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The Chemistry of Fluidized Bed Combustion Systems

  • ID: 3195209
  • Book
  • May 2020
  • 320 Pages
  • John Wiley and Sons Ltd
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Fluidized bed combustion (FBC) systems burn low–grade fuels in an efficient and environmentally friendly manner. By using a heated bed of sand–like material, fluidized within a rising column of air, these systems can burn many types and classes of fuel, including low grade coals and waste material. Suspending the fuel in the fluidized bed enhances both the rate and efficiency the combustion process and reduces emissions of sulphur and nitrogen oxides, to the point that these systems offer one of the most promising energy generation options available today.

Written as an authoritative guide for engineers and chemists in development programs in industry, academe, consulting and government laboratories, Chemistry of Fluidized Bed Combustion Systems will also be a key reference source for advanced courses in chemical and mechanical engineering.
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1 Introduction to FBC.

1.1 Fluidization (FPreto).

1.1.1 Onset and regimes.

1.1.2 Regime map.

1.1.3 Identification of conditions (T, P, particle size, density).

1.2 Bubbling Beds.

1.2.1 Bubbles and two–phase theory.

1.2.2 Splash zone.

1.2.3 Freeboard and elutriation.

1.2.4 Suspension density profile.

1.2.5 Temperature profiles and heat transfer.

1.3 Circulating Beds.

1.3.1 Dense zone and transition zone.

1.3.2 Clusters and wall layer in riser.

1.3.3 Exit geometry and suspension density profiles.

1.3.4 Hot cyclone and solids return.

1.3.5 Temperature profiles and heat transfer.

1.4 Combustion of fuels in fluidized beds (EJA, EMB).

1.4.1 Homogeneous and heterogeneous combustion.

1.4.2 Stages of combustion of a typical fuel.

1.4.3 Advantages of FBC. In–bed sulphur capture. Possible lower NOx emissions, and staging. Fuel flexibility.

2 Combustion of Carbonaceous Fuels (EJA).

2.1 The devolatilization process.

2.2 Combustion of volatiles.

2.3 Char combustion.

2.4 Fragmentation and attrition.

3 Combustion Characteristics of the Major Fossil and Waste Fuels.

3.1 Characterizing solids fuels (VM, Fixed C, Ash, coal ranks and variants).

3.2 Coal.

3.3 Petroleum coke.

3.4 High ash fuels.

3.5 Waste materials.

4 FBC Ash and Ash Behaviour.

4.1 FBC ash morphology.

4.2 Ash formation and classification.

4.2.1 Bed and fly ash.

4.3 Fate of mineral matter, i.e., stone and integral ash.

4.4 Ash chemical and phase composition.

4.5 Catalytic properties of fuel ash.

5 Sulphur Chemistry in FBC Systems (EJA).

5.1 Sulphur in coal and other fuels.

5.2 Thermodynamics of SO2 capture by calcium–based sorbents.

5.2.1 Effect of T.

5.2.2 Effect of CO2 pressure.

5.2.3 Effect of other solid phases (artificial sorbents, Al ?).

5.3 Sulphur capture by sorbents.

5.3.1 Reaction mechanisms.

5.3.2 Reaction kinetics.

5.3.3 Experimental evidence. Effect of Ca:S, particle size and porosity, T, P, sorbent type.

5.4 Sorbent particle structure.

5.4.1 Element/phase distribution.

5.4.2 Anhydritic layer, unreacted CaO core.

5.5 Sulphur capture by fuel ash.

5.6 Fuel ash sorbent interactions.

5.7 Formation and destruction of sulphides.

5.7.1 Characterization of reduced S forms.

5.7.2 Reactions responsible for sulphide formation (anhydrite, intermediates).

Formation in practical FBC systems (bed, cyclones, stripper coolers).

