Cogeneration Systems and Operation Practice Flashcards

A set of 100 question and answer flashcards covering cogeneration systems, cycles, components, environmental considerations, and operation procedures.

What is the definition of cogeneration?

The simultaneous production of more than one useful energy output from a single energy source.

What is another common name used for cogeneration?

Combined heat and power (CHP).

What are the three categories of cogeneration plant configurations?

Topping cycle, bottoming cycle, and combined cycle.

What is the typical thermal efficiency of a traditional simple cycle plant?

Approximately $40\%$.

Cogeneration can increase thermal efficiency to what percentage range?

Between $60\%$ and $90\%$.

What are the two primary electrical energy outputs mentioned for cogeneration plants?

Electricity and heat (for heating, cooling, or both).

In a topping cycle, what is the fuel energy used for first?

To produce electricity.

What type of steam turbine is used if all exhaust steam is directed to process heat?

A back pressure (noncondensing) steam turbine.

What type of steam turbine is used if only a portion of exhaust steam is needed for process heat?

An extraction-condensing steam turbine.

What is a trigeneration plant?

A plant where electricity, useful heat, and refrigeration are sourced from a single fuel input.

In a bottoming cycle, what is the primary use of fuel energy?

To meet process needs.

What is 'waste heat cogeneration'?

A process where the process releases heat without the combustion of fuel, which is then reclaimed for electricity generation.

In a basic oxygen furnace (BOF), what is the concentration of carbon monoxide ($CO$) in the off-gas?

$70\%$ to $80\%$.

What is the temperature of the off-gas exiting a basic oxygen furnace?

Around $1200^{\circ}C$ or hotter.

What is the autoignition temperature of carbon monoxide ($CO$)?

$609^{\circ}C$.

In the combustion method of BOF heat recovery, what pressure is the steam generated?

Around $4400\,kPa$.

What is the purpose of steam accumulators in a basic oxygen furnace bottoming cycle?

To provide steam capacity between intermittent steel production 'heats.'

What thermodynamic cycle combinations define a combined cycle?

Diesel-Rankine, Otto-Rankine, or Brayton-Rankine.

Which combined cycle combination is most common in thermal power generation?

The Brayton-Rankine combined cycle.

How much can combined-cycle technology improve overall thermal efficiency compared to a simple cycle?

From about $40\%$ to upwards of $70\%$.

What is the exhaust temperature range for a typical gas turbine?

Around $440^{\circ}C$ to $650^{\circ}C$.

What is the maximum exhaust gas mass flow mentioned for a gas turbine?

Upwards of $500\,kg/s$.

Where is the economizer located in an HRSG relative to the gas stream?

Downstream of the evaporator section, in the coolest gas stream.

Where is the superheater located in an HRSG relative to the gas stream?

Upstream of the evaporator section, in the hottest gas stream.

A HRSG operating at $14\,MPa$ has a saturation temperature of approximately what?

$337^{\circ}C$.

Describe the arrangement of a triple-pressure HRSG.

A high-pressure (HP) HRSG closest to the gas turbine, followed by an intermediate-pressure (IP) HRSG, and then a low-pressure (LP) HRSG furthest downstream.

What is a '$2 \times 1$ arrangement' in a combined-cycle plant?

Two gas turbines feeding a single HRSG.

What is the total turn down capability of a $2 \times 1$ arrangement?

Close to $85\%$.

Why are duct burners used in cogeneration plants?

To increase steam production and modulate plant electrical output.

Why do duct burners not require additional combustion air?

Because gas turbine exhaust contains ample excess oxygen for complete combustion.

What characterizes a single-shaft combined-cycle power plant?

Two prime movers (gas and steam turbines) drive a single generator with all shafts aligned and coupled.

What is the function of a sliding base for a steam turbine in a single-shaft plant?

To facilitate rotor removal from the generator during major maintenance.

What does 'SSS coupling' stand for?

Synchronous self-shifting coupling.

