Colloquia

Samenvatting

The transition towards sustainable energy has led to increasing interest in biomass combustion technologies. Moving-grate furnaces are widely used for their ability to efficiently handle heterogeneous biomass fuels. However, uncertainties regarding the combustion behavior of alternative feedstocks, particularly non-woody biomass, pose challenges for both operational efficiency and sustainability. The 15 MW Bemmel CHP plant, operated by HoSt, currently utilizes woody biomass, but future sustainability goals necessitate exploring alternative feedstocks. To assess the feasibility of such alternatives, numerical modeling serves as a crucial tool for understanding fuel bed conversion dynamics and optimizing furnace performance.

This thesis develops a one-dimensional transient numerical model to simulate the fuel bed conversion process in a moving-grate furnace, replicating the operating conditions of the Bemmel CHP plant. The fuel bed is treated as a two-phase porous medium, where the solid fuel matrix interacts with gas flowing through its pores. A walking-column approach is employed, tracking conversion in a fixed reference frame as the fuel progresses along the grate. The key thermochemical conversion processes—drying, pyrolysis, and char combustion—are explicitly modeled. The model incorporates essential transport mechanisms, including mass transfer between the solid and gas phases, convective heat transfer, conduction within the solid, and radiative heat transfer from the overbed region.

The model results are analyzed for feedstocks with varying moisture content (30%–55%) to assess their impact on fuel conversion. Higher moisture levels lower peak bed temperatures, from a maximum of 950°C at 30% moisture to 700°C at 55% moisture. Prolonged drying in wetter fuels extends the full conversion length from 65% to 90% of the grate. The effects of feedstock size on fuel conversion are also evaluated, showing that smaller particles (5 mm) enhance char conversion by providing more reaction sites, achieving full conversion at 70% of the grate length, whereas larger particles (15 mm) extend conversion to 95%. Increased porosity further reduces char gasification and oxidation rates, lowering the maximum char conversion rate from 5.40 kg/m³ at 38% porosity to 2.86 kg/m³ at 57%.