the shear stress w. is almost impossible to measure. But for laminar flow it is possible to calculate a theoretical value for a given velocity, fluid and pipe dimension.

In laminar flow the paths of individual particles of fluid do not cross, so the flow may be considered as a series of concentric cylinders sliding over each other - rather like the cylinders of a collapsible pocket telescope.

As before, consider a cylinder of fluid, length L, radius r, flowing steadily in the centre of a pipe.

We are in equilibrium, so the shearing forces on the cylinder equal the pressure forces.

By Newtons law of viscosity we have , where y is the distance from the wall. As we are measuring from the pipe centre then we change the sign and replace y with r distance from the centre, giving

Which can be combined with the equation above to give

In an integral form this gives an expression for velocity,

Integrating gives the value of velocity at a point distance r from the centre

At r = 0, (the centre of the pipe), u = umax, at r = R (the pipe wall) u = 0, giving

so, an expression for velocity at a point r from the pipe centre when the flow is laminar is

Note how this is a parabolic profile (of the form y = ax2 + b ) so the velocity profile in the pipe looks similar to the figure below

What is the discharge in the pipe?

So the discharge can be written

This is the Hagen-Poiseuille equation for laminar flow in a pipe. It expresses the

discharge Q in terms of the pressure gradient ( ), diameter of the pipe and the viscosity of the fluid.

We are interested in the pressure loss (head loss) and want to relate this to the velocity of the flow. Writing pressure loss in terms of head loss hf, i.e. p = ghf

This shows that pressure loss is directly proportional to the velocity when flow is laminar.

It has been validated many time by experiment.

It justifies two assumptions:

1. fluid does not slip past a solid boundary 2. Newtons hypothesis.

Thermal radiationFrom Wikipedia, the free encyclopediaJump to: navigation, search

This diagram shows how the peak wavelength and total radiated amount vary with temperature according to Wien's displacement law. Although this plot shows relatively

high temperatures, the same relationships hold true for any temperature down to absolute zero. Visible light is between 380 and 750 nm.

Thermal radiation in visible light can be seen on this hot metalwork. Its emission in the infrared is invisible to the human eye and the camera the image was taken with, but an infrared camera could show it (See Thermography).

Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation.

Examples of thermal radiation include visible light emitted by an incandescent light bulb, infrared radiation emitted by animals and detectable with an infrared camera, and the cosmic microwave background radiation. Thermal radiation is different from thermal convection and thermal conduction--a person near a raging bonfire feels radiant heating from the fire, even if the surrounding air is very cold.

Sunlight is thermal radiation generated by the hot plasma of the Sun. The Earth also emits thermal radiation, but at a much lower intensity and different spectral distribution because it is cooler. The Earth's absorption of solar radiation, followed by its outgoing thermal radiation are the two most important processes that determine the temperature of the Earth.

If a radiation-emitting object meets the physical characteristics of a black body in thermodynamic equilibrium, the radiation is called blackbody radiation.[1] Planck's law describes the spectrum of blackbody radiation, which depends only on the object's temperature. Wien's displacement law determines the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the radiant intensity.[2]

In engineering, thermal radiation is considered one of the fundamental methods of heat transfer, although a physicist would likely consider energy transfer through thermal radiation a case of one system performing work on another via electromagnetic radiation, and say that heat is a transfer of energy that does no work. The difference is strictly one of nomenclature.

Contents

[hide] 1 Overview

o 1.1 Surface effects 2 Properties 3 Interchange of energy 4 Radiation Heat Transfer 5 Radiative power

o 5.1 Constants o 5.2 Variables

6 See also 7 References

8 External links [edit] Overview

Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero.[3] It represents a conversion of thermal energy into electromagnetic energy. Thermal energy is the collective mean kinetic energy of the random movements of atoms and molecules in matter. Atoms and molecules are composed of charged particles, i.e. protons and electrons and their oscillations result in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy and carrying entropy away from the body through its surface boundary. Electromagnetic radiation, or light, does not require the presence of matter to propagate and travels in the vacuum of space infinitely far if unobstructed.

The characteristics of thermal radiation depends on various properties of the surface it is emanating from, including its temperature, its spectral absorptivity and spectral emissive power, as expressed by Kirchhoff's law.[3] The radiation is not monochromatic, i.e. it does not consist of just a single frequency, but comprises a continuous dispersion of photon energies, its characteristic spectrum. If the radiating body and its surface are in thermodynamic equilibrium and the surface has perfect absorptivity at all wavelengths, it is characterized as a black body. A black body is also a perfect emitter. The radiation of such perfect emitters is called black-body radiation. The ratio of any body's emission relative to that of a black body is the body's emissivity, so that a black body has an emissivity of unity.

Absorptivity, reflectivity, and emissivity of all bodies are dependent on the wavelength of the radiation. The temperature determines the wavelength distribution of the electromagnetic radiation. For example, fresh snow, which is highly reflective to visible light (reflectivity about 0.90), appears white due to reflecting sunlight with a peak wavelength of about 0.5 micrometres. Its emissivity, however, at a temperature of about -5°C, peak wavelength of about 12 micrometres, is 0.99.

The distribution of power that a black body emits with varying frequency is described by Planck's law. At any given temperature, there is a frequency fmax at which the power emitted is a maximum. Wien's displacement law, and the fact that the frequency of light

is inversely proportional to its wavelength in vacuum, mean that the peak frequency fmax is proportional to the absolute temperature T of the black body. The photosphere of the Sun, at a temperature of approximately 6000 K, emits radiation principally in the visible portion of the electromagnetic spectrum. Earth's atmosphere is partly transparent to visible light, and the light reaching the surface is absorbed or reflected. Earth's surface emits the absorbed radiation, approximating the behavior of a black body at 300 K with spectral peak at fmax. At these lower frequencies, the atmosphere is largely opaque and radiation from Earth's surface is absorbed or scattered by the atmosphere. Though some radiation escapes into space, it is absorbed and subsequently re-emitted by atmospheric gases. It is this spectral selectivity of the atmosphere that is responsible for the planetary greenhouse effect, contributing to global warming and climate change in general.

The common household incandescent light bulb has a spectrum overlapping the black body spectra of the sun and the earth. A portion of the photons emitted by a tungsten light bulb filament at 3000K are in the visible spectrum. However, most of the energy is associated with photons of longer wavelengths; these do not help a person see, but still transfer heat to the environment, as can be deduced empirically by observing a household incandescent light bulb. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal.

Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a tiny spot by using reflecting mirrors. Concentrating solar power takes advantage of this fact. In many such systems, mirrors are employed to concentrate sunlight into a smaller area. In lieu of mirrors, Fresnel lenses can also be used to concentrate heat flux. Either method can be used to quickly vaporize water into steam using sunlight. For example, the sunlight reflected from mirrors heats the PS10 solar power tower, and during the day it can heat water to 285°C (558.15K) or 545°F.

[edit] Surface effects

Lighter colors and also whites and metallic substances absorb less illuminating light, and thus heat up less; but otherwise color makes small difference as regards heat transfer between an object at everyday temperatures and its surroundings, since the dominant emitted wavelengths are nowhere near the visible spectrum, but rather in the far infrared. Emissivities at those wavelengths have little to do with visual emissivities (visible colors); in the far infrared, most objects have high emissivities. Thus, except in sunlight, the color of clothing makes little difference as regards warmth; likewise, paint color of houses makes little difference to warmth except when the painted part is sunlit.

The main exception to this is shiny metal surfaces, which have low emissivities both in the visible wavelengths and in the far infrared. Such surfaces can be used to reduce heat transfer in both directions; an example of this is the multi-layer insulation used to insulate spacecraft.

Low-emissivity windows in houses are a more complicated technology, since they must have low emissivity at thermal wavelengths while remaining transparent to visible light.

[edit] Properties

There are four main properties that characterize thermal radiation:

Thermal radiation emitted by a body at any temperature consists of a wide range of frequencies. The frequency distribution is given by Planck's law of black-body radiation for an idealized emitter. This is shown in the right-hand diagram.

The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases. For example, a red hot object radiates mainly in the long wavelengths (red and orange) of the visible band. If it is heated further, it also begins to emit discernible amounts of green and blue light, and the spread of frequencies in the entire visible range cause it to appear white to the human eye; it is white hot. However, even at a white-hot temperature of 2000 K, 99% of the energy of the radiation is still in the infrared. This is determined by Wien's displacement law. In the diagram the peak value for each curve moves to the left as the temperature increases.

The total amount of radiation of all frequencies increases steeply as the temperature rises; it grows as T4, where T is the absolute temperature of the body. An object at the temperature of a kitchen oven, about twice the room temperature on the absolute temperature scale (600 K vs. 300 K) radiates 16 times as much power per unit area. An object at the temperature of the filament in an incandescent light bulb--roughly 3000 K, or 10 times room temperature—radiates 10,000 times as much energy per unit area. The total radiative intensity of a black body rises as the fourth power of the absolute temperature, as expressed by the Stefan–Boltzmann law. In the plot, the area under each curve grows rapidly as the temperature increases.

The rate of electromagnetic radiation emitted at a given frequency is proportional to the amount of absorption that it would experience by the source. Thus, a surface that absorbs more red light thermally radiates more red light. This principle applies to all properties of the wave, including wavelength (color), direction, polarization, and even coherence, so that it is quite possible to have thermal radiation which is polarized, coherent, and directional, though polarized and coherent forms are fairly rare in nature.

These properties apply if the distances considered are much larger than the wavelengths contributing to the spectrum (most significant from 8-25 micrometres at 300 K). Indeed, thermal radiation here takes only traveling waves into account. A more sophisticated framework involving electromagnetic theory has to be used for lower distances (near-field thermal radiation).

°C Subjective color [2]

480 faint red glow

580 dark red

730 bright red, slightly orange

930 bright orange

1100 pale yellowish orange

1300 yellowish white

> 1400 white (yellowish if seen from a distance through atmosphere)

[edit] Interchange of energy

Radiant heat panel for testing precisely quantified energy exposures at National Research Council, near Ottawa, Ontario, Canada.

Thermal radiation is an important concept in thermodynamics as it is partially responsible for heat transfer between objects, as warmer bodies radiate more heat than colder ones. Other factors are convection and conduction. The interplay of energy exchange is characterized by the following equation:

Here, represents spectral absorption factor, spectral reflection factor and spectral transmission factor. All these elements depend also on the wavelength . The spectral

absorption factor is equal to the emissivity ; this relation is known as Kirchhoff's law of thermal radiation. An object is called a black body if, for all frequencies, the following formula applies:

In a practical situation and room-temperature setting, humans lose considerable energy due to thermal radiation. However, the energy lost by emitting infrared heat is partially regained by absorbing the heat of surrounding objects (the remainder resulting from generated heat through metabolism). Human skin has an emissivity of very close to 1.0 .[4] Using the formulas below then shows a human being, roughly 2 square meter in area, and about 307 kelvins in temperature, continuously radiates about 1000 watts. However, if people are indoors, surrounded by surfaces at 296 K, they receive back about 900 watts from the wall, ceiling, and other surroundings, so the net loss is only about 100 watts. These heat transfer estimates are highly dependent on extrinsic variables, such as wearing clothes (decreasing total thermal "circuit" conductivity, therefore reducing total output heat flux.) Only truly "grey" systems (relative equivalent emissivity/absorptivity and no directional transmissivity dependence in all control volume bodies considered) can achieve reasonable steady-state heat flux estimates through the Stefan-Boltzmann law. Encountering this "ideally calculable" situation is virtually impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "grey" approximations will get you close to real solutions, as most divergence from Stefan-Boltzmann solutions is very small (especially in most STP lab controlled environments).

If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared; e.g., most household radiators are painted white despite the fact that they have to be good thermal radiators. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature[5] (meaning the term "black body" does not always correspond to the visually perceived color of an object). These materials that do not follow the "black color = high emissivity/absorptivity" caveat will most likely have functional spectral emissivity/absorptivity dependence.

Calculation of radiative heat transfer between groups of object, including a 'cavity' or 'surroundings' requires solution of a set of simultaneous equations using the Radiosity method. In these calculations, the geometrical configuration of the problem is distilled to a set of numbers called view factors, which give the proportion of radiation leaving any given surface that hits another specific surface. These calculations are important in the fields of solar thermal energy, boiler and furnace design and raytraced computer graphics.

