Performance Optimisation of a Residential Wood Log Burning Stove
Azenic, A
Date: 27 March 2023
Thesis or dissertation
Publisher
University of Exeter
Degree Title
EngD in Sustainable Materials and Manufacturing
Abstract
Governments from around the world are committed to tighten the legal limits regarding pollutant
emission levels from residential wood burning appliances. The European Union has decided to
implement new Ecodesign pollutant emission limits from 1st January 2022, to which the United
Kingdom is still committed, even after Brexit. This ...
Governments from around the world are committed to tighten the legal limits regarding pollutant
emission levels from residential wood burning appliances. The European Union has decided to
implement new Ecodesign pollutant emission limits from 1st January 2022, to which the United
Kingdom is still committed, even after Brexit. This is the motivation for this doctorate project,
whose aim is to minimise pollutant emission levels from commercial residential wood log burning
stoves to below the required Ecodesign limits and, if possible, to below the pollutant levels
from wood stoves present in the market today. Towards achieving this aim the relevant scientific
and industrial literature has been reviewed, adequate formulae for calculation of important
physical quantities have been derived, novel design of a down-draft gasification stove has been
developed through iterative stove design, manufacturing, assembling, testing and analysis, the
test data of the eight developed gasification stove variations and of seven extant conventional
(up-draft) stoves have been analysed, and the developed gasification stove design has been
optimised and compared to the performance data of the comparable counterparts. Statistical
design of experiments was a method used for finding the statistically significant and influential
design factors, or parameters, of the developed gasification stove design. The four
tested design factors were i) primary chamber base area, ii) nozzle cover geometry, iii) secondary
chamber glass area and iv) geometry of secondary chamber outlet (hole arrangement).
Through analysis of variance it was shown that first two played the most influential and statistically
significant role in emission factors of carbon monoxide and organic gaseous compounds.
Distinct qualitative features of the tested gasification stove variants have been identified, tested
and discussed: i) primary chamber sooting regime, ii) blow-back, iii) char bed channeling, iv)
instability of secondary flame ignition, and v) the potential for leakage of air and, more importantly,
of fuel rich pyrolytic gases into the otherwise clean flue gas stream. Important findings
and conclusions of this project are: the tested gasification stove design is overall superior to the
extant conventional designs in terms of pollutant emission levels and thermal efficiency; analysed
conventional stoves can emit low particulate matter levels (2 to 15 mg/Nm3 @ 13% O2) if
the maximum volume fraction of CO2 in flue gases is below 12%; such limit in maximum CO2
concentration can be achieved by increasing the combustion chamber volume; alternatively,
low pollutant levels can be achieved through increased slimness of the combustion chamber,
even for increased CO2; tested gasification stoves feature two distinct regimes of elevated pollutant
emissions: oxygen deficient and char bed channeling regime; oxygen deficient regime
can be avoided if CO2 in flue gases is kept below the effective mixing limit of a nozzle , whereas
the char bed channeling regime can be minimised through adequate primary chamber design,
wood log size, count and moisture content; two design factors in tested gasification stoves that
were found to be most significant and influencing on the pollutant emissions during the char
bed channeling regime were the surface area of a primary chamber stove base and the geometry
of nozzle cover; the optimum gasification stove configuration featured following pollutant emission levels: CO = 36.1 ppm1, THC = 4.581 mgC/Nm3, PM = 10.021 mg/Nm3 (in some
configurations down to 5.621 mg/Nm3) and efficiency = 92%. With such performance values,
possibly excluding the PM, it is believed that the developed gasification stove is the lowest
polluting one to come into the market, thus successfully achieving the project’s aim.
Doctoral Theses
Doctoral College
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