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

Introduction and Definitions of Cogeneration

Combined Heat and 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 typically involves the simultaneous production of electrical or mechanical energy and useful thermal energy from a single source of fuel.

Input Energy Sources

According to the documentation by Er. Rajiv Parashar and Simons Green Energy, the input energy can come from various sources:

• Natural gas

• Gasoline

• Diesel

• Oil

• Coal

• A variety of waste fuels derived 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.

• Driving rotating equipment such as motors, fans, pumps, and compressors.

Secondary Type of Energy Output

The secondary output is heat energy, which is often a byproduct of combustion. It is utilized for:

• Direct process applications.

• Production of steam.

• Heating.

• Production of chilled water through absorption refrigeration.

System Configuration Categories

Cogeneration plant configurations are categorized into three distinct cycles:

1. Topping Cycle: The fuel energy is first used to produce electricity, and the rejected heat is then used for process or heating applications.

2. Bottoming Cycle: The fuel is first consumed to provide high-temperature heat for an industrial process, and the waste heat from that process is then used to generate electricity.

3. Combined Cycle: This identifies a system that simultaneously utilizes two different thermodynamic cycles (e.g., Brayton and Rankine) to produce electricity from a single fuel source.

Equipment Combinations by Cycle

• Topping Cycle: Typically utilizes a steam generator and a steam turbine OR a gas turbine and a Heat Recovery Steam Generator (HRSG) OR an internal combustion engine (spark-ignited or diesel).

• Bottoming Cycle: Utilizes an HRSG and a steam turbine to capture waste heat from an industrial process.

• Combined Cycle: Employs a gas turbine, an HRSG, and a steam turbine in sequence.

Purpose and Efficiencies of Cogeneration

The fundamental purpose of cogeneration is to reduce energy costs by increasing overall thermal efficiency.

Efficiency Comparisons

• Traditional Plants: Conventional plants using boilers, gas turbines, or internal combustion engines alone typically exhibit an overall efficiency of less than $40\%$.

• Cogeneration Plants: Cogeneration can increase the overall energy efficiency of a plant to a value between $60\%$ and $90\%$.

The Cogeneration Principle Fuels

The plant can run on a vast array of energy sources including:

• Natural gas

• Bioethanol

• Vegetable oil

• Heating oil

• LPG

• Coal

• Biomass

• Biogas

• Municipal waste

• Waste heat

• Geothermal

• Concentrated Solar Power (CSP)

• Nuclear energy sources

• Fuel Cells

Advantages of Cogeneration Systems

Micro (Local) Level Advantages

• Significant reduction in total energy costs.

• Systems are more compact and require less maintenance than conventional plants of similar capacity.

• Much shorter start-up times compared to traditional power plants.

• Highly flexible configurations and sizes to meet specific industrial facility requirements.

Macro Level Advantages

• Utilization of cleaner-burning fuels reduces the load on heavier polluting, coal-fired generation facilities.

• Reduction in peak demand on utility grids.

• Reduction in the emission of greenhouse gases.

• Preservation of global energy reserves.

• On-site electrical production reduces the need for constructing large, expensive utility plants.

• Reduction in transmission line losses due to local production.

Applications of Cogeneration

Commercial and Institutional Area

Commonly applied in schools, hospitals, hotels, universities, colleges, prisons, and malls. These facilities often require electrical demand for $24$ hours a day. Their heating requirements can usually be met with low-temperature heat recovery units. Relevant production costs considered include:

• Cost of fuel.

• Cost of electricity.

• Cost of emissions penalties.

Industrial and Large Scale Area

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

Centralized Heating and Cooling

Localized central plants situated in industrial areas or city centers that supply electrical, heating, and cooling requirements for multiple buildings or small industrial plants.

Detailed Breakdown of Topping Cycles

In a topping cycle, fuel is first used to generate electricity. This occurs in two main ways:

1. Fuel is burned in boilers to generate steam for steam turbines that drive AC generators.

2. Fuel is burned in gas turbines or internal combustion engines to directly drive AC generators.

Back Pressure Steam Turbine

This is a simple-cycle cogeneration system where fuel (gas, oil, coal, or waste) is burned in a boiler. The turbine exhaust steam is controlled at a specific pressure to be used for refinery processes or other industrial needs.

