Optimizing process parameters in water- and acid-washing pretreatment of rice straw

Ash-related issues such as slagging and fouling are major difficulties when agricultural waste is used in combustion applications. Washing pretreatment is a proven technology to improve fuel quality by reducing ash content and changing ash composition. The objective of this research was to analyze and optimize the washing pretreatment of rice straw to improve combustion properties. Thermogravimetric analysis was conducted to analyze the thermal decomposition behavior of rice straw. Further, solid yield was measured, and proximate analysis, ash composition analysis, and scanning electron microscopy were conducted for raw and washed rice straw. The thermal stability of the acid- and water-washed rice straw samples was much higher than that of the raw samples.

The optimum water-washing conditions were 30 min of washing at 47 °C with a liquid-to-solid ratio of 23 reducing 25.24% of ash content. For acid washing, the optimized condition was 30 min of washing with a liquid-to-solid ratio of 40 reducing 30.53% of ash. Water washing removed almost all the Cl and a significant portion of K and P, yet failed to lower the risk of fouling and bed agglomeration propensities to an acceptable level even though the slagging risk was low. Owing to the nearly complete removal of Cl, K, P, and Mg, acid washing was found to be effective for rice straw with some water-insoluble K, Mg, and P contents and significant Ca content, and it was validated by the observation of surface morphology.