Ash Deposition And Bed Particle Agglomeration In Circulating Fluidized Bed During High-sodium Zhundong Coal Combustion

Zhundong coal in Xinjiang region of China has an estimated reserve of 390 Gt,but its Na content is generally high,leading to severe ash fouling and slagging on heat transfer surfaces during combustion in a boiler,limiting the utilization of Zhundong coal.Circulating fluidized bed（CFB）combustion technology has the potential advantages of large-scale utilization of Zhundong coal due to its characteristics of wide-fuel adaptability and low-temperature combustion.However,ash deposition on heat transfer surfaces and bed particle agglomeration are still incurred during the practical operation of high-sodium Zhundong coal in CFB,being a technical bottleneck for the‘long,safe and stable’operation of CFB boilers.This involves key scientific issues including the migration and transformation mechanisms of Na,and the interaction mechanisms between Na and bed particles,etc.In terms of these key scientific issues,this thesis summarizes ash deposition and bed particle agglomeration characteristics during Zhundong coal combustion in a 0.2 t d^-1 CFB system,and investigates the effect of temperature,gas-solid flow,etc.on the deposits in detail.It clarifies the interaction mechanisms between coal ash and bed particles,and establishes the intrinsic relationship between Na migration and transformation and ash behaviors.As a consequence,the effective mitigation strategies including combustion temperature,flue gas recirculation,alternative bed material and co-combustion on ash deposition and bed particle agglomeration are proposed and verified.This aims at providing theoretical basis and basic engineering application data for the large-scale utilization of high-sodium Zhundong coal.The main work and conclusions are as follows:（1）In terms of the ash deposition characteristics,characterizes the morphology and chemistry of these deposits on probes installed at the top of the furnace,the cyclone outlet,and tail flue.Results show that a thin white ash layer was formed on the surface of the probe（P1）at the top of the furnace,and its composition was mainly enriched in Ca/Na sulphates.The strong gas-solid disturbance in the CFB furnace was the main reason why the ash particles were difficult to deposit,and it is also one of the main advantages of CFB boilers compared with pulverized coal furnaces.A sintered ash layer with height of 3 mm was formed on the windward of probe（P2）at the cyclone outlet.The inner layer was mainly enriched in Ca/Na sulphates,and the outer layer was Ca/Na aluminosilicates,indicating the presence of severe ash deposition.The ash deposits on the probes（P3-P4）of tail flue were relatively loose,and these compositions were similar to fly ash.The presence of low melting-point Ca/Na-based sulphates and these eutectics was the main reason for the severe ash deposition during CFB combustion burning Zhundong coal.（2）With respect to the mechanism of bed particle agglomeration,characterizes the surface and cross-section of bottom ash,clarifying the formation mechanism of coating layer.Results show that bottom ash consisted of unagglomerated particles and coarse-grained agglomerates with larger than 1 mm in size,indicating severe bed particle agglomeration.These unagglomerated bottom ash particles were coated by a layer of sub-micron particles enriching in Ca/Mg/Fe aluminosilicates but less Na,forming a coating layer.The agglomerated particles were formed by the sintering of multiple bed particles through a‘neck’structure enriching in Ca/Na/Si eutectics,which increased the particle size of the bottom ash.In-situ ash fusion analysis of environmental scanning electron microscopy（ESEM）shows that the coating layer enriching in Ca/Mg/Fe and the‘neck’structure consisting of Ca/Na/Si had melted significantly at 1100℃,confirming their low-melting points.The presence of the sticky coating layer and low melting-point Ca/Na-based minerals promoted bed particle agglomeration.