Trigen-Peoples District Energy Company - Executive Summary
Trigen-Peoples District Energy Company provides steam/heating, chilled water/air conditioning for the Chicago Metropolitan Pier & Exposition Authority's (MPEA) McCormick Place Exhibition and Convention Center. Trigen-Peoples is a company jointly owned by Trigen Energy Corporation and Peoples Energy Corporation (the parent company of Peoples Gas in Chicago).
Trigen Energy Corporation, the leading thermal sciences company in North America, develops, owns and operates commercial and industrial energy systems. Trigen uses its expertise in thermal engineering and proprietary cogeneration processes to convert fuel to various forms of thermal energy and electricity at more efficient conversion rates than conventional processes. Trigen combines heat and power generation, producing electricity as a by-product, for use in its facilities and for sale to customers. The Company serves more than 1,500 customer facilities with energy produced at 45 plants in 30 locations, includin
g industrial plants, electric utilities, commercial and office buildings, government buildings, colleges and universities, hospitals, residential complexes and hotels. Trigen's mission statement is: "Provide heating, cooling and electricity with half the fossil fuel and half the pollution of conventional generation." Additional information on Trigen Energy Corporation can be found on their web site: www.trigen.com
Trigen-Peoples District Energy Company integrated existing MPEA heating and cooling equipment with a Thermal Energy Storage (TES) system and three Trigeneration (combined heating, cooling and power) systems. The TES system, the largest chilled water storage tank in North America, (8.5 million gallons) stores cold water produced at night and discharges it to meet daylight peak cooling loads. Fewer chillers are required to meet peak demands and they can be operated continuously, maximizing their production efficiency. The three Trigeneration systems combine a gas turbine,
a motor/generator, a heat recovery steam generator and an ammonia screw compressor chiller. Their design also allows the equipment connected to the gas turbine to produce any combination of electricity, steam or chilled water while the gas turbine runs continuously. Steam absorption chillers utilize steam produced by the Trigeneration systems in the summer.
The efficiency improvements of the integrated facility result in substantial economic and environmental benefits. By using the same fuel twice to produce electricity and other energy products and maximizing the use of all the possible energy from the fuel, the facility is able to achieve fuel conversion efficiencies of 91%. Operational savings of $1 million per year are projected over the life of the project. Additionally, emissions of CO2 are reduced annually by 24,327 tons, SOx by 126 tons annually, and NOx by 59 tons annually when compared to the separate production of these same energy products (steam, cooling and electricity
) by conventional means. By increased efficiency in the cooling process and use of ammonia as a refrigerating agent, 6,500 refrigeration tons of potential ozone depleting refrigerant (CFCs and HCFCs) has been displaced.
Ammonia Refrigeration System
Trigen-Peoples uses pressurized liquid anhydrous ammonia as a refrigerant for portions of its district cooling system. The ammonia is used to cool a chilled water system, which in turn is distributed and cools re-circulated air systems. Trigen-Peoples' ammonia system contains approximately 30,000 pounds of anhydrous ammonia.
At ambient temperature and pressure, ammonia is a colorless gas with a characteristic, irritating odor. Ammonia is highly soluble in water. Ammonia is the third highest-volume chemical produced in the United States, with eighty percent of the ammonia produced used in the manufacture of fertilizers. The EPA defined toxic endpoint for ammonia is 0.14 mg/L (approximately 200 ppm). OSHA's Immediately Dangerous to Li
fe and Health (IDLH) limit is 300 ppm (the facility utilizes 250 ppm as its IDLH point). The OSHA Personal Exposure Limit (PEL) for an 8-hour average is 50 ppm.
Trigen-Peoples incorporates Process Safety Management and Risk Management for its ammonia refrigeration process. Its dedication to safety is reflected in zero lost-time accidents since equipment installation and beginning of operations in 1996. The facility has never had an ammonia leak resulting in an impact on a public or environmental receptor; however, there has been one ammonia discharge requiring leak notification to outside agencies but which did not require emergency response action by these agencies. A written report was submitted to the agencies and is available for review.
Trigen-Peoples' ammonia system utilizes state of the art design, control and protective devices, and systems. All ammonia equipment is located at the central power plant; there is no ammonia distribution piping. All equipment, construction,
operations and maintenance activity meets or exceeds the requirements and/or recommendations of over 13 different regulatory codes and industry guidance regarding ammonia refrigeration systems. Trigen-Peoples only utilizes ammonia compatible products which are designed to meet or exceed the conditions (pressure, temperature, and function) found in their refrigeration system. Trigen-Peoples incorporates a management system to ensure all modifications or other changes to the system meet these stringent criteria. Additionally, the management system ensures only qualified contractors (installation, welding, and other specialists), operators and maintenance personnel are allowed to work on or with Trigen ammonia systems.
Ammonia control systems include redundant monitoring and equipment shut down circuitry. This includes computer-controlled operations, alarms, and/or automatic equipment shut down due to changing process parameters. The computer system is backed up with an independent
"hard wire" system that can independently shut down equipment due to out of range process values. Finally, if it becomes necessary, an operator can activate a "master shut down" switch from the control room or from a remote location.
A preventative maintenance program supplemented by daily (minimum) equipment inspections maintains system mechanical integrity. Problems identified during equipment inspections are addressed immediately. The preventative maintenance program includes regularly scheduled maintenance activities such as equipment testing, clearance checks, vibration checks, periodic overhauls, vessel and pipe inspections and certifications, and lubrication oil and refrigerant quality testing.
