Application and Benefits of Combustion Management to Fired ... · PDF fileApplication and...

Application and Benefits of Combustion Management to Fired Heaters

A White Paper


A White Paper

TI 53A90A01-01E-A


Sections

Introduction

Managing Combustion in Natural Draft Fired Heaters

How It Works

CASE STUDY: A Major Gulf Coast Refinery

The Recommended Best Industry Practice

The Technology

The Installation Process

Figures

Figure 1: Diagram of a Typical Fired Heater

Figure 2: Features and Benefits of TDLS Technology

Figure 3: TDLS Technology for Combustion Safety and Optimization

Figure 4: Basic Functional Capabilities of CombustionONE

Figure 5: Refiners Participating in the API 556 Task Team

Figure 6: CombustionONE - An Integrated Solution

1

2

5

6

7

8

9

2

3

4

5

7

8


CombustionONE solution unites a TDLS analyzer

with a dedicated control system and a safety system

certified to meet FM NFPA and SIL 2 standards.

The intrinsic value gained by adopting this new

combustion technology can be summarized by the

following:

Best Industry Practices

which can only be satisfied with new technology

Increased safety

because both air and fuel are continuously controlled

Improved thermal efficiency

as excess air is always optimized .............................

Longer fired heater life

(asset reliability) since heat is not concentrated at the

bottom of the convection section

Lower greenhouse gas emissions

through a more efficient use of fuel

Second only to raw materials costs, energy is the leading cost pressure currently affecting manufacturers. New analysis techniques, such as tunable diode laser spectroscopy (TDLS), can improve efficiency, maximize throughput, reduce emissions, and improve safety and reduce energy in combustion processes.

ARC INSIGHTS, INSIGHT# 2009-50MP, November 2009

Introduction

While fired heaters are used throughout refining and

petrochemical processes as the source of process

heat, they carry inherent risks and costs that make

operating without current technologies problematic.

To address safety concerns, Industry standards

are upgrading their recommended practice for

Instrumentation, Control, and Protective Systems for

Fired Heaters and Steam Generators. While not yet

required by regulation, plants not meeting the best

Industry Practice guideline, will be at added risk in

the event of an incident on a fired heater. Since many

natural draft fired heaters do not meet this guideline

with existing instrumentation and control systems,

an upgraded system consisting of recommend

technologies will be needed. Further, since most

natural draft fired heaters have only automated control

of the fuel supply, not air, excess air is often applied to

the combustion process, reducing thermal efficiency.

According to ARC INSIGHTS, “Second only to raw

materials costs, energy is the leading cost pressure

currently affecting manufacturers. New analysis

techniques, such as Tunable Diode Laser Spectroscopy

(TDLS), can improve efficiency, maximize throughput,

reduce emissions, and improve safety and reduce

energy in combustion processes.”

This paper highlights and describes the application

of a new combustion management system by

Yokogawa called CombustionONE. By incorporating

Tunable Diode Laser Spectroscopy (TDLS)

technology, this new system has the ability to

simultaneously control air and fuel supply to fired

heaters by measuring average gas concentrations

across the high temperature radiant section. Cross

sectional averages that measure O2 and CO at high

temperatures has only recently become reliable

with the introduction of the TDLS technology. The

1


Managing Combustion in Natural Draft Fired Heaters

The air supply for most fired heaters is natural

draft – not forced air - and these heaters normally

lack the extent of automation as the other process

units in the plant. Natural draft fired heaters, as the

name implies, use flue gas buoyancy to support

combustion. These heaters can be either cylindrical

or box type (see Figure 1 below). The buoyancy of

the flue gas (combustion product) relative to the

surrounding air is determined by the product of the

average density of the flue gas and the height of the

heater. Furnaces are designed to run at a pressure

of -0.05” to -0.1” WC at the top of the radiant

section, whether a heater is natural draft, induced

draft or forced draft. Figure 1 provides a simplified

diagram of a typical fired heater.

Because of the lack of air control, operators will typically allow excess air into the fired heater, reducing its thermal efficiency.

The low level of control on most fired heaters

is due, at least in part, to the historical lack of

reliable, effective instrumentation and automation

technology to simultaneously measure and control

the fuel, gas concentrations, and the air/fuel ratio.

