- Email: [email protected]
Active Control of Biomass Fluidized Bed Combustion Environments
Copyright 10 IFAC Control Applications and Ergonomics in Agriculture, Athens, Greece, 1998
ACTIVE CONTROL OF BIOMASS FLUIDIZED BED COMBUSTION ENVIRONMENTS D.E. Ventzas,
and
P. Tsiakaras
Dept of Mechanical & Industrial Eng, University of Thessaly, Pedion Areos, 38334, Volos Email: [email protected]
ABSTRACT: In the present work. a scheme for industrial active control of a biomass fluidized bed combustor - heater system is proposed, modeled and simulated. It presents the instrumentation and control of a biomass fluidized bed combustor - heater system. Different control architectures are considered, based on overall fluidization and combustion control dynamics rather than control theory alone. Classical multivariable control based on modem instrumentation is applied on the fluidized bed combustion reactor, in close conformity with fluidization and combustion dynamics. if an optimal result must be achieved. Copyright @19981FAC KEYWORDS: Active control, biomass, flUidized bed, combustion, fluidization, dynamics.
IN1RODUCTION
simulate a biomass fluidized bed combustorheater system for applied active control.
It is well known that biomass is a byproduct of many biological, industrial and agricultural processes (Collins, 1991). Furthermore biomass is an energy source, potentially renewable. resulting in no net CO2 emission associated with its use. In plants, photosynthesis stores solar radiation as chemical energy in biomass carriers. Biomass is not sufficiently used, because it is believed that it bas low heating value. Biomass facilities are constrained by biomass availability (seasonality, transport costs is 50 % of total cost), capital investment and different kinds of biomass use flexibility, products design and energy concentration (10-50 times less than oil), storage areas (Williams, 1983). The paper models and
BIOMASS and COMBUSTION Biomass is a byproduct of many biological and industrial processes, such as sawdust, wood, vineyard and herbal industrial wastes, groundnut shell, cotton stalk, bushes, olive bask, other agricultural, forestal and residential residues; wastes and good mixes with other fuels are included. Municipal solid wastes are managed in incinerators (EPI, 1998). Every solid fuel has unique physical characteristics resulting in special overall conditions when burning. Hot and cold units for fluidized bed reactors simulate and experimentally resolve the operation and control of biomass reactors. The combustion
251
and O2 . Bio-oil and tar are highly corrosive, while costly post-processing by hydrogen impoves its properties. The distinction of the above processes is not clear, since they coexist, ego before gasification and burning we have pyrolysis, while in most biornass reactors preheating occurs due to partial oxidation of biomass (Kinetics, 1993). Most industries require heat energy of a high and exact temperature; few of them have at their disposal combustible solids for firing, but some burning chambers work with a wide range of fuels . Turbulence combustion (combined) towers with 3 (primary, secondary, tertiary) or more streams of air achieve increased capacities. The combustor turbulence and the thermal inertia affect efficiency, char and emissions. The high efficiency of a fluidized bed combustor makes it particularly well suited to problem fuels with low Btu value and high moisture characteristics. In typical units, the carbon burnout pecentages within the combustor are well in excess of 99 %. Large non-combustible material from the bottom of the bed, should be removed by self cleaning mechanisms. Emissions from a fluidized bed unit are inherently lower than conventional technologies because (Valk, 1995): a. low combustion temperatures and low excess air reduce the formation of NOx b. injecting limestone into bed, ammonia into the vapor space abates SOx and NOx c. high combustion efficiency results in flue gases that contain low amounts of CO
chamber is either the bed space or the end of the conveying pipe. Biomass is collected, transported, stored, pre-processed (drying, size reduction, shape formation, H2S removal); then it thermally reacts and used in energy applications. Biomass thernal conversion consists of: a. Pyrolysis: This is a thermal treatment at 350 - 700 ° C in inert atmoshere, that produces charcoal, tar and a gas mixture of low heat value. The rate of reaction is slow or immediate (flash); this depends on the degree of biomass preheating; flash pyrolysis has rate of heating greater than 1000°C/s and high gas poduction by fast cracking of its cellulosic matrix. The modem units recover vapours from the pyrolysis for further secondary burning (Froeling GmbH, 1997). b. Gasification: Thermal teatment at 750 1000°C, in the presence of steam and O2 , transforms biomass in a mixture of CO, H2, CxHy , CO2, etc. Gasification is driving biornass to maximum cracking and transformation to combustible CO and H2 gases. Partial gasifiers use as fluidizing medium a mixture of air and recycled flue gas, while above-bed secondary air is achieving temperatures as high as 1300°C. c. Burning: Thermal treatment in the presence of enough O2 with heat release. Direct burning is characterized by low heating value and high waste emissions and residuals. d. Liguidification: Biomass thermal treatment at 200 - 450°C and high pressure (300 bar) in the presence of H2 , CO and catalysts, produce organic liquids with low content of H20 cyclone
from turbine generator
deaerator
economizer
feed pumps
Fig. 1. Ex'temal fluidized-bed combustor (with fuel mixture and air preheat) for repowering plant
252
