(a) Rice and wheat

Residues from rice and wheat are mainly exemplified by chaff and straw. Of these, chaff generally refers to those derived from rice. This is because wheat chaff never drops in harvesting, and it can be processed without being removed. Rice chaff is almost uniform in shape and size, and is suitable for processing and transportation. Rice chaff, however, has a hard structure and less suitable for fermentation, due to its large amounts of lignin and silica (SiO2). Most rice chaff is used as fuel for combustion. However, the silica contained in rice chaff as much as 10 to 20 wt%, which may damage incinerators during combustion, raising some considerations on its practical use. On the other hand, straw similarly contains lignin, silica and so forth, but may be more fermentable than the chaff, and it is used as energy sources both for combustion and fermentation.

(b) Corn and rhizomic crops

Corn residues are discharged not only as residues in fields (leaves, stems, etc.), but also as corncobs after processing. Kernels of corn are abundant in starch, and it is used for ethanol production through fermentation in the USA.

Rhizomic crops may have leaves and stems as residues.

(c) Sugarcane

All parts of sugarcane other than the stems, such as cane tops (top portion with only a low sugar content), leaves and roots, are removed before the stems are transported to the sugar mill, leaving them as residues in the fields.