Production of Biochar by Pyrolysis of Food Waste Blends for Negative Carbon Dioxide Emission

Abstract

A negative carbon emission scenario via pyrolysis of three different food waste blends was investigated. A tube reactor was utilized for pyrolysis runs at temperatures of 650 °C, 725 °C, and 900 °C, while the carbon inventory was prepared. The blend of rice and french fries resulted in the highest char yield, being 212 g/kg at 650 °C pyrolysis temperature. In this case, each kg of food waste can correspond to 536 g of captured or removed CO₂ from the air. The blend of roast pork and breaded chicken showed significantly less carbon removal potential of 348 g_CO2/kg_sample measured at 650 °C pyrolysis temperature, compared to rice and French fries. A higher pyrolysis temperature resulted in lower char yields, but, on the other side, it resulted in a higher carbon content of char. Additionally, higher pyrolysis temperature resulted in lower carbon capture potential within the temperature range utilized in this study. The heating value of dry pyrolysis gas was between 12.0–16.6 MJ/Nm³ and 10.3–12.3 MJ/Nm³ during the heat-up and constant temperature period, respectively. Based on the results, negative CO₂ emission can be reached via pyrolysis of food waste with the benefit of capturing carbon in solid form, and therefore, this method can be considered a promising and alternative method to treat food waste.

1. Introduction

The generation of food waste is one of the biggest challenges that humans are facing as globally around 1.3 billion tons of food is wasted, which represents around one-third of food production [1,2]. In contrast, food production accounts for around 26% of global greenhouse gas emissions, from which around 6% come from food losses [3,4]. Other studies report even higher emissions of 8–10%, which can be associated with food that is not consumed [5]. Additionally, global warming might be reduced by more than 2 °C according to the “Project Drawdown” solution by avoiding food waste alone [6].

Recently, several methods have been available to treat food waste, including food donation, animal feed, composting, thermal or biological conversion, or landfilling [3,7]. Landfilling the food waste represents an outstandingly dominant portion of such waste streams with about 35% contribution [8]. On the other side, it is estimated that globally 1.2 trillion USD can be associated with food loss [9] causing significant economic impact. Therefore, lowering the amount of food waste generated is desirable from various aspects but alternative treatment methods might also be applied such as pyrolysis, which can be considered as negative carbon emission if food waste is thermally treated in the absence of oxygen and the solid char with high carbon content is stored underground or utilized as soil additive [10]. This approach fits well into the possible negative carbon emission methods, which include (1) afforestation, reforestation, (2) soil management (including biochar), (3) BECCS—bioenergy with carbon capture and storage, (4) DACCS—direct air capture and storage, (5) enhanced weathering, and (6) ocean fertilization [11].

Being carbon negative, or in other words, climate positive, means that more CO₂ is captured from the air than emitted. Although the thermochemical conversion of various food wastes was investigated and presented in the literature, the possibility of reaching negative carbon emissions was not the focus. One of the first investigations on this topic was published by Shule et al. [12]. In this study, the organic fraction of municipal solid waste after an anaerobic digestion process was pyrolyzed, and Aspen Plus V11 software was used to calculate the energy requirements and carbon negativity achievements. It was concluded that 1 ton of dry sample can provide 151.4 kg CH₄ and 355.6 kg negative CO₂ equivalent storage by combining anaerobic digestion, pyrolysis, catalytic reforming, methanation, and carbon storage. Elkhalifa et al. [13] published a review about biochar production by food waste pyrolysis, but alternative treatment methods are also summarized, such as composting, anaerobic digestion, or landfilling. It was concluded that the biochar obtained from pyrolyzed food waste can be applied to improve soils through enhancement of the retention of water and nutrients. Grycová et al. [14] pyrolyzed waste cereals and waste peanut chips at 800 °C in a retort inserted into a tube furnace and provided information about the process mass balance and elemental composition of solid residues. Around 22–25 wt% of solid residues were presented with 80.0–84.3 wt% carbon content. Opatokun et al. [15] published some properties of food waste across countries and regions over the world. It can be concluded that the total carbon content of food waste varies between 42.3 and 55.5 wt%. Additionally, by increasing the pyrolysis temperature from 300 to 700 °C, the carbon content of solid remains increases from 66.7 to 74.6 wt%, respectively. Poudel et al. [16] reported a similar carbon content of mixed food waste being 47.57 wt% on a dry basis. The food waste including vegetables, grains, and meats was collected from a university cafeteria. Gupta et al. [17] pyrolyzed canteen food waste in a muffle furnace at 500 °C, containing mainly rice, noodles, pasta, meat, vegetables, and bones. Additionally, rice itself was investigated due to the fact that a significant amount of cooked rice is also wasted. Based on the findings, biochar from mixed food waste and rice food waste contained 70.9 wt% and 66.2 wt% carbon, respectively. The study presents SEM pictures of the obtained biochars, which gives morphological insights into the structures of such pyrolysis chars.

