When investigating steam systems, the boiler is one of the
primary targets for energy-efficiency improvement. There are many tools used in
the evaluation and management of boiler performance. One of the most useful
tools is boiler efficiency. Boiler efficiency describes the fraction of fuel
energy that is converted into useful
steam energy. Of course, the fuel input energy that is not converted into useful steam energy represents the losses of the boiler operation. Boiler investigations generally examine the losses by identifying the avenues of loss, measuring the individual loss and developing a strategy for loss reduction.
steam energy. Of course, the fuel input energy that is not converted into useful steam energy represents the losses of the boiler operation. Boiler investigations generally examine the losses by identifying the avenues of loss, measuring the individual loss and developing a strategy for loss reduction.
There are many avenues of loss encountered in boiler
operations. Typically, the dominant loss is associated with energy leaving the
boiler with combustion gases. The temperature of exhaust gases is an indication
of their energy content. Ensuring that the heat-transfer surfaces of the boiler
are clean is a major point of focus for managing the thermal energy in exhaust
gases. Energy can be recovered from exhaust gases by transferring thermal
energy from the high-temperature gases to boiler feed water, or to the
combustion air entering the boiler.
Another aspect of exhaust-gas energy management, which is
the focus here, is combustion management. It should be noted that the
temperature- and combustion-related attributes of exhaust gases are
interrelated – they combine to represent the stack loss of the boiler. Again,
this is typically the dominant loss for the boiler. Stack loss is dependent on
the operating characteristics of the boiler, the equipment installed and the
type of fuel burned in the boiler. Stack loss generally ranges from as much as
30 percent for a green-wood-fired boiler, to 18 percent for a typical
natural-gas-fired boiler, to 12 percent for an oil-fired boiler, to as low as 9
percent for a coal-fired boiler. It must be pointed out that the stack-loss
range is wide for any given fuel.
To address your question, we will examine the combustion of
a simple fuel – methane (CH4). The chemical equation for the reaction of
methane with oxygen (O2) is presented below:
In a perfect world, methane will react with oxygen to
release energy and form carbon dioxide and water. In this perfect arrangement,
each molecule of fuel would find the exact amount of oxygen needed to cause
complete combustion. In the case of methane, one molecule of methane must find
two molecules of oxygen in order to produce a complete reaction.
In the real world, however, the combustion process does not
proceed in a perfect manner. A fuel molecule may encounter less oxygen than is
required for complete combustion. The result will be partial combustion; the
exhaust gases will then contain some unreacted fuel and some partially reacted
fuel. Generally, these unburned fuel components are in the form of carbon
monoxide (CO), hydrogen (H2) and other fuel components that may include the
fully unreacted fuel source, which in this case is methane.
When unburned fuel is found to be part of the combustion products,
a portion of the fuel that was purchased is consequently discharged from the
system, unused. It is also important to note that unburned fuel can accumulate
to a point where a safety hazard could result. Unburned fuel can burn in a part
of the boiler not designed for combustion – under certain conditions, the
materials can even explode. Additionally, these chemicals are typically toxic
in nature, presenting health and environmental hazards.
CH4 + 2O2 → αCO2 + βH2O + γCO + δH2 + εCH4 + ζO2
Energy Release
Unburned fuel presents negative safety, health,
environmental and economic impacts to the boiler operations. As a result, it is
imperative to manage the combustion process in order to maintain these
components at minimum levels. Fortunately, the complex interrelations of the
combustion process can be managed with two fairly simple principles. The first
principle of combustion management is based in the fact that unreacted fuel
components are undesirable in the exhaust gases of the boiler, but the presence
of unreacted oxygen presents minimal safety, health and environmental concerns.
Furthermore, as long as the burner is appropriately mixing the oxygen and fuel,
the presence of extra oxygen in the combustion zone essentially ensures that
all of the fuel will react completely. The fan energy required to move ambient
air into the combustion zone is generally minimal when compared to the cost of
fuel.
Combustion management principle No. 1, stated simply, is
provide more oxygen than you theoretically need to ensure that all of the fuel
burns up. As a result, when combustion management principle No. 1 is applied to
the example chemical reaction, rather than two molecules of oxygen being
supplied for every one molecule of methane, each molecule of methane may be
provided three or four molecules of oxygen. This will ensure that all of the
fuel is burned. The extra oxygen that is added to the combustion zone, however,
enters at ambient temperature and it exits the boiler at flue-gas temperature.
The flue-gas temperature of a typical boiler could be anywhere in the range of
300 to 500 degrees Fahrenheit range. As a result, the extra oxygen could have
entered the boiler at 70 degrees F and exit at 400 F. The extra oxygen reached
this temperature by receiving fuel energy – in other words, fuel was purchased
to heat the extra oxygen. Additionally, ambient air contains almost four
molecules of nitrogen (3.76 to be exact) for every one molecule of oxygen. As a
result, every amount of excess air brings with it a huge amount of nitrogen.