Trigen-Peoples operations and maintenance personnel are members of the International Union of Operating Engineers (Local 399) and carry a current Stationary Engineers License. Additionally, they have refrigeration equipment operation and maintenance training and all are licensed re
frigerant reclaimers. Initial ammonia specific training and a continuing education program including industry developments and refresher training support this initial qualification.
All ammonia-related activities (operations, maintenance and construction) are conducted under the aegis of the General Manager. The Plant and Controls Engineer coordinates all facility activity. Direct involvement of the Chief Engineer and the Assistant Chief Engineer (Safety Officer) is found in all ammonia-related activities and personnel utilize facility specific operation procedures in their operation and maintenance of the ammonia equipment.
Every effort is made to prevent ammonia leaks. However, the possibility of a leak does exist and thus Trigen-Peoples has developed an Emergency Ammonia Leak Response plan. Trigen personnel are thoroughly trained in ammonia response and have the necessary equipment maintained on-site to respond immediately and effectively to an ammonia release. Employee
s undergo initial response training and yearly refresher courses that include practice response scenarios utilizing the required protective equipment under adverse conditions. Personnel safety is of primary importance. An important component of the response plan is to, if necessary, shut down all equipment and await backup personnel before attempting to enter a potentially unsafe area. Essentially, production is sacrificed in order to achieve safety.
The key to leak mitigation is the ability to identify leaks as early in their development as possible. Ammonia has a very low odor threshold; in other words, it can be 'smelled' at very low concentrations. All ammonia equipment is staged in a room adjacent to the operations control room and personnel are often entering the area. A small ammonia leak in the room is immediately detected by area personnel and promptly investigated. Ammonia equipment located on the roof of the facility is physically inspected three times a day.
Peoples also utilizes ammonia sensors located throughout the ammonia equipment areas and ventilation air stream. This system is linked with the computer-control system. If a sensor detects an ammonia leak, a series of alarms will activate, alerting the operator not only to the situation but its severity. It also alerts the operator to a changing situation. Upon detection of an ammonia concentration equal or greater than 250 ppm, the system will automatically shut down all equipment in the central power plant (not just the ammonia system equipment) and activate a mandatory evacuation of all non-responding personnel. The ventilation system design mitigates the potential impact of an ammonia release on the public and the environment by dilution, increased atmospheric mixing and the elevated release point.
In the event of a major release, Trigen-Peoples would promptly link with the Chicago Fire Department for removal of the entire ammonia inventory or isolated sections of ammonia inven
tory. This system is called the "fireman's dump". The ammonia is mixed with water and discharged to the sewer system. The ammonia/water discharge ultimately enters one of the city's water systems in which ammonia will not adversely impact the environment. The system is required by the City of Chicago Building Code.
Impact of an Ammonia Release
Trigen-Peoples has identified its worst case scenario as that of an eight-inch pipe failure on the line connecting a set of three evaporative condensers to the pilot thermosiphon receiver. The connection is constructed so that failure of the line would include ammonia liquid contained in the evaporative condensers and the line. This scenario is highly unlikely due to the materials used in the piping (meets all applicable standards and codes), the construction techniques utilized (welding standards, certified welders, etc.), the design of the line (e.g. no vibration or mechanical stress are placed on the line) and the line's protection from
The total volume included in the worst case scenario is 8,059 pounds. As is required, the scenario assumes a wind speed of 1.5 meters/second (3.4 miles/hour), an Atmospheric Stability Class F and an ambient air temperature of 770F (250C). The hypothetical release is calculated as occurring in an urban surrounding (many obstacles in the immediate area), at ground level (although the release would occur atop a two-story building) with no building mitigation. The ammonia volume is discharged over a ten-minute period, resulting in a release rate of 806 pounds of ammonia liquid per minute.
Trigen-Peoples utilized the EPA approved RMP*Comp v1.03 software program for modeling its 'worst case' ammonia discharge scenario. This model estimates the distance to Toxic Endpoint (0.14 mg/L or 200 ppm) to be 1.1 miles (1.8 kilometers). There would be an offsite impact to this release.
It should be noted that RMP*Comp v1.03 software program is general and conservative. In 1996 Trigen-Pe
oples conducted a "Dispersion Modeling for an Ammonia System" study for its operation. This study utilized the DEGADIS v.2.1 software program (a more sophisticated modeling software program). An identical 'worst case' scenario results in an impact radius less than half that calculated by the RMP*Comp program (0.47 mile-DEGADIS compared to 1.1 mile-RMP*Comp). However, there would still be an offsite impact to the release.
The 'alternative release' scenario used is one of an evaporative condenser pressure-relief device (safety) malfunctioning. Again, the RMP*Comp v1.03 software program was utilized for dispersion modeling. As is required, the ammonia dispersion was based on a wind speed of 3 meters/second (6.7 miles/hour), an Atmospheric Stability Class D, at 770F (250C) ambient air temperature. The hypothetical ammonia gas release rate is 72 pounds/minute, at ground level (although the release would occur atop a two-story building), in urban surroundings, with no building mitigatio
The RMP*Comp model estimates the distance to Toxic Endpoint (0.14 mg/L or 200 ppm) for this 'alternative release' scenario to be 0.1 miles (0.2 kilometers) or 528 feet. There would be an offsite impact to this release. Release duration of fifteen minutes, one hour, four hours and eight hours was evaluated. All results showed a distance of 528 feet to the toxic endpoint. This illustrates both the impact of atmospheric mixing but also the limitations of the RMP*Comp model. Since the roof top equipment is inspected every shift, a leak duration of eight hours (480 minutes) is used in the facility's RMP Submittal.