**1. Introduction**

More than one billion tons of biomass will be available for bioenergy production in the USA by 2030 [1–3]. Among other resources, corn stover (CS) has a huge potential to contribute to the renewable energy portfolio, as more than 250 million dry tons of CS are produced annually in the USA [1,4]. With proper feedstock handling and preprocessing, CS could become an abundant source of bioenergy [5,6]. However, a significant amount of agricultural waste, including CS, is burnt or left unprocessed in the field due to high logistical costs. This results in serious environmental problems and economic forfeiture [7,8]. Therefore, the utilization of potential bioenergy from waste biomass sources such as CS needs to be considered with urgency.

Air classification (AC) is a low-cost preprocessing technology that has been developed particularly in agricultural processing and mining applications. In AC, the separations are performed based on a combination of material size, density, and drag properties, utilizing screens and air streams to effectively separate high-density fractions from lowdensity fractions. In general, large amounts of soil, rocks, and other foreign matters are incorporated into the feedstock during harvest and collection. The inorganics, dirt, rocks,

**Citation:** Islam, M.T.; Saha, N.; Hernandez, S.; Klinger, J.; Reza, M.T. Integration of Air Classification and Hydrothermal Carbonization to Enhance Energy Recovery of Corn Stover. *Energies* **2021**, *14*, 1397. https://doi.org/10.3390/en14051397

Academic Editor: Byong-Hun Jeon

Received: 28 January 2021 Accepted: 1 March 2021 Published: 4 March 2021

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and foreign materials are mostly dense materials compared to the lignocellulosic structure of biomass. Therefore, AC separates a significant amount of these exogenous inorganics and creates an enriched stream of biomass (often called the light ash fraction, LAF) and a soil-laden reject stream (also known as the high ash fraction, HAF) [9–11]. Lacey et al. [11] showed such separations for chipped forest residues after AC and reported remarkable ash (wt. %) reductions (>32 wt. %) in the final throughputs. Emerson et al. [9] also found quite reasonable reductions in ash (wt. %) for hybrid poplar (from 2.34 wt. % to 1.67 wt. %) and shrub willow (from 2.60 wt. % to 2.14 wt. %). Recently, Thompson et al. [2] showed that AC can effectively upgrade CS by removing 30 wt. % ash at a mild 7.5 Hz fan speed, or approximately 2.1 m/s counter-stream air flow of AC. It has also been reported that a large fraction of HAF byproduct is produced during AC, accounting for more than 12 wt. % of the initial CS. This HAF of CS contains more than 80 wt. % organic content that can be further utilized [2]. Instead, an estimated HAF disposal cost of USD 28.86 per ton has been suggested by Reza et al. [12] and Humbird et al. [13], accounting for 2.5 cents of the USD 2.15 per gallon minimum ethanol selling price. In order to make AC more economically viable, the further utilization of HAF needs to be implemented in conjunction with AC technologies and system-wide economics and sustainability for full material utilization. However, to the best of the authors' knowledge, no work has been done to date to utilize the waste energy in value-added fuel. Therefore, to make use of the vital unrecovered energy from this solid waste and integrate it with the AC technology, the HAF could be converted into essential products.

Hydrothermal carbonization (HTC) is a promising thermochemical process that transforms biomass, such as CS, into a carbon-rich solid product called hydrochar [5,14,15]. The reaction temperature varies between 180 and 260 ◦C, and the pressures are maintained above the saturation pressure. The ionic products of water increase three orders of magnitude (KH2O (573K)/KH2O (298K) = 103) [5,15,16] and the dielectric constant of water reduces from 78.5 (298 K, 0.1 MPa) to 27.1 (523 K, 5 MPa) [15,17]. As a result, biomass undergoes a series of reactions, i.e., hydrolysis, decarboxylation, dehydration, condensation polymerization, and aromatization, in subcritical water due to the increased reactivity and solvent-like properties [18,19]. For agricultural residues, the HTC reaction products can be divided into three streams: 41–90 wt. % solids with 80–95 wt. % of the original calorific value; 2–10 wt. % gas consisting mainly of CO2; the rest is process liquid [20–24]. The energy-dense solid hydrochar can be pelletized to improve the mass and energy density of the feedstock and reduce the cost of transportation, handling and storage [25–27].

During HTC, the low density and low viscosity of subcritical water, in combination with the acidic process liquid and the modification of the biomass structure, aid in the leaching of inorganics from biomass structures [18,28–31]. The reduction of ash is particularly important for hydrochars' application as fuel, as this can significantly reduce the ash-fusion temperatures, leading to slagging in fuel boilers [1]. Therefore, HTC could be used to recover the energy in the HAF stream while reducing ash from the hydrochar. However, HTC is an energy-intensive process as it requires high temperatures and high pressure. Therefore, an energy balance in the integrated AC–HTC is warranted to evaluate the net energy recovery from the integrated process compared to the individual AC and HTC processes. Therefore, the main goal of this work was to study the feasibility of three different scenarios, to determine the energy recovery from CS: 1. AC-only process; 2. HTC-only process; 3. Integrated AC–HTC process. First, the AC of CS was performed to separate LAF and HAF. Then HTC on HAF was performed at various temperatures and the hydrochars were pelletized with LAF. The energy recovery from the integrated AC–HTC process was compared with AC-only and HTC-only, wherein the original (ORG) CS was used for both technologies.
