Cogeneration Application Considerations
John A. Jacobs III, Technical Leader, Evaluation & Analysis
Martin Schneider, Senior Marketing Manager
Cogeneration or CHP (Combined Heat and Power). The terms cogeneration and CHP are used interchangeably in this paper and are defined as the combined simultaneous generation of heat and electrical energy with a common source of fuel. Common examples of cogeneration applications include pulp and paper mills, steel mills, food and chemical processing plants, and District Heating (DH) applications.
Since the beginning of the 20th century, cogeneration technology has been utilized by many industrial companies as an eco-friendly means to economically meet a plant’s combined heat and power demands. The volatility of fuel costs and electricity prices in deregulated markets—coupled with the need to secure reliable heat and power supplies, along with new environmentally based financial incentives—are driving the evolution of this technology. These key factors are causing many industrial companies, municipalities, developers and utilities to give even more consideration to cogeneration as an eco-friendly, profitable, and reliable means of addressing their specific generation needs while also meeting local environmental regulations.
In the past and certainly prior to 1960, most cogeneration applications were developed based on steam turbine cogeneration systems consisting of conventional fossil-fired boiler(s) in addition to an industrial type steam turbine and/or combinations of industrial type steam turbines. More recent factors have made gas turbine and engine based solutions highly desirable, including:
- Potential economic benefits resulting from higher power-to-heat ratios
- Rising fuel costs
- Operational flexibility
- Emerging environmental policies and incentives
- Increased focus and need for power security
- Availability of a wide range of system integration options coupled with attractive cogeneration system performance levels
These technological advances in the area of fuel flexibility, as well as gas turbine and engine product diversification/adaptation, have served as enablers to make some cogeneration opportunities feasible, while making others even more attractive.
Universal sensitivity to our environment and environmental considerations have led to the development of projects that not only minimize GHG (Green House Gas) emissions, but also help to displace GHG emissions from existing plants as well as other emissions sources. Thus, one of the more significant advantages for gas turbine, combined cycles and gas reciprocating engines is the potential for GHG reductions as compared to less efficient systems. This monetization of GHG reductions serves as a significant driver/incentive for the development of gas-turbine and gas-engine-based cogeneration applications.
Cogeneration applications range from industrial applications such as pulp and paper mills, steel mills, and chemical processing plants to commercial and civic-based applications like hospitals, universities and warehouses—thus encompassing a wide range of unique power-to-heat ratios. The variation of power-to-heat ratio combined with differences in grade/quality of heat (such as water, steam, and process heating/cooling) within the cogeneration application space are dictating both technology selection as well as system and product flexibility requirements.
The primary objectives of this paper are to:
- Review many of the technical considerations and alternative options associated with the development of cogeneration systems.
- Discuss some of the environmental benefits that are potentially available through cogeneration, and to introduce the concept of monetization (primarily surrounding CO2).
- Illustrate and provide the CHP performance characteristics associated with GE’s diverse gas turbine and reciprocating gas engine product portfolios that can ultimately be leveraged for project and technology screening purposes.
The technical parameters provided include—but are not limited to—power-to-heat ratio, equipment capacity (thermal/electrical) and efficiency/FCP (Fuel Chargeable to Power), and/or SFC (Specific Fuel Consumption) in the case of reciprocating gas engines. This paper reviews many of the technical, economical and environmental considerations in the development of cogeneration projects.
Cogeneration is frequently defined as the sequential production of necessary heat and power (electrical or mechanical) or the recovery of low-level energy for power production. This sequential energy production yields fuel savings relative to separate energy production facilities because both the heat and power requirements are satisfied from a common/single fuel source. The heat that would otherwise be wasted in the power production process is recovered and leveraged to provide process heat requirements (which otherwise would have to be generated with a separate fuel source), thus providing significant fuel savings.