This nitrogen enters at ambient temperature and receives fuel energy to exit
the boiler at flue-gas temperature.
This brings us to combustion management principle No. 2 – do
not use too much oxygen. In other words, combustion management requires that
extra oxygen be provided to the combustion zone in order to ensure that all of
the fuel is reacted; however, the amount of extra oxygen must be minimized to
reduce energy loss. The critical measurements required to manage the combustion
loss are the flue-gas oxygen content and the flue-gas combustibles
concentration. The flue-gas oxygen content is measured to allow the combustion
air flow to be modified to meet a set point. Combustibles concentrations are
measured to identify the minimum practical oxygen concentration. It should be
noted that combustibles concentrations can be elevated even though excess
oxygen is provided to the combustion zone. This situation is typically
indicative of a burner failure. A primary function of a burner is to mix the
air and fuel thoroughly to ensure complete combustion. If a burner component
has failed or is improperly adjusted, the mixing process can be ineffective.
This can result in unburned fuel and excess oxygen. Combustibles concentration
values vary based on fuel type, operating conditions, and burners. A typical
natural-gas-fired boiler will operate with combustibles concentrations less
than 50 parts per million (ppm), while a coal-fired boiler may operate with
combustibles concentrations greater than 200 ppm. Generally, combustibles
concentrations less than 100 ppm are considered negligible in terms of
efficiency impact. Baseline combustibles concentrations should be established
for each boiler.
The principles and measurements used in combustion
management outlined above indicate that combustion control should incorporate
flue-gas oxygen measurement, combustibles measurement and active control of
combustion airflow. Many boilers, however, do not incorporate these factors
into the control process. Common boiler-control strategies are based primarily
on steam-pressure control. As the process steam demand increases, the steam
pressure at the boiler deceases. The boiler steam-production controller will
measure this decrease in pressure and will increase fuel flow to the boiler. In
the case of a natural-gas-fired boiler, the controller will proportionally open
the fuel control valve. As the fuel flow is increased, the combustion air flow
must also increase in order to maintain safe and efficient combustion.
A very common and simple method of accomplishing the control
of combustion air flow is to mechanically link the air-flow control device to
the fuel-flow control device. This is commonly called positioning control
because the air-flow-control device will have a position that is based solely
on the position of the fuel-flow-control device. It should be noted that this
control does not incorporate any active oxygen or combustibles measurements.
Oxygen and combustibles measurements are only taken to establish the position
relationship between the fuel controller and the air controller. After the
position relationship is established, oxygen and combustibles measurements
cease.
Tuning the boiler is the act of re-establishing the position
relationship between the air and fuel. This tuning activity is completed in the
same manner that the original air-fuel control point positions were
established. The boiler is operated steadily at discrete fuel input positions
and the airflow control device position is redefined. The boiler will be
operated at discrete loads throughout the operating range of the unit. While
the fuel-flow controller is 100 percent open, for example, the position of the
air-flow controller is adjusted until an appropriate flue-gas oxygen content is
attained. Combustibles concentrations also should be measured to ensure proper
burner operation. The position relationship exercise is repeated over the
operating range of the boiler (95 percent load, 90 percent load, down to
minimum load). This retuning activity should be completed frequently to ensure
safe and efficient boiler operation.
It should be noted that when positioning control is used,
the oxygen content cannot be minimized because of many factors. One factor
influencing the airflow controller position is ambient temperature. Ambient
temperature is a concern because the combustion air fan is basically a constant
volume-flow device (for a given controller set point). If the position
relationship is established for a relatively cool inlet-air temperature, the
mass flow of air into the combustion zone could be dangerously low as the inlet
air temperature increases. As a result, positioning control can only attain
moderate efficiency.
Combustion control can be improved through the use of an
automatic oxygen trim system. This type of system continuously measures
flue-gas oxygen content and adjusts the combustion air flow to maintain a set
point. This type of control can be more precise and efficient than positioning
control because it continually tunes the air-fuel relationship. Combustibles
measurement and control can be added to allow the oxygen set point to be
minimized.
These control strategies take many forms, and there are many
variations of each; this description only outlines the primary composition of
the control. It should be noted that most boilers require higher flue-gas
oxygen content at lower loads. This generally results from the fact that mixing
is compromised in the burner at low loads. Additionally, flue-gas oxygen
content targets will be influenced by additional environmental controls, such
as nitrogen oxide control. When a boiler is equipped with nitrogen oxide
control, the minimum oxygen concentrations are somewhat higher than in boilers
without the nitrogen oxide control.