Abstract

The combustion of coal and/or biomass (sludge, wood waste, RDF, etc.) in a circulating fluidized bed has been a commercial topper for over 20 years, and references to principles and applications are numerous and widespread although few data are presented concerning the operation of large scale CFB-units. The authors studied the CFB-combustion at UPM-Kymmene (Ayr), a major paper mill relying for its steam production upon the combustion of coal (80–85%), wood bark (5–10%) and wastewater treatment sludge (5–10%). The maximum capacity of the CFB is 58 MW_th.

A complete diagnostic of the operation was made, and additional tests were performed to assess the operating mode. The plant schematics, relevant dimensions and process data are given. To assess the operation of the UPM-CFB, it is important to review essential design parameters and principles of CFB combustors, which will be discussed in detail to include required data, heat balance and flowrates, operating versus transport velocity, kinetics and conversion (including the possible effect of the Bouduard reaction if carbon is present).

Since the residence time in the riser and the cyclone efficiency determine the burnout of circulating fuel-particles, the UPM-CFB was subjected to a stimulus response technique using nickel oxide as tracer. Results illustrate the efficiency of the cyclone separation and the number of recycle loops for particles of a given size. Results will also be used to assess the cyclone operation and efficiency and to comment upon expected and measured carbon conversion.

Keywords: Circulating fluidized bed; Combustion; Coal; Biomass; Operation; Cyclone

Abbreviations: CFB, circulating fluidized bed; RDF, refuse derived fuel

Nomenclature

C_O2

concentration of O₂ (mol/m³)

d

particle diameter (μm)

d_pi

particle size of coal or biomass at the bottom of the riser (μm)

d₅₀

50% size of particle mass distribution, cut size of cyclone, respectively (μm)

D_O2

diffusion coefficient for O₂ (m²/s)

M_f

mass loss of solids (limited) (kg/s)

M_F

mass feed rate of solids (kg/s)

M_p

mass flow rate of product withdrawal (kg/s)

M_r

mass flow in the riser (kg/s)

M_R

mass recycle rate of solids (kg/s)

N

recycle ratio

p₁, p₂

primary air flow to riser (m³/s)

P

pressure (bar)

P_Bex, P_Eex

pressure at boiler and economizer exit, respectively (bar)

P_F

number of particles in the feed (1/s)

P_ID

pressure at the ID fan (bar)

P_R, P_r

number of particles in recycle stream and riser, respectively (1/s)

P_R1, P_R2, P_R3

pressure in the riser (bar)

R

universal gas constant (J/mol K)

s₁, s₂, s₃, s₄

secondary air flow to riser (m³/s)

t_vo

volatiles extinction time (s)

T

temperature (°C)

T_b

bed temperature (K)

T_Bex, T_Eex

temperature at the exit of boiler and economizer, respectively (°C)

T_H2O

temperature of the water after the economizer (°C)

T_ID

temperature at the ID fan (°C)

T_R1, T_R2

temperature in the riser (°C)

T_s

coal particle surface temperature (°C)

U

superficial gas velocity (m/s)

U_t

terminal velocity of particle (m/s)

U_TR

transport velocity (m/s)

X_A

conversion

average conversion

Greek letters

voidage in riser

ρ_M

molar density of carbon (mol/m³)

ρ_p

particle density (kg/m³)

τ

burnout time of particle (s)

Article Outline

Nomenclature

1. Introduction

2. The operational parameters of the UPM-CFB combustor

2.1. The design sequence

2.2. Data gathering

2.2.1. Principles

2.2.2. Applied to the coal/biomass combustor of UPM

2.3. The heat balance determines the net energy yield and flow rates

2.3.1. Principles

2.3.2. Applied to the coal/biomass combustor of UPM

2.4. The transport velocity determines gas velocity and riser diameter

2.4.1. Principles

2.4.2. Applied to the coal/biomass combustor of UPM

2.5. The residence time determines the height and recycle ratio

2.5.1. Principles

2.5.2. Kinetics determines burnout times

2.5.3. Complete combustion to CO₂

2.6. Operating conditions determine residence time

3. Operating results of the UPM-CFB combustor

3.1. Residence time distribution

4. Discussion

4.1. Cyclone performance

4.2. Riser conversion

5. Conclusions

References