A selective surface can be used when energy is being extracted from the sun. For instance, when a green house is made, most of the roof and walls are made out of glass. Glass is transparent in the visible (approximately 0.4µm<λ<0.8µm) and near-infrared wavelengths, but opaque to mid- to far-wavelength infrared (approximately λ>3 µm)[6][7]. Therefore glass lets in radiation in the visible range, allowing us to be able to see through it, but doesn’t let out radiation that is emitted from objects at or close to room temperature. This traps what we feel as heat. This is known as the greenhouse effect and can be observed by getting into a car that has been sitting in the sun. Selective surfaces can also be used on solar collectors. We can find out how much help a selective surface coating is by looking at the equilibrium temperature of a plate that is being heated through solar radiation. If the plate is receiving a solar irradiation of 1350 W/m2 (minimum is 1325 W/m2on July 4th and maximum is 1418 W/m2on January 3rd) from the sun the temperature of the plate where the radiation leaving is equal to the radiation being received by the plate is 393 K (248 °F). If the plate has a selective surface with an emissivity of 0.9 and a cut off wavelength of 2.0 µm, the equilibrium temperature is approximately 1250 K (1790 °F). Note that the calculations were made neglecting convective heat transfer and neglecting the solar irradiation absorbed in the clouds/atmosphere for simplicity, however, the theory is still the same for an actual problem. If we have a surface, such as a glass window, with which we would like to reduce the heat transfer from, a clear reflective film with a low emissivity coating can be placed on the interior of the wall. “Low-emittance (low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow”[8]. By adding this coating we are limiting the amount of radiation that leaves the window thus increasing the amount of heat that is retained inside the window.

[edit] Radiation Heat Transfer

The radiation heat transfer from one surface to another is simply equal to the radiation entering the first surface from the other, minus the radiation leaving the first surface.

• For a Black Body

[9]

Using the reciprocity rule, , this simplifies to:

[10]

Where σ is the Stefan–Boltzmann constant.

• For a Grey Body with only two surfaces the heat transfer is equal to:

However, this value can easily change for different circumstances and different equations should be used on a case per case basis.

[edit] Radiative power

Thermal radiation power of a black body per unit of solid angle and per unit frequency ν is given by Planck's law as:

or

where β is a constant.

This formula mathematically follows from calculation of spectral distribution of energy in quantized electromagnetic field which is in complete thermal equilibrium with the radiating object. The equation is derived as an infinite sum over all possible frequencies. The energy, E=h \nu, of each photon is multiplied by the number of states available at that frequency, and the probability that each of those states will be occupied. More at Planck's Law

Integrating the above equation over ν the power output given by the Stefan–Boltzmann law is obtained, as:

where the constant of proportionality σ is the Stefan–

Boltzmann constant and A is the radiating surface area.

Further, the wavelength , for which the emission intensity is highest, is given by Wien's Law as:

For surfaces which are not black bodies, one has to consider the (generally frequency dependent) emissivity factor ε(υ). This factor has to be multiplied with the radiation spectrum formula before integration. If it is taken as a constant, the resulting formula for the power output can be written in a way that contains ε as a factor:

This type of theoretical model, with frequency-independent emissivity lower than that of a perfect black body, is often known as a gray body. For frequency-dependent emissivity, the solution for the integrated power depends on the functional form of the dependence, though in general there is no simple expression for it. Practically speaking, if the emissivity of the body is roughly constant around the peak emission wavelength, the gray body model tends to work fairly well since the weight of the curve around the peak emission tends to dominate the integral.

[edit] Constants

Definitions of constants used in the above equations:

Planck's constant6.626 0693(11)×10−34 J·s = 4.135 667 43(35)×10−15 eV·s

Wien's displacement constant

2.897 7685(51)×10−3 m·K

Boltzmann constant

1.380 6505(24)×10−23 J·K−1 = 8.617 343(15)×10−5 eV·K−1

Stefan–Boltzmann constant

5.670 400(40)×10−8 W·m−2·K−4

Speed of light 299,792,458 m·s−1

[edit] Variables

Definitions of variables, with example values:

Absolute temperature

For units used above, must be in kelvin (e.g. Average surface temperature on Earth = 288 K)

Surface areaAcuboid = 2ab + 2bc + 2ac;Acylinder = 2π·r(h + r);Asphere = 4π·r2

Atmospheric Radiation

Contentsi. Introduction

ii. Thermal Radiation iii. The Greenhouse Effect iv. The Earth's Energy Budget v. The Radiative Environment

vi. Spectra of the Greenhouse Gases vii. The Global Warming Debate

viii. References

Introduction

Thermal Radiation

Thermal motion of the charged particles in matter causes electromagnetic radiation, called thermal radiation from its cause, though it is no different from other electromagnetic radiation. This implies that matter will also absorb electromagnetic radiation. In a closed system, emission and radiation will equilibrate at some temperature T. Since the emission and absorption properties do not depend on the arrangments, these properties will hold also when there is no equilibrium. A sample of matter at temperature T in space will cool gradually to 0K if there is no radiation present to absorb.

The emissivity e of a surface is the energy radiated per unit area per unit time, with units W/m2. It is usually assumed that the radiation is Lambertian, or diffuse. The absorptivity a of a surface is the fraction of incident radiation absorbed by the surface, also assumed to be Lambertian. The assumption of diffuse emission and absorption is not fundamental; it just makes the discussion simpler. Both e and a may be functions of the frequency or wavelength of the radiation. By considering radiation exchange in thermal equilibrium, Kirchhoff demonstrated that e/a was a universal constant at any frequency and temperature equal to the emissivity of a perfect absorber, or black body, with a = 1. That is, a good absorber is also a good emitter.

The spectrum of thermal radiation is illustrated at the right. The abscissa is the ratio x = hf/kT = hc/λkT, proportional to the frequency. hf is the quantum energy of radiation of frequency f, or wavelength λ = c/f, and kT is the average thermal energy per equivalent harmonic oscillator (two "degrees of freedom," one for kinetic energy and one for potential energy). k is Boltzmann's constant, the gas constant per molecule.

The amount of radiation decreases rapidly at both small and large frequencies with a maximum at x = 2.82. This remarkable and useful formula was discovered by Max Planck, and was the beginning of quantum theory.

The maximum of the curve occurs at x = 2.82, or hf = 2.82kT, from which we can find that λT = 5.102 x 106 nm-K. This is the peak of the emission per unit frequency interval. Since frequency and wavelength intervals are related by df = - dλ/λ2, the energy density per unit wavelength interval is proportional to x5/(ex - 1), where x = hc/λkT. The maximum of this curve is at x = 4.96, which gives λmaxT = 2.901 x 106 nm-K. The rule that λmaxT = constant is called Wien's Law, in either case. The maximum depends on which expression for the spectrum that you are using, and really has no absolute significance by itself, but is only useful for comparison. Although the wavelength interval result is more often quoted, the frequency interval result may be more meaningful. For solar radiation at 5750 K, the maxima are at 505 nm and 887 nm, respectively.

