Though I spends me time
In the ashes and smoke
In this whole wide world
There's no happier bloke

("Chim-Chim Cher-ee," by Robert and Richard Sherman written for the 1964 movie Mary Poppins)

Grab a cup of java before starting to read this one because Your Mentor is on a roll!

 

BERT THE CHIMNEY SWEEP IS A DEAD MAN

Bert the smiling Chimney Sweep character created for the Walt Disney movie Mary Poppins and his fellow dancing sweeper dudes were happy blokes until the buildup of soot in their bronchial passageways caused their slow, anguished deaths.

Soot is a black carbon byproduct of combustion that forms when hydrocarbons are incompletely burned due to insufficient oxygen.

Even Walt Disney cannot make soot something to smile about.

PTOA Readers and Students can deduce that the black soot blowing out of the stack on the left and the little pile of soot to the right are not what lungs were designed to handle.

The schematic to the left shows that a single speck of soot is smaller than the diameter of a human hair or even a speck of dust.

 

Seems a little odd that  something so small can choke a human being to death (and yet ... let us not forget about the Ebola virus).

The schematic to the upper right shows the toxic layers of a soot particle that forms when diesel is incompletely combusted.

The particle is even more toxic when heavier hydrocarbons and coal are used as firebox fuels.

When inhaled, the airborne soot particles lodge in the part of the lungs that help supply oxygen to the body.

Kind of karma that a problem that starts with incomplete oxygen for combustion ends up causing insufficient oxygen for breathing. What goes around, comes around!

 

ONE REASON THE ENVIRONMENTAL PROTECTION AGENCY WAS CREATED

Developing countries still use heavy hydrocarbons and coal to create heat in a combustion reaction because environmental and health concerns are low on the list of economic priorities.

The same disregard for the external problems caused by industrial development was prevalent in the USA from the start of the industrial revolution through the establishment of the Environmental Protection Agency (EPA) in 1970 by a bipartisan Congress and the Nixon  administration.

The EPA Clean Air Act (CAA) is one of the federal-level regulations Process Operators need to be aware of and maintain compliance to.

The CAA regulates the gaseous and particulate emissions from fired heaters and motor vehicles because of their direct tie to public health problems and global warming.

The business pages of a newspaper do not ever reveal that the EPA Clean Air regulations benefit the Plant Owner as well as Planet Earth.

For example:

PTOA Readers and Students should notice that the hydrocarbon escaping from the stack in the photo to the right is made of fuel that could be efficiently used in the combustion process were it not carelessly allowed to be emitted into the atmosphere as soot.

The misperception of beneficial regulations often results in Process Operators becoming mistakenly frustrated with the amount of paperwork required to certify that fired heaters are being properly operated and stay within compliance limitations.

During the moments where it seems like compliance paperwork is not "real work," stand back and look at the Big Picture:

Fired heaters are a necessity to achieve the process temperatures that convert feedstocks into valuable products that consumers use.

However, fired heaters also generate combustion products and flue gases that contribute to global warming and local health hazards.

Operating fired heaters within the permitted limits does not eliminate global warming; however, the regulations reduce needless degradation of the environment and local public health.

If the above arguments do not convince modern Process Operators to prioritize procedures related to environmental awareness ... keep this in mind:

Process Operators that stress about the cost of environmental compliance harming company profits can rest assured that the top echelon of the corporation will annually receive millions of dollars in compensation.

The real bottom line is that investments for environmental/health improvement and compliance help provide those grandiose salaries by recapturing and using the carbon that otherwise would have created soot.

So RIP, Bert.

Today's modern Process Operators are protected from eating dust just to make somebody like Mr. Banks more wealthy.

Modern and better informed Process Operators  realize they are on the front line with respect to protecting the local air quality for their kids and grand kids.

 

COMBUSTION GAS/FLUE GAS FLOW PATH
THROUGH THE RADIANT AND CONVECTION SECTIONS

Efficient fired heater operations begin with understanding how Radiant, Conduction, and Convection Heat Transfer are designed work throughout the structure.

Three typical styles of industrial fired heaters are shown below. The range of temperatures in these fired heaters are 1600-2000 °F (871-1093 °C). Reaction heater/furnaces that generate syngas operate 500-800 °F hotter.

PTOA Readers and Students that are reading the PTOA Segments in the intended sequence know that the Heat Transfer via Radiation is used to elevate the exterior Radiant Tube temperatures.

PTOA Readers and Students already know that the industrial problems associated with Radiant Heat Transfer include:

• Flameout which requires a shutdown and purge of the firebox.
• Flame Impingement which fatigues Radiant Section tubes.
• Coke Buildup on the interior of Radiant Section tubes which causes ...
• Hot Spots that fatigue Radiant Section tubes.

 

Soot buildup is a problem that impacts the Convection Section of the fired heater.

As shown in the graphic to the right, the Convection Section of the fired heater is above the firebox which contains the Radiant Section.

