Smouldering combustion plays a key role in wildfires in forests, grasslands, and peatlands due to its common occurrence in porous fuels like peat and duff. As a consequence, understanding smouldering behaviour in these fuels is crucial. Such fuels are generally composed of cellulose, hemicellulose, and lignin. Here we present an updated computational model for simulating smouldering combustion in cellulose and hemicellulose mixtures. We used this model to examine changes in smouldering propagation speed and peak temperatures with varying fuel composition and density. For a given fuel composition, increases in density decrease the propagation speed and increase mean peak temperature; for a given density, increases in hemicellulose content increase both propagation speed and peak temperature. We also examined the role of natural fuel expansion with the addition of water. Without expansion, addition of moisture content reduces the propagation speed primarily due to increasing (wet) fuel density. However, with fuel expansion similar to that observed in peat, the propagation speed increases due to the overall drop in fuel density. Finally, we studied the influence of fuel composition on critical moisture content of ignition and extinction: mixtures dominated by hemicellulose have 10% higher critical moisture content due to the increase in peak temperature.