Page 429 - Boiler_Operators_Handbook,_Second_Edition

414 Boiler Operator’s Handbook

equal you switch the standby fuel controller to auto- performance during that month can be used to predict
matic and then switch the leading fuel to manual. situations when less gas could be burned and increase
It doesn’t require an instant transfer because the the current rate so the gas is consumed by the end of
controller will simply adjust the two fuels in parallel. the month.
Control action will not be smooth with both fuels in Choice fuel firing can also be associated with
auto because every change in output produces twice the burning other combinations of fuels including a fuel
change in flow compared to firing one fuel; don’t leave produced by a process in your facility. Some of it may
both fuel controls in automatic unattended. It’s possible be sold to another facility. That case could involve burn-
to control two fuels in auto at once but, why? If you’re ing the excess fuel the other facility does not use. When
dual fuel firing there are other reasons, not one of which the other fuel is gas it is handled by pressure control,
involves auto operation of both fuels to maintain steam sensing the pressure of the generated gas and varying
pressure. To complete the transfer you reduce the firing the flow of gas to your fired equipment to maintain a
rate of the lead fuel manually until it is at low fire then constant pressure at the output of the process. It could
shut it down. also include a variable setpoint for the gas pressure de-
There’s nothing preventing you firing both fuels veloped by the process that generates the gas. When the
continuously as long as one is in manual control. It’s other facility uses more of the generated gas the pressure
convenient for burning down an oil tank while still fir- will attempt to drop so the controls reduce the amount
ing natural gas or firing natural gas at the maximum rate fired in your equipment to maintain the pressure. When
allowed by your supplier. the other facility uses less generated gas the pressure
I should have titled this section multi-fuel burning. will attempt to rise with the result of your burning more.
I’ve put in a couple of projects where we burned three So, what happens when neither facility needs
fuels simultaneously, gas, oil and a solid fuel. There are more of the generated gas? Your firing rate controls will
very few opportunities to do that so I stuck with the first reduce the firing rate of the main fuel which is the
“dual fuel” label. Don’t let that prevent you from con- only one normally controlled to maintain boiler output.
sidering firing more than two fuels on one burner; just When the main fuel firing rate is at minimum then the
keep in mind that only one can be operating on any one generated gas control is overridden to maintain the
automatic control signal. boiler output, reducing its use in your boiler. If there is
still too much the generated gas pressure will increase
and a relief valve will send it to a flare to simply burn it
FIRING RATE CONTROL—CHOICE FUEL FIRING and waste the energy. You should be able to note this oc-
curring (actually production should advise you that it’s
It is possible to fire two fuels and have both on going to happen) and look for ways to use the generated
automatic control, just different automatic controls. gas in other equipment to offset purchased fuel cost by
Modern microprocessor based controls allow dynamic utilizing all the generated gas that’s possible to use.
changes in controller gain so a fuel controller could fire If the production generates a liquid fuel then de-
oil and gas together. The question is, why would you viations in supply of the generated fuel can be absorbed
want to do that? Choice fuel firing is the incorporation in storage tanks. Automatic control for such applica-
of additional controls to meet fuel supplier’s criteria. tions is unusual. Production should be able to tell you
You could have a system that calculates the rate gas of any changes they make in production and you can
should be fired based on the amount of gas you are determine an optimum firing rate for the generated fuel
allowed to burn and the number of hours left in the based on production data and the level in the storage
month. The gas glut created by fracking ( a process of tanks.
fracturing gas laden rock in the earth that has produced Burning of waste gases, oils, and solid fuels gener-
the gas glut) will eventually diminish and this type of ated by the plant process operations can also require
firing may return. analysis of the fuel to determine its proper air to fuel
It’s easy with computerized control, you input ratio so it burns cleanly and efficiently. Liquid and solid
the amount of gas you’re allowed each month and the fuels can be stored and tested with the ability to adjust
controls do the rest. There are several parameters that air fuel ratio before firing it. Gas fuels, on the other hand,
the control system needs to make a decision about the are not easily stored. I had questioned one customer, a
gas firing rate at any time including the need to burn refinery, with why they didn’t burn gas that was flared
a minimum amount of fuel oil. Some history of facility in the boilers and was told the waste gas varied consid-