（3）Regarding the mechanism of combustion temperature,characterizes the morphology and chemistry of the bottom ash,fly ash and deposits,clarifying its mechanism on the migration and transformation of Na.Results show that when the bed temperature is 800 and 750℃,the low melting-point Na-based minerals such as Na₂SO₄ and Na₆Ca（SO₄）₄ were enriched in the bottom ash,forming an incomplete coating layer on the surface of bed particles;the P1 and P2 deposits consisted of granular sub-micron particles enriching in Ca/Na sulphates.Because the combustion temperature was lower than the melting temperature of Na-based minerals,the bottom ash,fly ash and deposits mainly existed in the forms of solid phase with low stickiness,and the degree of particle agglomeration and sintering was significantly reduced.Decrease in the combustion temperature can to some degree alleviate the ash deposition on heat transfer surface and bed particle agglomeration.（4）For the mechanism of flue gas recirculation（FGR）,characterizes the morphology and chemistry of the bottom ash,fly ash and deposits.Results show that when FGR was applied,the bottom ash was rich in Ca Si O₃ and Na Al Si₃O₈,presenting severe bed particle agglomeration;with the increase in flow rate of FGR,more Na/Fe-based minerals exhibited at the bottom ash,such as Na₆（Al Si O₄）₆ and Mg Fe₂O₄,inducing more severe bed particle agglomeration.Na contents in fly ash,P1 and P2deposits were significantly reduced,and mainly composed of sub-micron particles and their agglomerates with low sintering,significantly reducing the particle stickiness due to the lower flue gas temperature.Flue gas recirculation can alleviate ash deposition on heat transfer surfaces,but to some extent aggravate bed particle agglomeration.（5）With regard to the mechanism of alternative bed material,Mg-bearing steel slag and blast-furnace slag were used as bed materials,respectively,clarifying the mechanism of the formation of coating layer on bed particle agglomeration.Results show that when steel slag was used as bed material,the Na content in bottom ash was only 0.84 wt%,while it was enriched in fly ash particles,reaching 2.39 wt%,and existed in the forms of low melting-point(Na_0.8Ca_0.1)₂SO₄,Na₂SO₄,Na₂Si₂O₅ and Na Cl,which induced more severe ash deposition on heat transfer surfaces.However,the low melting-point Ca/Na silicates generated by the combustion of Zhundong coal still presented,inducing the presence of a small number of agglomerated particles.When the blast-furnace slag was used as the bed material,no agglomerated particles were found,which was mainly due to the low reactivity between blast-furnace slag and Na as well as its special surface structure,suppressing the formation of a sticky coating on the particle surface.Blast-furnace slag as an alternative bed material had the potential to alleviate bed material particle agglomeration,but aggravated ash deposition on heat transfer surfaces.（6）Pertaining to the mechanism of co-combustion of oil shale semi-coke with high Si/Al contents,clarifies the scavenging mechanism of oil shale semi-coke on Na.Results show that during co-combustion of Zhundong coal and oil shale semi-coke,Na vapors or aerosols can diffuse into the porous oil shale semi-coke particles and react with Si/Al in oil shale semi-coke;while the fine particles of Ca/Mg mainly attached on the surface of oil shale semi-coke.With the increase in the ratio of oil shale semi-coke,Na-based minerals in the bottom ash converted from（Na,Ca）Al（Si,Al）₃O₈ to Na Al Si₃O₈,inhibiting the occurrence of low melting-point Ca/Na silicates generated by the combustion of Zhundong coal and reducing the thickness of the coating layer,which can effectively alleviate the bed particle agglomeration.ESEM in-situ ash fusion analysis reveals that the melting temperature of oil shale semi-coke enriching in Na-based minerals was 1280℃,which was significantly higher than the CFB combustion temperature.It is insufficient to induce bed particle agglomeration.The occurrence of Na in fly ash changed from water-soluble Na to insoluble Na,effectively reducing the sintering and ash deposition property of fly ash.Na contents in P1 and P2deposits were significantly reduced,and exhibited in the form of high melting-point aluminosilicates,which did not induce obvious particle melting and sintering.Co-combustion of oil shale semi-coke and Zhundong coal can alleviate the ash deposition on heat transfer surface and bed particle agglomeration.