Duluth Steam Cooperative
Engineering Project
First draft
ME 3211 Thermodynamics
Chris Johnson
Cheng Vue
Mark Minter
Table of Contents
Background Information……………………………………………………………3
Baseline Utility Information………………………………………………………..4
Improvement Suggestion #1: (Insulation of Pipes)…………………………….6
Improvement Suggestion #2: (Electric Pumps)……….……………………..…8
Improvement Suggestion #3: (Introduce a dual purpose turbine)....…………...10
Improvement Suggestion #4: (Installation of a Feedwater Economizer)...…….11
Appendix……………………………………………………………………………13
Background Information:
Duluth Steam Power Plant was built in 1932 as a way to provide heat to over 200 offices, retail shops, hospitals and other buildings in the downtown Duluth area. The plant consists of four 100,000 lb/hr boilers, each having a maximum pressure of 425 psia but usually operate at 225 psia, which is the pressure necessary to meet the required loads. But once the steam reaches its customers they have to throttle the pressure down to three or four psia. When the plant was built there wasn’t much technology used to operate the pneumatic controls of each boiler. Currently the plant operates at 56.8% efficiency and produces over 733,423 klbs of steam. However, 136,407 klbs of steam are unaccounted for each year; this means 18% of the energy input to the plant is lost. Duluth Steam is also the leading contributor of CO2 emissions in the city of Duluth.
Problem Summary:
Currently Duluth Steam uses 22.3% of the steam it produces to run equipment within the plant. This means that only 77.7% of the steam it produces is used for steam heating. Our task is to explore ways of reducing the amount of steam used within the plant, which will reduce the amount of coal needed for operation. This will help the economics of Duluth Steam because they won’t have to purchase as much coal to burn. Also, this will reduce the amount of air emissions they produce and make the plant more environmentally friendly.
Baseline Utility Information
Table 1: Steam and Coal Accounting for January ’05 to May ‘06
Steam Accounting (klbs) / CoalMonth / Generated / Plant Use / Sold / Unaccounted / Tons / mmBtu / Cost
Jan '05 / 100,675 / 23,455 / 55,952 / 21,268 / 8,001 / 143,058 / $208,506.06
Feb '05 / 90,023 / 22,221 / 50,476 / 17,325 / 7,162 / 128,057 / $186,641.72
Mar '05 / 67,759 / 16,127 / 40,813 / 11,818 / 5,658 / 101,165 / $147,447.48
Apr '05 / 48,482 / 10,591 / 27,029 / 10,862 / 4,064 / 72,664 / $105,907.84
May '05 / 46,798 / 9,718 / 22,832 / 14,247 / 3,716 / 66,442 / $96,838.96
Jun '05 / 40,266 / 7,607 / 22,185 / 10,473 / 3,268 / 58,432 / $85,164.08
Jul '05 / 37,971 / 6,621 / 15,416 / 15,934 / 3,002 / 53,676 / $78,232.12
Aug '05 / 35,260 / 6,790 / 14,992 / 13,477 / 2,985 / 53,372 / $77,789.10
Sep '05 / 40,876 / 9,718 / 20,000 / 13,887 / 3,507 / 62,705 / $91,392.42
Oct '05 / 50,443 / 10,916 / 26,885 / 13,026 / 4,104 / 73,380 / $106,950.24
Nov '05 / 76,634 / 17,834 / 45,774 / 8,553 / 6,060 / 108,353 / $157,923.60
Dec '05 / 93,150 / 24,925 / 59,672 / 7,216 / 7,184 / 128,450 / $187,215.04
Jan '06 / 81,313 / 13,694 / 53,507 / 14,112 / 6,225 / 111,303 / $162,223.50
Feb '06 / 100,249 / 22,070 / 54,024 / 14,155 / 7,625 / 136,335 / $198,707.50
Mar '06 / 72,214 / 18,064 / 48,248 / 7,901 / 5,860 / 104,777 / $152,711.60
Apr '06 / 55,031 / 13,521 / 32,596 / 8,914 / 4,398 / 78,636 / $114,611.88
May '06 / 50,016 / 12,612 / 28,645 / 8,759 / 4,043 / 72,289 / $105,360.58
Jun '06
12 month total / 733,423 / 164,372 / 421,944 / 136,407 / 58,261 / 1,041,707 / $1,518,281.66
Reference: Supplemental Document to 'Project Development Progress Report' - August 14, 2006
Figure 1: Graphical representation of data from Table 1 (12-months)
Figure 1 visually displays the steam use in various aspects of the plant. There is a correlation between the steam lost (unaccounted) and the steam generated. As steam production increases from month to month, the plant uses and sells more steam. At the same time consumption and production are increasing, the unaccounted steam is decreasing. This means that during high demand months, the plant is operating more efficiently. This occurs because the boilers have a decrease in steam loss when operating at full capacity. Similar to an electric motor, when the motor is operating at full capacity it comes closer to operating at 100%. On the other side, when the motor is operating at half capacity it is much less efficient.
