Abstract
This entry discusses the integration of combined heat and power (CHP), otherwise known as cogeneration, with industrial processes. It builds on other entries in this topic that discuss the basics of CHP or cogeneration.
INTRODUCTION
This section discusses the integration of CHP, also referred to as cogeneration, with industrial processes. Topics discussed in this section include:
• Cogeneration unit location
• Industrial electric power systems
• Generator voltage selection
• Synchronous vs. induction generators
• Industrial thermal considerations
• Industrial process examples utilizing unique solutions
• Exhaust gas condensation solutions
• Cogeneration and interruptible fuel rates
• Future considerations
COGENERATION UNIT LOCATION
The location of the cogeneration unit is usually a function of the location of the following:
• Main electric switchgear
• Large electric branch circuit switchgear
• Natural gas lines or oil storage tank
• Thermal processes
• Exhaust stack
• Ancillary services, such as power, water, and sewer
• Utility relay and control location
A location that minimizes the cost of electric lines, piping, and other components required to supply the above services is the optimal solution. Industrial processes and branch circuit locations differentiate the location decision from nonindustrial sites. The cost of these components can have a substantial impact on the project cost.
INDUSTRIAL ELECTRIC POWER SYSTEMS
Industrial electric power systems typically involve multiple levels of voltage reduction through multiple levels of electric transformers. Most of these transformers are owned by the customer. By contrast, small commercial and residential customers are typically provided with one voltage supplied by a utility-owned transformer.
GENERATOR VOLTAGE
The generator voltage depends on the voltage of the industrial customer and the generator size. Manufacturers supply cogeneration units in multiple voltage sizes; however, the size range is generally limited to practical voltages. Because the size of the generator is inversely proportional to the voltage, there is a limitation to the minimum voltage based on the size and cost of the resultant generator.
Generator voltage should generally be matched to either the voltage after the main transformer or the voltage of the branch circuit, where most of the generator output will be utilized. In this manner, the electricity produced will travel through the least amount of transformers. Each time electricity travels through an electric transformer, approximately 2% or more of the electricity is given to transformer losses. When calculating the benefits of electricity production, these losses (or avoidance of these losses) should be considered. In nonindustrial applications, the cogeneration unit is typically installed at the electric service entrance. In industrial applications, the cogeneration unit can be installed in one of the branch circuits.
SYNCHRONOUS VS. INDUCTION GENERATORS
Generators are essentially electric motors in reverse; rather than using electricity, a generator produces electricity. The technology behind both can easily be described as an electric magnet spinning in a casing surrounded by wires.
A prime difference between synchronous vs. induction generators is that the synchronous generator is self-excited. This means that the power for the magnet is supplied by the generator and its control system. By contrast, an induction generator is excited by the utility. When utility power is lost, the loss of excitation power will theoretically shut down the induction generator. However, there is a chance that other electric system components could provide the excitation required to start the generator. Therefore, in addition to the electric utility engineers, a competent professional engineer should approve the design of the cogeneration installation.
Cogeneration systems require protective relays on the power system that will shut down the cogeneration installation in the event of a power interruption. They are not designed to run in power outages. They must be shut down or utility personnel will be endangered when they try to restore power to a nonpowered electric line. Many utilities have less stringent protective relay requirements for induction systems for the reasons explained above. The extra costs, however, can be prohibitive. It is important for owners to determine the costs of protective relays and other interconnection requirements before commencing construction. Many owners have been surprised after the fact and had to spend additional money to correct deficiencies.
Power factor is another consideratcion in the selection of induction vs. synchronous generators. Because induction generators use utility reactive power to excite the generator, the site power factor can degrade significantly. By using more reactive power while simultaneously reducing real (kW) power, there is a double effect on power factor. Why should an owner be concerned about power factor? Because many utilities measure and charge industrial users for reactive power and/or low power factor.