Extraction-Condensing Steam Turbine

Used when only a portion of the exhaust steam is required for process heat.

• Steam is extracted upstream of the condenser at an intermediate pressure using one or more turbine extraction valves.

• The remaining steam continues to perform work through final turbine stages until it reaches the condenser.

• Exhaust gas can be routed to an HRSG to produce more steam for the turbine.

Trigeneration Systems

Trigeneration refers to the sourcing of electricity, useful heat, and refrigeration from a single fuel input. If fuel is burned in a gas turbine or engine, waste heat is recovered for heating or absorption refrigeration (to produce chilled water).

SAGD Example (Steam-Assisted Gravity Drainage)

Used in enhanced oil recovery:

• Fuel is burned in a gas turbine.

• Exhaust heat is recovered by an HRSG to generate high-pressure steam at approximately $10\,MPa$.

• The steam is approximately $22\%$ wet and passes through a steam separator.

• The steam is injected underground to reduce the viscosity of oil/bitumen, allowing it to flow into a producer well as an oil-water emulsion.

Detailed Breakdown of Bottoming Cycles

In a bottoming cycle, the primary heat input is used for process needs. The process may release heat without additional fuel combustion, leading to "Waste Heat Cogeneration."

Waste Heat Cogeneration Principles

• Primary Purpose: Burn fuel for a process, then use waste heat to produce steam in an HRSG for electricity production.

• Fuel sources can include the incineration of municipal waste or wood waste.

• Steel-making (BOF) example: Waste heat is recovered from off-gas exiting a Basic Oxygen Furnace (BOF). The off-gas contains $70\%$ to $80\%$ carbon monoxide (CO) and is approximately $1,200^\circ C$.

• Both sensible heat and chemical energy are recovered through combustion or suppressed combustion methods.

Suppressed Combustion System in BOF

• Off-gas temperature near the furnace is $1450^\circ C$.

• Hood pressure is maintained between $-5$ and $+5\,mm$ water.

• A boiler with a radiation section and evaporator section cools the gas to $1000^\circ C$.

• Further secondary dust collection and induced draft fans cool it to $75^\circ C$ or even $67^\circ C$.

• When CO concentrations are low, gas is flared; when concentrations rise, a three-way valve diverts gas to a storage vessel through a water-sealed check valve.

• High-pressure steam can be stored in large steam accumulators to provide capacity between steel production "heats."

Combined Cycle Cogeneration (Brayton-Rankine)

This cycle combines two thermodynamic cycles—most commonly the Brayton-Rankine cycle—to improve thermal efficiency from approximately $40\%$ to upwards of $70\%$.

Brayton-Rankine Cycle Process

• An open cycle gas turbine (heavy-duty or aeroderivative) drives a generator.

• Fuel is burned in the combustor, and pressurized gas expands through the turbine.

• Exhaust temperatures typically range from $440^\circ C$ to $650^\circ C$.

• An HRSG extracts heat from this exhaust using economizer, evaporator, and superheater sections.

• Arrangement: The superheater is in the hottest gas stream (upstream of the evaporator), while the economizer is in the coolest gas stream (downstream of the evaporator).

Triple Pressure Combined Cycle

Features a triple-pressure HRSG supplying steam to 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 turbine exhaust.

• Low Pressure (LP) cylinder: Receives steam from the LP HRSG combined with IP turbine exhaust.

• All cylinders are on a single shaft driving a generator. Exhaust is condensed and returned via a regenerative feedwater heating system.

Plant Designs: Single-Shaft vs. Multi-Shaft

Single-Shaft Combined-Cycle

Gas and steam turbines are coupled to a single generator on one shaft, often using reduction gears or couplings.

• Benefits: Highest thermal efficiency, lowest capital costs, high operating flexibility (fast start-up), short construction times, and low maintenance costs.

• Disadvantage: The steam turbine may need removal for major generator maintenance (rotor removal).

• SSS Coupling: Single-shaft plants use Synchronous Self-Shifting (SSS) couplings.

  • Allows the gas turbine to start independently.

  • Allows the steam turbine to lock into the generator while it is in operation.

  • Allows the steam turbine to remain on barring gear while cooling down after the gas turbine stops.