effects of these transient behaviors (not the average motion) of bubbles on the erosion rate of in-bed tubes andlor walls is immediate, suggesting a predictive model for erosion rates (EPI, 1991). Under the same given fluidized bed condition, many significantly different fluidization behaviors could stochastically coexist; the behaviors could be grouped into normal and abnormal fluidization ; this is a typical bifurcation fractal case. The bubble interaction (coalescence) prevails more intensive at the bottom portion of the fluidized bed during the normal fluidization period and the interaction prevails more widely all over the entire bed during the abnormal fluidization period. The properties of both bubbles and emulsion phases generate high fluctuations of voidage and voidage peaks. The experimental setup design consists of a bed, a distribution chamber and one or multiple distributor grids, a temperature controller with its heater (the air temperature controls the bed temperature), a manometer (bed pressure) and a differential manometer (pressure drop across the bed), a thermometer (bed temperature) and the associated fluidizing air control actuators in order to achieve different temperatures and pressures profiles. Biomass reactor efficiencies are predicted to 50 %, with high power ranges and product temperatures. Wood to charcoal transformation gives the 35 - 45 % of the energy contained in the original biomass. Gasification methods result in 70-80 % heat recovery. The most valuable product from pyrolysis is the bio-oil, similar to the fuel oil, whose energy density is 5-10 times higher than that of the original biomass. Up to 90 % efficiencies have been reported, see fig . 4, (Kullendorf, 1985).
The lack of moving parts in an FBS reduces maintenance costs and down time, with availabilities above 98 %, that keep operating costs low for the difficult fuels they are burning. In cases of old power stations, repowering is attractive, especially with solid-waste disposal, by adding bubbling-bed fluidized combustors, see fig. 1, advanced pollution-control equipment, new fuel handling trains and modem distributed control systems and using the existing fired boilers as heat-recovery steam, generators. Bed temperature is maintained at approximately 900°C to minimize ash agglomeration and maximize sulphure capture, while flue gases are at 1000°C. The forced-draft fans combustors, that cary small unburned fuel particles into the cyclones, behave similar to a circulating-bed design. Combustion instabilities based on hysteretic behavior of highly nonlinear combustors, see fig . 2, (Isela, 1997) i.e. pulses of secondary fuel, based on a simple on/off control law, drive the transition between the two (hysteretic) burning modes present, i.e. the unstable and stable burning, thereby reducing pressure oscillations with minimal use of fuel ; the control parameters are the injection position, flow rate, pulses duration, pressure threshold used to trigger the secondary flow; the blowout limit should not be exceeded. The transition, from the unstable to stable zone and vice versa, see fig. 3, is extremely fast, barely a dozen cycles of the pressure oscillations (Daw, 1996). Stabilization can be achieved by secondary fuel , air or nitrogen injection in the recirculating zone. The bubble frequency and and velocity is derived from the cross-correlation function during dynamic changes in fluidization conditions. The
h [W.m-2.K 1]
height above grid
700 surface heat 600 transfer coefficient
W=35 kg pressure drop 150 across bed 100
40 30
50
20 10 [m1s J
[m1s J
v 0.1
0.2
0.3
0.4
0
v 0
0.1
0.2
0.3
0.4
(a) (b) Fig. 2. Non-linear characteristics of a fluidised bed (bed's pressure drop (a) and heat transfer coefficient (b) )
253
4000 Kpa I -
-
-
-
-
-
-
-
-
-
_ .... I
___________
I -
-
-
-
-
-
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-
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-
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-
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-
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-
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~-----------~-----------I------------I----- ------
I
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I
I
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{_~ IA~A~~;;k~l~ r:-fuYi lv i
I
fjf
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I I
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_
____ _
I I I
I
L _______ _
I I
I I
I I
I
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I I
I I
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__ L _________ __ 1_ _ _ _ _ _ _ _ _ _ _ _ 1_
I
_ _
_ _
I I _ _ _ _ _ _ , __ I
I I
o
1000
500
2000
1500
2500
Time
o
msec Fig. 3. Nonnal and abnonnal (unstable) fluidization bifurcation case for gas phase pressure in fluidized beds
fuel unburnt fuel fed
1.0 0.8 low bed temperature < 750 0 C 0.6 0.4 bed gas differential velocity
burn back
--------=--- I
0.2
[ mls 1
~-----0.8
poor fluidization 0.1
o
- - -_ _ _ 0.6
o
0.02
0.04 0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20 [mls]
Fig. 4. Fuel unburnt / fuel fed v.s. minimum fluidizing velocity ACTIVE CONTROL
In process control, the model of a system might change under two different cases, i.e. under control or without control; the first case is treated by the active control theory, while the latter is treated by the adaptive control theoy. Active control of instabilities, i.e. suppession of flame and combustion oscillations, is achieved by exciting downstream (in the combustion chamber) or upstream (in the premixing chamber) the flame by some external excitation method such as rotating valves, driver units, directed jets, small explosive loads, anti sound, acoustic pressure, (Halow, 1996) etc that trigger instability modes. At low flow rates of propane/air combustion mixture, strong self sustained vibrations at a frequency of 600 Hz, with
Active Control Technology (ACT) is a general control method, used in Avionics Internal Combustion engineering. In combustion systems control the relation of degree of instability, control system limitations and system adjustments is critical, due to the sensitivity of the fluidization (collapses, rarefication and condensation) process and fuel quality variations that should in any case retain reaction. An ACS is expected to exhibit higher robustness properties than a classical control scheme alone. There are different active control configurations. Open loop ACS ensure unaltered bed loading characteristics.