INDUSTRIAL THERMAL CONSIDERATIONS
Most cogeneration installations depend on almost full utilization of the thermal output of the plant. In order to utilize the full thermal output, the thermal usages and processes must be considered carefully. The thermal output each and every hour, as well as the temperature of the thermal output as it compares to the temperature of the industrial processes, must be examined. Industrial thermal considerations include:
• Steam vs. hot water
• The temperature of cogeneration thermal output and industrial processes
Steam vs. Hot Water
The thermal output of gas turbine installations is usually steam, whereas the output of reciprocating engines is generally hot water in the 200°F range. There are exceptions to this general rule. For industrial processes that require higher temperatures for thermal processes, a gas turbine selection may be necessary. As described below, care should be taken to ensure that the thermal output can be utilized. In other words, if 240°F thermal energy is needed, a reciprocating engine may not be able to meet the requirements unless properly designed.
Temperature of Cogeneration Thermal Output and Industrial Processes
As mentioned above, the temperature of the thermal output of the cogeneration system may not be utilized in all thermal industrial processes. Simply put, the cogeneration thermal output should be at a higher temperature that the industrial process temperature. Otherwise, the owner may realize less savings than anticipated.
The engineer must perform a thermal balance to ensure that the thermal output of the cogeneration system is less than the industrial process requirements for all hours. The author likes to ask what the thermal needs are in July at midnight. At that time, comfort heating requirements are nonexistent and many industrial thermal processes may be shut down.
A common error occurs when thermal balance is made on an annual, rather than an hourly, basis. Even if the annual thermal output of the cogeneration system matches the annual thermal input of the industrial thermal processes, there are usually hours when excess thermal output must be discarded.
Thermal storage is a potential solution for the hourly ups and downs of thermal energy requirements. The author has found, however, that this solution often falls short; if more than an hour or two of storage is needed, the amount of storage required is both too costly and impractical.
INDUSTRIAL PROCESS EXAMPLES UTILIZING UNIQUE SOLUTIONS
The author has been involved in many installations where finding a unique thermal application has rescued a financially challenged project. A few of these applications are explained below.
Paper Plant
In a paper plant, three thermal industrial processes were added to existing systems and a steam generation system was designed and added to the reciprocating exhaust gas stream. In general, a better solution is to reduce the cogeneration system size by 10%-20%, rather than to add a complicated steam generation system. A hot water recovery system on the exhaust gas stream is generally more cost effective. However, in this installation, a steam generation system was added. Exhaust gas was fed into a steam generator to produce steam directly at 100 psig. A secondary heat exchanger was added after the steam generators in order to recover additional energy.
The second unique application was a heat exchanger being added to a water pulping tank. Water and paper pulp are added in one of the first steps of the paper-making process. This water/pulp slurry was heated by direct injected steam originally. Water at 200°F was introduced to the tank via a piping serpentine installed in the tank. The water in the tank was heated to approximately 140°F. The large temperature difference allowed for the use of nonfinned piping of a reasonable length. It is very important to note that an energy balance needed to be considered. The steam used in this tank was low pressure steam that had been recovered by a flash steam energy recovery tank. Essentially, this steam was free, and if it could not be utilized elsewhere, the value of the cogeneration hot water would be zero. Because the steam was utilized elsewhere, the energy was useful.
The third unique thermal energy recovery system added to this paper plant was a hot air blower system. In the paper-making process, the water/paper slurry is eventually dried on a steam drum by being drawn over the drum by a continuous paper sheet. Steam in the drum dries off the water in the slurry, leaving only paper. By blowing hot dry air over the paper sheet, the paper dries faster. The first benefit is that less steam is needed because the hot air dries the paper. An even more important benefit is that the speed of the paper sheet in the process can be increased and more paper is produced with the same overhead and labor costs. This financial benefit can dwarf the cogeneration system savings. The physical limitations of the existing system were overcome by the addition of this production enhancing system.
