Ammonia Plant Reduces Gas Consumption
Editor’s note: Material for this article was compiled by the U.S. Department of Energy (DOE) as part of its Save Energy Now program. See the sidebar on p. 48 of this issue. As heavy consumers of steam and natural gas, ammonia plants are major energy users so any opportunity to reduce the gas bill can provide a critical boost to thin operating margins.
Editor’s note: Material for this
article was compiled by the U.S. Department of Energy (DOE) as part of
its Save Energy Now program. See the sidebar on p. 48 of this issue.
As heavy consumers of steam and natural gas,
ammonia plants are major energy users so any opportunity to reduce the
gas bill can provide a critical boost to thin operating margins. With
that in mind, Terra Nitrogen Co. took advantage of an opportunity to
participate in a DOE “Save Energy Now” plant study at its ammonia and
fertilizer plant in Verdigris, OK.
Terra Nitrogen Co. is a major producer of nitrogen
fertilizer products with annual revenues of more than $400 million. The
company’s facility in Verdigris, OK, is a highly integrated
manufacturing site producing 2.2 million tons of urea ammonium nitrate
solutions and 1.1 million tons of ammonia per year. Terra Nitrogen also
operates shipping terminals in Blair, NE, and Pekin, IL.
Because natural gas is the primary feedstock
for its hydrogen, which is combined with nitrogen to make ammonia, the
plant requires significant amounts of natural gas for production.
Moreover, natural gas is the primary fuel for the plant’s steam systems,
which provide critical support to the ammonia production processes. As a
result, natural gas costs account for most of the plant’s total
expenses, and the Verdigris plant’s management is committed to improving
its production and steam system efficiency. Natural gas cost for the
Verdigris plant was approximately $7.00/MMBtu during the implementation
period, which is the basis for savings calculations.
Terra Nitrogen undertook the study in early 2006
as part of the DOE assessment. Its main objective was to analyze natural
gas use in the plant’s steam system and identify opportunities for
energy savings. The assessment was performed by DOE energy expert
Veerasamy Venkatesan of VGAEC, Inc., and it identified some important
opportunities to improve efficiency with resulting reductions in gas
use.
Verdigris plant personnel implemented several of
the assessment’s recommendations to improve efficiency immediately. They
upgraded two turbines, installed a loop dehydrator on an ammonia plant,
and repaired failed steam traps and steam leaks. The aggregate annual
energy and cost savings resulting from implementing these first-level
measures is approximately 497,000 MMBtu and more than $3.5 million. With
project costs of around $3.1 million, the plant achieved a simple
payback of less than 11 months.
The energy assessment identified additional
opportunities that are still being implemented. The assessment results
were shared with three of the parent company’s U.S. plants.
Using new tools
Venkatesan is a
qualified specialist in the use of DOE’s steam system assessment tool
(SSAT) software. In addition to performing the evaluation, he introduced
the SSAT software to two plant employees and encouraged them to use it
in the future to analyze the steam system. This training enabled them to
learn the software and review the data to determine how best they could
improve the steam system’s efficiency.
Terra Nitrogen management has consistently
encouraged the Verdigris plant’s efforts to improve its energy
efficiency. Before the Save Energy Now assessment, plant management and
personnel had already installed a loop dehydrator in ammonia plant 1, so
they understood the potential for significant natural gas savings from
this project.
Assessment analysis performed using the SSAT
confirmed the scale of potential energy savings resulting from
installing a loop dehydrator and retrofitting the back-pressure turbine
with a condensing turbine in ammonia plant 2. When these and other
opportunities were uncovered and quantified in the assessment, the
plant’s personnel implemented them without hesitation.
After collecting data, the assessment team used
the SSAT to analyze it and identify potential energy efficiency
opportunities. Each opportunity was evaluated for technical and economic
feasibility and grouped into near-, medium-, and long-term projects
based on complexity and potential payback.
Near-term opportunities
Recover
flash steam from blowdown water—After flashing to a low-pressure header,
a substantial amount of blowdown water was being sent to a cooling
tower at 50 psig and 300 °F. Routing the blowdown water directly to a
deaerator could help generate more than 1,200 lb/hr of flash steam for
the plant. Estimated savings: 14,982 MMBtu or $105,000 per year.
Implement a steam trap maintenance
program—Although a steam trap audit was not performed during the
assessment, the team realized that some steam traps were poorly
positioned and some were not even operating. Estimated savings of better
trap installation techniques and maintenance: 12,264 MMBtu or $86,000
per year.
Implement a steam leak maintenance
program—Although few leaks were found, the assessment recommended
performing a leak audit and fixing all visible steam leaks. Estimated
energy and cost savings: 876 MMBtu or $6,000 per year.
Medium-term opportunities
Modify
synthesis loop—The assessment found that the plant 2 synthesis loop was
operating inefficiently, requiring large amounts of high-pressure steam.