Figure 2: Steam generated and seam used + sold by the plant (12-months)
As seen in Figure 2 for any given month, the total steam generated always exceeds that which is actually being consumed. The difference in the two lines shows the steam that is unaccounted for. By eliminating this difference, the plant will see an increase in overall efficiency. This can be accomplished many different ways. Some improvement suggestions are shown below.
Possible Improvements to Plant
Improvement Suggestion #1: Insulation of pipes:
Currently the pipes leading to downtown Duluth are made from piping that is insulated with asbestos and encased in a concrete conduit that is either square or rectangular. These pipes were installed in the 1930s and need to be replaced. Much of the original insulation has been damaged due to moisture that has come into contact with it. In some places it has gotten so bad that all of the insulation has actually fallen off.
Solution:
The pipes going to the downtown buildings would need to be reinsulated. However, these pipes are about 3 feet under ground, under some major streets and sidewalks. With new insulation technology we won’t have to dig up the streets, rather we can inject foam insulation into the concrete blocks through holes that have been dug. This will fill the air gap that is within the concrete blocks and seal out any ground water or moisture that may come in contact with the pipes. We will be doing this to only the oldest section of the pipes because lines that were direct buried would not work with this type of insulation. However, it will still account for approximately 57% of the steam in use because this is the amount of pipes we can reinsulate. This new foam will be able to stand a temperature of 400o F. Also, removable insulation covers can be fitted for the valves within the piping system. This makes it easily accessible to anyone who would need to use the valve.
Table 2: Approximate portions of insulation to be distributed.
Lineal Footage / Size of Pipe / Size of Concrete conduitSuperior Street / 1300 LF / 12” / 33 ½” x 27”
Superior Street / 470 LF / 14” / 28” x 25”
Superior Street / 1790 LF / 18” / 33” x 35”
1st Street / 1790 LF / 12” / 33 ½” x 27”
3rd Ave East / 140 LF / 8” / 24” x 30”
Estimated Benefits per year:
As Calculated by Thermal Sciences Technology:
Annual Fuel Savings: 94,974 mmbtu
Annual CO2 Avoidance: 9,869 metric tons
Distribution Piping Insulation Benefits:
· Restores the thermal efficiency of the underground piping system
· Prolongs the life of carrier pipe and surrounding concrete infrastructure
· Seals conduit and protects from intruding ground water
· Reduces potentially hazardous vapor on streets and sidewalks
· Minimal disruption of pedestrian and vehicular traffic during application
· Steam service to customers will not be interrupted at any time
Removable Insulation Cover Benefits:
· Restores the thermal efficiency of bare piping and fittings in manholes
· Eliminates frequent application of conventional insulations which are damaged easily
· Removed and replaced easily by personnel while working in manholes
· Steam service to customers will not be interrupted at any time
Improvement Suggestion #2: Electric Feed Pumps
Currently there are four Feed Pumps at Duluth Steam. One is powered with electricity and three are steam driven. Overall, 22.3% of steam produced is used to run equipment within the plant. If we can reduce the amount of steam the plant uses, we will then be able to burn less coal and reduce air emissions.
Solution:
The steam driven Feed Pumps would need to be replaced with electric ones. This would free up approximately 19,500 lb/hr of steam because it would no longer be needed to run this equipment. Because most of the steam they use in the plant comes from these machines, the percent of steam they need to run the plant would go down significantly.
From this steam that you save you could do one of two alternatives. This first alternative would be to sell the excess steam to other buildings in the downtown Duluth area. Doing this would help the plant make more money from the steam they produce instead of using it themselves. The second alternative is to cut down on the amount of steam they produce overall. This implies that the plant will consume less coal, making it more environmentally friendly and reduce the amount of steam purchased by the plant each year.
