Modeling Study On The Formation Of Initial Deposition Layer By Alkali-containing Species And The Influence Of Flue Gas Conditions

During biomass burning and co-firing with coal,the alkali-containing species released into the flue gas easily cause ash deposition on the heat exchange surfaces of the boiler.The deposition of alkali-containing species mainly forms the initial deposition layer,which is the key step in the formation of ash deposition.Aimed at the contributions of the two main mechanisms?i.e.,condensation and thermophoresis mechanism?to the formation of the initial deposition layer by alkali-containing species,the present work applied modeling approaches,and developed a condensation deposition model,a thermophoresis deposition model,and a deposition model with the combined mechanisms.The predictions of the models were compared for validation with experimental measurements and related model results in the literature.The validation showed that the three models are able to reasonably describe the condensation of alkali chloride gases and thermophoresis deposition of alkali sulfate aerosol particles.Considering the evolutions of the thickness and surface temperature of the deposition layer with location and time are important and necessary for accurately describing the initial deposition process.The models developed were applied to numerically investigate the deposition process of alkali?K,Na?gases and aerosol particles on the heat exchange surface and to explore the influences of the factors,including flue gas temperature and flow rate,concentrations of alkali gases and aerosol particles,tube wall temperature,and aerosol particles diameter,on the deposition process.The purpose was to clarify the formation process of the initial deposition layer under co-firing conditions and the influences of various factors on it.The numerical investigations show that?1?KCl is more prone to condensation deposition than NaCl;under the same conditions,the initial deposition rate of KCl is around five times of that of NaCl,and the deposition thickness is also larger than than from NaCl;higher temperature and velocity of flue gas and higher temperature of tube wall restrain the condensation deposition process while the greater concentration of alkali-containing gases is benefit for condensation.?2?Under the same conditions,thermophoresis deposition processes of K₂SO₄ and Na₂SO₄ are of a high similarity;the greater temperature and velocity of flue gas and the higher alkali-containing aerosol particle concentration favor the thermophoresis deposition while the higher tube wall temperature inhibits it.In addition,when the particle size of aerosol particles is<0.5?m,the initial deposition thickness is independent on the particle size distribution.?3?When the two mechanisms work together,both condensation and thermophoresis deposition may play a major role in the formation of the initial deposition layer.The contribution of KCl condensation is relatively larger,while the thermophoresis deposition is dominant at the later stage.During the formation of the initial deposition layer,the deposition was non-uniformly distributed on the tube surface.At the early stage the deposition layer on the windward surface is thicker than that on the leeward surface.As the deposition proceeds,the thickness of the leeward surface layer gradually increases and exceeds that of the windward surface.During the deposition process,"binaural"structure is observable in the 45°-90°region on both sides of the stagnation point of the windward surface of the tube.?4?Under the conditions of co-firing biomass and coal,the initial deposition rate and temperature of the outer surface of the deposition layer are greater when the relative concentration of chlorides is higher.The deposition rate then decreases under various working conditions.However,the deposition rate decreases more rapidly when the chloride relative concentration is higher and finally turns to be lower than that of the chloride relative concentration being lower.Regarding the impact of K/Na relative concentration,the deposition rate and outer surface temperature of the deposit layer are greater when the relative concentration of K-containing species is higher.Additionally,the time and position of the"binaural"shape appearing are all the same under the different relative concentrations of K and Na.Nevertheless,when the relative concentration of the K-containing species is lower,the thickness of the deposition layer is significantly lower than that for the higher K concentration.