POWER July 2020 - 42

NATURAL GAS
How to Optimize Your Facility
While Using Variable Quality
Fuel Gas
The composition and heating values of fuel gas can vary from one moment in time
to another. Using a Coriolis flow meter to obtain a mass flow measurement, rather
than relying on a standard volume flow measurement, allows for more precise
control of the air-to-fuel ratio, which improves efficiency, reduces emissions,
minimizes downtime, and increases safety.
Marc Buttler
E
very power generating facility
strives for smooth, consistent power
output by avoiding shutdowns or
upsets as much as possible. However,
variations in fuel gas are often unavoidable
because of different suppliers or different
sources from which the fuel gas
is received. These variations will impact
safety, as well as efficiency and emissions
of turbine generators, fired heaters,
and engines alike.
The air-to-fuel ratio (AFR) mix control
that feeds combustion requires continuous
monitoring and control to maintain
safe levels. If the level of oxygen present
in the combustors is too low, this
will result in unburnt hydrocarbons
that represent both a loss in fuel efficiency
and the potential for explosion.
Too much air in the mix will introduce
a higher volume of nitrogen, which also
reduces heating capacity and can increase
NOx
emissions (Figure 1). Any
excessive deviation from the AFR setpoint
can cause a trip in the system and
interrupted service. It should be noted
that, although a very conservative setpoint
can help prevent risks to safety,
such an approach is often inefficient and
can contribute to an elevated emission
of pollutants.
To optimize combustion control, which
is needed to ensure safe, smooth, and
consistent operations, requires live measurements.
The best process variables to
use for control systems input turns out to
be mass flow, and where possible, energy
flow. These measurements can be complicated
in situations where composition and
heating values of the fuel gas vary.
Maintaining consistency and safety is
the goal but is hindered at times by some
unique challenges, which will be explained
further below before different technology
42
1. Example showing 20% reduction in NOx by reducing excess oxygen from 6% to 4%. Courtesy:
Emerson
solutions are presented that can optimize
efficiency and improve safety.
Challenges of Gas Energy
Measurement
The two main challenges that can be
overcome by moving a setpoint closer
to optimum and safe operations are
better fuel efficiency and lower emissions.
Measurement is the primary
hurdle that must be overcome, regardless
of gas composition.
The first challenge to measurement
is that the gas energy content might be
unknown due to a composition measurement
error. This is not uncommon,

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POWER July 2020

Table of Contents for the Digital Edition of POWER July 2020

Contents
POWER July 2020 - Cover1
POWER July 2020 - Cover2
POWER July 2020 - Contents
POWER July 2020 - 2
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