Estimated Benefits per year:
Using information based of this website, http://www.liquimover.com/Assets/PDF/Boiler_Feed_Flyer_1003.pdf, we were able to convert the boiler feed pumps from steam driven to electric. Since each pump runs at a different flow rate, each pump will have a different annual cost. For this we assume that the flash lost of each new electric pump is 13.3%.
Table 3: Example Analysis of Electric Pump
Flow Rate (lb/hr) / 17,250Steam Cost ($/klb) / 2.06
Hours/day operation / 24
Days/year operation / 250
Flash Loss / 13.3%
Flash Loss (lb/hr) / 2,294
Flash Loss (lb/yr) / 13,750,000
Steam Cost ($6.02 x 13,750) / $82,700
Annual Conventional Feed Pump Electrical Cost / $2,030
Annual Boiler Feed Pump Operational Cost / $84,730
Pump 1: Steam Consumption is 9000 lb/hr = 9 klb/hr
This is the annual cost of running pump 1. These computations leads to the conversion from a steam powered pump to an electric powered pump:
Pump Cost = $70,000
Installation Cost = $20,000
Flow rate = 9000 lb/hr
Steam cost = $2.07 / klb
Flash loss =
Steam cost =
Annual electrical cost =
Annual feedpump operational cost =
Annual savings =
Pump 2: Steam Consumption is 6000 lb/hr = 6 klb/hr
This is the annual cost of running pump 2. These computations leads to the conversion from a steam powered pump to an electric powered pump:
Pump Cost = $70,000
Installation Cost = $20,000
Flow rate = 6000 lb/hr
Steam cost = $2.07 / klb
Flash loss =
Steam cost =
Annual electrical cost =
Annual feedpump operational cost =
Annual savings =
Pump 3: Steam Consumption is 4500 lb/hr = 4.5 klb/hr
This is the annual cost of running pump 2. These computations leads to the conversion from a steam powered pump to an electric powered pump:
Pump Cost = $70,000
Installation Cost = $20,000
Flow rate = 4500 lb/hr
Steam cost = $2.07 / klb
Flash loss =
Steam cost =
Annual electrical cost =
Annual feedpump operational cost =
Annual savings =
Table 4: Cost analysis of feedwater pumps
Pump 1 / Pump 2 / Pump 3Flow Rate (lb/hr) / 9000 / 6000 / 4500
Stream Cost per klb / $2.07 / $2.07 / $2.07
Flash Loss (klb/yr) / 7182 / 4788 / 3591
Annual Steam Cost / $14867 / $9911 / 7433
Annual Elec. Cost / $2030 / $2030 / $2030
Annual Operational Cost / $16897 / $11941 / $9463
Annual Savings / $94883 / $57623 / $46427
Total Pump Price (including installation) / $270,000
Total Annual Savings / $198,933
Payback Period (T. price / T. savings) / 1.36 years
Analysis of Results
With the conversion from steam powered pump to electric powered pumps led to an annual savings of $198,933. After installing the new electric pumps, the total cost of the proposed investment was $270,000. This led to a very good payback period of about 1.36 years.
Improvement Suggestion #3: Introduce a dual purpose turbine
Currently the Plant produces steam at a pressure of 225 psia to its customers in downtown Duluth. These customers need to throttle the steam down to roughly 3-5 psia so there could be a possible loss of energy (we took average pressure to be 4 psia). The suggestion is that a turbine be added prior to dispersing the steam to the buildings that would not only decrease the pressure of the outgoing steam but also create work in the form of electricity.
Leaving Plant:
P1 = 225 psia h1 = 1206.0 ρ1 = .4738
T1 = 400°F s1 = 1.5427
Entering Building:
P2 = 4 psia T2 = 400°F s2 = s1
h2 =
Energy and Cost analysis:
421,944 of steam sold
421,944 = 13.37
= 3869812.8
= 1131.147 kilowatts
Savings per hour = 1131.147 kilowatts=
Annual Savings = * 24 hours * 250 days = $441,120
Turbine Cost = $325,000 from www.powerplantsonline.com
Payback Period = = .737 years
Figure 3: Image of proposed turbine
Improvement Suggestion #4 / Installation of a Feedwater EconomizerExisting Condition
Flue gas exiting the plant leaves with temperatures ranging from 450F to 650F. Some amount of this heat could be salvaged and turned into some sort of energy to help improve the efficiency of the plant.
Proposed Solution: