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Increasing the heat transfer efficiency of a firebox by increasing wall to coil radiation is rapidly becoming accepted practice on ethylene furnaces. The development, however, is in it's relative infancy with some aspects of the technology not being fully appreciated. This report endeavours to clarify matters concerning the use of high emissivity coatings and their practical application.

1. PRIMARY BENEFITS:

2. SECONDARY BENEFITS:

3. DOWNSIDE:

4. THEORETICAL ASPECTS:

5. MATHEMATICAL MODELS:

6. PRACTICAL APPLICATION:

The benefit from an increase in wall to coil radiation depends on whether production is firebox limited or not.

1.1 Increased Production: Increased production is the main benefit when production is limited by either the combustion capacity of the firebox, the gas temperature at entry to the convection section, or some other factor such as insufficient furnace draft. In theory, the maximum attainable increase in production from refractory emissivity enhancement is about 6%. In practise, most plants will find the increase to be between 3% and 4.5%.

1.2 Reduced Fuel Input: Where production is limited by constraints upstream or downstream of the firebox then a reduction in fuel input is the main benefit. In theory, the maximum reduction in fuel fired to be achieved from a high emissivity coating alone is about 12%. In practice, most plants achieve a reduction of between 6% and 9%.

NB: The figures quoted concern brick lined furnaces. Results will generally be somewhat higher on ceramic fibre lined furnaces.

1.3 Influence of Shape Factor: The percentages quoted above cover quite a wide spread. This is because the geometry, or shape factor, of the firebox determines the maximum level of benefit far more than the absolute emissivity of the candidate coating material.

At first glance most fireboxes look about the same; simple rectangular boxes with a bank of coils in the centre. The shape factor in reality changes significantly from plant to plant. The internal height, breadth and width relationship of the firebox together with coil diameter and pitch coupled with the apportionment between wall and floor firing can determine the outcome of coating by more than a factor of two.

It is the difference in shape factor from plant to plant which explains the wide spread of results obtained by coating vendors as opposed to the merits or specific emissivity of a particular coating material.

1.4 Short Payback Periods: The reported payback, or return on investment period, from the use of a high emissivity coating is generally under six months on those plants which are limited to the fuel saving option. Payback with increased production is nearer six weeks.

There are a considerable number of secondary or spin-off benefits associated with high emissivity coatings. Having said this, a problem often arises when trying to quantify the secondary benefits in measurable or meaningful terms. This is partly because the secondary benefits tend to be given less attention in terms of monitoring and partly because by their nature it is frequently necessary to wait for a trend to develop over an extended period. However, it is possible to state the following;

2.1 Improved Temperature Uniformity: The improvement in temperature uniformity on a coated firebox can clearly be seen by eye. Quantifiable figures are scarce as the lack of temperature uniformity is rarely recorded in normal operation. However, the improvement is there to be seen and the general consensus from the process point of view is that coated fireboxes operate much closer to the original design assumption of a uniform firebox temperature.

2.2 Increased Coil Life: Coil life tends to be variable and it will likely take some years for this claim to be verified and quantified. The average coil skin metal temperature must increase after coating the refractories regardless of whether the increased wall radiation is used to increase production or save fuel. Users acknowledge this fact but say that the increase in temperature is too small to be noticed or accurately measured. Also, the "average" metal temperature is of little concern. It is the high "hot spots" which limit coil life and there is evidence that these are reduced in the after coating situation.

NB: Significantly larger benefits stand to be made from applying ceramic coatings to the coils. The technology lags a little behind the coating of refractory walls and roofs and the phenomena at work is somewhat different. However, impressive gains in the efficiency of heat transfer can be made. The subject will to be covered in a separate report (currently under construction).

2.3 Reduced Heat Loss: Here we have an apparent paradox. A body can only radiate heat it has first absorbed. High emissivity coatings increase the surface absorption of heat by refractories but heat loss through the wall is reduced. The explanation is that by increasing the overall heat transfer efficiency in the firebox then a given heat transfer task can be performed while operating at a lower average gas temperature. Casing plates run cooler after coating.

This aspect of coating is relatively easy to measure in the "before and after" situation. Reductions in the average outer casing metal temperatures of up to 18 deg C have been recorded.

2.4 Increased Refractory Life: This claim is considerably stronger based than that for increased coil life although more time is required to quantify by what amount. That a good quality coating offers hotface protection to the refractories is beyond question. However, a common cause of brick failure is distortion and collapse of the shelf plate and tie-back system. These metallic components run cooler in a coated wall situation and their service life, and that of the refractories, are indeed increased. There is ample evidence for this.