An oxygen (O2) sensor is typically required at the

stack base for thermal efficiency calculations, which

require total excess air. While operators attempt to

maintain ‘excess’ O2 in the furnace for safety, the

amount indicated from an existing sensor may be

incorrect due to tramp air. In fact, it is possible that

the burners may be starving for air, despite excess

oxygen at the stack base. Because of the lack of

air control, operators will typically allow excess air

Stack

Damper

Breeching

Process Fluid In

Process Fluid Out

Coil

Burner

Radiant section

Shield section

Convectionsection

Natural Air Draft

Figure 1: Diagram of a Typical Fired Heater

“Inefficient combustion can be attributed to the air/feul ratio. Too much excess air (air rich) results in loss of efficiency and increased NOx emissions, while too little air (fuel rich) is downright dangerous. Carbon monoxide measurements provides an indi-cation of fuel-rich conditions, while oxygen measurements indiate air-rich conditions. The optimal control point is the lowest pos-sible excess air value that does not cause the system to enter an unsafe condition or violate emissions limits.”

2


into the fired heater, reducing its thermal efficiency.

The lack of effective instrumentation to continuously

and rapidly measure O2 and CO in the combustion

chamber of fired heater introduces considerable

safety risk. Apart from the simultaneous control of

fuel and air concentrations, it is possible for fuel-

rich conditions to arise which increases the potential

explosion risk. Note that under fuel-rich conditions,

temperature/fuel controllers no longer work properly.

The detection of combustibles - primarily CH4 - in

the radiant section of the fired heater is recom-

mended by the American Petroleum Institute per API

556; however, traditional analyzer technology cannot

be installed in the radiant section due to the high

temperatures. As noted above, without accurate

measurements of CH4, O2 and CO concentrations,

operators tend to allow excess air than necessary

in the heater, reducing its thermal efficiency. To

properly control the combustion air, CH4 and CO

must be measured at the top of the radiant section

where combustion is completed, regardless of the

burner loading. Note that O2 and CO will coexist

within the flames, where the temperature may be as

high as 2,200 degrees F. Low NOx burners may use

delayed completion of combustion through staged

air/fuel mixing, or external recirculation of cooler

flue gas with combustion air, reducing peak flame

temperature. In either case, to effectively control the

combustion process it is essential to measure O2

Apart from the simultaneous control of fuel and air concentrations, it is possible for fuel-rich conditions to arise which increases the potential explosion risk.

and CO at the top of the radiant section, ideally one

foot below the roof tubes. This is where combustion

reaction is expected to complete under all heater

operating loading. This is an ideal application of

CombustionONE, which employs new TDLS tech-

nology. Using the TDLS in concert with a dedicated

controller, a cross section average O2 and CO

density can be measured to determine the right air/

fuel ratio, rather than a localized spot measure-

ment of these gases. Using an average O2 and CO

density produces safer burner control and greater

overall heater efficiency. Figure 2 lists the features

and benefits of TDLS technology, which has distinct

advantages over single point, in-situ analyzers that

may give false readings because of varying gas con-

centrations at different locations in the fired heater.

Fired heaters have two principal unsafe operating

conditions that must be avoided:

• Fuel Rich - where air is reduced, CO will be

produced by the burners and excess O2 will be

lower, which results in excess fuel (CH4) from

the burners

• Flame Out – where Loss of flame results in rapid

loss of gas temperature. O2 levels are high as

burner air is not reduced by combustion, and

un-combusted fuel (CH4) is present

Figure 2: Features and Benefits of TDLS Technology

Source: ARC INSIGHTS, INSIGHT# 2009-50MP, November 2009

Benefit

Sample conditioning not required

Real-time data for control

Interference-free analysis

Suitable for operations in harsh environments

In-situ analysis

Fast response

Tunable laser

Non-contact sensor

Feature

Low maintenanceOptical sensor

To properly control the combustion air, CH4 and CO must be measured at the top of the radiant section where com-bustion is completed, regardless of the burner loading.