If a is independent of frequency, then the total energy radiated per unit area per unit time is W = σT4, which is Stefan's Law. This relation was known long before Planck's formula, and is a consequence of classical thermodynamics. The value of σ was determined in terms of universal constants by Boltzmann. In calorie units, it is 8.14 x 10-11 cal/cm2-min-K4. In SI, it is 5.6705 x 10-8 W/m2-K4.

The Greenhouse Effect

The greenhouse effect is illustrated at the right with a diagram of a greenhouse, a building with a glass roof. Sunlight, 6000K short-wave radiation, mostly passes through the glass bringing an energy flux of W watt. The interior of the greenhouse emits radiation at a much lower temperature, say 300K, which is long-wave radiation. Assume the glass absorbs long-wave radiation completely (which is nearly a fact). When a steady state is reached, the glass is brought to a temperature T' at which it radiates an energy flux of W watt to the exterior. The same energy flux is re-radiated to the interior of the greenhouse, so the interior of the greenhouse must radiate an energy flux of 2W to the glass. This increased energy flux means that the interior must reach a temperature T > T' sufficient for this. In this way, the interior of the greenhouse assumes a higher temperature than it otherwise would if it absorbed W watt of short-wave radiation and re-emitted W watt of long-wave radiation.

Many simplifying assumptions have been made here, such as neglecting radiative transfer with the environment at some lower temperature T" <' T', but the general mechanism is

clear. Per square metre, W = σT4, if we assume the emissivity is 1. Actually, it is sufficient to assume that the emissivity is the same for all surfaces involved, not necessarily 1. Then, W = σT'4, and 2W = σT4. When the energy leaving the glass equals that absorbed, σ(T4 - σT'4) = σT'4 = W, or T4 = 2T'4, which means that T = 21/4T' = 1.189 T'. Since the glass must be hotter than its surroundings, the interior of the greenhouse is still hotter. If we know W, then we can find both T' and T.

A qualitative explanation of the terrestrial greenhouse effect may be useful here. The surface of the Earth is heated by short-wavelength solar radiation during the day, while the earth radiates continuously at long wavelengths. The equilibrium temperature of the Earth is determined by the equality of these energy fluxes, with the input constant and the output determined by the temperature of the Earth. The average surface temperature of the Earth is about 288K. If the atmosphere were transparent to infrared radiation, the radiation to space would exceed the solar input, and the Earth would cool. Equilibrium would be reached at a surface temperature of about 217K, or about -70°F. There would then be no liquid water, nor life, on the Earth.

However, convective processes establish a nearly linear decrease of atmospheric temperature from 288K at the surface to 217K at around 12 km altitude. The boundary temperatures are determined by radiation, but the lapse rate is not. This includes the densest part of the atmosphere, which will be the only part giving a significant contribution to radiative transfer, and in which water vapor is present in significant concentration. The water vapor absorbs long-wavelength radiation very strongly, and will be nearly opaque to it, so that it is emitted and absorbed continuously. The radiation will be emitted at the surface at a temperature of 288K, but will be emitted into space at 217K, as required for equilibrium. Water vapor is by far the most important contributor to this process, by means of its large concentration and strong rotational spectrum.

Carbon dioxide has a strong band near 15μ which is very well placed for greenhouse activity, another band at a shorter wavelength that is less important, and no rotational spectrum at longer wavelengths, since it does not have a permanent dipole moment. However, the 15μ band covers only a very limited part of the spectrum, and carbon dioxide is not present in large concentration (less than 400 ppm), so carbon dioxide will have a much smaller effect than water vapor. It should be realized that both the greenhouse effect and carbon dioxide are essential to life on Earth.

We can apply this to the earth as follows. The earth absorbs the short-wave sunlight falling on its projected disc of area πR2, and radiates long-wave radiation from its surface area 4πR2. The long-wave radiation per unit area must then be 1/4 the short-wave radiation incident per unit area. The incident short-wave radiation is about 1.94 cal/cm2-min, which works out to 1353 W/m2. Let's assume that half of this is absorbed by the earth, or 676 W/m2. The flux that must be radiated by every square metre is a quarter of this, or 169 W/m2, presuming that the earth reaches a uniform temperature. This is not nearly as bad an assumption as would be imagined, since the earth is actually at about the same temperature all over, thanks to the oceans and atmosphere. If the long-wave emissivity is assumed to be unity, also not a bad assumption, then 169 = σT4, or T =

235K or -38°C, if the atmosphere were transparent to long-wave radiation. This is not far from the truth.

The atmosphere is nearly transparent to short-wave radiation, but is relatively opaque to long-wave radiation, except in a few "windows" of transmission. Therefore, it acts like the glass of a greenhouse, absorbing and emitting long-wave radiation according to its temperature. Since it must emit the same amount of radiation to space that we assumed in the preceding paragraph, its effective temperature must be T' = -38°C. We now use our greenhouse theory to estimate the ground temperature at 1.189T' = 279K or +6°C. This is sufficient to melt the water and allow life to exist, as it does.

It is remarkable that our approximate estimate is close to actuality. The accurate calculation of the radiation balance of the atmosphere is extremely difficult because of the variability of the earth's surface, the differences in cloud cover, and other factors. In fact, in spite of computers, only simplified models are practical, and there is really not a good handle on this important factor, just enthusiastic scientists waving papers in the air and shouting at each other. The general effect has been well-known for a considerable time. Fortunately for us, it does not depend too greatly on the concentration of the trace gases responsible for the long-wave absorption, of which water vapor is the most important, and also quite variable.

If we think of greenhouses with multiple glass roofs, then each roof will contribute a factor 1.189, and pretty soon you are talking real insulation. Another glass roof to the earth would give a surface temperature of 332K or 59°C, which is downright uncomfortable. Three would give 122°C, and water would boil. It is not clear to me that carbon dioxide, with its absorption in closely limited bands, could ever be responsible for such extreme conditions.

The Earth's Energy Budget

The earth generates heat internally, and this internal source drives plate tectonics, the magnetic field, volcanic eruptions and earthquakes. Some energy also comes in the fast protons and electrons of the solar wind, which cause interesting ionospheric and auroral effects, and there is even a little from cosmic rays, which are mainly very energetic protons and photons. All of these sources of energy are inconsequential compared to the copious bath of radiation in sunlight, which extends from 0.15 μm in the far ultraviolet to 4 μm in the infrared. We can, therefore, safely neglect these inputs and concentrate on the solar input. The total power received on a surface normal to the direction of the sun outside the atmosphere at the earth's distance is 1.94 cal/cm2-min or 1353 W/m2. The spectrum is close to a black-body spectrum for 6000K, crossed by narrow absorption lines due to absorption in the chromosphere of the sun, the Fraunhofer lines.

