Enhancing fuel qualities of cassava crop residues by washing

• Washing can improve fuel quality of milled cassava stems.
• K, Cl, P content and risks for deposition, corrosion, emission reduced
• Within 5 minute time is important for washing effects.
• The temperature changes C and H only.
• Smaller milling grade increases amounts of organic (starch) washed away.

Cassava (Manihot esculenta Crantz) stems, being waste residues after harvesting starchy roots, are a potential biofuel resource. However high concentrations of ash and elements Cl, K, etc. in the stems may cause severe deposition, corrosion and particle emissions, in addition to slagging during combustion. This study tests washing by water as a pretreatment to reduce the problems. A 3-level full factorial designed experiment was conducted with washing time (< 1800 s) and temperature (20–40 °C) as factors and fuel characteristics as responses. The effect of milling particle size was also examined in a supplementary experiment.After washing, the net calorific value of the biomass tended to be higher, though not significant. Both washing time and temperature increased C but decreased H, while N and S content decreased with time only. A short washing of 50 s decreased the ash content by approximately 50% and Cl around 75%, followed by K and P that decreased to 50% after 5 min. Smaller milling size resulted in a larger amount of starch washed away, but no significant change in content of total ash and individual elements, except for Cl which was significantly higher in the smaller particles. The effect of washing on the ash composition is visualized in a ternary diagram, showing that the risk for slagging and fine particle emissions is reduced. A reduction in the risk of corrosion is also predicted as indicated by a relatively higher ratio of S/Cl and lower Cl/(K + Na). Thus, in addition to extraction of starch, the washing can also improve fuel quality of the residual biomass.