| Author | Bui Tuyen |
| Call Number | AIT Diss no.ET-96-3 |
| Subject(s) | Wood--Combustion
|
| Note | A dissertation submitted in partial fulfillment requirements for the Degree of Doctor of Engineering, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ET-96-3 |
| Abstract | An experimental and theoretical investigation of the combustion of wood, as a single
piece in air and a packed bed of pieces in a downdraft flaming pyrolysis reactor, was conducted.
The results have been integrated and embodied in a multi-stage gasifier which, as the
experimental tests showed, has better performance characteristics than conventional gasifiers
regarding to the tar content and gas composition.
The tar contamination of producer gas from conventional downdraft wood gasifiers, a
recurrent headache for gasifier designers, has been identified as unavoidable because oxygen is
completely consumed long before the wood pieces are completely devolatized. In a multi-stage
gasifier flaming pyrolysis process is conducted in a separate reactor section, the first stage of
gasifier, until the solid is completely carbonized. Another subsequent reactor section, the second
· stage, where a secondary air supply is provided, is used for combustion of the pyrolysis gas and
gasification of the residual char. This stage develops another temperature peak and thus is able
to crack any tar that might escape oxidation.
The direct phenomenological connection of the flame existence and its appearance over
wood with the surface gas flux (SGF) emitted from pyrolyzing wood was investigated. The
minimum flame sustaining SGF, which can also serve as the pilot ignition threshold, was
determined to be about 5x10·3 kg-m·2
-s·1
• The ranges of the SGF for stable-smokeless flames
were found to be quite narrow: from 15x10·
3 to 50x10·3 kg-m·2
-s·1
• At higher values of the SGF
the flame was smoky indicating that combustion was incomplete.
Combustion of single wood pieces in air was studied. There was found some critical size,
specimens smaller than which would be ignited, burn with a flame and, produce a carbonaceous
residue. Thin pieces burned more easily than thick ones but the duration of flaming combustion
depended on the specimen size. Pieces thicker than the critical size did not sustain a continuous
flame until volatiles were depleted, showing the need to use wood in relatively small sizes for
gasification in down draft gasifier to obtain low tar gas.
The opposed flow flame spread upward in a packed bed of wood pieces was studied
experimentally for downdraft mode using a transparent reactor made of high temperature glass.
The lower limit of supply air superficial cold velocity at which flame extinguished and the upper
limit when blow off occurred were found to be about 0.02 and 0.3 mis respectively. Flaming
pyrolysis of the bed was possible only at air supply within these limits. In the flame supporting
air supply range, the flame spread rates were experimentally measured.
Based on the knowledge gained from studies on combustion of single particle and packed
bed, a multi-stage gasifier was constructed and tested. Basic conditions for successful operation
of a multi-stage gasifier are favorable fuel size, air flow rate to maintain the flaming pyrolysis
in the first stage and, appropriate distance between primary and secondary air supplies to ensure
a sufficiently long residence time for complete wood devolatilization before the secondary air
supply. Also, a proper amount of air need to be supplied to the second stage to raise temperature
again; however, the solid consumption in the second stage should not cause a solid flow
exceeding the opposed flow flame spread rate in the first stage. Under the optimal operation,
a two-stage gasifier produced gas with tar content around 50 mg/Nm3 , about 80 times less than
-ii-
the best results that could be obtained with the similar size gasifier operating in conventional
one-stage configuration.
A one-dimensional mathematical model of a two-stage wood gasifier was developed for
steady ;state operation. The numerical solution correctly predicts behavior of the gasifier
qualitatively: two temperature peaks were developed, the first peak corresponded to the flame
front in the first stage and the second peak was due to combustion in the second stage. |
| Year | 1996 |
| Corresponding Series Added Entry | Asian Institute of Technology. Disertation ; no. ET-96-3 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Energy Technology (ET) |
| Chairperson(s) | Bhattacharya, Sribas C. |
| Examination Committee(s) | Loof, R.;Surapong Chiraratananon;Martin, J. |
| Scholarship Donor(s) | Asian Institute of Technology;The Government of Germany through the Deutsche |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1996 |