NB: The longest serving ethylene furnace in the petrochemical industry with a high emissivity coating applied has now served 5 years (as of July 1999) with zero brickwork and zero coating repair. Remedial work to the coating at the next overhaul is assessed as being approximately 15% of the total area.

2.5 Improved Oxygen Control: This is certainly true on older furnaces where the brickwork is in poor condition. Although the outer casing plates on ethylene furnaces are seam welded it is surprising how much tramp air is pulled through gaps and cracks in the refractory lining. Air ingress, particularly in the upper reaches of a firebox, gives a false reading to the oxygen monitor in the stack. Sealing the brickwork with a thick film coating significantly improves the situation and makes for a further gain in firebox efficiency.

2.6 Increased Yield: A typical reduction in coil inlet temperature after coating is 30 deg C. This delays the onset of cracking leading to a shorter and sharper crack which in theory should increase the yield. Again, this has proved difficult to quantify in the field but can be considered a move in the right direction.

2.7 Reduced Carryover of Refractory Dust: Surveys have shown that a high proportion of the debris found on the convection bank tubes comprises refractory dust. The problem is more apparent with brick linings as ceramic fibre dust tends to find it's way to atmosphere via the stack. Sealing the hotface surface of bricks does help prevent the deterioration of convection chamber efficiency. Sealing the hotface surface of ceramic fibre does help prevent potentially hazardous dust being emitted to the environment.

2.8 Reduced Emissions: A reduction in stack emissions is obvious as a 9% reduction in fuel fired equates closely to a 9% reduction in emissions. When going for a production increase, emissions are reduced in the sense that a net increase in production is obtained with no increase in the amount of fuel fired.

3.1 Steam Production: The only reported downside from the use of high emissivity coatings is a fall-off in steam production. There is also a fall-off in preheat of the incoming feedstock but this is quickly made up in the radiant chamber.

Any increase in the heat transfer efficiency of a firebox will result in a reduced mass flow of cooler gas at entry to the convection section. The situation being more pronounced on those plants which are limited to the fuel saving option. This could be an embarrassment on plants which run lean on steam and needs to be looked at closely. Having said this, the overall benefit from coating is such that it can often justify the make-up of steam loss elsewhere.

4.1 Total Hemispherical Emissivity: The values most commonly used, or assumed, in text books on furnace design relate to radiation at all wavelengths and in all directions; otherwise known as total hemispherical emissivity. Practical site measurements are usually made normal to the surface to give normal total emissivity. Both measurements can be considered, in layman's terms, as "average" emissivity with the typical values used in ethylene furnace design being hot gas 0.22, refractories 0.6 and coils 0.9. This has advantage in that it keeps calculations simple but it does make a rather large assumption that the bodies referred to are "grey" in behaviour. That being that emissivity and absorptivity are a fixed fraction of an ideal black body (at all wavelengths) and that the magnitude of radiation is solely dependant on temperature.

The total hemispherical (or average) emissivity of hot gas, refractories, and the coils are shown on Fig.1.

4.2 Spectral Emissivity: Spectral emissivity is a measure of the emissivity and absorptivity of a body at discrete wavelengths and is of particular importance when bodies exhibit "non-grey" behaviour. Hot gas and refractories fall into this category with their emissivities changing dramatically with wavelength. Attached Figs. 2, 3, & 4 show the spectral emissivities of hot gas, insulating firebrick and ceramic fibre. Note the contrast with Fig.1.

Of particular interest is the fact that hot gas is a selective emitter, both radiating and absorbing at selective frequencies but having "spectral windows" were no energy is emitted or absorbed. The emittance and absorption characteristics of the gas being equal and opposite.

Refractories, on the other hand, absorb and radiate at all wavelengths but their spectral emissivity is extremely low at the shorter wavelengths. Also, absorption and re-radiation are not equal and opposite. With the refractory wall being at a different temperature than the gas then the re-radiated energy is at a different wavelength from that received.

4.3 Heat Transfer to the Coils: The predominant mode of heat transfer in a firebox is wall to coil radiation. If the wall is highly reflective (low emissivity) then energy falling upon it from the gas is largely reflected back into the chamber with no change in wavelength. This reflected energy is re-absorbed by the gas. Of the energy absorbed by a furnace wall a high proportion is re-radiated with a change in wavelength. The re-radiated energy then passing unattenuated through the spectral windows in the gas to heat the coils. An increase in refractory emissivity results in a net increase in heat transfer to the coils.