3


• HighercostsasoperatorsincreaseO2flowtoavoidafuelrichatmosphere

• Unexpecteddemandforfuel,leadingtounsafecombustionconditions

• Greaterriskduringaprocessupset

• Riskofinadequateaircontrolmaynotbeassessedcorrectlyforprocessupset

• Wetsteamintroducedonstartup,requiringasteampurge,riskingignitionfailure

• Shorterlifeofthefinnedconvectionsectionwithafterburningduetopresenceofcombustibles

Before Combustion Management

of CombustionONETM, the embedded control and

safety systems will ensure that these conditions are

avoided or that combustion is extinguished and fuel

flow is interrupted automatically if these conditions

are detected. The TDLS analyzer technology will

reliably respond to all “O2 events”, where conven-

tional sensor technology will miss most such events.

• ReducedO2andloweroperatingcostsasthefuel-airmixtureiscontrolled

• Fuelislimitedtotheavailableairtopreventunsafefuelrichcombustion

• COandO2concentrationsaresafelycontrolledattheoptimumlevels

• Unburnedfuelisdetectedreadily,avoidingunsafecombustion

• Processupsetsarehandledwithcontrolledcombustionconditions

• Enforceddrainremovalfrompurgesteampreventsunsafeignitionattempt

After Combustion Management

Figure 3: TDLS Technology for Combustion Safety and OptimizationBy optimizing air

flow control, O2 concentration is typically reduced from 6% to 2%, increasing thermal efficiency of the furnace

Consequently, continually measuring percent O2 is

critical to improving heater efficiency and maintain-

ing safe operating conditions. When firebox con-

ditions are unacceptable, i.e. high levels of CO or

combustibles exist, the effective combustion man-

agement solution must rapidly detect the condition

and initiate the appropriate response. In the case

ConcentrationVariations

TDL Transmitter

Cool temperatures but CO reactionis nearing completion

Convection Tubing

TDL Receiver

Radiant Tubes(Radiant Zone)

4


inputs directly from the furnace and operates the

heater steam purge valves and fuel trip valves when

necessary. The simplified architecture diagram

below illustrates the key components comprising the

solution. Because the TDLS is a non contacting

measurement - never touching the flue gas - and

has no moving parts, the whole system enjoys very

high reliability. The TDLS analyzer technology within

CombustionONE has been operational on furnaces

since 2003 without incident and most of those units

have not required calibration. The analyzer has full

diagnostic capability, and if there is an issue, the

analyzer will alert the operator. Also, the measure-

ment signals from the TDLS are unaffected by the

Continuous measurements of percent O2 is critical to improving heater efficiency and maintaining safe operating conditions.

How It Works

Capable of measuring the average gas concentra-

tion across the radiant zone of the fired heater,

CombustionONE addresses both of the above less-

than-optimum conditions by simultaneously control-

ling the fuel and air (O2) supply based on 5 second

sample intervals. Measuring the gas concentration

in the radiant zone is also a requirement of API 556.

Measurements from the system include CO, CH4,

O2, H2O and temperature. Using an average gas

concentration produces safer burner control and

greater overall heater efficiency. By optimizing air

flow control, O2 concentration is typically reduced

from 6% to 2%, increasing thermal efficiency of the

furnace.

The combustion management system manages fuel

flow and arch draft through the existing plant DCS

via Modbus, and combustion airflow directly.

The safety system with CombustionONE receives

(TDLS)

Gas ConcentrationMeasurement

FieldInstrument

Safety System

CombustionONE

Control Systems

Control Element

Control Element

Burners

Burners

Refinery Heaters

Figure 4: Basic Functional Capabilities of CombustionONETM

5


6

be correlated to the output from the existing stack

gas analyzer to verify the results. Data collected

during this testing will be incorporated into the

future safety shutdown system. The modular proce-

dural automation capability in CombustionONE will

enforce safe operating conditions during start-up

and shut-down. Once the Safety Interlock System is

in place, CombustionONE will be able to detect and

interdict any unsafe operating conditions.

Natural draft heaters lack the capability to use

air to purge the heater. Instead, steam is used. If

the steam is not dry water will accumulate on the

burners/igniters, preventing ignition. The start-up

sequence, part of the CombustionONE solution, will

purge the condensate from the steam line, thus pro-

viding dry steam to purge the heater before ignition.

