DOI: 10.3303/CET24112046 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 03 July 2024; Revised: 30 July 2024; Accepted: 30 August2024 
Please cite this article as: Lim H.Y., Goh B.H.H., Chai P.V., Ang C.K., Chong C.T., 2024, Assessing the Energy Profitability of Biofuels 
Generated from Coffee Waste via Microwave Pyrolysis, Chemical Engineering Transactions, 112, 271-276  DOI:10.3303/CET24112046 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 112, 2024 

A publication of 

 

The Italian Association 

of Chemical Engineering 

Online at www.cetjournal.it 

Guest Editors: Selena Sironi, Laura Capelli 

Copyright © 2024, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-13-7; ISSN 2283-9216 

Assessing the Energy Profitability of Biofuels Generated from 

Coffee Waste via Microwave Pyrolysis 

Hui Yi Lima,b, Brandon Han Hoe Goha, Pui Vun Chaib,c, Chun Kit Angb,d, Cheng Tung 

Chonga,* 

aChina-UK Low Carbon College, Shanghai Jiao Tong University, Lingang, Shanghai 201306, China. 
bDepartment of Chemical and Petroleum Engineering, Faculty of Engineering, Technology and Built Environment, UCSI  

 University, Kuala Lumpur 56000, Malaysia 
cUCSI-Cheras Low Carbon Innovation Hub Research Consortium, Kuala Lumpur, Malaysia 
dMechanical Engineering Department, Faculty of Engineering, Technology & Built Environment, UCSI University. 

 ctchong@sjtu.edu.cn 

The potential of coffee waste to be valorised into useful bioenergy is investigated using a continuous microwave 

pyrolysis rig. Coffee waste of Arabica species was ground and pretreated prior to pyrolysis at elevated 

temperature under CO2 atmosphere. Valorisation of the coffee waste result in char, liquid, and gaseous 

products. Analysis shows that biochar yield is highest at 450 °C, while bio-oil recovery peaks at 550 °C. Gaseous 

product results in highest yield at 650 °C. Energy analysis indicates that biochar contains the highest energy 

density, while bio-oil's energy density decreased with temperature due to cracking of volatiles. Energy 

profitability was shown to be negative across all temperatures, with highest deficit at 450 °C due to the formation 

of non-recoverable products such as water. Higher energy profitability was achieved at 650 °C as more volatiles 

could be recovered in gaseous form. The study shows that microwave pyrolysis is effective in valorising coffee 

waste and essential to enhance the energy recovery from coffee waste. 

1. Introduction 

Coffee is one of the most widely consumed beverages globally with significant economic value. With millions of 

cups consumed daily, coffee ranks as the second most favoured drink after water and stands as the second 

most traded commodity following petroleum (Lee et al., 2021). The coffee industry produced substantial 

amounts of waste that often end up in landfills that poses environmental concerns such as groundwater 

contamination, pollution from polyphenols, tannins, and caffeine, and requiring considerable space for disposal 

(Sermyagina et al., 2021). Each tonne of processed green coffee yields approximately 650 kg of coffee waste, 

leading to the accumulation of dark coloured waste (Mata et al., 2018). During the 2020-2021 period, global 

coffee consumption reached nearly 10 M t, leading in the production of about 6.5 M t of coffee waste (Ordieres 

and Cultrone, 2022). Microwave pyrolysis is a highly developed thermochemical process that break down 

biomass into biogas, bio-oil, and biochar without the presence of oxygen. This method is particularly 

advantageous as it achieves high fuel conversion efficiency up to 95.5 % and prevents the primary pollutants 

such as volatile organic compounds (VOCs), carbon monoxide (CO), and particulate matter (PM) undergo 

chemical reactions, leading to the formation of secondary pollutants. Utilising coffee waste as a feedstock for 

biofuel production offers the added benefit of not competing with food supplies and also provides an 

environmentally friendly substitute traditional lipid source such as soybeans and rapeseed. Prior research has 

demonstrated the feasibility of converting various types of waste into biofuels through pyrolysis. Horse manure 

pyrolysis in a microwave has been recorded to yield a negative energy profit due to the presence of non-

energetic products such as water (Mong et al., 2020). Empty fruit bunches by microwave pyrolysis results in an 

overall energy deficit of -81.17 %, primarily due to its higher oxygen content. In contrast, waste tire valorisation 

by microwave pyrolysis achieves a 20.66 % total energy profit because the raw feedstock has a high calorific 

value (Mong et al., 2022). Despite these findings, there is a notable lack of data on the microwave pyrolysis of 

