Author Topic: New features for performance enhancement of experimental Model Bubbling Fluidize  (Read 2813 times)

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Author : Raji T.O, Oyewola O.M, Salau T.A.O
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
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Abstract—  Fuel flexibility and capacity to burn broad spectrum of fuels at high combustion efficiency with minimum emissions of greenhouse gases are few of the key advantages fluidized bed combustion technology has over other existing combustion technology. This report examines the design, development and testing of an experimental model Bubbling fluidized bed combustor. Three unique features to enhance performance of this system were suggested and comprehensively discussed; inert bed’s temperature regulating unit, an integrated unit that enable Fluidizing air pre-heating as well as Biomass feeding pipe’s cooling and segmentations of the Combustor body into modules /partitioning of these modules into lower and upper section. The results of the test run with Palm kernel shell and Coconut shell show that the system performance is enhanced and that the temperature is well regulated  as observed in the thermal distribution. It is therefore proposed that the present Bubbling Fluidized bed combustor could be beneficial to development of commercial sizes for power generation in Nigeria and Africa sub region.

Index Terms:- Bubbling, Fluidized Bed, Biomass, combustion, design analysis, Experimental model, enhancement, Performance, Renewable energy,
Bubbling fluidized bed combustor (BFBC) have different components functioning in unison to burn wide variety of fuels in an efficient and environmentally friendly manner. It employed strong stream of fluidizing air with approach velocity Vo such that Vo is greater than the minimum fluidizing velocity Umf and less than the full fluidization velocity Uff ; Umf≤Vo ≤Uff .; at this stage the fluidization regime is  characterized by bubbles formation and vigorous mass turbulence, the bed particles  exhibits property of fluid and assumes appearance of a boiling liquid;  the bed at this point is said to be in Bubbling Fluidized Stage.  This fluidization characteristics and the selected feed rate are essentially the basic criteria that determined the dimension of any BFBC and capacity of its auxiliary equipment e.g. Blower, the Biomass feeder, cyclone separator etc. When Vo<Umf  (minimum fluidizing velocity)  the bed material remain a fixed bed (packed bed), at the other extreme when Vo≥Ut (terminal velocity) the bed mobilizes and transition to Circulating Fluidized Bed (CFBC) occurred see fig 1.

Raji, T,O is currently pursuing  PhD degree program  in the Department of Mechanical engineering, University of Ibadan, Ibadan. Nigeria. Email:

Oyewola O.M  is  a Reader and the current Acting Head; Department of mechanical engineering, University of Ibadan, Ibadan. Nigeria. Email:

SalauT.A.O is a senior lecturer in the same Department.   Email:
Fixed, Bubbling & Fast Fluidized Beds: As the velocity of a gas flowing through a bed  of particles increases, a value is reached when the bed fluidizes and bubbles form as in a boiling liquid. At higher velocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled to maintain a stable system.
Fig.1 A schematic drawing shows transition from packed bed to circulating bed.[15].

Fluidized Bed Combustion technology (FBC) has been shown to be a versatile technology capable of burning practically any waste combinations with low emissions ([1],[4]) it has gone beyond  being a mere idea to a proven technology for efficient combustion of difficult to burn wastes and biomass.
                          Gas cyclone
Flue gas analyzer