What is one role of an SSS coupling during gas turbine start-up?

It allows the gas turbine to start up and produce electricity quickly and independently of the steam turbine.

Why does an SSS coupling allow a steam turbine to remain on barring gear?

Because the steam turbine takes longer to cool down after the gas turbine has stopped.

Compared to multi-shaft plants, name three benefits of single-shaft plants.

Greater thermal efficiency, lower capital costs, and a smaller footprint.

Name three shared balance of plant (BOP) components in a single-shaft design.

The generator, the generator transformer, and the electrical bus.

What defines a multi-shaft combined-cycle power plant?

Two or more gas or steam turbines on separate shafts, each driving a dedicated generator.

When is 'multi-shaft' a popular configuration?

When flexibility is required or when adding cogeneration to an existing simple cycle plant.

What does 'internal use power generation' refer to?

A plant that generates all its own power on-site and is not connected to the electrical grid.

In a 'base loaded system', how is electrical production set?

Electrical production is fixed at its maximum production.

How does a 'set electrical demand system' differ from a base loaded system?

The electrical supply from the utility grid is fixed, and the cogeneration system responds to changing plant needs.

What is the function of a diverter in thermal load control?

It restricts exhaust gas flowing to the HRSG by venting it to the atmosphere during low thermal demand.

At what point does a duct burner typically fire in a diverter-equipped system?

After the diverter has repositioned to allow maximum exhaust gas through the HRSG and thermal requirements still increase.

What is the primary heat source for a heat recovery steam generator (HRSG)?

Gas turbine exhaust or internal combustion engine exhaust.

Where is the horizontal type of HRSG gas flow most common?

North America.

Where is the vertical type of HRSG gas flow most common?

Europe.

What type of circulation is typically used in vertical gas flow boilers?

Forced recirculation.

What are three typical functions of duct burners in a fired HRSG?

To increase steam production, control superheated steam temperature, and control steam quality.

What is the pressure of the high-pressure drum in the sketched triple-pressure HRSG (Figure 20)?

$7000\,kPa$.

What is the pressure of the intermediate-pressure drum in Figure 20?

$2400\,kPa$.

What is the pressure of the low-pressure drum in Figure 20?

$350\,kPa$.

What is a specialized type of HRSG that lacks boiler drums?

A once-through steam generator (OTSG).

What is the purpose of 'pigging' in heavy oil recovery OTSGs?

To remove carbonaceous deposits, scale, and silica from the tubes.

Why do OTSGs in SAGD processes produce steam that is $78\%$ dry?

So that dissolved solids remain concentrated in the water portion to prevent boiler deposits.

What role does a large steam separator play downstream of an OTSG?

It ensures only dry, saturated steam goes to the wellhead while impurities are concentrated in the water blown off.

What determines the environmental impact of a cogeneration system?

The type and amount of fuel used and the type of prime mover.

Which nitrogen oxides ($NO_x$) formation is primarily due to high combustion temperature?

Nitrogen oxides produced when fuel burns in air.

What concentration of $NO_x$ is considered an acceptable limit for modern combustors?

$20\,ppm$ or less.

How do steam and water injection reduce $NO_x$ formation?

By lowering the combustion temperature.

What is a disadvantage of water injection regarding component purity?

The water must be ultra-pure to prevent deposits in the combustor or on the turbine blades.

What are DLN combustors?

Dry low $NO_x$ combustors that use proprietary techniques to reduce emissions without steam or water injection.

What does the term SCR stand for in emission control?

Selective catalytic reduction.

What chemical is injected prior to a catalyst in an SCR system?

Ammonia ($NH_3$).

What is the optimal temperature range for SCR applications?

Between $350^{\circ}C$ and $450^{\circ}C$.

To what levels can SCR reduce $NO_x$ emissions?

From $150\,ppm$ to less than $10\,ppm$.

Why is fuel sulfur content a concern for heat recovery systems?