254
injected into an incipiently fluidized bed identifies its dynamic response to step changes and controls average bubble size at various heights. The instability initiation or suppression of the transient behavior of oscillations is obvious in P*Q. p. Q (where P = pressure. Q = heat release) as a complex combination of standing and traveling waves. It has been observed that both the pressure signal and the heat release, present peaks at the same frequency (poinsot, T, et al, 1989). ACS in furnaces, including fluidized combustors, integrates operation, control and results in: a. stability augmentation on processes with reduced inherent stability b. increased range performance and adjustments c. gust disturbances compensation d. control over coupling effects (uncoupling and introduction of desirable coupling) e. subsystems design and control integration f. task tailoring and data base integration g. fault tolerance h. automatic start-up and shut-down i. back-up uncertainties (biomass quality,etc)
maximum air pressure of 200 Pa has been observed. Combustor boundaries may be considered as rigid walls (zero pressure) or pressure release (the exhaust). Active controller acts as an instability amplifier with filters, designed by phase shift techniques. Combustion instabilities (longitudinal and radial ones) occur in poweplants, exhibiting large amplitude oscillations of the flow, variations of the pressure, mechanical vibrations of the combustor walls, that lead to total loss of the system; active methods deminish, damp or suppress combustion oscillations. The nonsteady heat release can be measured by OH or C2 radical light emission (Howard, 1983). Here an injection method of PRBS pulsating air and/or fuel gas flow is suggested, under a wide set of different operating conditions, based on identification methods and control theory. The gain (intensity or energy) of the injected air and its phase compared to the inside combustor phenomena and their influence on the combustor stability is still under investigation. Bubbling gas
HEATER
REACTOR
FLUIDIZER
Fig. 5. Integrated (Active) Fluidized Bed Control System
REACTOR MODELLING, SIMULATION
CONTROL
and
flow dynamics and the combustion dynamics, see fig. 5. The critical parameters considered in fluidized bed reactors identification and modelling work are the primary air flow, powder load, porossity, bed's &, bed's velocity, air velocity, solids' velocity and the combustion chamber's characteristics, i.e. the bed temperature, thermal load, bed pressure, their profiles and the overall efficiency. In all cases we
All the reactions and the optimization of the yield in fuels are controlled by the bed (fluidizing air) heating speed and the contact time. Anaerobic of biomasses, are digestion processes advantageous for large installations. We attempted to model and present two interrelated parts of the fluidized bed, namely the
255
FBS have demonstrated the ability to operate under a wide range of load conditions. The thennal "fly-wheel" effect on the bed material allows swings in moisture and heating content of the fuel to be absorbed by the system without negative impact. Conversely, the low fuel inventory present in the unit makes it very responsive to varying loads. The fluidized bed also maintains efficiency during system turndown. The system was identified and the controller design and stability investigated, see fig . 7. The intrinsic physical complexity of the fluidized bed's process itself lies in the fact that its parameters are susceptible to changes, i.e the biomass flow is not homogeneous (Ventzas, 1996).
try to avoid bubbling (fluid boiling) conditions.