271

mailto:ctchong@sjtu.edu.cn


coffee waste, particularly concerning its energy recovery potential. Hence, this study focuses on evaluating the 

pyrolysis characteristics of coffee waste, the yield and energy content of the pyrolysed products, and the overall 

energy profitability of the thermochemical conversion process. 

2. Material and methods 

2.1 Feedstock preparation and characterisation  

In this study, coffee waste of Arabica species was collected from a local café located in Pudong, Shanghai, 

China. The collected sample was evenly distributed on a metal pan and dried at a temperature of 60 °C for 36 

h in a furnace. The specific drying temperature was selected to prevent the breakdown of lipids which can occur 

at temperatures exceeding 60 °C because of the fatty acid oxidation. Additionally, maintaining this temperature 

inhibits the growth of mould during the storage phase. The coffee waste exhibited a moisture loss of 13.68 wt% 

after drying. Characterisation of coffee waste was performed via ultimate analysis. The ultimate analysis was 

performed using an elemental analyser (Elementar, UNICUBE) to determine the proportions of sulphur, 

nitrogen, oxygen, hydrogen and oxygen in the sample. 

2.2 Operating condition and test apparatus 

Microwave pyrolysis of coffee waste was performed using a continuous microwave pyrolysis device (CY-

CP1100C-S model). The microwave pyrolysis device is shown in Figure 1. The main component of the 

microwave pyrolysis reactor consists of a microwave generator that operating at a frequency 2.45 GHz with a 

maximum output power 7 KW. A quartz-glass reactor, measuring 450 mm in height and 60 mm in diameter, was 

used to hold the feedstock inside the microwave cavity. The coffee waste with a consistent weight of 100 g is 

transported forward by the rotary screw that maintain the rotational speed of the screw at 3 r/min to ensure a 

consistent residence time of the coffee waste within the microwave pyrolysis zone are and a stable processing 

speed. The microwave pyrolysis reaction is carried out under CO2 atmosphere to maintain inert condition at the 

controlled flow rate of 2 L/min. The precise control was facilitated by a rotary flowmeter controller. The 

microwave pyrolysis was conducted at the temperature was set at the temperature of 450 °C, 550 °C, and 650 

°C. Each run of the pyrolysis process was conducted for 1 h. 

A flask under the condenser was used to collect the condensed vapour, which is the liquid product (bio-oil). In 

the meantime, moisture was removed by passing the uncondensed vapour over the silica gel bed. The resulting 

dry gases were collected in a gas bag for further analysis. The solid product was collected after the pyrolysis 

process for analysis. The condensed liquid product is purified via a filtration process to remove any solid 

residues or impurities prior to analysis. 

 

Figure 1: Microwave pyrolysis device schematic diagram 

2.3 Characterisation of biofuel 

The gravimetric approach was employed to quantify the product yields produced from the microwave pyrolysis 

process. The bio-oil collected in a flask and solid yield were quantified with a weight balance. To quantify the 

gas yield, a mass balance equation Eq(1) was utilised to quantify the end products.  

𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝑦𝑖𝑒𝑙𝑑 (𝑤𝑡%) =
𝐸𝑛𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 (𝑘𝑔)

𝐹𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 (𝑘𝑔)
× 100 % 

         

(1) 

The biogas produced utilised a gas chromatograph for analysis (Agilent GC 8860). The analysis allowed for the 

detection of various gaseous compounds such as CO, N2, O2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, C4H8 and 

C5H12 are detected. The high heating value of the biogas is calculated by using Eq(2).  