It can produce acidic condensate that is corrosive to heat exchangers and flues.

Which fuel type produces negligible $SO_2$?

Natural gas.

What is the typical power range for internal combustion (IC) engines used in CHP?

$20\,kW$ to more than $10\,MW$.

Name the four sources of recoverable heat from an internal combustion engine.

Hot surfaces, exhaust gases, lubricating oil, and engine cooling water.

What percentage of fuel energy is rejected by the engine coolant system in an ICE?

About $30\%$.

What percentage of the exhaust heat from an IC engine is recoverable?

$30\%$ to $50\%$.

What can the exhaust temperature of an IC engine reach?

$540^{\circ}C$ or higher.

What is a 'heat recovery muffler'?

A device that captures IC engine exhaust heat while providing sound attenuation.

What products can be produced by a heat exchanger recovering heat from IC engine coolant?

Hot water, hot oil, hot glycol, or low-pressure steam.

What grade of heat is typically rejected by the engine lubricating oil system?

Lower grade heat (below $70^{\circ}C$).

According to Figure 24, what is the temperature of the exhaust after the diverter but before the boiler?

$450^{\circ}C$.

What ASME code is used for the design of high-pressure steam boilers and heat recovery mufflers?

ASME code.

List the six main steps for starting a gas turbine with a HRSG topping cycle.

1. HRSG pre-start checks, 2. Gas turbine pre-start checks, 3. Generator pre-start checks, 4. Start gas turbine/HRSG, 5. Synchronize generator, 6. Running checks.

Name three items confirmed during HRSG pre-start checks.

Access doors are closed, valves are in start-up positions, and drum water levels are correct.

What is confirmed regarding the lube oil tank during gas turbine pre-start?

That the level in the lube oil tank is ready for start-up.

Which system must be confirmed ready for heavy duty gas turbines during pre-start?

The jacking oil system.

What is the boiler purge sequence during step 6 of the gas turbine start-up?

The turbine rotates at a speed that produces airflow through the unit for $20\,minutes$ or longer before light-off.

When do the igniters fire during the gas turbine start-up?

At the start-up speed after the purge completes.

At what speed does the jacking oil system disengage during the start-up sequence?

At the appropriate speed after the fuel valve opens to the start position.

What happens to vibration sensors as the rotor passes through critical speed?

The vibration system disengages.

Who must the operator call to obtain permission to synchronize the generator to the grid?

The electrical grid system control.

What running check should be performed on the instrument air system?

Check the system operation and dew point measurement.

Why must the combustion air inlet system be checked during operation?

To check for plugging and icing conditions.

In industrial topping cycles, what is traditionally done with exhaust steam instead of condensing it?

It is used for process heat.

What characterizes 'suppressed combustion' in steel making?

Minimizing air infiltration to prevent $CO$ combustion while recovering sensible heat.

What pressure is the high-pressure steam generated by the HRSG in the SAGD example?

About $10\,MPa$.

What is the wetness percentage of steam entering a separator in a SAGD plant according to step 8 description?

Around $22\%$ wet.

What are 'balance of plant' (BOP) components?

Supporting mechanical and electrical components such as transformers and lube oil systems.

What is the primary benefit of 'local' electric power production?

Reduced transmission line losses and reduced need for new transmission lines.

How does IC engine output react differently to atmospheric changes compared to gas turbines?

IC engine output is less affected by changes in atmospheric temperature and pressure.

What is an HRWH?

A heat recovery water heater.

Define the combustion method of BOF heat recovery.

$CO$ is allowed to combust in the top hood with combustion air, liberating heat for high-pressure steam generation.

What happens to the vibration sensors once the turbine is at idling speed for warm-up?

The vibration sensors re-engage.

What are the three cylinders in a compound steam turbine described for triple-pressure HRSGs?

$\text{High-pressure (HP)}$, $\text{intermediate-pressure (IP)}$, and $\text{low-pressure (LP)}$ cylinders.