Comprehensive Study Guide for Combined Heat and Power (CHP) and Cogeneration Systems

Definition and Energy Output of Combined Heat and Power (CHP)

Combined Heat & Power (CHP), also known as Cogeneration, is defined as the utilization of one form of input energy to generate two (or more) forms of output energy. This process allows for significantly improved energy efficiency by capturing thermal energy that would otherwise be wasted.

Input Energy Sources

Input energy can be derived from various sources, including:

Natural gas, gasoline, diesel, oil, or coal.
A variety of waste fuels from industrial processes.

Primary Type of Energy Output

The primary output is generally Mechanical Energy, which is utilized for:

Driving an alternator for electrical production (generating electricity).
Driving rotating equipment, such as motors, fans, pumps, and compressors.

Secondary Type of Energy Output

The secondary output is Heat Energy, which is a byproduct of combustion. It is utilized for:

Direct process applications.
Production of steam.
Heating.
Chilled water production via absorption refrigeration.

Cogeneration Plant Configurations and Cycles

Cogeneration plant configurations are categorized into three main cycle types based on how they sequence power and heat production:

Topping Cycle: Fuel is first used to produce electricity, and the resulting waste heat is used for process applications.
Bottoming Cycle: High-temperature heat is first used for an industrial process, and the remaining waste heat is then used to generate electricity.
Combined Cycle: Simultaneously utilizes two different thermodynamic cycles (typically the Brayton and Rankine cycles) to produce electricity from a single fuel source.

Equipment Combinations by Cycle

Cycle Type	Equipment Combination
Cogeneration Topping Cycle	Steam generator and steam turbine; Gas turbine and HRSG; Internal combustion engine
Cogeneration Bottoming Cycle	HRSG and steam turbine
Cogeneration Combined Cycle	Gas turbine, HRSG, and steam turbine

Purpose and Fundamental Principles

The central purpose of cogeneration is to reduce energy costs by increasing overall thermal efficiency. Traditional power plants using boilers, gas turbines, or internal combustion engines often have overall efficiencies of less than $40\,\%$ . In contrast, cogeneration can increase a plant's overall energy efficiency to values between $60\,\%$ and $90\,\%$ .

The Cogeneration Principle

A Cogeneration plant can be fueled by:

Bioethanol, Vegetable oil, Heating oil, LPG, Natural Gas, Biogas, Biomass, Coal, Municipal Waste.
It can also run on waste heat, geothermal, CSP (concentrated solar power), and nuclear energy sources.

Technological Components

Prime Movers: Engine/Generator, Gas Turbine/Generator, Steam Turbine/Generator, or Fuel Cell.
Outputs: Power (Electricity) and Heat.

Advantages of Cogeneration Systems

Micro (Local) Level Advantages

Significant reduction in total energy costs.
More compact design compared to a conventional plant of similar capacity.
Requires less maintenance.
Much shorter start-up times than conventional plants.
Wide range of configurations and sizes available to meet the requirements of almost any industrial facility.

Macro (Regional/Global) Level Advantages

Utilization of cleaner-burning fuels reduces the load on heavier polluting, coal-fired generation facilities.
Minimizes peak demand on utilities and subsequently reduces greenhouse gas emissions.
Preservation of energy reserves.
On-site electrical production reduces the need for constructing new, large, and expensive utility plants.
Local production results in a reduction of transmission line losses.

Areas of Application for Cogeneration

Cogeneration typically falls into three general application areas:

1. Institutional and Commercial Establishments

Schools, hospitals, hotels, universities, colleges, prisons, and malls.
These facilities often have a demand for electricity, heating, and cooling for $24$ hours a day.
Heating requirements can normally be met with a fairly low-temperature heat recovery unit.

2. Large Scale Industrial Processes

Pulp and paper mills, petrochemical plants, refineries, gas processing plants, and food processing plants.
These require high-temperature energy for specific processes or low temperatures from refrigeration units.

3. District Energy Centers

Localized central heating and cooling plants situated in industrial areas or city centers.
These supply electrical, heating, and cooling requirements for a number of buildings or small industrial plants.

Simple Cycle Cogeneration Systems

A simple-cycle cogeneration system uses a single thermodynamic cycle. Common types include:

Back Pressure Steam Turbine: Uses high-pressure steam from a boiler; turbine exhaust steam is controlled at a specific pressure for process use. Fuel sources include gas, oil, coal, or waste fuel.
Waste Heat Cogeneration.
Internal Combustion (IC) Engine Cogeneration.
Gas Turbine Cogeneration.

