The purpose of the author is to explain the technical possibilities for reducing CO2 emissions from solid fuels. The basics of steam generation are presented and the design principles of a conventional state-of-the-art steam power plant are explained.
List of figures xix 4.70 Boiler loss as a function of boiler outlet temperature and air conditions. List of Figures xxv 6.39 Influence of excess airflow on efficiency (Gohlke and Spliethoff. 2007).
List of Tables
List of Symbols
Indices
Motivation
Primary Energy Consumption and CO 2 Emissions
- Development of Primary Energy Consumption in the Past 40 Yearsin the Past 40 Years
- Developments Until 2030
For example, the reference scenario of the International Energy Agency (IEA) in 2006 assumes a growth of the world population to 8.1 billion to the year billion), an increase in gross national product of 4% at the beginning, stabilizes at 2 .9% per annum, and natural oil prices are just over $60 a barrel (2005 fair price). Fossil energy sources will continue to account for more than 80% of primary energy consumption in 2030, with crude oil still being the predominant source of energy, with a crude fraction of approximately 35%.
Greenhouse Effect and Impacts on the Climate
- Greenhouse Effect
- Impacts
- Scenarios of the World Climate
Overall, CO2 emissions over the years have led to an increase in CO2 concentrations in the atmosphere and thus to an increase in the CO2 reservoir in the atmosphere. The source of uncertainty lies on the one hand in the uncertainties of the climate model calculations.
Strategies of CO 2 Reduction
- Substitution
- Carbon Capture and Storage (CCS)
- Energy Saving
- Mitigation Scenarios
Carbon dioxide capture and transport to a storage location involves a marked reduction in efficiency of 8-10%. Primary energy serves to provide useful energy or energy services in the form of process heat, room heat, driving force or light.
Solid Fuels
Fossil Fuels
- Origin and Classification of Coal Types
- Composition and Properties of Solid Fuels
- Petrographic Analysis
- Reserves of Solid Fuels
The weight of ash (residues from combustion) is lower than the weight of the original mineral content. When determining the content of volatile substances, part of the decomposition products of combustion is taken into account.
Renewable Solid Fuels
- Potential and Current Utilisation
- Biomass from Farming and Forestry By-Products of Farming
- Wastes
- Refuse-Derived Fuels
- Sewage Sludge
- Considerations of the CO 2 Neutrality of Regenerative Fuels
- Comparison of Miscanthus and Hard Coal on a Greenhouse Gas Emissions Basis
- Fuel Characteristics of Biomass
- Biomass from Farming and Forestry Molecular Structure
- Waste
- Refuse-Derived Fuel (RDF)
- Sewage Sludge
Of the gross yield of straw, however, it is only a fraction that can be used for energy purposes – the fraction that remains after agricultural uses have been exhausted. When processing trunk wood, residues are produced especially at sawmills and when processing the timber. The total potential is 570 PJ/year, corresponding to a fraction of the primary energy consumption of 4%.
The distinction between recovery (or utilization) and disposal is based on the energy efficiency of the process. In late summer, perennial grass plants such as Miscanthus transfer nutrients (nitrogen, potassium, phosphorus) from the sprouts to the rhizome, as a result of which the nitrogen content in the above-ground plant material decreases (Lewandowski 1996). When straw is stored outdoors, most of the chloride is leached out by rain (Wieck-Hansen 1996).
The low fusion temperatures of herbaceous biomass can be attributed to the composition of the inorganic components of the ash. The fuel properties of residual waste vary greatly from region to region based on the relative proportions of material groups (such as plastic, paper, cardboard, wood and organic matter) in the waste. It can be noted that the calorific value of thermally dried digested sludge (range A) is generally below 11 MJ/kg.
