Uncatalyzed and acid-aided microwave hydrothermal carbonization of orange peel waste

Highlights

• Orange peel waste is suitable to be treated by microwave hydrothermal carbonization.

• Citric acid catalyst resulted in 30% higher hydrochar yield.

• Citric acid produces hydrochar with higher C content, calorific value and nanospheres.

• Citric acid catalyst promotes the formation of nanospheres in hydrochar.

Abstract

Orange, one of the most important fruit categories to be consumed across the world, when processed produces 50% of its weight as waste. Current waste management options for orange peel waste are inadequate to use the waste in wholesome and its disposal might lead to other environmental concerns. Here, we present microwave hydrothermal carbonization as an alternative to utilize the orange peel waste. Further, using citric acid to catalyze the microwave hydrothermal carbonization resulted in 30% higher maximal yield of hydrochar, and the hydrochar produced had better elemental, proximate and energy properties than hydrochar made during uncatalyzed microwave hydrothermal carbonization. Further, structural analysis revealed that citric acid promoted the formation of nanospheres during microwave hydrothermal carbonization. Taken together, microwave hydrothermal carbonization of orange peel waste using citric acid as a catalyst might not only help address the waste management concerns for orange peel waste, but also can produce end products of potential commercial value.

Introduction

Oranges being one of the important fruits, are either consumed directly or primarily processed into juices, or marmalades. Of the ~ 73 million tons of oranges that was produced globally in 2017 (FAOSTAT, 2018), 70% was processed into foods including juices (Martín et al., 2010). During juice production, it is estimated that about 50% of the fresh fruit weight after extraction of juice is deemed as waste, which is primarily composed of peel, seeds, and membrane residues (Braddock, 1995). Some of the major ways of managing this waste include cattle feed, composting, incineration, or land fill (Coma et al., 2017). Among these applications, orange waste is prominently used as Cattle feed supplement as it is shown to be associated with improved quality of the meat. However, serious drawbacks of this application include high demand for energy, pre-drying before valorization to reduce moisture content, ease of decay of waste, bitter taste, and development of illness from feed with high content of orange waste (Negro et al., 2017). Other uses such as composting, land fill, and incineration lead to several undesired environmental and health concerns owing to a high risk for microbial contamination and fermentation capability of the waste, and environmental pollution due to the release of greenhouse gases during incineration (Negro et al., 2017). Hence, waste valorization strategies that can utilize the waste in a wholesome manner are needed.

Hydrothermal carbonization (HTC) is gaining increasing attention as one such novel method of waste utilization owing to its ability for wholesome waste utilization. HTC is typically conducted by submerging the biomass in an aqueous medium carried out at relatively low temperatures (typically 180 to 250 °C) and autogenous pressure (2–5 MPa) for several minutes to several hours (Hoekman et al., 2011, Libra et al., 2011). The biomass is mostly converted into a solid fraction known as hydrochar, and a liquor with very little to no gas. As the biomass undergoes HTC, it becomes compacted, which facilitates the transportation, storage, and handling of the resulting hydrochar. Since orange waste is composed of 80–90% of moisture and is rich in polysaccharides and related polymers such as cellulose, hemicellulose, pectin, and lignin, it is highly suitable for HTC. Conventional HTC processes have been explored to treat orange waste to obtain hydrochar. At holding temperatures and times ranging from 180 °C to 260 °C, and 30 min to 20 h, respectively, the yield of hydrochar produced from orange waste fluctuated between 36 and 58%wt (Catalkopru et al., 2017, Erdogan et al., 2015, Fernandez et al., 2015). However, conventional HTC suffers from the disadvantages of conduction-mediated heating, which is slow and thus takes longer to reach the target holding temperature, and hence the total process time and energy requirement increases (Nüchter et al., 2004).

Microwave heating on the other hand overcomes these disadvantages and has garnered wide attention owing to its efficient heating principle which can heat the material from within by taking advantage of the dielectric heating phenomenon such as dipolar polarization or ionic conduction (Glasnov and Kappe, 2007). For instance, in our previous study we have reported that the time it takes to reach the target holding temperature, i.e., come-up time, was on average 3–4 times lower for MHTC when compared to our conventional HTC reactor that operates based on conduction mediated heating (for details of the reactor design, please refer to Kannan et al., 2018). One of the most commonly used conventional HTC reactors, the Parr reactor, was used for conventional HTC of orange waste with a heating rate of 3 °C/min, resulting in a come-up time of 157 mins to reach 200 °C holding temperature (Erdogan et al., 2015). On the other hand, the microwave reactor used in our study has been previously shown to reach 210 °C in ~ 15 mins (Kannan et al., 2018), thereby drastically reducing the come-up time. Thus, microwave heating not only lowers the residence time of the process, but also may potentially facilitate better energy balance and cost-benefit ratio due to lower power demands (Nüchter et al., 2004). Further, microwaves have been shown to promote new pathways during biomass valorization, albeit, the composition of the feedstock might profoundly influence such reactions (Luque et al., 2012). Currently, microwave hydrothermal carbonization (MHTC) has been shown to be effective to treat lignocellulosic biomass such as pine sawdust (Guiotoku et al., 2009), grapefruit peel (Semerciöz et al., 2017), and coconut shell (Elaigwu and Greenway, 2019), mixed biomass (containing lignocellulosic and non-lignocellulosic material) such as human biowaste (sewage sludge and human faecal sludge) (Afolabi et al., 2017), as well as non-lignocellulosic biomass such as fish waste and shrimp waste (Kannan et al., 2017a, Kannan et al., 2017b, Kannan et al., 2015). The suitability of orange waste for MHTC is largely unexplored.

The use of catalyst to aid in the HTC has been explored in many lignocellulosic wastes and has been shown to improve the yield and properties in many cases (Susanti et al., 2019). Citric acid has been reported in literature as the catalyst of choice for HTC of lignocellulosic waste (Susanti et al., 2019). This is because acid catalysts are known to accelerate the dehydration of sugars to produce 5- Hydroxy Methyl Furfural (5-HMF) which in turn is the precursor for hydrochar (Susanti et al., 2019). Also, in comparison to strong acids such as sulfuric acid and hydrochloric acid, citric acid poses little risk to the environment and is inexpensive (Susanti et al., 2019). The efficacy of using citric acid as a catalyst during MHTC is yet unknown.

We set out with three main objectives, 1. To assess the suitability of the orange peel waste (OPW) for MHTC to produce hydrochar, and to further optimize the MHTC process parameters (holding temperature and time) to maximize hydrochar yield. 2. To characterize the elemental, proximate, energy value and structural properties of the hydrochar produced from OPW. 3. To ascertain whether citric acid aided MHTC offered any advantages over uncatalyzed MHTC, in terms of offering better hydrochar yield and/or hydrochar with better properties. Our results, for the first time, show that OPW is a suitable material for MHTC to produce hydrochar of properties comparable to hydrochar produced from other biowastes. Further, for the first time, we found that the addition of citric acid during MHTC of OPW resulted in enhanced yield, and promoted the formation of nanospheres, while achieving elemental, energy, and proximate qualities comparable to hydrochar derived from uncatalyzed MHTC. These results collectively show that OPW could be valorized using MHTC to yield hydrochar with properties that may yield potential commercial value.