With the recent increases in gas and oil prices, advancements in gas-turbine and gas-reciprocating-engine fuel flexibility—combined with a worldwide desire to reduce GHG (Green House Gas) emissions, increase power security (through localization of power generation), and attractive cogeneration system efficiency levels have sparked renewed interest in cogeneration applications. Power can be cogenerated in topping or bottoming cycles. In a topping cycle, power is generated prior to the delivery of thermal energy to the process. Typical topping cycle examples include:
- Non-condensing steam turbine cycles (commonly used in the pulp and paper industry)
- Heat recovery and combined cycles (applied in many chemical plants), where exhaust energy for a gas turbine or heat from gas reciprocating engines provide thermal energy that is ultimately used to satisfy the process requirements
- Central heating/cooling applications that exist in urban locations where electric power stations also supply thermal energy (or similarly on a smaller scale, where heating/cooling requirements are recovered from gas turbine or gas reciprocating engines to satisfy localized, civic or commercial based CHP requirements)
In bottoming cycles, power is produced from the recovery of process thermal energy that would normally be rejected to the heat sink. Typical bottoming cycle examples include:
- Power generation resulting from recovery of excess thermal energy (combined cycle steam turbine output generation)
- Power generation derived from exothermic process reactions, and heat recovery from kilns, process heaters and furnaces. This paper focuses primarily on application considerations for topping cogeneration cycles.
For comparative purposes Figure 1 illustrates energy utilization effectiveness (the percent of total energy output from the cycle which is useful heat and/or power) for a typical non-reheat coalfired utility/industrial plant configuration (three-stage feed water heating with steam conditions of 1450 psig / 950°F [101 bar /510°C] steam conditions vs. a cogeneration facility utilizing the same fired boiler but with a non-condensing steam turbine generator that supplies steam to process. This diagram suggests that relative to the typical coal-fired power generation application (as previously defined) the energy utilization associated with an equivalent cycle with cogeneration can be improved by as much as 35%. This improvement in energy utilization is made possible because the process demand becomes the heat sink for the cogeneration cycle, thus eliminating energy losses associated primarily with the condenser.
This principal is further illustrated by Figure 2, which highlights the influence of decreasing the thermal energy to a process from a steam turbine cycle. As less steam is delivered to process, the electrical output ratio (relative to the electric output at 100% steam-to-process) increases, becoming a maximum of about 2.0 for the steam conditions noted if no steam is delivered to process. The overall efficiency decreases from 84% to 35% as process steam delivery is eliminated.
Similar performance benefits are also available in gas turbine and reciprocating gas engine cogeneration systems. For example, an Fclass technology gas turbine generator with feeding to an HRSG (which in turn provides process steam) can yield overall energy effectiveness levels between 80-85% depending upon process steam conditions. In comparison, the same F-class gas turbine in combined cycle (and producing power only) yields an overall energy effectiveness of between 50-55% depending upon the cycle design. This comparison is illustrated in Figure 3.
It is worthy to note that energy effectiveness as previously defined differs from efficiency/CHP efficiency in that CHP Efficiency is defined as the useful energy-out (combined heat and power) divided by energy-in (energy in the fuel), whereas energy effectiveness also accounts for the energy in the air. By comparison the efficiency/CHP efficiency for gas-turbine-based cogeneration plants are 90+% versus 80+% for a conventional steam plant based cogeneration system.
Figure 1 and Figure 3 clearly illustrate that from a fuel utilization perspective, cogeneration system performances are significantly better than typical steam turbine or gas turbine combined cycles that are designed to only produce power. Today, across the globe, many local governmental incentives have been established to help promote the development of new cogeneration applications with an objective of driving fuel utilization. One such example is the SPP (Small Power Plant) in Thailand. While such incentives are not new (for example, PURPA in the US), the underlying motivations can be different. More often than not, current incentives are borne out of a want, desire and need to reduce green house gas emissions, whereas the motivations of the past may have focused more on fuel utilization from an energy market perspective (deregulation and market competition). In support of today’s market drivers GE not only maintains a position of industry leadership in the areas of gas turbine and gas engine fuel flexibility and emissions capability, but also continues to evolve world-class advanced technology with focused research and development efforts in these areas.
Coincidentally, in the case of the aforementioned regulations a STAG (STeam And Gas) cycle qualification is/was to provide about 6% of its steam generation to process. At this operating condition, the overall performance approaches that of a conventional STAG power generation cycle. Later in this paper, tables are provided that define GE’s gas turbine and gas engine product characteristics, which in turn illustrate the wide application range and flexibility of these products to support cogeneration applications.
For purposes of the following discussions, “thermally optimized” cogeneration systems are defined as those developed using noncondensing steam turbine generators or condensing units operated at minimum flow to the condenser for cooling purposes.
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