Combustion control

Boiler Operations

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CARIBBEAN MARITIME INSTITUTE
Kingston Jamaica


Combustion Control System



Assignment submitted in partial fulfilment of the requirements for the degree Bachelor of
Engineer in Industrial Engineering

To
Mr. Andre Subron
By
Group Member
George Pecco (20152444)
Dane Johnson (20151255)
Dalene Coore (20151581)
Romarne Buddington (20151374)

November 21, 2016

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Table of Content

Introduction ……………………………………………………………………….. 3
Combustion Control Systems……………………………………………………… 4
Importance of Combustion Control Systems………………………………………. 6
Burner Combustion Control System……………………………………………….. 7
Applications of Burner CCs……………………………………………………….. 11
Advantages & Disadvantages of Burner CCs……………………… ……………... 12
References…………………………………………………………………………. 16

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Introduction
Combustion is the rapid oxidation of material (the fuel) to release energy (heat). The fuel can be
a solid, liquid or gas, and the amount of heat released is normally expressed in BTUs (British
thermal units); the amount of heat required to raise the temperature of 1 pound of water 1°F, or
Calories; the energy required to raise one Kg of water one degree Celsius.
As fuels are burnt, with just enough air to release the total BTUs in the fuel, the reaction is said
to be “stoichiometric” or burned “on ratio”—when combustion is complete, no free oxygen or
unburned fuel remains.
Gas and oil burners are everywhere. The power package boilers, start-up larger furnaces with
fluidized beds and grates, and heat many other processes. Larger burners have Combustion
Control Systems (CCSs) which should be tuned periodically.
To understand how to tune a gas and/or oil burner, it helps to first understand how and why they
work the way they do.

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Combustion Control System (CCS)

Larger burners are controlled with a combination of a Combustion Control System (CCS) and a
Burner Management System (BMS). A CCS is a key burner subsystem that is composed of
equipment that controls fuel-to-air ratio and firing rate. This equipment can be comprised of both
mechanical and electronic components, including actuators, flow control valves, linkages,
dampers, positioners, PLC’s; process loop controllers, oxygen analyzers, variable frequency
drives (VFD), as well as other instrumentation. All of these devices play important roles in the
performance and efficiency of the combustion system. Other control requirements are commonly
grouped into the category of combustion controls. After the CCS determines how much fuel, air,
and water to put into the boiler the BMS determines if there will be a fire or not, and is primarily
responsible to shut down the system if conditions become unsafe, as well as enforcing purge
requirements on re-start. (Faber Burner Company, 2016)

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Diagram of Combine CCS

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Importance of Combustion Control Systems

The Combustion Control system is need for multiple operation the most important function
however is to prevent the possibility of an accumulation of combustible gas followed by
accidental or improper ignition sequence resulting in an explosion. The Combustion Control
systems also monitor elements of the burner combustion system, these include:
 Safety and performance requirements of pulverisers, burners and igniters.
 Furnace safety standards and regulations
 Flame monitors and flame failure detection
 Start-up protection and sequencing
 Furnace supervisory controls and shutdown systems

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Burner Combustion Control System

Boiler burners are the functional component of boilers that provide the heat input by combustion
of a fossil fuel, including natural gas, with air or oxygen. They are available either as part of the
boiler package from the manufacturer, as stand-alone products or for custom installations.







Boilers are often the principal steam or hot water generator system used in industrial plant or
commercial heating. Consequently, they must be designed to operate efficiently and safely whilst
responding rapidly to any change in demand. Burner management systems must be equally
adaptive. Eurotherm Process Automation; a supplier of control and measurement instruments to
industrial and process markets, provides efficient, well implemented control techniques capable
of reducing operating costs whilst providing resources for greater flexibility in plant management
and control. Burner combustion control generally includes one or a combination of the following
methods:
 Oxygen trim
 Burner modulation
 Regulation of excess air
 Air/fuel cross-limiting
 Total heat control

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Oxygen trim
When a measurement of oxygen in the flue gas is available, the combustion control mechanism
can be vastly improved, since the percentage of oxygen in flue is closely related to the amount of
excess air. This is done by adding an oxygen trim control module, allowing:
 Tighter control of excess air to oxygen set point for better efficiency
 Faster return to set point following disturbances
 Tighter control over flue emissions
 Compliance with emissions standards

Burner modulation
A continuous control signal is generated by a controller monitoring the steam or hot water line.
Reductions in steam pressure or hot water temperature lead to an increase in firing rate. The
modulation in combustion control is introduced to ensure:
 Fuel and air requirements are continuously matched to the combustion demand
 Steam pressure or hot water temperature is maintained within closer tolerances
 Greater boiler efficiency
 Weighted average flue gas temperature is lower