Specialized Topping Cycle Systems

Extraction-Condensing Steam Turbine

This system is used when only a portion of the exhaust steam is needed for process heat.

Steam is extracted upstream of the condenser at an intermediate pressure.
Pressure is regulated by one or more turbine extraction valves.
The remaining steam continues through the final turbine stages to do work until it is exhausted to the condenser.
In a gas turbine configuration, exhaust gas is routed to a Heat Recovery Steam Generator (HRSG) to produce the steam for the turbine.

TriGeneration (CHCP)

Trigeneration is the combination of producing electricity, useful heat, and refrigeration from a single fuel input.

Fuel is burned in a gas turbine or IC engine.
Waste heat in the exhaust is recovered with a steam or hot water boiler.
The recovered heat is used for space heating, absorption refrigeration (chilled water), or industrial processes.

SAGD (Steam-Assisted Gravity Drainage) Application

A specific application of the topping cycle for enhanced oil recovery involves:

A gas turbine burning fuel to produce electricity.
Recovered exhaust heat goes into an HRSG which generates high-pressure steam at approximately $10\,MPa$ .
The steam is approximately $22\,\%$ wet and passes through a steam separator before injection into wells.
The steam reduces the viscosity of underground oil, allowing it to flow and be produced at the surface.

Bottoming Cycle and Waste Heat Cogeneration

In a bottoming cycle, the fuel source is primarily used to meet a process's heat requirements first.

The process may release heat without additional fuel combustion, referred to as Waste Heat Cogeneration.
Common examples include the incineration of municipal waste or wood waste.
Turbine exhaust steam from the HRSG can be used in feedwater heaters or for other purposes.

Application in Steel-Making (Basic Oxygen Furnace - BOF)

Waste heat is recovered from the off-gas as it exits the furnace.
Off-gas contains $70\,\%$ to $80\,\%$ Carbon Monoxide ( $CO$ ) and is approximately $1200^{\circ}\,C$ .
Recovery methods include the combustion method or the suppressed combustion (non-combustion) method.
In suppressed combustion, a three-way valve diverts off-gas to storage when $CO$ concentration is high enough; otherwise, it is flared.
High-pressure steam from the waste heat boilers can be stored in large steam accumulators to provide capacity between production "heats."
This saturated steam is then used in a saturated steam turbine to drive a generator.

Combined Cycle Cogeneration

This involves producing electricity and heat from a system utilizing two different thermodynamic cycles simultaneously with one fuel source.

Common cycle combinations include Diesel-Rankine, Otto-Rankine, or Brayton-Rankine.
The Brayton-Rankine combined cycle is the most common for thermal power generation.
This technology improves overall thermal efficiency from around $40\,\%$ (simple cycle) to upwards of $70\,\%$ (combined cycle).

Brayton-Rankine Cycle Mechanics

Brayton Cycle (Gas Turbine): An open cycle gas turbine drives a generator. Fuel is burned, and hot gas expands through the turbine. Exhaust temperatures range from $440^{\circ}\,C$ to $650^{\circ}\,C$ .
Rankine Cycle (Steam Turbine): Exhaust from the gas turbine enters the HRSG, which absorbs heat in economizer, evaporator, and superheater sections.
The superheater is located in the hottest gas stream (upstream), while the economizer is in the coolest stream (downstream of the evaporator).

Multi-Pressure and Single-Shaft Systems

Triple-Pressure Combined Cycle

Utilizes a triple-pressure HRSG arrangement and a three-cylinder steam turbine:

High-Pressure (HP) Cylinder: Receives steam from the HP HRSG.
Intermediate-Pressure (IP) Cylinder: Receives steam from the IP HRSG combined with HP exhaust.
Low-Pressure (LP) Cylinder: Receives steam from the LP HRSG combined with IP exhaust.
All cylinders are on a single shaft driving a generator. Exhaust from the LP cylinder is returned via a regenerative feedwater heating system.

Single-Shaft Combined-Cycle Power Plant

The gas turbine and steam turbine are coupled to a single generator on a single shaft.

The machines are joined by couplings or reduction gears.
Benefits: Highest level of thermal efficiency, very low capital costs, high operating flexibility (fast start-up), short construction times, and low O&M costs.
Disadvantage: Major maintenance on the generator requires the removal of the steam turbine rotor unless the design allows rotor removal without stripping the turbine.
SSS (Synchronous Self-Shifting) Coupling: Connects the steam turbine to the generator, allowing the gas turbine to start independently to produce power quickly, and allows the steam turbine to lock in once ready.