272



𝐻𝐻𝑉𝐺𝐴𝑆  (
𝑀𝐽

𝑁𝑚3) =
(12.63 × 𝑌𝐶𝑂𝑚𝑜𝑙 + 12.75 × 𝑌𝐻2𝑚𝑜𝑙 + 39.82 × 𝑌𝐶𝐻4𝑚𝑜𝑙 + 63.43 × 𝑌𝐶2−𝐶5𝑚𝑜𝑙)

100
 (2) 

where YC2-C5,mol, YCH4,mol, YH2,mol, and YCO,mol are the molar fractions of C2-C5 compounds, methane, hydrogen, 

and carbon monoxide. 

The energy content of the biogas is calculated by aggregating the energy content of individual gas species 

based on the volume via Eq(3).  

𝐺𝑎𝑠𝑒𝑜𝑢𝑠 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝐸𝑛𝑒𝑟𝑔𝑦 (𝑀𝐽) = 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝐻𝐻𝑉 (
𝑀𝐽

𝑚3) × 𝐺𝑎𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 (𝑚3) (3) 

The elements that make up the bio-oil including hydrogen, carbon, nitrogen, sulphur, and oxygen, was analysed 

using an elemental analyser (Elementar, UNICUBE). These values were then used to determine the higher 

heating value (HHV) of the liquid product as shown in Eq(4). 

𝐻𝐻𝑉𝐿𝐼𝑄𝑈𝐼𝐷  (
𝑀𝐽

𝑘𝑔
) =

(124.3 × 𝐻 + 34 ×  𝐶 + 6.3 × 𝑁 + 19.3 × 𝑆 − 9.8 × 𝑂)

100
  (4) 

A bomb calorimeter (SDC 712) was used to measure the heating value of solid product (biochar) heating value.  

The product energy of the liquid and solid product is calculated using Eq(5) which considers the product yield 

(wt%) and the higher heating value (HHV) of each product. 

𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝐸𝑛𝑒𝑟𝑔𝑦 (𝑀𝐽) = 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝐻𝐻𝑉(𝑀𝐽/𝑘𝑔) × 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝑦𝑖𝑒𝑙𝑑(𝑤𝑡%) × 𝐹𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘(𝑘𝑔) (5) 

The energy profit is calculated using Eq(6) that involves summing the energy values of the solid, liquid and gas 

products and comparing them to the initial feedstock energy to determine the overall profitability of the biofuel 

production process. 

𝐸𝑛𝑒𝑟𝑔𝑦 𝑃𝑟𝑜𝑓𝑖𝑡 (%) =  
∑ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝐸𝑛𝑒𝑟𝑔𝑦 (𝑀𝐽) − 𝐹𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 𝐸𝑛𝑒𝑟𝑔𝑦(𝑀𝐽)

𝐹𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 𝐸𝑛𝑒𝑟𝑔𝑦(𝑀𝐽)
 × 100 % (6) 

3. Result and discussion 

3.1 Feedstock physiochemical properties 

Table 1 shows the ultimate analysis of coffee waste. The ultimate analysis indicates that coffee waste has a 

high content of carbon (46.11 wt%) and oxygen (46.05 wt%), while hydrogen (5.73 wt%), nitrogen (1.97 wt%) 

and sulfur (0.14 wt%) constitute the rest. The high carbon and oxygen content in the coffee waste is attributed 

to the high lignocellulosic content, making it a potential feedstock for product valorisation via thermochemical 

conversion. Lo et al. (2017) reported that coffee waste has carbon (44.89 wt%), hydrogen (6.14 wt%), nitrogen 

(0.35 wt%), and oxygen (48.62 wt%) with an HHV of 16.78 MJ/kg. The measured higher heating value of the 

coffee waste prior to pyrolysis is 18.44 MJ/kg, which is comparable to other biomass such as paper film (18.1 

MJ/kg) (Yang et al., 2017), corn stover (16.82 MJ/kg) (Cong et al., 2018), and rice straw (16.16 MJ/kg) (Lo et 

al., 2017). The HHV value of coffee waste falls within the typical HHV range of crude biomass which is between 

15 and 20 MJ/kg (Chen et al., 2015). These comparisons indicate the potential of coffee waste as feedstock. 