As observed at the surface, direct sunlight, which is about 27% of that incident, has been modified by atmospheric absorption and scattering. The energy at wavelengths shorter than 0.29 μm has been cut off due to the creation of ozone and its strong absorption. However, the atmosphere is remarkably transparent to the remainder of the spectrum,

only a few weak lines due to oxygen (the diatomic molecule) and water vapor being evident. Scattering is stronger at shorter wavelengths, so the scattered radiation is noticeably blue, and makes the blue of the sky, while the longer wavelengths remain in the direct beam. This scattering is from density fluctuations in the upper atmosphere, not from individual

molecules as is sometimes asserted.

An estimate of the steady-state energy budget of the earth is shown at the left. The numbers represent energy in units of 1022 cal/year. The figures are rough estimates, but illustrate the relative magnitudes of the contributions. The sun provides 130 in the form of short-wave radiation (0.15 to 4.0μm) at the top of the atmosphere. Of this, 76 is scattered or reflected, and 19 is absorbed, leaving 35 to reach the surface as direct radiation. The absorbed portion includes interaction with ozone, and this energy heats the upper atmosphere. Of the scattered and reflected light, 25 also reaches the surface, while 51 is radiated into space as short-wave radiation. This makes the albedo of the earth to be about 0.39.

The atmosphere absorbs 141 from the surface, and re-emits 125 to the surface as long-wave (4-120μm) radiation. The net amount, 65, is radiated to space. The atmosphere receives 30 by condensation of water, and gives up 30 by evaporation, most of which is part of the hydrologic cycle. The surface receives 5 by turbulence, and releases the same amount of energy on the average by the same means. Finally, 14 is directly radiated from the earth to space through the "window" in the infrared spectrum from 8 - 12μm, and in the near infrared below 4 μm.

This diagram suggests the complexity of making an energy budget for the earth, because of the many things that must be taken into consideration. Dynamic changes are even more difficult to predict, because of the interrelation between factors. It shows the very important role of radiation in the energy budget, and the different parts played by short-wave and long-wave radiation.

The Radiation Environment

The solid and liquid surfaces of the earth are usually good absorbers and good radiators. An exception are cloud and fresh snow surfaces, where light is repeatedly reflected without absorption, and returned little reduced in intensity. The reflection may be 70-80%, and these surfaces appear white to the eye. Rock and vegetation reflect only 10%-30%. Short-wave radiation largely penetrates water, where it is absorbed almost totally. Only the part that is reflected at the upper surface, which depends on the angle of incidence, is not absorbed. The earth and the oceans appear dark from space, with a blue haze from scattering and white clouds making a definite contrast. The average cloud cover is about 52%, it is said.

Things are quite different in the long-wave spectrum. Here, water has strong absorption from 4 - 8 μm, and beyond 25 μm, so that now clouds and snow are black, not white. Snow and thick clouds radiate like black bodies at long wavelengths, so exchange of energy by radiation is very important. Clouds may be warmed by radiation from below, while they are cooled at their tops by radiation into space. This creates instability (a large lapse rate) so thunderstorms can continue to boil during the night. Liquid water, interestingly, strongly absorbs everything except the visible. It is even blacker with long waves, but the energy does not penetrate as far as short waves will, so absorption and radiation of infrared takes place only in superficial layers. Cooling is rapid at the surface, which may produce the convection that keeps the surface waters well mixed.

Spectra of the Greenhouse Gases

The discussion of the greenhouse effect showed that the absorption of long-wave radiation in the atmosphere was important to its explanation. The major constituents of the atmosphere, the diatomic gases nitrogen and oxygen, and the 1% of argon, do not interact with electromagnetic radiation in the long-wave spectrum at all. The only trace gases present in significant amounts are water vapor and carbon dioxide. Water vapor has a permanent dipole moment, and so a strong pure rotation spectrum beginning at about 25 μm and extending with greater and greater absorption to longer wavelengths. It also has a vibration-rotation band for the bending mode at around 6.3 μm, and for an asymmetric stretching mode at 2.66 μm.

The pure rotation spectrum of carbon dioxide is at much too long wavelengths to play a role, but there is a strong bending mode band at 14.7 μm, as well as an asymmetric stretching vibration at 4.26 μm. The 14.7 μm band is at a critical wavelength, and has a considerable effect in spite of the low concentration of carbon dioxide. Carbon dioxide must be considered in atmospheric radiative transfer, but it is much less important than water.

It is usual to express absorption in terms of Beer's Law, which is the integrated form of dI/dx = -Iadx, I = Ioe-ax, where a is the linear absorption coefficient. Sometimes it is convenient to use the optical density, which is the product of the density and the distance, d = ρx, expressed in g/cm2. Then, if a' = a/ρ, I = Ioe-a'd. The advantage is that a' is not a function of the density and distribution of the absorber, only on the number of absorber molecules present.

Beer's Law is almost useless for expressing the absorption of radiation in the atmosphere. The spectra consist of sharp, separated absorption lines. If you start with a uniform radiation spectrum, first the centres of the lines are absorbed and removed from the beam, then the wings of the lines, and in a complex way that depends on the details of the spectrum (which for a long time were not accurately known). Attempts to use an average absorption, as if the atmosphere were a "grey" body failed utterly. This is a place where computers can help greatly when a detailed absorption curve is available, and many good calculations have been carried out. We won't go into this in detail here, only point it out lest it be thought that things were straightforward and easy.

The Global Warming Debate

There is evidence that the average temperature of the Earth is increasing. The rate of increase is slow, and is certainly not unreasonable. The average temperature has fluctuated rather widely in recent geological history. In fact, it is generally assumed that we are in an interglacial era, and that the temperature is changing is less remarkable than if it remained unchanged. The reasons for continental glaciation are still quite unknown, and prediction is not possible. It is somewhat remarkable that permanent ice still persists at polar latitudes and high altitudes, since this does not appear to be typical in geologic history. At present, then, it would be reasonable for the Earth's temperature either to decrease or to increase, since it is at a rather intermediate level, perhaps cooler than normal, so an increase would not be surprising.

The argument current among some scientists, politicians and the general public (not remarkable for geologic knowledge) is that the increase in temperature is caused by carbon dioxide emitted into the atmosphere by human activity, and that restriction of coal burning by electrical utilities, together with some less effective measures, will reduce the carbon dioxide concentration and solve the problem. It is indeed an inconvenient truth that this simple argument is rubbish.