Multi-Shaft Combined-Cycle Power Plant

Uses two or more gas turbines or steam turbines on separate shafts.

Application: Popular when operational flexibility is required, or when power and steam loads vary significantly.
Maintenance: Easier to perform maintenance on individual sections without shutting down the entire system.

Control Strategies in Cogeneration

Selection of a control strategy depends on the primary function of the system and the facility type.

Electrical Load Control

Grid-Connected Systems:
- Base Loaded System: Electrical production is fixed at maximum; utility grid covers changes in demand.
- Set Electrical Demand: Utility supply is fixed; the cogeneration system fluctuates to meet changing site demand.
Separated (Island) Systems: The system supplies all site requirements. Excess heat may be exhausted to the atmosphere if site heat demand is low.

Thermal Load Control

Fixed Minimum Production: Heat production is fixed at the site's minimum requirement; the prime mover is at full load. Additional heat needs are met by auxiliary boilers.
Varying Requirement: Heat production changes to meet process needs. Power is either purchased from or sold to the utility grid to balance electrical changes.

Diverter and Duct Burner Control

Diverter Valve: Used during start-up or low thermal demand to vent exhaust gases to the atmosphere rather than the HRSG.
Duct Burner (Auxiliary Firing): Modulates to provide additional thermal energy once the diverter is fully engaged to the HRSG. This increases steam production and controls superheater temperatures.

Heat Recovery Steam Generator (HRSG) Varieties

Also known as Waste Heat Recovery Boilers (WHRB) or Turbine Exhaust Gas (TEG) boilers.

Circulation Types: Can be Forced or Natural circulation.
Vertically Fired HRSG with Horizontal Drum: Uses a configuration of superheater, evaporator, and economizer tubes. Employs a boiler circulation pump (forced circulation).
Horizontally Fired HRSG with Vertical Drum: Often used in large, triple-pressure plants producing steam at various levels (e.g., $7,000\,kPa$ , $2,400\,kPa$ , and $350\,kPa$ ).
Once-Through Steam Generator (OTSG): Used for SAGD processes. They provide high-pressure saturated steam ( $10\,MPa$ ). These boilers produce very wet steam (about $78\,\%$ dry) to keep dissolved solids concentrated in the water phase to prevent scaling. "Pigging" is used to remove deposits from the tubes.

Environmental Concerns and Mitigation

Nitrogen Oxides ( $NO_x$ )

Emissions depend on combustion temperature, pressure, geometry, and air/fuel mixture.
To reduce $NO_x$ in natural gas combustion, steam can be injected into the combustion zone (this reduces thermal efficiency).
Selective Catalytic Reduction (SCR / DeNOx): Uses Ammonia ( $NH_3$ ) and a catalyst at an optimal temperature of $350^{\circ}\,C$ to $450^{\circ}\,C$ to convert $NO_x$ into Nitrogen ( $N_2$ ) and water ( $H_2O$ ).

Carbon Dioxide ( $CO_2$ )

Cogeneration reduces primary fuel consumption by about $35\,\%$ , proportionally reducing $CO_2$ emissions compared to separate heat and power generation.

Sulphur Dioxide ( $SO_x$ )

Emissions vary with the sulphur content of the fuel. Natural gas has negligible sulphur, but diesel and biogas require emission controls.
Stainless steel heat exchangers may be necessary for fuels with sulphur.

Internal Combustion Engine (ICE) Cogeneration

Fuels: Natural gas, oil, gasoline, or diesel.
Components: Paired with an electrical generator and a Heat Recovery Water Heater (HRWH).
Heat Recovery Sources: Heat is recovered from hot surfaces, exhaust gases ( $540^{\circ}\,C$ or higher), lubricating oil, and engine cooling water.
Efficiency: Approximately $70\,\%$ to $80\,\%$ of fuel energy can be recovered. Roughly $30\,\%$ to $50\,\%$ of exhaust heat is recoverable.
Heat Recovery Muffler: Captures exhaust heat while providing sound attenuation. It utilizes a modulating bypass diverter to control exhaust flow.

Start-Up Procedures (SOPs)

The standard start-up for a gas turbine with an HRSG topping cycle consists of six major steps:

HRSG pre-start checks.
Gas turbine pre-start checks.
Generator pre-start checks.
Start gas turbine / HRSG.
Synchronize generator to the electrical grid.
Running checks.