We suppose that, see fig. 6: 1. There is a minimum value of air flow that must be exceeded in order to achieve solids fluidization and transportation; this is simulated by a dead space nonlinerarity with exponential rise 2. The pressure drop in the combustion pipe section, is linearly dependent on the transportation air flow 3. For low fluidizing air flow rates the pressure drop in the combustion pipe section (or chamber), and the overall fluidized bed is higher. 4. The beds porrossity is p = 1 - [ i1P / L . p} 5. Expert knowledge and combustion dynamics is represented by look-up tables of flows and temperatures and process characteristics (Ventzas et al, 1998)
Scope
x = Ax+Bu y
=Cx-+Du
State- Spa ee
Tr.nsport Delay
State-Spaee1
Random Number
Sine Wave
Look·Up Table
Variable Transport Delay
Soope1
MAT LAB Fen
Fig. 6. Block diagram of a fluidizing bed reactor (combustor lower and fluidizer upper)
The control task to increase reactor efficiency, i.e. optimal combustion conditions in a pyrolytic reaction process (Berruti et ai, 1988), is achieved by control schemes such as single loop, cascade and multivariable control combined with expert fluidized bed reactor and combustion knowledge, integrated to an Active Control Scheme (ACS). ACS integrates operation, control and design (AGARD, 1988) resulting in stability augmentation on processes with reduced inherent stability, increased range and performance, tuning and disturbances compensation, control over coupling effects, with fault tolerance and ACS backs-up uncertainties in robustness. combustion and fluidization, due to the sensitivity
of the fluidization process (collapses, rarefication and condensation) and fuel quality variations, since in any case we should retain combUstion. Operator induced upsets are performed; failures are simulated, offering certification processes. Catalytic combustion lowers combustion temperature, controls flue gases and increase efficiency; such a scheme needs a series of fluidized beds with escalating reaction temperatures. The fluidized bed reactor consists of the bed, its pIp 109, the measuring instrumentation and control. These devices are modeled for ideal instrumentation hardware performance and
256
physical limitations are not considered here.
various control schemes are applied. Sensors controller elements
PL ANT
MFRcx:
~[I +~ _ ~
S
0 SP V2
>0-.4 -
V1 [rnJs)
A1 [Itls)
SPV 1 +
[10.5 *,_,0,
·.6]
13'"s+ 1
-9.8 *e-5O ,
1+s
~
1
-R2 100.9
~
V2 [rnJ~
A2 [Itls)
.
... .. )
(a)
Fig. 7. Block diagram ofMIMO closed loop system (a) Closed loop air flow response of tuned MIMO control system (b) of a fluidized bed; the flow dynamic side
Fluidized beds, Proceedings of the 5th Wold Congress of Chemical Eng (AIChE), San Diego, 1996, July 14-18, vol VI, pp. 291-296 Howard, 1. R, Fluidized Beds. Combustion and Applications, Applied Science Publishers, 1983 Isella, G, Seywert, C, Culick, et ai, A fUrther note on Active Control of Combustion Instabilities based on Hysteresis, Combust. Sci. and Tech, 1997, vol. 126, pp.381-8 Kinetics Technology International SpA, Advanced Pvrolysis Technology High Energy Fuels from Biomass, 1993 Kullendorff, A, Anderson, S, A General Review on Combustion in Circulating Fluidized Beds, Proc. of the 1st Internat. Confer. on Circulating Fluid. Beds Techn, France, 1985 NATO-AGARD, A Technical Evaluation Report (Or the Symposium on Active Control Systems - Review. Evaluation and PrOjections, AGARD Confer Proc. No. 384, Toronto, Canada, October 1984 Poinsot, T, et al,Suppression of Combustion Instabilities. by Active Control, Journal of Propulsion, Vol. 5, No. 1, 1989, pp. 14-20 Valk, M, Atmospheric Fluidized Bed Coal Combustion: Research. Development and Application, Elsevier Science, 1995 Ventzas D.E. , Tsiakaras P, Expert Knowledge for Suboptimal Industrial Furnace Overall Control. Intern. Conf. on CONTROL '98, lEE Ventzas, D, Modelling and MIMO Controller Design of Dense Powder Fluidised Beds, ISATRA13 , v. 35, 1996, Elsevier Sci Ltd, pp. 105-119 Williarns, F, A, Combustion Theorv, Benjamin Cummings, Palo Alto, CA, 1985
CONCLUSIONS Biomass reaction and application in energy production depends on its availability and cost of use, but gives solutions to biological wastes management. An active control scheme integrating multivariable control with expert fluidized bed reactor and combustion knowledge was designed and simulated, based on fluidized bed identification and modelling results. The response is optimal under the simulation conditions and can easily transfered into real laboratory, even industrial conditions.