Available studies investigate the pyrolysis of food waste from many aspects; however, the role of carbon negativity was not in the focus, and related studies are scarce. The lack of comprehensive understanding of food waste pyrolysis on the environmental performance including carbon footprint can be originated from the fact that, to date, only a limited number of such pyrolysis plants are operating across the globe [18,19]. Although the reduction in food waste should be the primary desired goal, the amount of food waste is expected to increase in the future [20]. Therefore, the investigation of food waste treatment methods from various aspects is necessary for exploiting the possibilities.

Biochar produced from the pyrolysis of biomass [21,22,23,24] is a similar approach to that of food waste char in the negative carbon scenario pathways. Negative CO₂ emissions can be achieved through the deposition of pyrolysis cokes but incorporation into the soil should also be mentioned as a potential use as soil amendments. Li et al. [25] published a comprehensive study that discusses the effects and benefits of incorporating pyrolysis coke (biochar) from biomass into the soil. According to their findings, coke can enhance soil fertility, nutrient retention capacity, and water-holding capacity. Its porous structure provides a habitat for beneficial microorganisms, improves soil health, and promotes plant growth. Additionally, it is characterized by high chemical and microbial stability. In addition to these advantages, a critical review published by Yaashikaa et al. [26] points out that despite these positive properties, the number of studies on such characteristics is still limited. Therefore, the investigation of long-term interactions between different cokes and soil types remains essential.

The main goal of this study is to investigate the pyrolysis of various food waste streams collected from a canteen located in Miskolc (Hungary) and estimate the magnitude of negative carbon emission potential if the solid pyrolysis char is stored and its carbon content is not converted back to carbon dioxide. Additionally, a possible treatment method scenario is proposed for food waste streams and from negative carbon emission prospects.

2. Materials and Methods

Food waste samples were collected from a canteen located at the University of Miskolc (Hungary) and three different blends were prepared for pyrolysis with composition summarized in

Table 1. Ultimate, ash, and moisture analysis of food waste blends used for pyrolysis experiments.

Base Material	Sample ID	Concentration on Dry Basis, wt%						Moisture, wt%
		Carbon	Hydrogen	Nitrogen	Sulfur	Oxygen	Ash
Roast pork and breaded chicken	RPBC	54.84	7.49	11.56	0.77	22.14	3.20	55.25
Rice and french fries	RFF	49.86	6.93	1.78	1.06	38.50	1.87	49.47
Rice and roast pork	RRP	53.84	7.51	9.87	1.37	24.89	2.52	61.76

. Each blend contains an equal amount of individual samples on a weight basis. Ultimate analysis of waste blends was performed based on EN ISO 16948:2015 (determination of total carbon, hydrogen, and nitrogen) and MSZ 24051:2001 (determination of sulfur content using high-temperature tube furnace combustion method) standards.

Pyrolysis runs with 50 g samples performed at three different temperatures of 650 °C, 725 °C, and 900 °C in a measurement system (depicted in Figure 1) containing a tube reactor. The heating procedure can be divided into two parts: (1) the heat-up period, where the sample is heated from ambient to the desired temperature, and (2) when the sample is kept at the desired temperature until the specific gas yield drops below 1 l/kg. The reactor was purged with argon before each experiment to eliminate the presence of the air. Specific gas yield, gas composition, and temperature were monitored during the experiments. The dry gas composition (H₂, CO, CO₂, H₂S, and hydrocarbons including CH₄, C₂H₆, and C₂H₄) was measured by gas chromatography with a sampling interval of 3 min (model: Dani Master; TCD detector with 3 columns: Restek RT-Q-Bond 30 m × 0.32 mm × 10 μm, Restek RT-Q-Bond 15 m × 0.53 mm × 20 μm, and Restek RT-Msieve 5A 30 m × 0.53 mm × 50 μm). The condensable content of the wet pyrolysis gas was collected and analyzed with the Parr 6200 isoperibolic bomb calorimeter for HHV.