ResultsCombustionONE has been operational since June,

2010 and the TDLS analyzers continue to operate

reliably with no maintenance needed. The operators

have been able to reduce the percent O2 by 1% to

1.5%, thus making the heater more efficient. The

furnace is now near optimum operating point – using

minimum excess air. The TDLS measurements have

been verified by the existing stack gas analyzers,

but with a percent O2 reading of 1% to 1.5% lower

than the stack gas analysis because the measure-

ments are taken in the radiant section. Furnace con-

ditions can now be controlled (or shutdown) quicker

since CombustionONE is taking concentration

measurements at five second intervals in the radiant

section. If there were to be an excess concentra-

tion of CO or CH4 in the furnace, these gases can

be detected earlier versus the conventional stack

gas analyzer, enabling the heater to be shut down

sooner and avoiding unsafe conditions.

presence of other gases in the flue gas, unlike sen-

sor based technology. TDLS uses a path average

measurement, as opposed to the traditional point

measurement, making the value of concentration

much more accurate.

CASE STUDY: A Major Gulf Coast Refinery

ChallengesThe most critical times of heater operation are at

start-up and shutdown. Recognizing that a much

faster, more reliable analyzer is required to measure

O2, CO and CH4 concentrations, a major Gulf Coast

refining operation has adopted the CombustionONE

solution for combustion management. Since

CombustionONE measures gas concentration in

the radiant section of the fired heater, the system

is expected to improve heater safety and overall

operational efficiency.

SolutionCombustionONE was installed with two Tunable

Diode Laser Spectroscopy (TDLS) analyzers for

measurement of O2, CO and CH4 concentrations

in the radiant section of the heater. A dedicated

CombustionONE controller uses these

measurements while feeding these values to an

existing DCS for monitoring and future control of the

heaters. The dedicated control hardware is equipped

to receive additional signals from the heater, which

will be used to control the airflow in the burners in

a subsequent phase of the project. Space has also

been allotted for a future safety shutdown system.

The CombustionONE system will be tested during

upset conditions and the response of combus-

tion gases will be analyzed to confirm the desired

response to unsafe conditions. This data can then


Composed of representatives from a cross-section

of the refining and petrochemicals industry, the API

Task Team continues to refine the API 556 guideline.

Figure 6 lists the refining companies who have pro-

vided representatives to the API556 Task Team.

The Recommended Best Industry Practice

Over two years in development, API 556, to be pub-

lished by the American Petroleum Institute, is a rec-

ommended practice for “Instrumentation, Control,

and Protective Systems for Fired Heaters and Steam

Generators”. This guideline specifically applies to

gas fired heaters and steam generators in petroleum

refinery, hydrocarbon-processing, petrochemical,

and chemical plants. API 556 specifically states the

following regarding the use TDLS technology:

Not covered in this recommended practice are the

following:

• Oil fired and combination fired heaters

• Water tube boilers which consist of single or

multiple burners and are designed for utility

operation or where the primary purpose is steam

generation

• Fired steam generators used to recover heat

from combustion turbines (HRSG)

• Oven and furnaces used for the primary purpose

of incinerating, oxidizing, reduction or destruc-

tion of the process medium (covered by NFPA

86).

• Water bath or oil bath indirect fired heaters.

• CO boiler, ethylene furnace and other specialty

heaters

Laser based technology for combustion control (oxygen trim to air or air/fuel ratio controller) is a design consideration for heater applications where a single sample point will not provide a representative sample. It has a response time of ≤ 5 seconds and can measure across a radi-ant section up to 98.4 ft. (30 m). It is not an ignition source to flue gas and requires no reference air.

API 556

Figure 5: Refiners Participating in the API 556 Task Team

• Valves• Analyzers• ProtectiveSystem• Instrumentation

andAnalytical

• Analyzers• Design• Burners• Control• Safety• Valves

• ChairmanSCOICS• Combustion/general• General

Instrumentation

• ChairmanSCOICS• HeaterDesign

• ProtectiveSystem• Control,protective• GeneralInstrumentation• ProcessControl• HeaterDesign

• Analyzers• Controls

• GeneralInstrumentation• ProcessControl

CITGO

BP

Chevron

ConocoPhillips

Valero Corp.