Thermodynamics Fundamentals
Cycles
- Carnot Cycle
- Joule–Thomson Process
- Clausius–Rankine Cycle
The thermal efficiency (the ratio of useful work to input heat) is calculated for the Carnot cycle as follows:. hence the thermal efficiency of the reversible Carnot cycle, also called the Carnot factor, depends only on the constant temperatures of heat input and output. Of the equations of state for the isentropes of the process. 3.11) and by transforming it, one gets the following expression for the thermal efficiency of the ideal Joule–Thomson process: The efficiency of the ideal Joule–Thomson process is therefore only dependent on the pressure ratio.
However, an increase in the pressure ratio also results in an increase in the temperature T2. Irreversibilities in the compressor and turbine are defined using the isentropic compressor efficiency. In the turbine, the steam provides mechanical work, whereas work must be added to raise the pressure in the feed water.
The thermal efficiency of the Clausius – Rankine cycle thus becomes greater the higher the thermodynamic mean temperature of the heat supply and the lower the thermodynamic mean temperature of the heat extraction. In the Clausius-Rankine cycle, feed water preheating, evaporation and superheating inevitably result in a lower average temperature of the heat input, so the efficiency of the Rankine cycle is lower than the Carnot factor. As in any technical plant, there are losses in steam-electric power plant processes, which make a reversible course of the Clausius-Rankine cycle impossible.
Steam Power Cycle: Energy and Exergy Considerations
- Steam Generator Energy and Exergy Efficiencies
- Energy and Exergy Cycle Efficiencies
- Energy and Exergy Efficiency of the Total Cycle
The efficiency of the evaporator is determined mostly indirectly - by the losses of the evaporator. The cycle efficiency ηth, in contrast to the efficiency of the lossless process ηth,0, is reduced due to frictional expansion losses in the turbine. The efficiency of the turbogenerator, unlike the thermal efficiency of the cycle, also takes into account the losses that occur in the turbine and the generator.
Losses due to the sensible heat of flue gases and due to radiation are taken into account both in energy and exergy efficiency. Analogous to the energy efficiency of the Clausius–Rankine cycle:. 3.48) it is possible to define the exergy efficiency:. 3.49). This efficiency determines how much of the exergy absorbed in the steam generator is converted into useful work.
The exergy loss of the feed pump, eL12, is small in contrast to the exergy loss of the steam turbine, eL34. The exergy loss of a steam turbine depends on the isentropic efficiency of the turbine. However, there are clear differences when considering the efficiency of the steam generator and the thermal efficiency of the cycle.
Steam Power Stations for Electricity and Heat Generation
Pulverised Hard Coal Fired Steam Power Plants .1 Energy Conversion and System Components.1 Energy Conversion and System Components
- Design of a Condensation Power Plant
- Reference Power Plant
The ease of grinding pulverized coal is adjusted according to the requirements of the roaster. Complete fuel combustion is achieved by injecting secondary air, preheated to 300–400◦C, into the furnace. In air heaters, the flue gases transfer their residual heat to the combustion air, during which they are cooled to the temperature of the steam generator outlet gas.
In the steam generator, the energy released in combustion is transferred to the steam-water cycle, and the enthalpy of the steam is converted into mechanical work by the turbine. The steam-water cycle is an essential parameter in the overall design of the power plant. The thermodynamic data of the water-steam cycle are the basis for the configuration of the steam generator and the turbine and determine the efficiency of the power plant.
The high-pressure feed water pump sets the working pressure in the water-steam section of the boiler and transports the feed water to the boiler inlet through the high-pressure preheaters, which are heated by steam from the high-pressure turbine extraction stages. The superheater heats the steam coming out of the evaporator to the superheater outlet temperature, i.e. the turbine inlet temperature level is slightly lower, in relation to the amount of temperature drop in the connected high pressure steam lines.
