Condensing Boilers; Saving Energy and Reducing Emissions

In my previous post, we looked at how taking
a broader, life-cycle cost perspective on a boiler replacement
project
can make an it more palatable for an Owner by
demonstrating that there is actually a payback associated with a
project that they were obligated to take on simply to ensure they
could continue to operate their core business in the manner
intended. So far, we’ve looked at the impact of simply complying
with current efficiency codes and standards and the reduction in
maintenance cost associated with the new boilers. But for the San
Diego Marriott Facilities Engineering team leaders …

… and the crew they work with …

… efficiency codes and standards represent the
minimum level of acceptability.
Since 2005, after becoming the first facility to initiate MRCx and
MCCx (Marriott’s internal brand of retrocommissioning and ongoing
commissioning), the leadership team and the technicians working
with them have steadily reduced the facility’s utility consumption
at the rate of 5-6% per year, garnering a
SANDEE award
in the process.

At the same time, they have put a lot of sweat and elbow grease
into improving their O&M practices to the point where most of
their 23 plus year old mechanical rooms literally sparkle. And all
the while, they have been dealing with the issues that inevitably
occur as the mechanical equipment in the facility reaches the end
of it service life, begins to fail, and must be replaced. On top of
all of that, the crew donates some of their free time to do
for community service, working as a team to repair and improve
orphanages and other facilities that are in need in their area. The
boiler project we are discussing is just one example of many the
many projects they have been juggling or will be juggling into
their busy schedules.

Given their drive to continue to improve efficiency and embrace
the latest technology where appropriate, the Marina Marriott team
asked me to help them look at what the options might be for further
enhancing the benefits they could achieve via the boiler
replacement project. An obvious place to start was to look at
improving boiler efficiency beyond what was required by the
California Title 24 Efficiency Code. 

The boilers that were the basis behind their project budget were
guaranteed to deliver 85% efficiency, which is the basis for the
discussion
in the previous post
.  While this is already several
percentage points better than code would require, the team knew
that condensing boilers had the potential to deliver efficiencies
in the mid to high 90% range, literally doubling their savings.

Of course, the condensing boilers represent a more complex
solution technically, and as a result, represent approximately a
30% cost premium to acquire them. But a first pass through the
project’s numbers based on gaining an additional 10% in efficiency
and taking the potential condensing boiler cost premium into
account  revealed that their simple
payback could drop from a little under 8 years
to slightly over
5 years if the condensing boiler solution where implemented.

In other words the significant increase in savings associated
with the improvement in efficiency not only covered the cost of the
boilers required to deliver it, it paid for the project sooner.
From the Owners perspective, this means that a little more cash
front will let them make more money sooner, all while saving energy
and reducing emissions.

Emission reductions are an important part of the equation. 
Based on emission
metrics on the Climate Trust’s web site
,  the project as
originally envisioned would reduce the Marina’s CO2 emissions by nearly 130 metric tons;  burning
less carbon based fuel means generating less emissions with carbon
in them.  Doubling the boiler efficiency also has the
potential to double the reduction in emissions.

But, there’s a catch, which is evident in the following figure
taken from the ASHRAE Equipment
Handbook
.

As you can see from the illustration, boiler efficiency is a
function of entering water temperature. In general terms colder
water entering the boiler can remove more heat from the products of
combustion before they leave the boiler, improving
efficiency. 

There is a catch here too. As you can see from the curve, there
is an inflection point in the efficiency vs. heat transfer line in
the range of 130 to 140°F entering water temperature (see
footnote 1). The inflection point is created because when operating
in this temperature range, the boilers will begin to condense water
out of the flue gas. The reason that there is water in the flue gas
is that one of the by-products of the chemical reaction that occurs
when a fossil fuel is burned is water vapor. This water vapor
represents energy, and condensing it to a liquid removes the latent
heat of vaporization and makes it available to the system served by
the boiler, thus improving the over-all combustion efficiency.

This is a good news/bad news proposition. 

The good
news
is that condensing the water vapor out of
the flue gas can significantly improve boiler efficiency, lowering
fuel consumption and reducing emissions. This phenomenon is the
fundamental principle behind the efficiency gains that are achieved
by condensing boilers.

The bad
news
is that the water condensed out of the
flue gas can combine with other products of combustion like carbon
and sulfur to create acidic liquids that will quickly corrode
conventional boiler and flue construction. That is why
the O&M (Operation and Maintenance) manuals for most
conventional boilers indicate that you will void the warranty if
you operate the boiler below a certain minimum entering water
temperature, typically in the range of 140°F for gas fired
equipment.

So a few bottom lines: 

Condensing boilers represent a significant
opportunity to reduce fuel consumption and emissions
,
but these improvements will only be realized if you can operate the
system at the low entering water temperatures required to drive the
boiler into condensing mode.

As a result of the first bullet, a project that
retrofits conventional boilers with condensing boilers needs to
include the engineering and adjustments necessary
to
allow the system to operate at a reduced water temperature.

As a result of the first bullet, one can not simply
assume that the savings potential to be achieved by the system will
be equal to original gas consumption divided by the peak
efficiency
achievable by the new boilers. While this
may be true if the boilers are in fact operated continuously at the
water temperature that generates that efficiency, it will overstate
the savings if the water temperature varies with time or load,
especially if the water temperature never drops to the point where
it will allow the boiler to condense and generate its maximum rated
efficiency.

Mixing condensing boilers with conventional boilers
on the same system requires some careful planning and
engineering.
If the system is operated at a
temperature that leverages the condensing boiler efficiency
benefits, the conventional boilers could be ruined in short order.
On the other hand, if the system is operated to protect the
conventional boilers and comply with their operating limitations,
then the maximum benefit to be achieved by the condensing boilers
may not be realized. In both cases, success is dependent upon a
system configuration and control strategy that will protect the
conventional boilers while leveraging the condensing boiler
benefits.

Operating condensing boilers brings new operating
issues to the table that may not have existed with conventional
boilers.
One is the potential for increased system
complexity as implied by the previous bullets. The other is that
the flues will need to be arranged to deal with corrosive
condensate. Usually, this implies special materials of construction
as well as a collection system that recovers the condensate and
neutralizes its acidity before discharging it to the sewer
system.

None of these are reasons to not consider condensing boilers if
you are replacing existing equipment. But they
are reasons that you want to take
some time to assess the situation so you can make an informed
choice and include the necessary funds in your budget to cover all
of the bases.

In the next post, I’ll take a look at some of the thing the
Marina Marriott team looked at as they considered upgrading to
condensing boilers.

Senior Engineer – Facility Dynamics
Engineering

1 – The exact temperature at which flue gas condensation occurs
is a function of the type of fuel being burned. This graph
represents the condensation point for the products of combustion
from burning natural gas, which generally has a low sulfur content.
Fuels like oil and coil can contain significant amounts of sulfur,
the byproducts of which can condense out at higher temperatures (I
believe the range is about 200-350°F, but you should check me
on that as I seldom have to deal with it) and generate things like
sulfuric acid in addition to the carbonic acid associated with
condensing water out of the flue products in a gas fired
boiler.

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One Response to Condensing Boilers; Saving Energy and Reducing Emissions

  1. Good article very much. Useful for people who did not know.
    Thank you for sharing knowledge.
    Webblog and would like to introduce air conditioning(Thai).
    Recommend that your first forum. http://www.watcharaaircon.com.

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