Modelling of biomass combustion and ash deposition in an 88 MW grate-fired boiler

Nazari, Shakerullah

4. term

Energy Engineering, Master

2020

46

Denne afhandling undersøger forbrænding og askedannelse i en 88 MW ristfyret biomassekedel (EV3) ved hjælp af CFD. Modelleringsstrategien er todelt: (1) en empirisk model af de termo-kemiske processer i brændselslejet på risten, baseret på antagne konverteringsrater fra målinger og erfaringer, som beregner temperatur, hastighed og gassammensætning ved udløb fra risten; og (2) en CFD-baseret forbrændings- og strålingsmodel for selve forbrændingskammeret, hvor de beregnede gasbetingelser indgår som randbetingelser sammen med øvrige grænseflader. For at vurdere askedannelse kobles forbrændingssimuleringerne til en iterativ model for aflejringsvækst, der skal estimere asketykkelse på blandt andet varmevekslere. Simuleringerne (ANSYS Fluent 19.2) valideres mod målinger, og sammenligningerne viser relativt betydningsfulde afvigelser. En hovedårsag er, at modellen antager en helt ren kedel, mens målingerne stammer fra drift med aflejringer, hvilket kan give afvigende randbetingelser; desuden er askedannelsen ikke medtaget i de stationære simuleringer. Strømningskonturerne indikerer to lokale recirkulationszoner i kammeret, hvilket tyder på god sammenblanding mellem forbrændingsgas og sekundærluft. Askedannelsesmodellen fungerede ikke som planlagt for EV3, og der foreligger derfor ingen direkte askedannelsesresultater for denne kedel. Bearbejdede resultater fra tilsvarende simuleringer på en anden kedel viser dog, at askedannelse kan øge temperaturen ved varmevekslernes hjørner. Arbejdet bidrager med indsigt i fribordforbrænding og fremhæver behovet for mere realistiske randbetingelser og robust kobling til aflejringsmodeller for at forbedre nøjagtigheden.

This thesis investigates combustion and ash deposition in an 88 MW grate-fired biomass boiler (EV3) using CFD. The modeling strategy has two parts: (1) an empirical bed model of thermo-chemical processes on the grate, based on assumed conversion rates from measurements and experience, which computes the temperature, velocity, and gas composition at the grate exit; and (2) a CFD-based combustion and radiation model for the furnace, using these gas conditions as boundary conditions together with other surfaces. To assess ash deposition, the combustion simulations are coupled to an iterative deposition growth model intended to estimate ash thickness on components such as heat exchangers. The simulations (ANSYS Fluent 19.2) are validated against measurements and show relatively significant deviations. A main reason is that the model assumes a completely clean boiler, whereas measurements were taken during operation with deposits, leading to non-identical boundary conditions; additionally, ash deposition is not accounted for in the steady-state simulations. Flow contours reveal two local recirculation zones in the furnace, indicating good mixing between combustion gases and secondary air. The ash deposition model did not work as planned for EV3, so no direct ash deposition results are available for this boiler. Processed results from similar simulations on another boiler indicate that ash deposition can raise temperatures at heat exchanger corners. The work provides insights into freeboard combustion and highlights the need for more realistic boundary conditions and robust coupling to deposition models to improve accuracy.

[This summary has been generated with the help of AI directly from the project (PDF)]

Biomass ; kedel ; Forbrænding ; CFD ; Modellering

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