From the burners to the stack exit, the flow path of combustion products/flue gases moves through these sections of a fired heater:

• Radiant Section which has the Radiant Tubes and is also called "the firebox."
• A Bridgewall is a slanting roofed section of a "Cabin Style" firebox that separates the Radiant Section from the Convection Section. A "Cabin Style" firebox is shown in the schematic that starts this section.
• Shock Bank/Crossover Tubes are the lowest tubes levels in the Convection Section and experience both Radiant and Convection Heat. The label "Shock Bank" does not appear in the graphic.
• Convection Section with Finned Convection Tubes.
• Breeching Area above the Convection Section and below the Stack which collects the flue gases and funnels them into the Stack.
• Stack and Stack Damper and opening to the atmosphere.

 

The below photo is taken from the vantage point of looking straight up from the firebox floor of a "Vertical Cylindrical" style fired heater.

The lowest level of the Convection Section tubes is visible. These Shock Bank/Crossover Tubes are fabricated of a metal that can endure the heat of radiation and has sufficient room to expand upon heating.

PTOA Readers and Students have already figured out that the beams of radiant heat have diffused into waves by the time the heat reaches the Convection Section tubes above the Shock Bank Tubes.

The waves of convected heat make the outside temperature of the Convection Section tubes hot; however the Convection Section tube temperatures are still several hundred degrees cooler than the Radiant Section tube temperatures.

The process fluid is preheated in the Convection Section prior to flowing into the  Radiant Section as shown in the graphic to the right.

The typical design of the Convection Section is intended to transfers 30% of the heat needed to attain the  desired process temperature at the fired heater outlet.

PTOA Readers and Students already know that the heat transfer that occurs in the Convection Section simultaneously decreases the temperature of the exiting flue gases. Beside increasing the heater efficiency, cooling the flue gases simultaneously decreases global warming.

 

SOOT INHIBITS CONVECTIVE HEAT TRANSFER

The formation of soot is evidence of inefficient operations for two reasons:

• The energy escaping in the form of soot could efficiently be used to create radiant heat in the firebox and eliminate wasted fuel costs.
• Conductive Heat Transfer ... designed to heat up the process fluid 30% in the Convection Section tubes ... is inhibited by soot buildup.

The Convection Section tubes are finned because adding fins creates more surface area (A) available for Heat Transfer via Convection.

Sooty Convection Section heater tubes like those shown below decrease the rate of Heat Transfer via Conduction because:

• Clogged fins reduce the exposed surface area, A.
• The thickness of the physical barrier separating the hot and cold areas is effectively increased with soot buildup made of a carbon which has a very low conductivity factor "k."

Tube Tech International once upon a time had a website that showed the impressive work they did cleaning of hard to reach Convection Section tubes using robots.

The same section shown above was cleaned by one of  their robots and is shown below:

The ability for these Convection Section tubes to conduct heat and preheat the process stream has been restored:

• The elimination of soot decreases the (effective) thickness of the pipe through which Conductive Heat Transfer takes place and removes the insulating layer made of carbon.
• The cleaned fins provide more surface area through which to conduct heat.

 

Fired heaters that use heavy hydrocarbons and coal to generate radiant heat have Soot Blowers installed that clean Convection Section tubes with steam.

PTOA Readers and Students must realize that soot can form in the Convection Section of any process industry equipment that creates radiant heat by the combustion reaction.

Soot that accumulates on the hot side of the economizers installed in package boilers decreases the preheating of Boiler Feed Water (BFW) which will require the burners to increase firing to make saturated steam.

Soot that accumulates on the hot side of superheaters installed in package boilers reduces the 'degrees of superheat' that can be added to saturated steam to generate superheated steam. Less degrees of superheat will reduce the capacity of the steam to perform useful work.

Soot that accumulates on the hot side of the various preheaters and superheaters that flow thought the ducts of a syngas-generating Steam Reformer make the process more costly to operate by inhibiting heat transfer used for preheating and superheating.

 

TAKE HOME MESSAGES: Soot is carbon black powder that is created by incomplete combustion of hydrocarbon fuels.

Soot buildup impacts the Convection Section tubes in a fired heater which are typically designed to transfer 30% of the total heat duty into the process fluid flowing through the fired heater.

Soot inhibits conductive heat transfer in the Convection Section heater tubes by clogging the fins and by increasing the effective thickness of the tube wall with a coat of insulating carbon.

Processing facilities that still use heavy hydrocarbons and coal for the combustion reaction fuel will use Soot Blowers to clean Convection Section tubes with steam.

Inhalation of soot clogs the bronchioles of the lungs and impairs breathing.

In the USA, Soot and other fired heater emissions are regulated under the Clean Air Act. Operating fired heaters to maintain compliance to permitted emissions is the job of Process Operators.

Outside Process Operators and Board Operators are in the driver's seat with regard to minimizing local hazardous air emissions and global warming associated with fired heaters.

Understanding how Radiant, Conduction, and Convection Heat Transfer are designed to work in a fired heater is essential to operating a fired heater efficiently.

The Convection Section of the firebox is above the Radiant Section.

The flow of flue gases/combustion products proceeds through the fired heater in this order: 

• Radiant Section Firebox
• Bridgewall
• Shock Bank/Crossover Tubes
• Finned Convection Section Tubes
• Breeching
• Stack, Stack Damper, Flue Gas Outlet

 

©2015 PTOA Segment 00072
Process Industry Equipment Troubleshooting Operations
Process Industry Regulation Compliance

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