Multi-Shaft Combined-Cycle

Consists of two or more turbines on separate shafts.

• Benefits: Ideal for varying power and steam loads. Convenient for maintenance on individual sections without shutting the whole plant. This is the common choice when adding cogeneration to existing simple cycle plants.

Control Strategies and Load Management

Control strategy selection depends on the facility's primary function and size.

Electrical Load Control

1. Base Loaded System: The cogeneration system produces electricity at a fixed maximum level. Any changes in demand are met by the utility grid.

2. Set Electrical Demand: The utility grid supply is fixed, and the cogeneration system modulates its output to meet changing demand.

3. Island Mode: The system is separate from the grid and must supply all site requirements, including peak loads and emergencies. Excess heat may be flared/vented, or auxiliary firing may be used if heat is insufficient.

Thermal Load Control

1. Fixed Minimum Thermal Production: Thermal output is fixed at the site's minimum requirement with the prime mover at full load. Auxiliary boilers meet demand peaks.

2. Varying Thermal Requirement: The system modulates to match process needs. The prime mover output varies, and electrical deficits/surpluses are managed by buying or selling power to the utility grid.

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 increase gas temperatures entering the HRSG to produce more steam when process demands exceed the prime mover's waste heat capacity.

Heat Recovery Steam Generator (HRSG) Detailed Design

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

Types of HRSG

• Vertically Fired with Horizontal Drum: Uses horizontal tubes. Forced circulation with internal recirculation. Includes superheater, evaporator, and economizer sections.

• Horizontally Fired with Vertical Drum: Large triple-pressure systems.

  • HP Steam: $7,000\,kPa$.

  • IP Steam: $2,400\,kPa$.

  • LP Steam: $350\,kPa$.

• Once-Through Steam Generator (OTSG): Can be fired or unfired. Provides saturated steam at $10\,MPa$ for SAGD.

  • Saturated steam is used for its high latent heat during condensation.

  • Boilers produce $78\%$ dry steam to keep dissolved solids concentrated in the liquid phase to prevent tube damage.

  • Pigging: Used to remove carbonaceous and scale deposits.

Environmental Concerns and NOx Mitigation

Environmental impact depends on the fuel and prime mover. Considerations include Nitrous oxides ($NO_x$), Carbon dioxide ($CO_2$), and Sulphur dioxide ($SO_2$).

Nitrogen Oxides (NOx)

• Formation depends on combustion temperature, pressure, geometry, and air/fuel mixture.

• To reduce $NO_x$ in natural gas burning, steam can be injected into the combustion zone (though this reduces thermal efficiency).

• Selective Catalytic Reduction (SCR / DeNOx): This process uses Ammonia ($NH_3$) and a catalyst.

• Optimal Temperature for SCR: $350^\circ C$ to $450^\circ C$.

• The reaction converts $NO_x$ and $NH_3$ into Nitrogen ($N_2$) and water ($H_2O$).

Other Emissions

• CO2: Cogeneration reduces primary fuel consumption by about $35\%$, thereby reducing overall $CO_2$ output.

• SOx: Varies directly with the sulfur content of the fuel. Natural gas is negligible. For diesel or biogas, stainless steel heat exchangers are required to handle corrosive condensates.

Internal Combustion Engine (ICE) Cogeneration

Commonly selected for smaller facilities.

• Fuels: Natural gas, oil, gasoline, or diesel.

• Advantages: Capable of intermittent operation; not affected by ambient temperature changes (unlike gas turbines).

• Heat Recovery Sources:

  1. Hot surfaces.

  2. Exhaust gases ($540^\circ C$ or higher; $30\%$ to $50\%$ is recoverable).

  3. Lubricating oil.

  4. Engine cooling water.

• Recovery Rate: Approximately $70\%$ to $80\%$ of total fuel energy is recoverable.

• Components: Heat recovery mufflers provide both heat capture and sound attenuation. They must be designed to ASME code.

Start-Up Procedures (SOPs)

For a gas turbine with an HRSG topping cycle, the procedure includes six main steps:

1. HRSG pre-start checks.

2. Gas turbine pre-start checks.

3. Generator pre-start checks.

4. Start gas turbine / HRSG.

5. Synchronize generator to the electrical grid.

6. Running checks.