REFERENCES Berruti, F, et al, A Generalized Gas-Solid Reaction Model (Or Circulating Fluidized Beds- An Application to Wood Pyrolysis, Proceedings of 1st Int. Conf. on Circulating Fluidized Beds Technology, France, 1988 CoUins S, Bubbling-bed Combustors achieve True Co-firing of RDF. Wood. Coal, Power, 1991 Daw, C. S, Finney, Measuring and Controlling Chaotic Dynamics in a Slugging Fluidized Bed, Proc.3d Exp.Chaos Confer, 1996, pp.8391 EPI, Benefits of Fluidized Bed Combustion, 1998 EPI, Bubbling-bed Combustors Achieve True CoFiring ofRDF. Wood. Coal, Power, 1991 Froling Ges. m. b. H, Biomass Combustion, Warme furs Leben, Heizkessel, Austria, 1997 Halow,1. S, Daw, C. S, Finney, C.E.A, Nguyen, K, Interpretation of Acoustic Signals from
257
ACTIVE CONTROL OF BIOMASS FLUIDIZED BED COMBUSTION ENVIRONMENTS D.E. Ventzas,
and
P. Tsiakaras
Dept of Mechanical & Industrial Eng, University of Thessaly, Pedion Areos, 38334, Volos Email: [email protected]
ABSTRACT: In the present work. a scheme for industrial active control of a biomass fluidized bed combustor - heater system is proposed, modeled and simulated. It presents the instrumentation and control of a biomass fluidized bed combustor - heater system. Different control architectures are considered, based on overall fluidization and combustion control dynamics rather than control theory alone. Classical multivariable control based on modem instrumentation is applied on the fluidized bed combustion reactor, in close conformity with fluidization and combustion dynamics. if an optimal result must be achieved. Copyright @19981FAC KEYWORDS: Active control, biomass, flUidized bed, combustion, fluidization, dynamics.
IN1RODUCTION
simulate a biomass fluidized bed combustorheater system for applied active control.
It is well known that biomass is a byproduct of many biological, industrial and agricultural processes (Collins, 1991). Furthermore biomass is an energy source, potentially renewable. resulting in no net CO2 emission associated with its use. In plants, photosynthesis stores solar radiation as chemical energy in biomass carriers. Biomass is not sufficiently used, because it is believed that it bas low heating value. Biomass facilities are constrained by biomass availability (seasonality, transport costs is 50 % of total cost), capital investment and different kinds of biomass use flexibility, products design and energy concentration (10-50 times less than oil), storage areas (Williams, 1983). The paper models and
BIOMASS and COMBUSTION Biomass is a byproduct of many biological and industrial processes, such as sawdust, wood, vineyard and herbal industrial wastes, groundnut shell, cotton stalk, bushes, olive bask, other agricultural, forestal and residential residues; wastes and good mixes with other fuels are included. Municipal solid wastes are managed in incinerators (EPI, 1998). Every solid fuel has unique physical characteristics resulting in special overall conditions when burning. Hot and cold units for fluidized bed reactors simulate and experimentally resolve the operation and control of biomass reactors. The combustion
251
and O2 . Bio-oil and tar are highly corrosive, while costly post-processing by hydrogen impoves its properties. The distinction of the above processes is not clear, since they coexist, ego before gasification and burning we have pyrolysis, while in most biornass reactors preheating occurs due to partial oxidation of biomass (Kinetics, 1993). Most industries require heat energy of a high and exact temperature; few of them have at their disposal combustible solids for firing, but some burning chambers work with a wide range of fuels . Turbulence combustion (combined) towers with 3 (primary, secondary, tertiary) or more streams of air achieve increased capacities. The combustor turbulence and the thermal inertia affect efficiency, char and emissions. The high efficiency of a fluidized bed combustor makes it particularly well suited to problem fuels with low Btu value and high moisture characteristics. In typical units, the carbon burnout pecentages within the combustor are well in excess of 99 %. Large non-combustible material from the bottom of the bed, should be removed by self cleaning mechanisms. Emissions from a fluidized bed unit are inherently lower than conventional technologies because (Valk, 1995): a. low combustion temperatures and low excess air reduce the formation of NOx b. injecting limestone into bed, ammonia into the vapor space abates SOx and NOx c. high combustion efficiency results in flue gases that contain low amounts of CO
chamber is either the bed space or the end of the conveying pipe. Biomass is collected, transported, stored, pre-processed (drying, size reduction, shape formation, H2S removal); then it thermally reacts and used in energy applications. Biomass thernal conversion consists of: a. Pyrolysis: This is a thermal treatment at 350 - 700 ° C in inert atmoshere, that produces charcoal, tar and a gas mixture of low heat value. The rate of reaction is slow or immediate (flash); this depends on the degree of biomass preheating; flash pyrolysis has rate of heating greater than 1000°C/s and high gas poduction by fast cracking of its cellulosic matrix. The modem units recover vapours from the pyrolysis for further secondary burning (Froeling GmbH, 1997). b. Gasification: Thermal teatment at 750 1000°C, in the presence of steam and O2 , transforms biomass in a mixture of CO, H2, CxHy , CO2, etc. Gasification is driving biornass to maximum cracking and transformation to combustible CO and H2 gases. Partial gasifiers use as fluidizing medium a mixture of air and recycled flue gas, while above-bed secondary air is achieving temperatures as high as 1300°C. c. Burning: Thermal treatment in the presence of enough O2 with heat release. Direct burning is characterized by low heating value and high waste emissions and residuals. d. Liguidification: Biomass thermal treatment at 200 - 450°C and high pressure (300 bar) in the presence of H2 , CO and catalysts, produce organic liquids with low content of H20 cyclone
from turbine generator
deaerator
economizer
feed pumps
Fig. 1. Ex'temal fluidized-bed combustor (with fuel mixture and air preheat) for repowering plant
252