The solid-to-solid carbon conversion (η_s-s) was calculated with the following formula:

$${\eta }_{s\text{-}s}=100·{\displaystyle \frac{m_{sr}·c_{c,sr}}{m_f·c_{c,f}}},$$

(1)

where c_c,sr and c_c,f are the carbon concentrations of the solid residue and fuel used, respectively, while m_sr and m_f are the weights of the solid residue and fuel used, respectively. The solid-to-gas conversion (η_s-g) was calculated by

$${\eta }_{s\text{-}g}=100-{\eta }_{s\text{-}s},$$

(2)

where the portion of carbon appearing in wet gas is represented. The higher heating values of the gases were calculated based on the measured concentrations expressed as

$$HH{V}_{gas}=\sum _{i=0}^n{c}_i\ast HH{V}_{c_i},$$

(3)

where c_i is the concentration and HHV_ci represents the HHV value of the combustible component [27]. The char yield was computed using the following equation:

$$M_s={\displaystyle \frac{m_{sr}}{m_{wf}}}·1000,$$

(4)

where m_wf is the weight of wet fuel. The higher heating values of the chars generated were calculated as follows:

$$HH{V}_{char}=c_C·HH{V}_C+c_H·HH{V}_H,$$

(5)

where c_C and c_H are the concentrations of carbon and hydrogen, respectively. The gas flow rate was evaluated from two gas composition measurements combined with a known amount of CO₂ gas injection (Figure 1). The CO₂ with a purity of 99.995 Vol% was injected into the gas stream through a mass flow controller (type: Sensirion SFC5500). The gas flow rate can be calculated by measuring the CO₂ concentration before (c_IN) and after (c_OUT) the CO₂ injection, and the formula is expressed as

$$V_{syngas}={\displaystyle \frac{V_{C{O}_2}(100-c_{OUT})}{c_{OUT}-c_{IN}}},$$

(6)

where V_CO2 is the volumetric flow rate of the added CO₂. The CO₂ concentration was measured with an Agilent 490 micro GC gas chromatograph.

3. Results

3.1. Pyrolysis Process

Table 2. Yield and composition of pyrolysis solid residues at temperatures investigated in this study.

Sample	Temp, °C	ηs-s, %	ηs-g, %	Char Yield, g/kgsample	Composition of the Dry Samples, wt%
					Carbon	Hydrogen	Nitrogen	Sulfur	Oxygen	Ash
RPBC	650	38.7	61.3	149	63.77	2.91	3.52	<100 ppm	17.28	12.52
RPBC	725	28.5	71.5	101	69.22	1.95	2.42	<100 ppm	1.85	24.56
RPBC	900	25.3	74.7	85	73.11	1.17	2.18	<100 ppm	1.87	21.67
RFF	650	58.1	41.9	212	69.01	5.19	1.41	<100 ppm	18.33	6.06
RFF	725	37.1	62.9	115	81.31	1.75	0.68	<100 ppm	7.63	8.64
RFF	900	29.3	70.7	87	84.79	1.88	0.54	0.48	4.99	7.32
RRP	650	44.8	55.2	152	60.66	1.98	5.44	<100 ppm	21.33	10.59
RRP	725	34.6	65.4	89	80.04	1.20	0.62	<100 ppm	3.02	15.12
RRP	900	31.5	68.5	80	81.10	1.09	0.50	0.68	2.49	13.13

summarizes the carbon conversion, char yield, and char composition as a function of different pyrolysis temperatures. Typically, meals (roast port and breaded chicken) resulted in char with lower carbon content and higher ash content compared to vegetables (rice and French fries). Additionally, temperature increases elevated the char carbon content but resulted in less overall char yields and lower carbon conversion. Therefore, from a solid-to-solid carbon conversion point of view, it is better to use lower temperatures, which also has the benefit of lower energy requirements. As only a part of the carbon is converted into biochar, the remaining carbon should appear in the gas phase, forming mainly CO, CO₂, and C_xH_y.