5.7.3 Destruction of sulphides.

5.8 Increasing sorbent utilization.

5.8.1 Sorbent reactivation. hydration. effect of inorganic salts.

5.8.2 Effect of carbonation.

5.8.3 Sorbent selection and sizing.

6 Nitrogen Oxides in FBC Systems (FWinter).

6.1 NOx formation in FBC.

6.1.1 Thermal NOx and prompt NOx.

6.1.2 NOx derived from fuel–N (char and volatiles).

6.2 NOx destruction in FBC.

6.2.1 Interactions with char.

6.2.2 Interactions with CO.

6.2.3 Interactions with limestone and ash.

6.2.4 Strategies for NOx control.

6.3 N2O formation in FBC.

6.3.1 From char.

6.3.2 From volatiles.

6.4 N2O destruction in FBC.

6.4.1 Gas phase processes.

6.4.2 Heterogeneous processes.

6.4.3 Strategies for N2O control.

7 Trace Pollutants (EJA, FWinter, FPreto).

7.1 Chlorine and other halogens.

7.1.1 Cl in coal and waste materials.

7.1.2 Equilibrium considerations (HX, X, X2).

7.1.3 Effect on free radical concentrations and CO emissions.

7.2 Behaviour of metals.

7.2.1 Metals and alloys, non–reactive and reactive, volatile (high–T metal chem).

7.2.2 Metals in waste incineration and their fate.

7.2.3 Effect of chlorides.

7.2.4 Trace metals in FBC ashes.

7.3 Polycyclic aromatics.

7.3.1 General characterization.

7.3.2 Formation.

7.3.3 FBC emissions levels.

7.4 Dioxins and furans (FPreto).

7.4.1 Structure and toxicity, TEQ determination.

7.4.2 Formation/destruction mechanisms.

7.4.3 Emissions levels from fuel combustion and waste incineration.

8 Ash Disposal Issues (EJA, EMB).

8.1 Reactions in landfill.

8.2 Analysis of ash disposal issues.

8.3 Interactions between desulphurization products and water, hydration of the CaO/CaSO4 system.

8.4 Long term interactions between fuel ash and desulphurization products, formation of aluminosulphates and silicates.

8.5 Ash disposal technologies.

8.5.1 Conventional hydration processes.

8.5.2 Advanced hydration process(es).

8.5.3 AWDS technologies.

9 Experimental Techniques (?).

9.1 The basics.

9.1.1 Steady–states (thermal and chemical).

9.1.2 Minimum operating times.

9.1.3 Exit streams (gas and solids).

9.1.4 Gas analysers.

9.1.5 Statistical sampling theory.

9.2 Laboratory scale.

9.2.1 Single particle (TGA?) and batch experiments.

9.2.2 The problem of temperature profiles.

9.2.3 The problem of scale and mixing.

9.3 Probes (?).

9.3.1 Issues in using probes. Location, resolution and response. Averaging and sample times. Segregation and flowfield.

9.3.2 Solids sampling. Suction (isokinetic?). Traps.

9.3.3 Gas sampling. Gas conditioning. Flowrate and sampling times.

9.3.4 Solid–state oxygen sensor. Principle of operation. Interference and catalytic activity. The problem of interpretation.

9.3.5 Thermocouples. Bulky and micro. Radiation error.

10 Mathematical Modelling (?).

10.1 Transport processes.

10.1.1 Hydrodynamic models.

10.1.2 Mixing models.

10.2 Standard rector models for fluid beds.

10.2.1 Two–phase gas balance.

10.2.2 Population balances for particles.

10.3 Uncertainty and sensitivity of parameters.

10.4 Stochastic effects.

10.5 Sample calculations.

10.5.1 For BFBC and CFBC, to illustrate major effects.

10.5.2 Decoupled combustion and pollutant reactions.

10.5.3 Simplified chemistry or recommendations.

11 Biomass Combustion in FBC (FPreto).

1 Biomass in FBC Systems.

11.2 Co–firing Issues.

12 Trends and Outlook
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Ben Anthony is Group Leader on the FBC Gasification Project at Natural Resources Canada
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