However, the nitrogen content in coffee waste is slightly higher compared to most energy crops and fossil fuels 

which typically have nitrogen contents below 1 wt% (Ebeling et al., 1985).  The sulfur and nitrogen content in 

coffee waste is in trace amount, which is advantageous for the bio-oil and syngas production via thermochemical 

conversion, as these compounds will not migrate and contaminate the bioproducts. Thus, low emissions of SOx 

and NOx can be expected when using these products as fuel (Mong et al., 2019). 

Table 1: Physiochemical properties of coffee waste sample 

Ultimate Analysis (wt%) 

Carbon 46.11 

Hydrogen 5.73 

Oxygen 46.05 

Nitrogen 1.97 

Sulphur 0.14 

3.2 Product yield 

Figure 2 shows the product yield generated by microwave pyrolysis of coffee waste at a constant CO2 flow rate 

of 2 L/min, and at different pyrolysis temperatures. At pyrolysis temperature of 450 °C, the yield of biochar is 

273



highest at 42 %, followed by bio-oil at 34 %, and biogas at 24 %. As the temperature rises to 550 °C, the biochar 

yield remains relatively constant at 43 %, while the bio-oil yield peaks at 42 %, and the biogas yield decreases 

to 15 %. At the elevated temperature of 650 °C, the biogas yield increases significantly to 43.20 %, the yields 

of biochar and bio-oil decrease to 33 % and 23.80 %. These observations indicate that higher pyrolysis 

temperatures favor the production of biogas, while lower temperatures are more conducive to biochar 

production. The peak bio-oil yield at 550°C suggests that this temperature is optimal for maximising bio-oil 

production.  

The observed trends are aligned with the results reported by Jouhara et al. (2018), in which the yield of biochar 

decreases with increasing temperatures. The temperature at which pyrolysis occurs is 450 °C, the energy 

supplied is insufficient to fully decompose the coffee waste, resulting in higher yield of biochar caused by the 

carbonisation process (Setter et al., 2020). The rising in biogas yield at higher temperatures is attributed to 

secondary reactions such as decarboxylation, dehydration, and condensation of pyrolytic vapors, which convert 

bio-oil into smaller gaseous compounds. The yield of bio-oil increases with temperature elevation from 450 °C 

to 550 °C. This increase is due to the primary decomposition of cellulose, hemicellulose, and lignin within the 

coffee waste. However, further heating to 650 °C results decrease in bio-oil yield, which is likely due to 

secondary reactions that convert bio-oil into smaller gaseous compounds.  

 

Figure 2: Product distribution from coffee waste microwave pyrolysis carried out at various temperatures. 

3.3 Analysis of energy recovery 

The energy density of the pyrolysed products from coffee waste via microwave pyrolysis were determined to 

assess their potential as biofuels. Figure 3 illustrates the higher heating value (HHV) of the biogas, bio-oil and 

biochar at different pyrolysis temperatures. The results indicate that biochar consistently exhibits a significantly 

higher HHV compared to biogas and bio-oil. Specifically, the HHV of biochar increases from 26.7 MJ/kg at 450 

°C to 28.3 MJ/kg at 650 °C. This increase in energy density with temperature due to the use of CO2 carrier gas 

promotes the carbonisation process, which reduces the carbon proportion in carboxylate species and raises the 

aromatic carbon content in the biochar, thereby enhancing its thermal stability (Wang et al., 2022). These 

chemical processes generate a dry and dense carbon structure by breaking down the functional groups that 

include oxygen and developing alkyl-aryl C-C bonds. In comparison, other biomass sources such as canola 

straw and wheat straw exhibit energy density ranging 22.3-26.9 MJ/kg (Nzediegwu et al., 2021). Biochar derived 

from microwave pyrolysis of coffee waste contains high energy density, making it a potential solid fuel with high 

energy content. 

