Enhanced Hydrothermal Carbonization of Spent Coffee Grounds for the Efficient Production of Solid Fuel with Lower Nitrogen Content

Abstract

As an increasingly abundant lignocellulosic biomass with high moisture content, spent coffee grounds (SCG) are an ideal feedstock for hydrothermal carbonization (HTC). The few examples that have converted SCG into a solid fuel hydrochar have yet to address the char's prohibitively high nitrogen content (∼3 wt %)−a barrier to commercialization due to NOx emission regulations. In this work, an alkaline pretreatment is presented that reduces the N content prior to carbonization under regimes optimized for maximum calorific value (HHV), energy yield, and a conflation of both fuel properties. Characterization of hydrochars and secondary chars, the extractable volatile phase, revealed the highest calorific value for the secondary chars (max HHV = 40.69 MJ/kg) and a minimum N content of 0.1 wt % in hydrochars derived from the alkali pretreated feedstock. Char thermal stability and oxidative reactivity were also determined, with pretreated primary chars exhibiting superior combustion reactivities, ignition, and burnout temperatures. Management of HTC process water is both an environmental and operational challenge due to the presence of phytotoxic organic components and the energy expenditure associated with heating large volumes of water. To this end, the process water was recirculated for up to five cycles resulting in an increase in HHV, energy, and solid yield as well as compositional change of the char products.

Recovery of Nitrogen from Low-Cost Plant Feedstocks Used for Bioenergy: A Review of Availability and Process Order

“…The final solid residuals were converted to hydrochar using HTC. 334 HTC-char after N recovery had a higher HHV content (32−37 MJ/kg vs 29−36 MJ/kg) and a higher burnout temperature (518 °C vs 452 °C). Arauzo et al described a biorefinery approach for brewer's spent grains in Figure 2.…”

“…The extraction of proteins prior to thermochemical processing (pyrolysis, HTL, and hydrothermal carbonization (HTC)) has been investigated. − Massaya et al described a multiple-product process for spent coffee grounds. A series of hydrothermal processes were used to obtain an antioxidant aqueous extract containing chlorogenic acids, polyphenolics, and polysaccharides.…”

“…Proteins (21.8–32.8 wt %) were then recovered from the residual cake using alkaline extraction and acid precipitation. The final solid residuals were converted to hydrochar using HTC . HTC-char after N recovery had a higher HHV content (32–37 MJ/kg vs 29–36 MJ/kg) and a higher burnout temperature (518 °C vs 452 °C).…”

Liquid Hot Water (LHW) and Hydrothermal Carbonization (HTC) of Coffee Berry Waste: Kinetics, Catalysis, and Optimization for the Synthesis of Platform Chemicals

“…Previous research has been performed regarding the use of coffee (mainly SGC (spent coffee grounds) and silverskin) (Scheme 1) and different hydrothermal processes to obtain platform molecules, such as biochar [25], polysaccharides (Cellobiose, Glucose, Xylose, Galactose, Arabinose, and Maltose) [26], and activated carbon [20] using HTC; via LHW polyphenols, hydroxymethylfurfural, feruloylquinic acid, epicatechin [27], and antioxidants (DPPH, ABTS, and HPLC) [28] were obtained. It is worth noticing that little to no information was found on the hydrothermal valorization of coffee berry waste (made up of husk, mucilage, parchment, and silverskin) in order to present a possibility for sustainable management of coffee wastes.…”

Biomass Waste Carbonization in Piranha Solution: A Route to Hypergolic Carbons?

“…This motivates on-going research interest for utilizing this waste as a resource for biogas/biofuel production or for the extraction of useful chemicals [7][8][9][10]. Bread and spent coffee wastes have also been employed as starting material for the development of advanced functional carbon materials with interesting properties, often produced via conventional pyrolytic or hydrothermal carbonization methods [11][12][13][14][15][16]. However, to the best of our knowledge, no report on bread and coffee biomass waste carbonization in piranha solution has been published so far.…”