1. Introduction
Coal dependence has led to significant air pollution as the combustion of coal releases various air pollutants (trace metals, chlorine, greenhouse gases etc.) that are hazardous and toxic to the surrounding life and environment. In 2016, U.S. coal combustion accounted for 26.3% of the CO
2 produced from fossil fuel combustion and contributed to nearly 69% (CO
2 equivalent basis) of the greenhouse gases produced from the electrical power generation sector [
1]. In the same year, the electrical power sector also contributed to 43.8% of the total SO
2 released, of which 92% was contributed by coal combustion [
1]. The mitigation of these pollutants while meeting the electricity demand are essential for keeping up with the continual growth of the world’s economy as well as maintaining its environmental health. In the U.S., nearly 261 billion tons of coal are available in recoverable coal reserves (as of 2012) which is estimated to last for more than 150 years based on an average production rate of 1.5 billion tons per year [
2]. Such resource availability allows for U.S. coal-fired power plants account for approximately 40% of yearly electricity generation (per 2015) while 2040 projections expect total consumption to be 705 million short tons (an approximate energy equivalent of 13.49 quadrillion) [
3].
In 2017, biomass was the highest primary energy producer among renewable energies at 45.6% while accounting for 5.8% of the U.S. total primary energy produced (fossil fuels, nuclear, and renewables) [
4,
5]. Biomass utilization as a fuel can minimize greenhouse gas emission as the emitted carbon dioxide was originally used for plant growth via photosynthesis [
4]. Thus, in an attempt to reduce coal-fired plant emissions in the form of toxic metals, sulfur dioxide, and nitrogen oxides, co-firing of biomass and coal has been implemented [
6,
7]. Despite the reduced environmental impacts from co-firing, significant combustion setbacks are still experienced. Overall energy content is reduced due to co-combustion of higher heating value of coal (e.g., 24–30 MJ kg
−1) with lower heating value of biomass (10–22 MJ kg
−1) [
8,
9,
10,
11,
12,
13]. Additionally, coal densities (e.g., 700–900 kg m
−3 for bituminous coals) vary with biomass densities (e.g., 100–600 kg m
−3) which can result in density partioning and overall lack of homogeneity in the coal-biomass mixture [
8,
9,
10,
11,
12,
13]. Lastly, biomass processing (e.g., milling, drying, etc.) prior to co-firing are necessary installments that can be costly due high moisture content. In fact, a 2004 EERE study discussed that minor boiler modifications could be performed for existing coal plants in order to co-combust with biomass, however, feedstock handling for drying, grinding, and overall homogeneity would have to be maintained [
14].
Hydrothermal carbonization (HTC) has been extensively researched for biomass pretreatment and low rank coal dewatering [
15,
16,
17,
18,
19]. HTC uses subcritical water (temperatures 180–260 °C) to convert biomass into a carbon-rich and hydrophobic solid fuel, known as hydrochar. Co-Hydrothermal Carbonization (Co-HTC) is the simultaneous treatment of two feedstocks in the presence of subcritical water. Co-HTC can promote the synergistic interaction between the two feedstocks during treatment, which would not occur when the feedstocks are separately treated (i.e., HTC). Shen et al., demonstrated de-chlorination could be improved with Co-HTC [
20]; as biomass intermediates produced during treatment provided more phenolic compounds and short chain organic intermediates to react with chlorine groups in PVC. Zhang et al., showed increased organic and carbon retention rates for the Co-HTC of sewage sludge and pine sawdust as opposed to the individual HTC of each respective feedstock [
21]. It was hypothesized that Mailard reactions between sawdust sugar derivatives and sewage sludge protein formed insoluble hydrochar. More applicable to improved co-firing, Nonaka et al., and Saba et al., evaluated the Co-HTC of biomass-coal blends with varying feedstock compositions and varying reaction temperature, respectively. Nonaka et al., found that Co-HTC favored more polymerization reactions and that more thermally stable hydrochars were produced from higher coal ratios [
22]. Furthermore, heating value and chemical characterization did not change notably with changing feedstock compositions. Meanwhile, Saba et al., observed hydrochar homogeneity and additional biomass degradation (via mass yield) catalyzed by Co-HTC with coal, resulting in lower pH media [
23]; low pH would then encourage more leaching of inorganic content and sulfur. As expected, both studies found increased HHV of produced hydrochars than that of the blended and untreated feedstocks. Overall, hydrochar can potentially serve as a better option for co-firing than biomass; co-treatment with coal can have significant advantages as well.