We have noted above that by far the most effective greenhouse gas is water vapor. Some very small increase in its atmospheric concentration, perhaps caused by human activity, would also cause an increased greenhouse effect, and an increase in the Earth's average temperature if the greenhouse effect is indeed responsible for climate. Exactly the same argument can be made for water vapor as for carbon dioxide. For example, burning natural gas produces large quantities of the principal greenhouse gas, while if coal is burned to produce the same amount of heat, only the much less effective carbon dioxide is emitted, turning the usual argument on its head. The atmosphere is no more a closed system for water vapor than it is for carbon dioxide, and what is added may not end up in the atmosphere after all. In fact, agriculture could be responsible for much water vapor, and since agriculture increases at the same rate as population, this would provide an anthropogenic source as well.

Not only is water vapor not mentioned in connection with global warming, neither is the effect of population, except peripherally. If global warming is anthropogenic, then the only means of preventing it would be a significant reduction in human numbers, which seems politically impossible. It is another inconvenient truth that there appears to be no way for human population to be self-limiting until resources are exhausted and starvation does the job. Russia seems to be the only major country expecting a decrease in population (which they are doing all possible to avoid). This is valid even in the carbon dioxide picture. Predictions are now being made for times when the population will certainly exceed the resources, as soon as 2050, when the population will (hypothetically) have doubled. How much limitation of carbon dioxide can be realized in this case?

More carbon dioxide and warmer weather are good news for plants (they survive and give us food even with the small amount of carbon dioxide available in the atmosphere).

Such conditions are maintained in some actual greenhouses to increase crop yield, but any positive consequences of global warming or increased carbon dioxide are extremely unpopular with the enthusiasts.

None of the proposals for controlling climate can be expected to have any measurable effects whatever, as good as they may be for conservation and efficiency.

References

E. W. Hewson and R. W. Longley, Meteorology Theoretical and Applied (New York: John Wiley & Sons, 1944).

F. A. Berry, Jr., E. Bollay and N. R. Beers, eds., Handbook of Meteorology (New York: McGraw-Hill, 1945).

Return to Weather Index

ID="Postscript">Composed by J. B. CalvertCreated 29 May 2007Last revised 21 July 2009

Primary Metals

Table of Contents  Industry Overview  Steel Making Industry   Ferrous & Non-Ferrous Foundries  Aluminum Smelting & Refining 

Copper Processing  Lead Processing  Zinc Processing  Glossary

2 The Steel Making Industry

Background

Steel is an alloy of iron usually containing less than 1% carbon. It is used most frequently in the automotive and construction industries. Steel can be cast into bars, strips, sheets, nails, spikes, wire, rods or pipes as needed by the intended user. Based on statistics from The 1992 Census of Manufacturing, 1,118 steel manufacturing facilities currently exist in the United States. Steel production is a $9.3 billion dollar industry and employs 241,000 people.

The process of steelmaking has undergone many changes in the 20th century based on the political, social and technological atmosphere. In the 1950s and 1960s, demand for

high quality steel encouraged the steelmaking industry to produce large quantities. Large, integrated steel mills with high capital costs and limited flexibility were built in the U.S. (Chatterjee, 1995). Integrated steel plants produce steel by refining iron ore in several steps and produce very high quality steel with well controlled chemical compositions to meet all product quality requirements.

The energy crisis of the 1970s made thermal efficiency in steel mills a priority. The furnaces used in integrated plants were very efficient; however, the common production practices needed to be improved. The large integrated plants of the 1950s and 1960s tended to produce steel in batches where iron ore was taken from start to finish. This causes some equipment to be idle while other equipment was in use. To help reduce energy use, continuous casting methods were developed. By keeping blast funraces continually feed with iron ore, heat is used more efficiently.

As environmental concerns have gained importance in the 1980s and 1990s, regulations have become more stringent, again changing the steelmaking industry. In 1995, compliance with environmental requirements was estimated to make up 20-30% of the capital costs in new steel plants (Chatterjee, 1995). Competition has also increased during the period do to decreasing markets and increasing foreign steel production plants. The competition has forced steelmaking facilities to reduce expenses and increase quality.

To meet these changing needs, just-in-time technology has become more prominent and integrated steel plants are being replaced with smaller plants, called mini-mills, that rely on steel scrap as a base material rather than ore. Mini-mills will never completely replace integrated steel plants because they cannot maintain the tight control over chemical composition, and thus cannot consistenly produce high quality steel. Mini-mills have a narrower production line and cannot produce the specialty products produced by integrated plants. Although technology continues to improve, in the mid 1990s, mini-mills captured less than half of the quality steel market.

Steel Production from Iron Ore

Steel production at an integrated steel plant involves three basic steps. First, the heat source used to melt iron ore is produced. Next the iron ore is melted in a furnace. Finally, the molten iron is processed to produce steel. These three steps can be done at one facility; however, the fuel source is often purchased from off-site producers.

Cokemaking

Coke is a solid carbon fuel and carbon source used to melt and reduce iron ore. Coke production begins with pulverized, bituminous coal. The coal is fed into a coke oven which is sealed and heated to very high temperatures for 14 to 36 hours. Coke is

produced in batch processes, with multiple coke ovens operating simultaneously.

Heat is frequently transfered from one oven to another to reduce energy requirements. After the coke is finished, it is moved to a quenching tower where it is cooled with water spray. Once cooled, the coke is moved directly to an iron melting furnace or into storage for future use.

Ironmaking

During ironmaking, iron ore, coke, heated air and limestone or other fluxes are fed into a blast furnace. The heated air causes the coke combustion, which provides the heat and carbon sources for iron production. Limestone or other fluxes may be added to react with and remove the acidic impurities, called slag, from the molten iron. The limestone-impurities mixtures float to the top of the molten iron and are skimmed off, see Figure 1, after melting is complete.

Sintering products may also be added to the furnace. Sintering is a process in which solid wastes are combined into a porous mass that can then be added to the blast furnace. These wastes include iron ore fines, pollution control dust, coke breeze, water treatment plant sludge, and flux. Sintering plants help reduce solid waste by combusting waste products and capturing trace iron present in the mixture. Sintering plants are not used at all steel production facilities.

Steelmaking with the Basic Oxide Furnace (BOF)

Molten iron from the blast furnace is sent to a basic oxide furnace, which is used for the final refinement of the iron into steel (Figure 1). High purity oxygen is blown into the furnace and combusts carbon and silicon in the molten iron. The basic oxide furnace is fed with fluxes to remove any final impurities. Alloy materials may be added to enhance the characteristics of the steel.

The resulting steel is most often cast into slabs, beams or billets (USEPA, 1995). Further shaping of the metal may be done at steel foundries, which remelt the steel and pour it into molds, or at rolling facilities, depending on the desired final shape.

