Preparation and fuel properties of biochars from the pyrolysis of exhausted coffee residue

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Abstract

Exhausted coffee residue (ECR), a processing by-product from the soluble coffee industry, was evaluated as a potential feedstock for preparing biochar fuel. Its thermochemical characteristics were first investigated using the standard methods, showing that the biomass obviously comprises a large percentage of volatile matter and less amount of ash. Its molar ratio of hydrogen to carbon (H/C) is about 1.59, which is close to the cellulose (H/C = 1.67) and hemicellulose (H/C = 1.60). Under the pyrolysis temperature of 673–973 K and the heating rate of about 10 K/min studied, the yields and calorific values for the resulting biochar products reached to the maximum at the condition of around 673 K. The calorific value (around 31.9 MJ/kg) of the optimal biochar from ECR was relatively high as compared to that of coal. However, the resulting biochar has greater nitrogen content, which may contribute to greater nitrogen oxides (NOx) emissions during combustion or co-firing. On the other hand, the resulting biochars were characterized using helium-based solid density (true density), showing that the density increased with increasing pyrolysis temperature in accord with the conversion of low-density disordered carbon to high-density turbostratic carbon.

Highlights

► Under the pyrolysis temperature of 673-973 K and the heating rate of about 10 K/min studied, the yields and calorific values for the coffee residue-derived biochars reached to the maximum at the condition of around 673 K. ► The calorific value (around 31.9 MJ/kg) of the optimal biochar was relatively high as compared to that of coal. ► The true densities for the resulting biochars increased with increasing pyrolysis temperature in accord with the conversion of low-density disordered carbon to high-density turbostratic carbon.

Introduction

The reuse of renewable bioresources as energy sources or alternative fuels in replacement of fossil-based feedstocks has received much attention in recent years. This transition from non-renewables to renewables is mainly due to the energy crisis and environmental issues (e.g., global warming). The energy supply from domestic biomass resources not only enhances fuel diversification, but also reduces the air pollution because the biomass resource contains relatively low contents of sulfur and heavy metals in comparison with fossil fuels such as coal [1]. In this respect, agricultural residues contain large amounts of lignocellulosic constituents (i.e., cellulose, hemicellulose and lignin) and thus possess high-energy contents [2]. Therefore, they can enrich available carbon sources in the production of biomass energy based on benefits of both energy utilization and environmental protection. For these reasons, there is an increasing interest in co-firing biomass with coal in the power plants and industrial utility boilers [3], which have been designed with the best available control technologies for hazardous or toxic air pollutants emitted from flue stack. However, the success of coal co-firing was limited to about 20% biomass mix mainly due to the contents of moisture, ash and chlorine in the biomass [3].

Coffee is one of the most important agricultural commodities in the world because it is a popular brewed drink prepared from roasted bean. However, the processing of coffee often generates significant amounts of solid residues. Among them, exhausted coffee residue (denoted as ECR), or called spent coffee ground, is inevitably generated from soluble coffee production during the extraction process, where the roasted and grinding coffee is introduced into percolators in countercurrent treatment with the pressurized hot water to extract the favors (soluble materials) in the soluble or instant coffee manufacturing factory [4]. The solid final product is thus obtained, and the insoluble residue (a slurry containing spent coffee grounds) is screw pressed. The residue weighs approximately 50% of the total input mass of coffee feedstock, representing a significant bioresource from the coffee-derived products processor. In Taiwan, for example, the annual generation of ECR from the food processors was estimated as over 6000 metric tons based on about 12,000 metric tons of imported coffee (not roasted and not decaffeinated) in 2009. Because of its high organic contents such as carbohydrate, protein, fiber, caffeine, polyphenols, tannins, and pectins, ECR was suitable as an animal feed, soil conditioner, and organic fertilizer [5], or further reused as a potential feedstock to produce useful products such as enzymes, organic acids, flavours and aroma compounds [6]. On the other hand, ECR is characterized chemically by a high content of carbon source. In this respect, it is similar to various agricultural processing by-products used as good biomass fuels in residentials and mills [2].

However, the use of raw biomass residue as a fuel often faces several problems, which may limit its wider application in the power generation. These include large bulk volume, high moisture content, low heating value and energy density, hygroscopic nature, and smoke during combustion, making it low in combustion efficiency [7], [8]. In view of overcoming some of the aforementioned limitations of biomass material, thermochemical conversion is a promising route to convert lignocellulosic biomass to fuel, chemical, and renewable power [9]. Pyrolysis is generally described as the thermal decomposition of the organic components (long polysaccharides) in biomass in an inert atmosphere at mediate temperature to yield biochar (charcoal). As a result, this process improves the thermochemical properties of biomass, and produces a hydrophobic solid product with an increased energy density.

In view of reusing ECR as a biomass precursor for the production of pyrolysis products, several research works have been performed on this [10], [11], [12], [13], [14], [15]. However, these focused on the preparation of activated carbon using chemical and physical activation. Although a few studies paid attention to the use of coffee husk (another coffee residue) as an energy source [16], [17], very little work on the pyrolysis of ECR is available in the published literature. Data on the fuel properties (especially in true density) of resulting biochars are also limited. In this study, the thermochemical characteristics of ECR were investigated using the standard methods. Then, the pyrolysis of ECR was studied in a tubular fixed-bed system. A series of biochar products were thus prepared at the heating rate of about 10 K/min up to the prescribed temperature (i.e., 673–973 K), and further analyzed using chemical and physical techniques to determine its possibility of being a potential source of renewable fuels.

Section snippets

Materials

The biomass sample used as the feedstock for its characteristic analysis and experimental pyrolysis runs was exhausted coffee residue (denoted as ECR), which was obtained from a local soluble coffee factory in southern Taiwan. This bioresource sample was first closely stored in grass bottles. Prior to the thermochemical characteristics analyses and pyrolysis experiments, each prepared sample was dried at about 373 K for at least 24 h.

Proximate analysis

According to the American Society for Testing and Materials

Characterization of exhausted coffee residue

The data in Table 1 indicated the results of proximate, ultimate, and chemical composition analyses for ECR. It is evident that the biomass comprises a large percentage of volatile matter (i.e., 79.52 wt%). The contents of fixed carbon and ash in the sample are only 8.23 and 0.73 wt%, respectively. It is well known that fixed carbon is the solid combustible residue that remains after a particle sample is heated and the volatile matter is expelled. Thus, it is used as an estimate of the amount of

Conclusions

The reuse of the coffee milling by-products as energy sources has received much attention in recent years due to the economic consideration, energy crisis and environmental issue. In this work, the thermochemical characteristics of exhausted coffee residue (ECR) have been analyzed according to the standard methods. The results showed that molar ration of hydrogen to carbon (H/C) for the biomass was about 1.59, which is relatively low in comparison with those (2.2–2.4) of hydrocarbon fuels, but

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