Car Parts Plant
A solution considered in a car parts plant was to direct the engine exhaust directly in a parts drying process. The 1200°F exhaust gas temperature was to be directed into a large process heater, where a 300°F temperature was maintained. One ironic outcome of this solution was that project economics were actually impaired because condensation of the exhaust gas could not be implemented. The benefits of exhaust gas condensation will be described below.
Boiler Plant Air and Water Reheat
Opportunities exist at the central boiler plant at many industrial plants. The boiler water is preheated in a makeup tank and/or condensate tank. Both returning condensate water at 180°F (or less) and makeup water at 60°F represent opportunities to use cogeneration thermal output.
At both of these temperatures, an added opportunity to condense the exhaust gas, as explained later, also exists.
A less practical but potential opportunity exists to preheat the combustion air to the boiler. Each 40°F increase in the boiler air temperature equates to approximately a 1% drop in boiler fuel usage. The boiler air-fuel ratio and the combustion air fan speed must often be reset. This solution is often impractical, costly, and it provides marginal returns.
Fuel Switch on an Heating, Ventilating, and Air Conditioning System
A unique and profitable solution can be to utilize excess cogeneration thermal output for a new usage. At one facility, the author’s company replaced an electric resistance coil in a heating system with a hot water coil. The electric rates were approximately four times the gas rates. In effect, the thermal output had four times the value and effectively boosted the electrical efficiency of the plant. The economic benefit of the displaced electric energy was almost equal to the fueling costs of the cogeneration plant, even though it only amounted to about a quarter of the thermal output of the cogeneration plant.
EXHAUST GAS CONDENSATION SOLUTIONS
The energy that is contained in the input fuel is generally not fully recovered. The more energy that has been recovered from the exhaust gas stream, the lower the temperature of the exiting exhaust gas stream. Approximately 10% of the entire energy that is contained in the fuel can only be recovered if the water in the exhaust is condensed. The same amount of energy that it takes to boil water is available if the reverse process of condensation is conducted. Converting the water vapor in the exhaust gas stream into water releases 10% additional energy. To condense the water in the exhaust, water at a temperature less than 200°F is generally needed.
If exhaust gas is condensed, plastic or stainless steel exhaust stack materials must be used. Care must be taken in design because an exhaust temperature that is too high can melt plastic exhaust stacks. Regular steel cannot be used because sulfur in the fuel and nitrogen in the air can cause the production of sulfuric and nitric acid in the condensate. Both of these compounds are extremely corrosive to normal steel.
COGENERATION AND INTERRUPTIBLE FUEL RATES
In the 1980s and 1990s, when natural gas prices at the well head were low, cogeneration was viewed as a natural way to increase gas companies’ market share. A cogeneration system with a 40% thermal efficiency would result in twice as much, or more, gas usage as an 80% efficient boiler. Further discounts were offered for interruptible fuel rates, wherein a customer agrees to reduce their gas usage upon notification from the gas company. Gas companies were able to offer these interruptible discounts because they were using spare distribution pipe capacity during nonwinter months. In the late 1990s, natural gas fuel prices began to abruptly increase. Many discount gas rates have disappeared since then.
FUTURE CONSIDERATIONS
In the future, gas rates may decrease, making cogeneration more cost effective. Cogeneration gas rates and inter-ruptible gas rates may return, even if for only a few months each year.
Increases in the efficiencies of reciprocating engines, gas turbines, and microturbines are already taking place and should continue. The cost of technologies, such as fuel cells and alternative energy technologies, are also decreasing. Some of these technologies may become cost competitive in the near future. Fuel cells already have higher electrical efficiencies than most present cogeneration technologies, but the cost is presently much higher than turbine and reciprocating engine technology.
CONCLUSION
The integration of CHP or cogeneration into industrial processes offers unique opportunities to optimize revenue and energy savings. The installation of new industrial thermal processes can make a financially challenged cogeneration project feasible. Condensation of the exhaust gas stream can both produce more revenue and increase overall system efficiency by more than 10%. Future advances in technologies, such as fuel cells, may offer additional options.