Steam Generators
- Flow and Heat Transfer Inside a Tube
- Evaporator Configurations
- Natural Circulation
- Forced Circulation
- Once-Through Systems
- Steam Generator Construction Types
- Single-Pass Boilers and Two-Pass Boilers
- Operating Regimes and Control Modes
- Operating Regimes
- Primary, Secondary and Tertiary Control
- Constant-Pressure and Sliding-Pressure Operation
- Impacts on the Turbine by Sliding-Pressure or Constant-Pressure Operation
- Impacts on Circulation or Once-Through Steam Generators by Sliding-Pressure or Constant-Pressure Operation
- Start-Up
Both the temperature of the steam flow and the temperature of the tube subsequently increase. In the evaporation region, this configuration can lead to the formation of a transition zone where only part of the. Based on the operating system of the power plant, the number of start-ups must be specified in the design phase.
In sliding pressure operation, the turbine output and steam flow are adjusted by the pressure at the boiler outlet. Disadvantages are changes in the boiling temperature in the evaporator due to the pressure changes. The influence of the control mode on the temperature in the high pressure section is shown in fig.
This means that in sliding pressure operation, the steam generator determines the dynamic power performance. By means of a short increase in the feed water flow, the pressure can be accelerated and cooling of the superheater ensured. The effect of different control modes on the heat rate is described in Sec.
Design of a Condensation Power Plant
- Requirements and Boundary Conditions
- Fuel
- Operating Regime
- General Conditions and Official Directives
- Efficiency
- Costs
- Serviceability
- Thermodynamic Design of the Power Plant Cycle
- Heat Balance of the Boiler and Boiler Efficiency
- Design of the Furnace
- Cross-Sectional Area Heat Release Rate
- Burner-Belt Heat Release Rate
- Calculation of the Flue Gas Cooling
- Design of the Steam Generator and of the Heating Surfaces
- Impact of the Live Steam Pressure
- Design of the Evaporator
- Evaporators with Vertical Internally Rifled Tubes
- Design of the Convective Heating Surfaces
- Air Preheater
- Design of the Flue Gas Cleaning Units and the Auxiliaries
- Design of the Flue Gas Cleaning Units
- Design of the Auxiliaries
The design of the furnace, steam generator and other components depends on the fuel. The results of these calculations are the mass flow data needed for furnace and steam generator design (Stultz and Kitto 1992). The design of the furnace partly determines the type of construction and the size of the steam generator.
The components in the flue gas path following the furnace are the convective heating surfaces of the superheater, reheater and economizer. This property depends on the mass flow density and fluid friction of the fluid involved. The units in the flue gas path following the furnace are the convective heating surfaces of the superheater, reheater and economizer.
As the temperature decreases, the pipe spacing in the direction of the flue gas flow narrows. 140 4 Steam power plants for the production of electricity and heat Energy losses of flue gases of the evaporator. The combustion air temperature (air preheating temperature) depends on the requirements of the combustion chamber.
Possibilities for Efficiency Increases in the Development of a Steam Power Plant
- Increases in Thermal Efficiencies
- Increasing the Live Steam and Reheater Steam Conditions, Single or Double Reheating and Reheater Spraying
- Influence of Feed Water Preheating
- Lower Heat Dissipation Temperatures – Optimisation of the “Cold End”
- Reduction of Losses
- Internal Turbine Efficiency and Losses
- Steam Generator Losses
In this case, reheating increases the medium temperature of heat addition from the steam generator. If the cold reheat temperature is lower, at least part of the heat addition in the reheater results in lower efficiency. With the live steam pressure and superheater outlet temperature remaining constant, preheating the feed water increases the average temperature of the heat input.
The increase in feed water outlet temperature is against the limiting factors regarding the steam generator design. Because of the pumps required, direct contact heaters are only used in the feedwater tank for deaeration. Low discharge steam temperatures and pressures in the condenser can be imposed by low cooling medium temperatures.
Losses in the exhaust steam are taken into account in the internal efficiency of the turbine. With the heating and buoyancy of the air, a convective flow is formed in the cooling tower - this. Final temperature difference (TTD) of the capacitor: In the example, the difference is tC−tW2=1.5◦C.