Biomass feeder                        Upper section   2900

Distributor plate                       lower section       
air in via G


   nine thermocouples (T1 – T9) arranged    axially along the combustor body.         
G   Fluidizing air pre-heater/Biomass feeding    pipe’s cooling attachment.
Lower section is module 1& 2
Upper section is module 3, 4& 5
Fig 3: Schematic drawing of the developed BFBC.
Biomass resources like woods, grasses, plant and animal wastes are the leading sources of energy generation in Nigeria contributing about 37% of energy demand. With annual turnover of 144million tonnes/year [3] it is particularly popular among the rural dwellers and small section of urban populace who generally employ method of open air burning of the biomass, which limit the thermal efficiency of the combustion to the lowest possible. Apart from firewood which is used for domestic cooking other agricultural and silvicutural wastes like Coconut shell, Oil palm solid wastes, cassava sticks, maize stems etc, are generally left wasted in the farm. One of the key agricultural crops in Nigeria is palm tree. It is found predominantly in southern Nigeria especially in the wet rain forests and savannah belt. It also exists in the wet parts of North central Nigeria, in areas like Southern Kaduna, Kogi, Kwara, Benue, Niger, Plateau, Taraba and Nasarawa States as well as the Federal Capital Territory (FCT) [17]. Solid waste from palm tree comprises of empty fruit branches (EFB), palm press fibres (PPF) and palm kernel shell (PKS) this waste collectively account for 48% of the original palm fruit branches, PKS alone account for 8% [4]. In Nigeria virtually every part of this wonder tree is traditionally useful for one thing or the other, however PKS is not been maximally utilized, only an insignificant portion of it is used for cooking or domestic processing vast majority of it is left unused in the farm creating environmental nuisance, since it could not rot and is useless for agricultural cultivation. It is worthy of note that even the use of EFB and PPF as local broom and domestic cooking fuel is fast reducing with modernization, as plastic brooms and modern way of cooking is now taking predominant share. Considering about 2.5million hectares of palm trees cultivated yearly [17], a huge quantity of PKS and other palm waste components which could otherwise be used for energy generation is  wasted, a huge loss considering the aggregate energy generation possible if such biomass could be fired with appropriate technology.
The potential of agricultural waste as fuel for energy generation has been investigated by many researchers. Srinivasa Rao et al [1] investigated the effect of secondary air injection on combustion efficiency of sawdust in a BFBC with an enlarged disengagement section, maximum combustion efficiency of  99.2% efficiency was observed at 65% excess air. Suthum P [4] examined the characteristics of palm waste when combusted in BFBC with modularly constructed combustion body of diameter 150mm. The study showed that oil palm waste could be burnt successfully in a BFBC, it was discovered that the relationship between excess air and combustion efficiency is such that CE increases with EA; reach a maximum value for a particular feed rate, then starts to fall: this was explained with the fact that beyond the maximum point the EA promotes higher elutriation of unburnt fuels particle. A maximum CE of 92.47 was achieved at 50% excess air. Rosyida P et al[2] reported that the use of air staging is beneficial to reduction of CO emission when palm waste is combusted in a BFBC, a maximum combustion efficiency of 89%  was achieved for palm fiber. Achieving high CE when biomass is used as fuel is not always the norm for instance an investigation conducted by [16] achieved less than 32% thermal efficiency in several experiments using inclined grate burner to combust PKS.
The foregoing results confirmed that Biomass could be combusted at higher efficiency and with lower emission of NOx and CO in BFBC than in conventional combustion technology such as grate burner. 
Furthermore it could be seen from the above examples that each literature employed BFBC with different modification for instance from the suggestion that increased residence time promote volatile combustion, [1] employed a BFBC with an  enlarged freeboard section to achieve more than 99% CE of sawdust. Clearly it could be seen from  all the examples cited, that modifications to BFBC were employed to optimize its performance; a confirmation that further modifications and features may be imperative to making FBC a more efficient and more environmentally friendly method for combusting fuels. 
Three performance enhancing features targeted at addressing potential problems of BFBC were examined in this work.

The features discussed here are added as alternative solution to key issues normally associated with BFBC especially when Biomass is used as fuel. Features suggested and examined in this work include the following:
i. Inert Bed Temperature Regulating Unit (ITRU); In BFBC, temperature is generally and deliberately kept below 950oC, the bed temperature being always lower (often 650-800oC); this is to limit formation of atmospheric NOx and to prevent ash fusion a condition that is detrimental to fluidization of inert particles.  Conventional approach employed water cooled coil to limit the bed temperature to acceptable level, water cooled coil immersed in bed apart from being costly, impose additional technical complication and could potentially affect fluidization characteristic of inert bed; in the present work an electronic feedback system was employed, it senses the temperature of the inert bed and via an electro-mechanical mechanism, controls the biomass feeder and fluidizing air supply as necessary. The ITRU comprises of Temperature controller, a type-K thermocouple, and two 40Amps contactors. The circuit is constructed in such a way that the biomass feeder motor is de-activated and activated as necessary to ensure the inert bed temperature is maintained at the preset temperature. See fig 6b.

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