Complex Study of Straw Suitability for the Production of Nonindustrial Straw Pellets

Nonindustrial straw pellets should comply with limitations on the content of ash, chlorine, nitrogen, sulfur, and heavy metals, and have a high melting temperature of ash. To produce such pellets, the properties of straw can be improved by leaching. In known papers, the completion of chlorine washing-out was not controlled. Aims of the paper were to study ash solubility at leaching of straw until completion of chlorine removal and to make a conclusion on studied straw suitability for the production of nonindustrial pellets. Aims were achieved by straw soaking with heating to 100 °C and subsequent plug flow flashing with control of leaching completion by the absence of chlorine in leachate; studying the ash, chlorine, nitrogen, sulfur, and heavy metals content of straw; studying the thermal behavior of ash at heating; determining the initial deformation temperature (IDT) of ash; and comparing the properties of original and leached straw with the specification of straw pellets. Straw leaching until completion of chlorine washing-out provided decreasing chlorine, nitrogen, and sulfur contents below limitations, and the ash content decreased from 7.15 to 3.93% at water leaching to 4.29% at leaching with a 10% solution of acetic acid. In the ternary diagram, the composition of straw ash shifted from a zone of low melting eutectics to zones of high-melting tridymite and cristobalite. The IDT of the original straw ash was 847, 1250 °C after water leaching, and above 1275 °C after leaching with an acetic acid solution. Monitoring the absence of chlorine in the leaching liquid can be applied as a control parameter for straw leaching completion. The original straw was not suitable for the production of nonindustrial pellets because of the high contents of Cl, S, and Cr and the low IDT of ash. All indexes of straw were improved due to leaching, but the Cr content was above limitation. Producers of pellets need to assess straw suitability as to heavy metal content both in the original and leached states.