effects of these transient behaviors (not the average motion) of bubbles on the erosion rate of in-bed tubes andlor walls is immediate, suggesting a predictive model for erosion rates (EPI, 1991). Under the same given fluidized bed condition, many significantly different fluidization behaviors could stochastically coexist; the behaviors could be grouped into normal and abnormal fluidization ; this is a typical bifurcation fractal case. The bubble interaction (coalescence) prevails more intensive at the bottom portion of the fluidized bed during the normal fluidization period and the interaction prevails more widely all over the entire bed during the abnormal fluidization period. The properties of both bubbles and emulsion phases generate high fluctuations of voidage and voidage peaks. The experimental setup design consists of a bed, a distribution chamber and one or multiple distributor grids, a temperature controller with its heater (the air temperature controls the bed temperature), a manometer (bed pressure) and a differential manometer (pressure drop across the bed), a thermometer (bed temperature) and the associated fluidizing air control actuators in order to achieve different temperatures and pressures profiles. Biomass reactor efficiencies are predicted to 50 %, with high power ranges and product temperatures. Wood to charcoal transformation gives the 35 - 45 % of the energy contained in the original biomass. Gasification methods result in 70-80 % heat recovery. The most valuable product from pyrolysis is the bio-oil, similar to the fuel oil, whose energy density is 5-10 times higher than that of the original biomass. Up to 90 % efficiencies have been reported, see fig . 4, (Kullendorf, 1985).
The lack of moving parts in an FBS reduces maintenance costs and down time, with availabilities above 98 %, that keep operating costs low for the difficult fuels they are burning. In cases of old power stations, repowering is attractive, especially with solid-waste disposal, by adding bubbling-bed fluidized combustors, see fig. 1, advanced pollution-control equipment, new fuel handling trains and modem distributed control systems and using the existing fired boilers as heat-recovery steam, generators. Bed temperature is maintained at approximately 900°C to minimize ash agglomeration and maximize sulphure capture, while flue gases are at 1000°C. The forced-draft fans combustors, that cary small unburned fuel particles into the cyclones, behave similar to a circulating-bed design. Combustion instabilities based on hysteretic behavior of highly nonlinear combustors, see fig . 2, (Isela, 1997) i.e. pulses of secondary fuel, based on a simple on/off control law, drive the transition between the two (hysteretic) burning modes present, i.e. the unstable and stable burning, thereby reducing pressure oscillations with minimal use of fuel ; the control parameters are the injection position, flow rate, pulses duration, pressure threshold used to trigger the secondary flow; the blowout limit should not be exceeded. The transition, from the unstable to stable zone and vice versa, see fig. 3, is extremely fast, barely a dozen cycles of the pressure oscillations (Daw, 1996). Stabilization can be achieved by secondary fuel , air or nitrogen injection in the recirculating zone. The bubble frequency and and velocity is derived from the cross-correlation function during dynamic changes in fluidization conditions. The
h [W.m-2.K 1]
height above grid
700 surface heat 600 transfer coefficient
W=35 kg pressure drop 150 across bed 100
40 30
50
20 10 [m1s J
[m1s J
v 0.1
0.2
0.3
0.4
0
v 0
0.1
0.2
0.3
0.4
(a) (b) Fig. 2. Non-linear characteristics of a fluidised bed (bed's pressure drop (a) and heat transfer coefficient (b) )
253
4000 Kpa I -
-
-
-
-
-
-
-
-
-
_ .... I
___________
I -
-
-
-
-
-
-
-
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-
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-
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-
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-
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-
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~-----------~-----------I------------I----- ------
I
---J- -lA! y'
I
I
L--------L---------! ---
{_~ IA~A~~;;k~l~ r:-fuYi lv i
I
fjf
I
I
I
I
I
I I
I I
I I
I
I
I
I
I
__ L ___________ ,___________ _ ,__
_
____ _
I I I
I
L _______ _
I I
I I
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I
I
__ L _________ __ 1_ _ _ _ _ _ _ _ _ _ _ _ 1_
I
_ _
_ _
I I _ _ _ _ _ _ , __ I
I I
o
1000
500
2000
1500
2500
Time
o
msec Fig. 3. Nonnal and abnonnal (unstable) fluidization bifurcation case for gas phase pressure in fluidized beds
fuel unburnt fuel fed
1.0 0.8 low bed temperature < 750 0 C 0.6 0.4 bed gas differential velocity
burn back
--------=--- I
0.2
[ mls 1
~-----0.8
poor fluidization 0.1
o
- - -_ _ _ 0.6
o
0.02
0.04 0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20 [mls]
Fig. 4. Fuel unburnt / fuel fed v.s. minimum fluidizing velocity ACTIVE CONTROL
In process control, the model of a system might change under two different cases, i.e. under control or without control; the first case is treated by the active control theory, while the latter is treated by the adaptive control theoy. Active control of instabilities, i.e. suppession of flame and combustion oscillations, is achieved by exciting downstream (in the combustion chamber) or upstream (in the premixing chamber) the flame by some external excitation method such as rotating valves, driver units, directed jets, small explosive loads, anti sound, acoustic pressure, (Halow, 1996) etc that trigger instability modes. At low flow rates of propane/air combustion mixture, strong self sustained vibrations at a frequency of 600 Hz, with
Active Control Technology (ACT) is a general control method, used in Avionics Internal Combustion engineering. In combustion systems control the relation of degree of instability, control system limitations and system adjustments is critical, due to the sensitivity of the fluidization (collapses, rarefication and condensation) process and fuel quality variations that should in any case retain reaction. An ACS is expected to exhibit higher robustness properties than a classical control scheme alone. There are different active control configurations. Open loop ACS ensure unaltered bed loading characteristics.