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Regulation of Excess Air
Gas, oil, coal burning and other systems do not mix the fuel and air completely even under the
greatest degree of achievable conditions, and complete mixing may be a lengthy process. In
order to ensure complete combustion and reduce heat loss, excess air has to be kept within a
suitable range and hence the regulation of excess air provides:
 A better boiler heat transfer rate
 An advance warning of flue gas problems, e.g. excess air coming out of the zone of
maximum efficiency
 Substantial savings on fuel

Air/fuel cross-limiting
A cross-limiting combustion control strategy ensures that there can never be a dangerous ratio of
air and fuel within a combustion process. This is implemented by always raising the air flow
before allowing the fuel flow to increase, or by lowering the fuel flow before allowing the air
flow to drop. Cross-limiting combustion control is highly effective and can easily provide:
 Optimization of fuel consumption
 Safer operating conditions by reducing risk of explosion
 Fast adaptation to variations in fuel and air supplies
 Satisfaction of the plant demand for steam

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Total heat control
In situations where combustion is not the principal heat source and when several factors
contribute to the total heat to be generated by a boiler, a control loop can be introduced in order
to monitor and manage the generated heat.

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Application

Combustion control burners can typically be found controlling the following fired applications:
 Power boilers
 Process heaters
 Heat recovery steam generators
 Reformers
 Incinerators
 Fluidized bed boilers
 Driers & Kilns
 Thermal oxidizers
 Cabin heaters
 Duct burner

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Advantages and Disadvantages

When we talk about combustion control we talk about “combustion control Schemes”, these
schemes have various advantages and disadvantages that are as follows: (Zuidema, july 22
,2015)
Jackshaft Control Scheme
Advantages:
 Jackshaft controls don’t require any measuring equipment (airflow or fuel flow
transmitters)
 Jackshaft controls are mechanically linked, which allow very fast load changes
 Jackshaft controls are simplistic with minimal hardware, which demands less
maintenance and upfront costs
 Jackshaft controls are inherently safe because the fuel and air are mechanically linked
together. This decreases the likelihood of a problem occurring with mechanical
failure significantly.
Disadvantages:
 Jackshaft controls don’t have feedback and/or measurements, so the excess air is
often setup higher than optimal efficiency
 Jackshaft controls have no way of compensating for ambient changes in temperature
or heating value for fuel

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 Jackshaft controls allow adding O2 trim as an option; however, this is usually
complex and costly with higher maintenance costs and reduces the inherent safety of
the system
 Jackshaft controls have Limitations with firing different fuels because only one fuel
can be fired at one time and only one fuel can be set optimally

Parallel Control
Advantages:
 Parallel controls can adapt to O2 more efficiently than Jackshaft controls can
 Parallel controls have a more complex control scheme that allows burners to fire
multi-fuel at their optimal excess air.
 Parallel controls give the end user more flexibility in the fuel/air ratio.
 Parallel controls allow more sophisticated burners can be used.
Disadvantages:
 Parallel controls don’t measure the fuel and air. Therefore, end users must have
further training and operate at higher levels
 Parallel controls cannot compensate for ambient changes in temperature or heating
value of the fuel.
 Parallel controls require more sophisticated and demanding maintenance and tuning
to ensure that the feedback signal matches the demand signal

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 It is costlier to install a parallel control systems than to install jackshaft control
systems, and the parallel control system does not have a substantive efficiency gain
compared to jackshaft control system with O2 trim
 Parallel controls have a higher risk of failure due to the more complex system
Fully Metered control
Advantages:
 Provide precise fuel/air ratios at all firing rates for optimal efficiency
 Can correct fuel/air ratio for ambient changes in air temperature and fuel heating
values
 Give the end user more flexibility over the fuel/air ratio
 Allow multi-fuels can be fired at their optimal efficiency
 Allow simultaneous fuel firing can be done at the optimal efficiency
 Have a slightly higher cost than parallel controls
Disadvantages:
 Have a high upfront cost, especially compared to jackshaft controls
 Require higher levels of maintenance and setup because of adding transmitters and
selection of equipment (fuel valves, dampers, etc.)

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Burners Control diagrams

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References
Charter Tech Limited. (2016). Burner Management System. Charter Tech , http://www.charter-
tech.com/pages/burnermanagement.html.
Faber Burner Company. (2016). Combustion Control Systems (CCS). www.faberburner.com,
http://www.faberburner.com/industrial_burner_products/combustion-control-systems/.
Schneider Electric. (2016). Burner Combustion Control for Boilers. Eurotherm,
ttp://www.eurotherm.com.au/burner-combustion-control-for-boilers.
Zuidema, B. H. (july 22 ,2015). Advantages and Disadvantages of Combustion Control Schemes.
ADVANCED BOILER CONTROL SERVICES ,
https://boilercontrols.wordpress.com/2015/07/22/advantages-and-disadvantages-of-
combustion-control-schemes/.