BOF Pollution Sources and Prevention Opportunities

Different types of pollution result from the different steps in steel production. Below, the pollution sources and the possible pollution prevention opportunities are discussed for each process.

Pollution Sources and Prevention for Coke making

Coke production is one of the major pollution sources from steel production. Air

emissions such as coke oven gas, naphthalene, ammonium compounds, crude light oil, sulfur and coke dust are released from coke ovens. Emissions control equipment can be used to capture some of the gases. Some of the heat can be captured for reuse in other heating processes. Other gases may escape into the atmosphere.

Figure 1: The Steel Making Process (EPA, 1995)

Water pollution comes from the water used to cool coke after it has finished baking. Quenching water becomes contaminated with coke breezes and other compounds. While the volume of contaminated water can be great, quenching water is fairly easy to reuse. Coke breezes and other solids can usually be removed by filtration. The resulting water can be reused in other manufacturing processes or released.

Reducing Coke Oven Emissions

Pollution associated with coke production is best reduced by decreasing the amount of coke used in the iron melting process. The smaller the volume of coke produced, the smaller the volume of air and water emissions. However, process modifications in actual coke production are not widely available and are very expensive.

One fairly economical method of reducing coke oven pollution is to reduce the levels of coke used in blast furnaces. A portion of the coke can be replaced with other fossil fuels without retrofitting the furnace. Pulverized coal can be substituted for coke at nearly a 1:1 and can replace 25 - 40% of coke traditionally used in furnaces (USEPA, 1995). Pulverized coal injection is used worldwide to reduce coke use and, thus, coke emissions (Chatterjee, 1995).

Pulverized coal injection may affect the final steel products. Pulverized coal may reduce gas permeability of the metal and unburnt coal particles may accumulate in the furnace, decreasing efficiency. Thus, it may not be possible to substitute pulverized coal for coke in the production of high quality steel.

Other alternative fuels such as natural gas, oil or tar/pitch can be used to replace coke using similar process modifications. The reduction in emissions is proportional to the reduction in coke use.

Air and water emissions may also be reduced by using a non-recovery coke battery. In traditional plants, by-products are recovered from the blast furnace. In non-recovery batteries, coke oven slag and other by-products are sent to the battery where they are combusted. This technique consumes the by-products, eliminating much of the air and water pollution. Non-recovery coke batteries do require replacement or retrofitting of traditional coke ovens. This process modification does reduce pollution, but can be expensive.

A third method for reducing coke oven emissions is the Davy Still Autoprocess. The

process uses water to remove ammonia and hydrogen sulfide from coke oven emissions prior to cleaning of the oven.

Cokeless Ironmaking

Cokeless ironmaking procedures are currently being studied and, in some places, implemented. One such procedure is the Japanese Direct Iron Ore Smelting (DIOS) process (Figure 2). The DIOS process produces molten iron from coal and previously melted ores. In this process, coal and other ores can produce enough heat to melt ore, replacing coke completely (USEPA, 1995).

In addition to reducing coke use, the DIOS process could cut the costs of molten iron production by about 10%, reduce emissions of carbon dioxide by 5 - 10% and increase flexibility by improving the starting and stopping capabilities of the steel mill (Furukawa, 1994). However, the DIOS process remain very expensive and requires extensive process modification. In 1995, this method was still being tested and economic feasibility will be determined from those tests.

The HISmelt process, named after the HISmelt Corporation of Australia, is another cokeless iron melting process being tested. In this process, ore fines and coal are manipulated to melt iron ore. In 1993, the process could produce eight tons of iron ore per hour using ore directly in the smelter. Process modifications are expected to increase the efficiency to 14 tons per hour. Commercial feasibility studies were performed in 1995. Midrex is expected to be the U.S. distributor of the process.

A final cokeless iron melting process is the Corex or Cipcor Process. This process also manipulates coke to produce the heat required to melt iron. A Corex plant is operational in South Africa. Posco of Korea has a Corex plant operating at 70% capacity in 1996 and is expected to continue progress (Ritt, 1996). India is also expected to build a plant in 1997. The process integrates coal desulfurization, has flexible coal-type requirements, and generates excess electricity that can be sold to power grids (USEPA, 1995). Further testing is being conducted to determine actual commercial feasibility in the U.S.

Figure 2. The DIOS Process (Furukawa, 1994)

Iron Carbide Steel Production Plants

Iron carbide production plants can be an alternative to the Basic Oxide Furnace. These plants use iron carbide, an iron ore that contains 6% carbon rather than 1.5-1.8% of regular iron ore . The additional carbon ignites in the presence of oxygen and contributes heat to the iron melting process, reducing energy requirements (Ritt, 1996). While these types of plants do not reduce pollution on site, they do reduce the electricity requirement for steel production, reducing polution from the power plant.

Pollution Sources and Prevention in Ironmaking

Slag, the limestone and iron ore impurities collected at the top of the molten iron, make up the largest portion of ironmaking by-products. Sulfur dioxide and hydrogen sulfide are volatized and captured in air emissions control equipment and the residual slag is sold to the construction industry. While this is not a pollution prevention technique, the solid waste does not reach landfills.

Blast furnace flue gas is also generated during ironmaking. This gas is cleaned to remove particulates and other compounds, allowing it to be reused as heat for coke furnaces or other processes. Cleaning gas for reuse can produce some air pollution control dust and water treatment plant sludge, depending on the method used. The dust can be reused in sintering processes or landfilled.

Pollution Sources and Prevention in the BOF

Slag is a major component of the waste produced in BOFs. Because of its composition, this slag, unlike that from the blast furnace, is best used as an additive in the sintering process. As its metallic content is lower, it does not make a good raw material for the construction industry.

Hot gases are also produced by the BOF. Furnaces are equipped with air pollution control equipment that contains and cools the gas. The gas is quenched and cooled using water and cleaned of suspended solids and metals. This process produces air pollution control dust and water treatment plant sludge.

Steel Production from Scrap Metals

Steelmaking from scrap metals involves melting scrap metal, removing impurities and casting it into the desired shapes. Electric arc furnaces (EAF) are often used (Figure 1). The EAFs melt scrap metal in the presence of electric energy and oxygen. The process does not require the three step refinement as needed to produce steel from ore. Production of steel from scrap can also be economical on a much smaller scale. Frequently mills producing steel with EAF technology are called mini-mills.