254
injected into an incipiently fluidized bed identifies its dynamic response to step changes and controls average bubble size at various heights. The instability initiation or suppression of the transient behavior of oscillations is obvious in P*Q. p. Q (where P = pressure. Q = heat release) as a complex combination of standing and traveling waves. It has been observed that both the pressure signal and the heat release, present peaks at the same frequency (poinsot, T, et al, 1989). ACS in furnaces, including fluidized combustors, integrates operation, control and results in: a. stability augmentation on processes with reduced inherent stability b. increased range performance and adjustments c. gust disturbances compensation d. control over coupling effects (uncoupling and introduction of desirable coupling) e. subsystems design and control integration f. task tailoring and data base integration g. fault tolerance h. automatic start-up and shut-down i. back-up uncertainties (biomass quality,etc)
maximum air pressure of 200 Pa has been observed. Combustor boundaries may be considered as rigid walls (zero pressure) or pressure release (the exhaust). Active controller acts as an instability amplifier with filters, designed by phase shift techniques. Combustion instabilities (longitudinal and radial ones) occur in poweplants, exhibiting large amplitude oscillations of the flow, variations of the pressure, mechanical vibrations of the combustor walls, that lead to total loss of the system; active methods deminish, damp or suppress combustion oscillations. The nonsteady heat release can be measured by OH or C2 radical light emission (Howard, 1983). Here an injection method of PRBS pulsating air and/or fuel gas flow is suggested, under a wide set of different operating conditions, based on identification methods and control theory. The gain (intensity or energy) of the injected air and its phase compared to the inside combustor phenomena and their influence on the combustor stability is still under investigation. Bubbling gas
HEATER
REACTOR
FLUIDIZER
Fig. 5. Integrated (Active) Fluidized Bed Control System
REACTOR MODELLING, SIMULATION
CONTROL
and
flow dynamics and the combustion dynamics, see fig. 5. The critical parameters considered in fluidized bed reactors identification and modelling work are the primary air flow, powder load, porossity, bed's &, bed's velocity, air velocity, solids' velocity and the combustion chamber's characteristics, i.e. the bed temperature, thermal load, bed pressure, their profiles and the overall efficiency. In all cases we
All the reactions and the optimization of the yield in fuels are controlled by the bed (fluidizing air) heating speed and the contact time. Anaerobic of biomasses, are digestion processes advantageous for large installations. We attempted to model and present two interrelated parts of the fluidized bed, namely the
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FBS have demonstrated the ability to operate under a wide range of load conditions. The thennal "fly-wheel" effect on the bed material allows swings in moisture and heating content of the fuel to be absorbed by the system without negative impact. Conversely, the low fuel inventory present in the unit makes it very responsive to varying loads. The fluidized bed also maintains efficiency during system turndown. The system was identified and the controller design and stability investigated, see fig . 7. The intrinsic physical complexity of the fluidized bed's process itself lies in the fact that its parameters are susceptible to changes, i.e the biomass flow is not homogeneous (Ventzas, 1996).
try to avoid bubbling (fluid boiling) conditions.