ExxonMobil

Company Contribution

Marathon AshlandPetroleum

7


The CombustionONE combustion management

system is an integrated, self-contained system that

can be rapidly installed on any fired heater. Four

principal components that comprise the solution are

shown in Figures 7-10 and described as follows:

• TDLS technology for gas concentration mea-

surements on 5 second intervals

• A dedicated system for control of fuel and air

volumes based on a fired heater model

• An OSHA compliant safety system to prevent

unsafe conditions from persisting

• Sensing and actuation for additional measure-

ments and air flow control as needed

Two TDLS systems (transmitters and receivers)

are typically installed in the radiant section of the

fired heater, which is the most accurate location for

optimum combustion management. At minimum,

one unit measures O2 and the other CO. Additional

TDLS units can be installed to measure the products

of combustion, such as CH4. Since fired heaters

have differing configurations, capacities, environ-

mental and process conditions, custom mounting

brackets are built to hold and position the laser

across the radiant section. This is the best location

to obtain the most accurate gas concentration and

temperature measurements.

The dedicated controller is used to simultaneously

control fuel and air volumes based on five second

sampling measurements of average gas concentra-

tions across the radiant section from the TDLS.

The Technology

8

TDLS for Gas Concentration

Sensing and Actuation

Safety Interlock System

Dedicated ControllerClass 1, Division 2 Rated Enclosure

Figure 6: CombustionONE - An Integrated Solution


Since the combustion management solution

is completely self-contained and requires little

integration with existing control systems and

instrumentation, the installation phase is typically

straightforward. Where mechanical modifications

and/or upgrades to the fired heater are necessary,

Yokogawa acts as the prime contractor, partnering

with engineering companies and furnace

manufacturers to provide a single source for

the project. The installation and commissioning

processes occurs in 8 to 12 weeks, depending

on the size and complexity of the fired heater.

Installations on multiple fired heaters on a site or

across a fleet can also be easily accommodated.

A systematic installation plan that schedules the

installation across multiple fired heaters offers

greater economy of scale, while reducing the

time to realize the benefits from the combustion

management solution.

Embedded with the controller are proprietary fired

heater combustion strategies that direct fuel flow

at the valve and air volume through the burner

registers.

An OSHA compliant safety system is required on a

fired heater to meet regulatory requirements. The

safety system will safely shut down the fired heater

if the main controller fails or if unsafe conditions

persist in the fired heater. The embedded safety

system in CombustionONE is certified to meet

the requirements of NFPA, FM, ISA S84.01 and

IEC61508 and SIL3.

Smart (self diagnosing) multi-variable transmitters

and valve positioners provide reliable and accurate

performance and feedback of the combustion man-

agement solution. Additional SIL 2 transmitters are

sometimes added on the fired heater to provide air

flow measurements at multiple locations, ensuring

safety and optimum performance.

As a best practice, Yokogawa recommends

commencing the installation of a combustion

management system with a discovery session that

includes a site review. The initial discovery session

will produce:

• The specific fired heaters on-site where com-

bustion management can be of highest value

• A note of the changes needed to each fired

heater that require detailed engineering

• Order of installation for the combustion manage-

ment solution

• A general project time-line and budget

The Installation Process

9

Represented by:Yokogawa Corporation of America2 Dart Road, Newnan, GA 30265-0928Phone: 770-254-0400 Fax: 770-254-0928

12530 W. Airport Blvd., Sugar Land, TX 77478Phone: 281-340-3800 Fax: 281-340-3838

http://www.yokogawa.com/us

Yokogawa Canada, Inc.Bay 4, 11133 40th Street SE, Calgary, AB T2C 2Z4Phone: 403-258-2681 Fax: 403-258-0182http://www.yokogawa.com/ca

Yokogawa de Mexico, SA de CVAv. Urbina No. 18Fracc. Parque Industrial NaucalpanNaucalpan de JuarezEstado de México, C.P. 53489Phone: (55) 5955-7400 Fax: (55) 5955-7417http://www.yokogawa.com/mx

For More Information

Contact Us: [email protected]

Improving and Sustaining the Combustion Asset

1CombustionONETM

Application and Benefits of Combustion Management to Fired ... · PDF fileApplication and...

Documents

Transcript of Application and Benefits of Combustion Management to Fired ... · PDF fileApplication and...