03 Power generation by combustion

Cyclic Processes

• Electricity generated by thermal engines

• Heat can never be converted completely into mechanical work —> 2. law of thermodynamics

• Heat must always be dissipated (output)

—> Production of work in the cycle is only possible when there is a heat feeded and dissapated

• Steam power plant

• Steam engine (saturated steam generator)

• ORC process

• Stirling engine

Fuel heat = Power + useful heat + heat losses + boiler losses + electrical personal need

efficiency (el) is not the same as efficiency (fuel)

—> efficiency (fuel) output heat

1: near vacuum

1-2: water —> boiling

2-3: steam and water —> evaporating

3-4-5: steam —> expand in the turbine

4: live steam

left line: saturated liquid line

right line: saturated vapor line

By changing the live steam parameter (4)

• Average temperature (heat input) and efficiency increases by..

  • increased live steam temperature

  • increased evaporation pressure (live steam pressure)

By reheating

• Reheating: First high pressure turbine —> heating —> middle/low pressure turbine —> condensator

• Increases average temp. level of the heat input and efficiency

• Reduces the water content

By regenerative feedwater preheating

• Used medium: Steam from the turbine

• Heat exchanger before the steam generator

—> Changes the mass flow in the condenser (less steam)

—> Changes feedwater inlet temperature/enthalpy (increase of enthalpy)

• Efficiency is improved with CHP

• Electrical efficiency decreases with heat output

• Thermal efficiency dependent on live steam / back pressure and temperature (CHP ≈ 25 bar; 20 %)

• Disadvantage is condenser reduces temperature to ambient temperature

—> The CHP coefficient is the ratio of the generated electrical power Pel or electrical work to the available heat

Distinctions:

• Back-pressure steam turbine (constant)

• Extraction condensing steam turbine (variable)

• Extraction condensing steam turbine with disengageable low pressure part (particularly variable)

• Heat consumers are connected directly to the generator —> no condenser —> heat consumer serve as “condenser”

• Problem: Only power generation when you have a heat consumer —> electrical power depends on available heat —> higher electrical power —> higher available heat

  
  

• Condenser and heat consumer connected to the steam generator —> More flexible heat dissipation

• Condenser is always working

• Higher investment costs

• Condenser don’t need to work all the time —> even if the heat consumer doesn’t need heat

• High pressure —> clutch —> low pressure turbine

• Rankine process —> not that much overheating

• Organic working fluid

• Thermal use at low temperature level (low evaporation and low heat input)

• Biomass furnaces —> thermal oil as medium

• No direct evaporation of flammable liquids —> no thermal overload of the working fluid

  
  

Advantages

• Can be operated at low temperatures (100C) and low pressures (10 bar)

• Simple construction

• “High“ efficiencies —> low power and low temperatures

• Good part-load performance

Disadvantages

• “Only” electrical efficiencies of 10 – 15%

• Quite high costs due to heat exchangers (small temperature differences) —> Big heat exchanger needed because of low heating temperatures

• Suitable for gaseous and liquid fuels, flue gases from solid fuels

• Low pollutant and noise emissions due to adjustable external combustion

Key problems

• Tightness

• Heat transfer (contamination of heating surfaces)

• Low efficiency —> potential for increases

Aim

• Co-firing allows the generation of electricity from biomass with the efficiencies of large coal-fired power plants (> 40 %)

• 10 - 20 % of coal can be substituted

Advantages

• Additional investments are significantly lower than for new plants (steam generator, turbine and exhaust gas cleanup are available)

• Low electricity generation costs —> lower investment costs

• Improves the economic efficiency of coal-fired power plants (e. g. for waste wood or co-firing of sewage sludge)

• Uses the existing, efficient exhaust gas cleanup

• CO2 reduction potential is very high in the short term

Concepts

• Co-firing in coal burners (simplest)

• Grate-firing under the steam generator

• External grate firing and flue gas inlet

• Pre-gasification

• Co-firing in fluidized beds