The Optimisation of Fluidised Bed Combustion for Emissions and NOx Control Farooq Sher*, Hao Liu, Chenggong Sun, Colin E. Snape
[email protected] Background Biomass plays an important role in renewable energy generation. The carbon in biomass used as fuel does not contribute to greenhouse gas emissions. Unlike most other renewable energy sources biomass can be stored and used on demand to give controllable energy. Fluidised bed combustion has been recognised as a promising technology for energy production from biomass [1].
Objectives
Flue gas
Design and optimisation of bubbling fluidised bed (BFB). Study of fluidisation velocities in the BFB. Study of emissions characteristics like NOx, SOx, CO2 etc. Study of air staging to control NOx. Study of air and oxy-fuel combustion using O2/N2 , O2/CO2 ratios.
Gas sampling probe
Heat extraction probe
Water out Water in
P-6
Pressure tapings (P)
T-7 S-4
Cyclone
Experimental Setup
P-5
T-6
S-3
A pilot scale bubbling 20 kW bubbling fluidised bed (BFB); consists of main reactor, cyclone, screw feeding system and other auxiliaries shown in Fig. 1. Garside 14/25 silica sand with diameter (d32) 0.78 mm was used as bed material. Primary air flow rate was 300 L/min, biomass pellet used in the size range of 10-20 mm.
P-4
T-5
S-2
Thermocouples (T)
T-4
Air staging points (S)
P-3
P-2
Results
T-3
Gas analysers 950
A
900
T2 T3 T4 T5 T6 T7
b
850
a
o
Temperature ( C)
o
Temperature ( C)
B
d
c
700
600 Distance from distributor
800
Data taker
750
Computer
650
Distribution plate Gas flow controls
550
2000
4000
6000
8000
O2 2
10000 12000 14000 16000
3
4
5
6
7
8
21
V-3
Inverter
350
C 15
250
CO2 CO 1x10 NOx Corrected
200
9
150
6
100
3
50
NOx Corrected at 6% O2(ppm)
300
NOx Corrected at 6% O2 (ppm)
300
18
Air
D
Staging Level 1 Staging Level 2
0 3
4
5
6
Oxygen in Flue Gas (Vol%)
7
8
F-1 V-1
Air preheater
CO2
250
V-2
200
Fig. 1 Schematic diagram of bubbling fluidised bed (BFB) . 150
Discussion
100 50
0 2
Geared motor
V-4
Oxygen in Flue Gas (Vol%)
Time elasped after Feeding (Sec)
12
P-1
T-1
500 0
Screw feeding system
700
600
400
Flue Gas Composition (Vol%)
5cm 74cm 98cm 123cm 148cm 173cm
T-2
T2 5 cm T3 74 cm T4 98 cm T5 123 cm T6 148 cm T7 173 cm
500
Feeder air
Thermocouple distance from distributor
Main heaters
900
800
F-2
S-1
0 2
3
4
5
6
7
8
Oxygen in Flue Gas (Vol%)
Fig.2 Non air staging (A) Continuous profiles under different conditions: SR = 1.45: 1.30; 1.15; 1.10, (B) Temperatures against oxygen in flue gas, (C) Exhaust gas composition, (D) NOx against oxygen in flue gas under air staging.
Conclusions BFB is successfully designed, fabricated and setup for biomass combustion. Smooth operation was observed even after five hours of continuous feeding. The temperatures in the reactor were evenly distributed. Air staging combustion was found to be effective for reduction of NOx emissions.
Three different types of biomass fuels including wood, miscanthus and straw pellets were tested. The results presented here are from straw pellets combustion. Temperatures show increasing trend with a decrease in oxygen concentration in flue gas. CO2 and CO concentrations decrease with an increase in oxygen concentration in flue gas, whereas NOx concentration increases with an increase in oxygen concentration in flue gas. An average 30% reduction in NOx level was achieved with air staging experiments. Stable temperature profiles with low CO concentrations were achieved [2].
References 1. Slade, Raphael, et al. Energy from biomass: the size of the global resource. Imperial College Centre for Energy Policy and Technology and UK Energy Research Centre, London (2011). 2. Carroll, J. P., et al. Air staging to reduce emissions from energy crop combustion in small scale applications. Fuel 155 (2015): 37-43.
Cleaner Fossil Energy and Carbon Capture Technologies Research Group University of Nottingham University Park, Nottingham NG7 2RD, UK