4.4 Refractory Emissivity: The spectral region of interest on ethylene furnaces operating at 1200 deg C is the infrared band 1.5 to 6.4 microns where 90% of the energy is transmitted.
At the peak transmission wavelength of 2 microns on the Planck distribution curve, the spectral emissivity of insulating firebrick and ceramic fibre is in the region 0.3 and 0.2. So there is ample scope for refractory emissivity enhancement starting from a relatively low base. However, increases in heat transfer from an increase in refractory emissivity are not linear. At low refractory emissivity values small increases in emissivity bring large reductions in the gas temperature and fuel input rate for a given heat transfer task. At high refractory emissivity values increases in emissivity produce very little change in gas temperature or the fuel input rate. A Law of Diminishing Returns is at work and once a refractory emissivity value of about 0.8 is reached in the short wavelength region of the spectrum then further gains do not quite cease but they do become extremely difficult to detect.

A problem currently arises in that most coating vendors continue to quote either total hemispherical emissivities or total normal emissivities which give an artificially high value. Refractory emissivity values quoted without reference to wavelength are misleading.

Another problem arises over a preoccupation with physical colour. Some vendors make a major selling point of having a black, or near black, coating and misuse the term "black body coating". Visible colour is not the dominant factor in the infrared region of the spectrum. Strange things happen in the quantum world of electromagnetic radiation. Some white radiator paints have a higher emissivity than black paint. Also, human skin has a total hemispherical emissivity of about 0.9 in the infrared. The drive for a visibly black coating has often led to the use of metallic oxide pigments with their attendant problems of health, safety, and final disposal.

4.5 Measurement of Emissivity: No International Standards exist for the measurement of emissivity at either high temperature or at discrete wavelengths. There is no approved test apparatus and no approved test method. Test houses go about measurement in different ways so absolute values are difficult to obtain. The subject is a minefield through which coating vendors are free to stroll without fear of explosion.

5.1 Computer Predictions: A problem arises when trying to predict a change in heat balance from a change in refractory emissivity. Most plants have access to a mathematical model of their furnaces. Aside from a handful of universities and research establishments, no models are programmed with the selective emission characteristics of hot gas or the change in the spectral emissivity of refractories with wavelength. Consequently, the majority of mathematical models in present use cannot accept the phenomena at work let alone predict the outcome. This can be inhibiting for those who do not like to go where the computer cannot go.

6.1 Thin film versus Thick Film Coatings:

A debate continues over the merits of thick film against thin film coatings but the answer is simple and straightforward. Ceramic fibre linings require thin film spray-on coatings as they are ill-suited to support anything other. With brick linings it depends on the condition of the brickwork.

If the brickwork is in good condition and there is no evidence of air infiltration then a sprayed thin film coating is all that is required to increase the surface emissivity. On older furnaces where the brickwork is in poor condition with evidence of cracks, spalling, high heat loss and air infiltration (see Fig. 5) then a thick film coating is the superior solution. Photographs of thin and thick film coatings appear in Figs 6,7 & 8. Aside from increasing emissivity, a thick film coating will last longer. Thick films, say 2mm to 3mm, are far better equipped to improve refractory life, reduce heat loss and seal against unwanted air ingress. The decision whether to spray or hand plaster a thick film coating varies from plant to plant. It being a question of on-site experienced judgement.

6.2 Surface Preparation: The surface preparation of refractories is extremely important prior to coating as it has a direct bearing on the integrity of adhesion and the service life of a coating. Beware coating vendors who follow a "get in, coat, and get out" policy. This has led to some major disappointments.

6.3 Dry-out & Cure: Most modern coatings do not have a critical dry-out or cure requirement after application. This has attraction in that normal plant re-commissioning procedures can be followed on start-up. However, this point is worth checking with vendors as it is not always the case.

6.4 Installation: Coating vendors insist on applying and repairing their own coatings with their own "skilled" installation crews. Apart from adding to the initial cost this also renders the subsequent upkeep and maintenance of a coating more expensive. Coatings do require routine attention and maintenance to give of their best over an extended period. A stitch in time saves nine! Routine maintenance, if not the initial installation, is best handled by plant personnel or the local approved refractory contractor.

Brakeglen Ltd offer this service. Your plant personnel or local refractory contractor can be trained to install and maintain high emissivity coatings.