The bio-oil exhibits peak energy density at 450 °C. As the temperature rises, the energy density in bio-oil 

decreases due to the increased cracking of volatiles, which reduces the presence of heavier compounds in the 

liquid yield. The product distribution shifts towards gaseous product outputs at higher temperatures because the 

extra energy causes further volatiles to break and decompose into light hydrocarbon gases. The lower heating 

value of bio-oil from CO2 pyrolysis is attributed to the presence of water within bio-oil that formed due the water-

gas shift reaction (H2 + CO2 → H2O + CO). High concentrations of CO2 in the pyrolysis atmosphere have the 

potential to react with hydrogen molecules to produce water and CO (Mong et al., 2022). Despite its lower 

calorific value, coffee waste derived bio-oil mainly contains oxygenated compounds such as ketones, phenols 

and pyridine that can be extracted for biochemical production (Del Pozo et al., 2020). The presence of 

oxygenated compounds in the bio-oil contributes to lower energy content in the liquid yield. 

The energy density in biogas increases with rising pyrolysis temperatures, as higher temperature facilitates 

volatile substances to break down into smaller gaseous components and enhancing the energy density of the 

gaseous product. Higher temperatures favor the production of biogas with higher energy density, lower 

temperatures are more conducive in producing bio-oil with higher energy density. Biochar consistently 

274



demonstrates high energy density across all temperatures, making it a viable solid fuel option. This trend 

highlights the temperature as an important factor influencing the yield and energy density of the resulting 

products. 

 

Figure 3: Higher heating value of products conducted at different pyrolysis temperatures. 

By calculating the energy content of each pyrolysed product, the energy profitability of the microwave pyrolysis 

of coffee waste was determined. Figure 4 shows the energy profit radar plot for the pyrolysed coffee waste 

products at 450 °C, 550 °C, and 650 °C. The analysis reveals that all three cases result in negative energy 

profit, indicates that the overall energy in the final products is lower than the initial coffee waste. Among the 

tested temperatures, the highest energy profit occurs at temperatures of 650 °C. Conversely, a temperature of 

450 °C shows the highest energy deficit. The energy deficit is partly due to the water formation which is a by-

product and therefore not contributing to the energy profit. The generation of bio-oil containing high oxygenated 

compounds results in low heating value and reducing the energy density. The energy lost during microwave 

pyrolysis due to non-recoverable products such as water. In the case of coffee waste, the energy deficit ranges 

from -27 % to -34 %. Despite the negative energy profit, coffee waste shows promise as a biomass feedstock 

for energy production. The energy profitability is significantly influenced by the nature of the by-products formed 

and the pyrolysis conditions.  

 

Figure 4: Energy profit for microwave pyrolysis of coffee waste at temperature of 450 °C, 550 °C and 650 °C. 

4. Conclusions 

This study examined the potential of coffee waste as a biomass feedstock for energy production through 

microwave pyrolysis. The ultimate analysis of coffee waste has a significant amounts of carbon and oxygen. 

The microwave pyrolysis process yielded biochar, bio-oil, and biogas with their respective yields varying 

according to the pyrolysis temperature. Biochar yield was highest at 450 °C, bio-oil yield peaked at 550 °C, and 

biogas yield increased significantly at 650 °C. The energy density analysis indicated that biochar consistently 

exhibited the highest energy density, making it a viable solid fuel. However, the analysis revealed a negative 

energy profit across all tested temperatures, with the highest energy deficit observed at 450 °C and the highest 

energy profit at 650 °C. The energy deficit was primarily due to the formation of non-recoverable products such 

as water, which absorb heat and reduce the overall energy output. The findings highlight that both the pyrolysis 

temperature and the composition of the feedstock are critical factors influencing the energy efficiency and 

275



profitability of the process. Despite the current challenges, coffee waste shows promise as a sustainable 

biomass feedstock for clean energy production, offering a potential alternative to solid fossil fuels. 

Acknowledgments 

The authors acknowledge the financial support from the research fund for International Excellent Young 

Scientists (52250610220) awarded by the National Natural Science Foundation of China (NSFC). 

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	Assessing the Energy Profitability of Biofuels Generated from Coffee Waste via Microwave Pyrolysis