Pollution Sources and Material Recovery

Gaseous emissions and metal dust are the most prominent sources of waste from electric arc furnaces. Gaseous emissions are collected and cleaned, producing EAF dust or sludge. The remaining gas contains small quantities of nitrogen oxide and ozone and is usually released. The EAF dust or sludge composition varies depending on the type of steel being manufactured. Common components include iron and iron oxides, flux, zinc, chromium, nickel oxide and other metals used for alloys. If the dust or sludge contains lead or cadmium, it is listed as a hazardous waste (RCRA K061) (USEPA, 1995).

In 1996, 500kg of EAF dust were produced for each tonn (UK weight measurements) of crude steel production. In the United Kingdom, 70% of that dust is sold to other companies, 20% is recycled in-plant and 10% is landfilled. Although it is a relatively small proportion of the total volume of waste, the landfilled EAF dust amounts to 50 kg per tonne of crude steel produced (Strohmeier, 1996).

Recycling and recovery of EAF dust can be difficult because of the alkalinity and heavy metal (zinc and lead) content. The dust can be landfilled, but, because of the fine nature, it may leach into ground water. Several processes have been developed to recover the zinc, lead and other heavy metals from EAF dust. Although not pollution prevention, metal recovery is almost always profitable if the zinc content of the dust is 15 - 20% of the total volume. It can be marginally profitable with lower levels of zinc. Other metals such as chromium and nickel can also be reclaimed and sold.

After the heavy metals have been removed, the dust is composed primarily of iron and iron oxides and may be remelted. If the metal content is sufficient, the dust can be reused in the blast furnaces. If it is not sufficient, the dust can be sold to other industries for use as raw materials in bricks, cement, sandblasting or fertilizers.

Energy Optimizing Furnaces (EOF)

EOF was developed to replace the electric arc and other steelmaking furnaces. The EOF is an oxygen steelmaking process. Carbon and oxygen react to preheat scrap metal, hot metal and/or pig iron. These furnaces reduce capital and conversion costs, energy consumption and environmental pollution, while increasing input flexibility (Chattergee, 1995).

Steel Forming and Finishing

After the molten metal is released from either the BOF, EAF or EOV, it must be formed into its final shape and finished to prevent corrosion. Traditionally, steel was poured into convenient shapes called ingots and stored until further shaping was needed. Current practices favor continuous casting methods, where the steel is poured directly into semi-finished shapes. Continuous casting saves time by reducing the steps required to produce the desired shape.

After the steel has cooled in its mold, as further detailed in Chapter 3, continued shaping is done with hot or cold forming. Hot forming is used to make slabs, strips, bars or plates from the steel. Heated steel is passed between two rollers until it reaches the desired thickness.

Cold forming is used to produce wires, tubes, sheets and strips. In this process the steel is passed between two rollers, without being heated, to reduce the thickness. The steel is then heated in an annealing furnace to improve the ductile properties. Cold rolling is more time consuming, but is used because the products have better mechanical

properties, better machinability, and can more easily be manipulated into special sizes and thinner gauges.

After rolling is completed, the steel pieces are finished to prevent corrosion and improve properties of the metal. The finishing process is detailed in the Pollution Prevention and the Metal Finishing Industry manual.

Pollution Sources and Prevention from Steel Forming

The primary wastes produced in the metal forming process include contact water, oil, grease, and mill scale. All are collected in holding tanks. The scale settles out and is removed. It can be reused in sintering plants or, if the metal content is sufficient, may be sold as a raw material elsewhere. The remaining liquid leaves the process as waste treatment plant sludge. As the waste results in a small portion of pollution produced by steelmaking, pollution prevention and process modification opportunities are not a priority.

References

Chatterjee, Amit. "Recent Developments in Ironmaking and Steelmaking." Iron and Steelmaking. 22:2 (1995), pp. 100-104.

Frukawa, Tsukasa. "5000 Daily Tons of Direct Iron-Ore Smelting by 2000." New Steel. 10:11 (November, 1994), pp. 36-38.

McManus, George, ed. "Replacing Coke With Pulverized Coal." New Steel. 10:6 (June, 1994), pp. 40-42.

Ritt, Adam. "DRI comes to the Gulf Coast." New Steel. January, 1996, pp. 54-58.

Strohmeier, Gerolf, and John Bonestell. "Steelworks Residues and the Waelz Kiln Treatment of Electric Arc Furnace Dust." Iron and Steel Engineering. April, 1996, pp. 87-90.

U.S. Department of Commerce. 1992 Census of Manufacturers — Blast Furnaces, Steel Works and Rolling and Finishing Mills. 1992.

USEPA. "Profile of the Iron and Steel Industry." EPA/310-R-95-010, U.S. Environmental Protection Agency. Washington, D.C., September 1995.

Annotated Bibliography

Andres, A. and J.A. Irabien. "The Influence of Binder / Waste Ratio on Leaching Characteristics of Solidified / Stabilized Steel Foundry Dust." Environmental

Technology. 15 (1994), pp. 343-351. This article discusses effective methods for steel dust stabilization.

Andres, A., et al. "Long-term Behavior of Toxic Metals in Stabilized Steel Foundry Dust." Journal of Hazardous Waste Materials. 40 (1995): pp. 31-42. This study describes the leaching properties of heavy metals in steel dust.

Berry, Brian. "Hoogovens Means Blast Furnaces — And Clean Air." New Steel. December, 1994. pp. 26-30. Pulverized coal injection, particularly in Holland, are discussed.

McManus, G.J. "The Direct Approach to Making Iron." Iron Age. July, 1993. pp. 20-23. Direct ironmaking, Corex plants and other alternatives to the BOF are discussed.

Mohla, Prem. "New Ductile Iron Process Meets the Challenge of the 1990's Head On." Foundry Management and Technology. 121:4 (April, 1993), pp. 52-56. Discusses alternative production processes to help reduce pollution.

Schriefer, John. "Hot Iron Without Coke - And Blast Furnaces." New Steel. August, 1995, pp. 50-52. Corex, direct iron-ore smelting and HISmelt processes are all alternatives to the coke oven and blast furnace. Examples are discussed in this article.   

Case Study

Metal Recovery from Electric Arc Furnace DustCase Study: CS616 North Carolina Department of Natural Resources and Community Development, July, 1989

The Florida Steel Company of Charlotte, North Carolina produces significant amounts of baghouse dust with a high zinc from their steel smelting operations. The air pollution control system on their electric arc furnaces capture the zinc-rich dust. Rather than disposal, the furnace dust is sent directly to a zinc smelter for metal recovery.

At this writing, 2700 tons per year were sent to the zinc smelter for recovery at a cost of $61 per ton. By allowing the zinc to be reused, Florida Steel saves $130,000 per year over the cost of landfilling.