Reversing the circulation in the ammonia condensing loop would improve
the ammonia plant’s efficiency and reduce high-pressure steam demand.
The assessment estimated that this measure would increase the ammonia
plant’s efficiency by 0.4% and lower high-pressure steam demand by
approximately 20,000 lb/hr. Estimated energy and cost savings: 0.4 MMBtu
per ton of output, yielding natural gas savings of 274,000 MMBtu or
$1.9 million per year.
Turbine upgrade—Ammonia plant 2 uses two
back-pressure turbines to let down 545 psig steam to 50 psig used for
some low-pressure steam applications. The back-pressure turbines powered
methyldiethanolamine pumps and were supplemented by hydraulic turbines.
The assessment found that excess 50 psig steam was being vented and
recommended that the existing turbines be upgraded with more efficient
condensing turbines. The recommended condensing turbines could reduce
high-pressure steam demand and low-pressure venting. Estimated energy
and cost savings: 178,000 MMBtu or $1.2 million per year.
Improve operation of condensing turbines—The
vacuum in the surface condensers of the condensing turbines in ammonia
plant 1 is maintained at between 24 and 26 in. of Hg, depending on the
season. Installing an absorption chiller powered by low-level waste heat
to cool the supply-side cooling tower water could increase the vacuum
by an additional 0.5 in. of Hg. Estimated energy and cost savings:
170,000 MMBtu or $1.2 million per year.
Long-term opportunities
Build a
high-pressure natural gas pipeline—The plant’s local utility delivers
natural gas to the Verdigris plant at 185 psig. Because the plant
requires natural gas at 550 psig for its processes, it operates
steam-driven gas compressors to raise the level. The assessment explored
the possibility of building a high-pressure gas pipeline from the plant
and connecting it to a high-pressure pipeline owned by the plant’s
natural gas utility to eliminate the compressors. Estimated energy and
cost savings: 851,000 MMBtu or nearly $6 million per year.
Improve auxiliary boiler efficiency—Efficiency of
the auxiliary boiler in ammonia plant 1 could be improved by reducing
the stack temperatures from 400 to 320 °F. This could be done by
installing an air preheater on the stack to recover some heat. Estimated
annual energy and cost savings: 135,000 MMBtu or $945,000.
Taking action
Verdigris plant
personnel implemented two of the most important recommendations in the
assessment soon after it was conducted and then began working on several
others.
They upgraded the back-pressure turbines with
condensing turbines and installed a loop dehydrator on ammonia plant 2.
Each of these two measures resulted in annual energy savings of 228,000
MMBtu, for a combined savings of 456,000 MMBtu per year. The annual
energy cost savings resulting from implementing the two measures is just
under $3.2 million.
In addition, the plant hired a consultant to
audit and repair broken or poorly functioning steam traps, and purchased
an infrared leak detector to detect and repair steam leaks.
These “low hanging fruit” projects were easy to
implement with straightforward solutions and had very short payback
periods. Others were not so clear once costs and other trade-off issues
were included, but still merited additional thought in hopes of finding
creative but practical approaches.
After Verdigris plant personnel carefully
reviewed other opportunities uncovered in the assessment, they decided
to pursue some additional steps to improve steam system efficiency. They
examined the boiler in ammonia plant 1 and found that the boiler’s
coils were dirty and one was leaking. Cleaning and repairing the coils
could improve process efficiency by an estimated 0.3 MMBtu/ton.
After evaluating the condensing turbines in
ammonia plant 1, they decided to defer installing an absorption chiller,
and instead do an overhaul of the condensing turbines by changing
rotors, cleaning cooling units, and replacing low-pressure steam ejector
nozzles during a 2007 plant maintenance shutdown. Other recommended
measures either had lengthy paybacks or were too difficult to implement.
For example, many difficult permitting and right of way issues were
associated with the high-pressure natural gas pipeline, and it would
have required renegotiating the plant’s contract with its natural gas
utility.
Lessons learned
Independent,
alternative evaluations of industrial process systems can confirm many
known opportunities to reduce energy consumption, as well as uncover
important additional opportunities that help to achieve significant
energy cost savings. Employees at Terra Nitrogen’s Verdigris plant were
highly knowledgeable about the steam system’s energy use, and they
regularly performed steam balance analyses. They also knew from prior
experience that efficiency gains could result from upgrading steam
system components. However, a system-level analysis using the SSAT
quantified these and similar opportunities and uncovered new ones that
were highly compelling.
This energy assessment’s results persuaded
management to carry out many of the recommendations and encouraged
performance of more rigorous future analyses. DOE software tools such as
the SSAT and AIRMaster+, the Fan System Assessment Tool (FSAT),
MotorMaster+, the Process Heating Assessment and Survey Tool (PHAST),
the Pumping System Assessment Tool (PSAT), and 3EPlus can all be used to
analyze industrial systems and processes and generate energy efficiency
opportunities
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