*2.2. Methods*

#### 2.2.1. Air Classification of Corn Stover

The ORG CS intended for HTC were sampled using mechanical sampling procedures and equipment according to Solid Biofuels Sampling ISO 18135:2017. A super sack of the deconstructed CS bale was divided using a custom rotary splitter that consists of a conveyor and eight bins mounted on a rotating table. In this unit, a feed hopper accommodates approximately 120 L of sample and a live-bottom style belt feeder slowly dispenses the samples to 8 sample bins below. The 45 L bins rotate over 100 times during each splitting operation and make at least one full rotation for each belt flight to ensure the samples are representative of the original bulk solid. Both the belt feeder and the rotary table are equipped with speed control devices (variable frequency drive and/or DC potentiometer) to adjust the processing parameters according to the sample feed behavior to ensure analytical splitting (an image of AC can be found in the supplementary information (Figure S1)). AC separates the samples into two fractions as the material is passed over a screen-covered fan: LAF and HAF. The HAF is blown upward and removed while the LAF (air classified material) remains. Air classification was performed on CS feedstocks using a 2× Air Cleaner equipped with an Iso-flo dewatering infeed shaker (Key Technologies, Walla Walla, WA, USA). CS was air classified using a fan speed of 1.8 m/s. Both light and heavy fractions were collected after classification for downstream processing and analyses.