HTC energetics have shown to be promising as the heat of reaction was shown to be exothermic by Funke and Ziegler [
24]. For HTC of glucose, cellulose, and wood at 240 °C and 6 h heats of reactions were determined via digital scanning calorimetry to be approximately −1 MJ kg
−1 for each feedstock. Increasing reaction severity through temperature (>310 °C) and time (64 h) resulted in a minimum heat of reaction of −2.4 MJ kg
−1 as a result of the production of lower energy products CO
2 and H
2O [
24]. Although consideration for reaction by-products were considered, all carbon losses from experimental mass balance was assumed to have gone into the gas phase as CO
2. These results show potential for alleviating energy input for a continuous process. In fact, utilization of these results allowed McGaughy et al., to perform an energy audit on a simplified continuous HTC plant that treats food waste [
25]. Energy output to input ratio (EOIR) were 2.95–4.91 depending on HTC reaction temperature.
Although HTC treatment shows promising fuel upgrading and energy savings from decreasing energy duty for the feedstock drying step, process economics should also be considered for feasibility. Li et al., performed a techno-economic models for rice husk to fuel conversions via HTC, pyrolysis, and anaerobic digestion and compared them to the direct combustion of rice husks for heat and power [
26]. With considering operating costs and negligible differences in fixed capital costs, HTC of rice husks proved to be a more advantageous process compared to the other two as it can have higher solids loading, lower utilities costs, and lower process water costs. For 1-ton rice husk conversion, HTC cost per MJ was
$0.013 MJ
−1 (
$153 MWh
−1), having 81% and 38% savings compared to fossil fuel oil costs and direct combustion of rice husks (when not incorporating capital costs into the HTC costs), respectively. This cost is still 4.5 times more than the cost of Central Appalachian Coal at
$33.7 MWh
−1 [
27]. Further heat integration of HTC process liquid can further benefit the process economics in this study with more temperature optimization of HTC. Additionally, considering capital costs, maintenance costs, and the time value of money contributes to a more accurate model for commercial design.
Lucian et al., did process modeling for the HTC of grape marc and off-specification compost from raw processing to pelletized hydrochar [
28]. Of the various process parameters, process optimization from their data occurred at the shortest residence time of 1 h (from 1, 3, and 8 h times), a reaction temperature of 220 °C, and a slightly higher dry biomass-to-water treatment ratio of 0.19. Treatment at 220 °C from 180, 220, and 250 °C produced a high enough higher heating value (HHV) without excess energy input, which allowed for enough heat integration and lowered hydrochar drying costs as samples were more hydrophobic and had lower mass yields. Most electrical costs came from pelletization and reactor feed pumping, while most thermal costs came from HTC reaction heating. HTC reaction heating input was significantly larger for lower dry biomass-to-water ratio as there is less energy efficiency from heating more water content, which is why the higher ratio of 0.19 yielded better optimization. Overall pellet production cost was
$171.1 per ton of hydrochar and overall breakeven cost was equal to
$218 per ton; these costs were considered competitive with the cost of wood pellets (
$163.5–218 per ton) but not with coal costs. Co-treatment can remove pelletization costs as coal is not pelletized for traditional coal firing and heat integration is essential for viability. Shorter retention times have shown to be effective for hydrochar conversion and can increase revenue by allowing more product to be produced. To the author’s knowledge, process economics have not been performed for a Co-HTC system, regardless of the feedstocks treated. Therefore, the main objective of this study is to evaluate techno-economic viability of Co-HTC of bituminous coal with miscanthus, while taking into consideration of capital costs and overall operating and maintenance costs over the plant lifetime. Heat integration will also be taken into consideration to reduce energy costs and to improve overall plant design.