1. Introduction
Currently, there are about 203 Millions households in Europe,
mostly equipped with a single heating system (boilers). A signifi-
cant part of heating units is clearly obsolete and need immediate
replacement [1]. The heat production with old-fashioned boilers
can cause serious health and ecological problems. Pollution of
environment by household heating boilers that are fired with coal
and is called low emission. The combustion process in such appliances
frequently runs without appropriate supervision, which results
in excessive emission of substances that are hazardous to
humans and the environment. Effects of that emission are clearly
visible in urban agglomerations, where a huge number of such
boilers operate within relatively small areas. Flue gas from such
small heating units is usually released to the atmosphere by means
of stacks that are of low height and as such do not distribute the
pollutants over larger areas. The resulting air pollution is comparable
with that generated by the industry. During winter, the
emission of hazardous substances is often a dominant factor of
urban air pollution. The problem of low emission can be resolved in
several ways. One of the promising ideas is using of more efficient
heating units. When considering pollutants emission, higher effi-
ciency of the boiler advantages in two ways. First of all, less fuel is
used for producing the same effect. Additionally, better combustion
means less carbon monoxide in the flue gases which indicates less
pollutants emitted to atmosphere.
The technology of small-scale coal-fired (biomass-fired) boilers
with retort burners appeared on the domestic market in mid 90's.The major advantages of these boilers, as compared to units of older
design, include relatively high efficiency (80%e90%), semiautomated
operation and meeting requirements of emission standards
that enable operation of such boilers in EU countries.
Typically, numerical simulation of such boilers focuses only at
the combustion process [2]. The presence of water pipes or jacket is
accounted for through boundary conditions (BC) of Robin's type or
using the concept of porous zones. The heat exchangers are
analyzed separately using numerical modelling [3] or black box
approach [4]. When considering large scale boilers such assumption
is feasible. The situation is, however, different for small units,
where the water flow strongly influences temperature in the wall
separating the heated water and the flue gas. Additionally, small
scale boilers often operate at lower than nominal (design) power,
which leads to abnormal increase of temperature of the walls. Long
overheat of the latter over time may led to a burn-through of a wall
and a failure of the boiler. That is why the present study is aimed at
modelling of both combustion and water flow occurring inside a
solid e fuel e fired retort boiler.
A most straightforward way would be to include both phenomena
in a single numerical simulation model. However, different requirements
on used models, mesh, materials etc. leads to a
conclusion, that decoupling of the combustion from water flow is a
more efficient approach. The obvious advantage of decoupling is a
simplification and reduced size of both models. This allows not only
for better, finer mesh of the model but also for reduction of the
computational effort and thus improvement of the accuracy of the
results. The disadvantage is that the additional effort is required to
couple the water and flue gas zones through BC. Finding of a proper
BC requires conducting several simulations for both separate models.
The major objective of this study is to gain a comprehensive
understanding of the combustion process that takes place in smallscale
coal/biomass fired retort boilers. The development of a
methodological approach for modelling of the combustion process
that takes place in both, the fixed-bed and in the combustion
chamber above the retort may help to improve the conversion of
chemical energy stored in fuel into heat and increase a thermal
efficiency of the heating units.
Separate simulation of water flow allows for studying of the
influence of the water distribution pattern on combustion. The
results will allow for pointing out the deficiencies in the organization
of the water flow and help to design more efficient
constructions.