We suppose that, see fig. 6: 1. There is a minimum value of air flow that must be exceeded in order to achieve solids fluidization and transportation; this is simulated by a dead space nonlinerarity with exponential rise 2. The pressure drop in the combustion pipe section, is linearly dependent on the transportation air flow 3. For low fluidizing air flow rates the pressure drop in the combustion pipe section (or chamber), and the overall fluidized bed is higher. 4. The beds porrossity is p = 1 - [ i1P / L . p} 5. Expert knowledge and combustion dynamics is represented by look-up tables of flows and temperatures and process characteristics (Ventzas et al, 1998)
Scope
x = Ax+Bu y
=Cx-+Du
State- Spa ee
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Fig. 6. Block diagram of a fluidizing bed reactor (combustor lower and fluidizer upper)
The control task to increase reactor efficiency, i.e. optimal combustion conditions in a pyrolytic reaction process (Berruti et ai, 1988), is achieved by control schemes such as single loop, cascade and multivariable control combined with expert fluidized bed reactor and combustion knowledge, integrated to an Active Control Scheme (ACS). ACS integrates operation, control and design (AGARD, 1988) resulting in stability augmentation on processes with reduced inherent stability, increased range and performance, tuning and disturbances compensation, control over coupling effects, with fault tolerance and ACS backs-up uncertainties in robustness. combustion and fluidization, due to the sensitivity
of the fluidization process (collapses, rarefication and condensation) and fuel quality variations, since in any case we should retain combUstion. Operator induced upsets are performed; failures are simulated, offering certification processes. Catalytic combustion lowers combustion temperature, controls flue gases and increase efficiency; such a scheme needs a series of fluidized beds with escalating reaction temperatures. The fluidized bed reactor consists of the bed, its pIp 109, the measuring instrumentation and control. These devices are modeled for ideal instrumentation hardware performance and
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physical limitations are not considered here.
various control schemes are applied. Sensors controller elements
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Fig. 7. Block diagram ofMIMO closed loop system (a) Closed loop air flow response of tuned MIMO control system (b) of a fluidized bed; the flow dynamic side
Fluidized beds, Proceedings of the 5th Wold Congress of Chemical Eng (AIChE), San Diego, 1996, July 14-18, vol VI, pp. 291-296 Howard, 1. R, Fluidized Beds. Combustion and Applications, Applied Science Publishers, 1983 Isella, G, Seywert, C, Culick, et ai, A fUrther note on Active Control of Combustion Instabilities based on Hysteresis, Combust. Sci. and Tech, 1997, vol. 126, pp.381-8 Kinetics Technology International SpA, Advanced Pvrolysis Technology High Energy Fuels from Biomass, 1993 Kullendorff, A, Anderson, S, A General Review on Combustion in Circulating Fluidized Beds, Proc. of the 1st Internat. Confer. on Circulating Fluid. Beds Techn, France, 1985 NATO-AGARD, A Technical Evaluation Report (Or the Symposium on Active Control Systems - Review. Evaluation and PrOjections, AGARD Confer Proc. No. 384, Toronto, Canada, October 1984 Poinsot, T, et al,Suppression of Combustion Instabilities. by Active Control, Journal of Propulsion, Vol. 5, No. 1, 1989, pp. 14-20 Valk, M, Atmospheric Fluidized Bed Coal Combustion: Research. Development and Application, Elsevier Science, 1995 Ventzas D.E. , Tsiakaras P, Expert Knowledge for Suboptimal Industrial Furnace Overall Control. Intern. Conf. on CONTROL '98, lEE Ventzas, D, Modelling and MIMO Controller Design of Dense Powder Fluidised Beds, ISATRA13 , v. 35, 1996, Elsevier Sci Ltd, pp. 105-119 Williarns, F, A, Combustion Theorv, Benjamin Cummings, Palo Alto, CA, 1985
CONCLUSIONS Biomass reaction and application in energy production depends on its availability and cost of use, but gives solutions to biological wastes management. An active control scheme integrating multivariable control with expert fluidized bed reactor and combustion knowledge was designed and simulated, based on fluidized bed identification and modelling results. The response is optimal under the simulation conditions and can easily transfered into real laboratory, even industrial conditions.
REFERENCES Berruti, F, et al, A Generalized Gas-Solid Reaction Model (Or Circulating Fluidized Beds- An Application to Wood Pyrolysis, Proceedings of 1st Int. Conf. on Circulating Fluidized Beds Technology, France, 1988 CoUins S, Bubbling-bed Combustors achieve True Co-firing of RDF. Wood. Coal, Power, 1991 Daw, C. S, Finney, Measuring and Controlling Chaotic Dynamics in a Slugging Fluidized Bed, Proc.3d Exp.Chaos Confer, 1996, pp.8391 EPI, Benefits of Fluidized Bed Combustion, 1998 EPI, Bubbling-bed Combustors Achieve True CoFiring ofRDF. Wood. Coal, Power, 1991 Froling Ges. m. b. H, Biomass Combustion, Warme furs Leben, Heizkessel, Austria, 1997 Halow,1. S, Daw, C. S, Finney, C.E.A, Nguyen, K, Interpretation of Acoustic Signals from
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