District Energy - 2010 Second Quarter

of condensate returned represents an average of 67.3 percent with a generated bill credit of $34,654. Additionally, the amount returned represents a savings of 7,868 MMBtu (78.7 ktherms) of fuel. (The energy savings rate is based on average difference between heat energy received from condensate less heat energy available from regular source water.)

Ever since the condensate return system has

been in place, the stadium and convention

center have seen both energy and cost savings.

The water, chemicals and energy savings made possible
by our new condensate return system make this statement
a correct one: “Sometimes the old way is the right choice,
even today.”

Reinstallation of Back-Pressure Turbines

As Citizens saw with its ‘return to condensate return,’ the basic processes of district energy dating back to its modest beginnings are still relevant today. Many of our current district energy systems were founded to utilize the byproduct of providing electricity to customers. Generated steam was often considered waste until it was captured for heating or process load, which still holds true for many new engineering designs today.

In the past few years, we have been hearing more and more about efficiency, sustainability (“meeting the needs of the present without compromising the ability of future generations to meet their own needs”) and cogeneration – the simultaneous production of electricity and mechanical power from a single heat source (using the steam twice). The term “cogeneration” today is usually used to refer to combined heat and power.

The mechanical power portion of the cogeneration
equation is sometimes forgotten. A large number of IDEA
members and industrial users have boilers that provide steam
for heating or processing. The steam also generates energy
through small turbines that are used to drive equipment.
This process happens prior to the steam moving into the
distribution system. This type of turbine is often referred to as
a “back-pressure turbine.” The back pressure goes to either a
deaerator tank for heating purposes and O removal or to the
2
steam distribution system as a salable or usable product. This
same type of cogeneration can be used to drive pumps, fans,
compressors or other pieces of equipment required to run
integrated systems.

Taking care of basics ties in here, as that may include
using a blowdown heat exchanger to recover the heat going

Courtesy Citizens Thermal Engineering Group.

Two 1.7 MW back-pressure turbines take 600 psig steam and reduce it to 250 psig, which provides cogeneration and sustainability, improving the efficiency of the steam produced.

to waste from the boiler blowdown or other areas, for example. Other basic improvement measures may involve managing water treatment to remove both scale inside tubes, cleaning soot and ash from the outside of tubes, reviewing oxygen settings for efficient combustion, providing adequate insulation of piping and equipment, or reviewing the heat balance of your plant and total system in light of changes in operation or equipment over the years.

This leads us to our second project – installing two back-pressure turbines to produce 1.7 MW (cogeneration) each, instead of utilizing a PRV. It is another instance of looking back to the system basics of the past to improve our operations for the future.

In 2007 and 2008, as Citizens investigated system data
from the past, we noted that our system used to have a back-
pressure turbine rated at 11 MW. It had been installed in 1936
and removed during the 1980s. This back-pressure turbine was
used to take 600 psig steam and reduce the steam to 250 psig.

In its place is a full set of PRVs that was installed to provide 250 psig steam to the distribution system. With our cost of electricity increasing over the years and our escalating electrical load, we needed to install approximately 3-4 MW of additional generating capacity to meet our existing peak electrical load. We have an existing 5 MW turbine that supplies power on our side of the meter. But during outages or required maintenance to this unit, we would set a new demand, which involved a ratchet that required us to live with the additional electric cost for the next 11 months.