Typically, the HHV of the chars lies between 22.7 and 30.0 MJ/kg, representing significant energy content. The HHV of the chars was calculated based on the ultimate analysis of solid residues. However, if a negative carbon emission scenario is the desired goal, then the energetic utilization of the char should be at least partially avoided, meaning that this carbon cannot be emitted into the atmosphere in the form of CO₂. It is worth noting that although the initial material contains sulfur between 0.77 wt% and 1.37 wt%, the sulfur content of solid remains was below the detection limit in most cases, indicating that rather, sulfur appears in the gas phase.

Pyrolysis gas analysis is important from an energetic utilization point of view because the gas can be combusted to provide heat for the overall process. From a carbon point of view, part of the initial carbon is converted into solid biochar, while another part appears in the gas phase in the forms of C_xH_y, CO, and CO₂. Most of the hydrocarbons evolved during the heat-up period, while H₂ and CO are the most dominant components in pyrolysis gas at constant temperatures.

Table 3. Average gas concentrations at different pyrolysis temperatures. The temperature given during the heat-up period represents the final temperature. C₂H_x is the sum of C₂H₄ and C₂H₆ components.

Period	Sample	Temp, °C	H2	CO	CH4	C2Hx	CO2	H2S
Heat up	RPBC	650	16.57	12.83	10.2	6.31	20.73	0.52
	RPBC	725	11.19	10.81	10.97	7.5	25.58	0.58
	RPBC	900	32.06	20.26	6.34	5.49	17.89	<0.01
	RFF	650	27.4	27.78	5.07	8.8	23.26	0.13
	RFF	725	17.36	27.53	10.26	7.55	31.38	<0.01
	RFF	900	27.13	32.44	6.23	9.87	16.21	0.76
	RRP	650	16.34	26.27	10.28	8.6	34.09	0.57
	RRP	725	19.61	24.41	7.87	6.02	12.99	0.11
	RRP	900	24.51	12.21	11.71	10.73	16.28	<0.01
Constant temperature	RPBC	650	36.1	29.9	5.84	1.46	8.9	<0.01
	RPBC	725	39.3	37.7	4.32	1.08	5.6	<0.01
	RPBC	900	48.2	38.1	1.92	0.48	3.7	<0.01
	RFF	650	38.3	28.1	3.36	0.84	23.2	<0.01
	RFF	725	42.9	40.2	2.32	0.58	10.9	<0.01
	RFF	900	50.2	40.1	1.44	0.36	2.8	<0.01
	RRP	650	41.4	23.3	4.24	1.06	10.1	<0.01
	RRP	725	45.8	37.2	2.56	0.64	6.4	<0.01
	RRP	900	47.1	37.1	1.68	0.42	3.1	<0.01

summarizes the gas composition measurements in each case investigated. In general, higher temperatures resulted in lower hydrocarbon content but increased hydrogen and carbon monoxide in either heat-up or constant temperature periods. Additionally, H₂S was measured only in the heat-up period; therefore, the combustion of gases evolved during the heat-up will result in flue gas with significant SO₂ content, which needs to be treated before emission. The generated syngases were combusted without using external flame support during the experiments.

Figure 2a shows the specific dry gas yield at different temperatures, highlighting the portion of pyrolysis gas that evolved during the heat-up period. Higher temperatures resulted in higher specific gas yields, which is in agreement with the lower amount of char. The HHV of the gas during the heat-up period (Figure 2b) lay between 12.0 and 16.6 MJ/Nm³ under all nine measurements and the relatively high heating value can be elucidated by the significant hydrocarbon presence in the pyrolysis gas. The HHV of pyrolysis gas at constant temperature (Figure 2c) is generally lower compared to the heat-up period, typically laid between 10.3–12.3 MJ/Nm³, which is a result of CO + H₂ rich pyrolysis gases (64–66 Vol% at 650 °C, 77–83 Vol% at 725 °C, 84–90 Vol% at 900 °C).

3.2. Negative Carbon Emission Scenario

Pyrolysis char is considered the main product of the pyrolysis process in recent investigations. Basically, the carbon in char can originate from the atmospheric carbon dioxide as a result of global food chain cycling, starting with photosynthesis. Usually, the global warming potential of carbon dioxide is not evaluated for organic waste since CO₂ emitted during decomposition is already absorbed as food by plants and is thus regarded to be biogenic in origin [28]. Therefore, if the carbon content of the char is not converted back to carbon dioxide, then this amount of carbon can be considered as carbon removed from the atmosphere, also constituting negative carbon emission as carbon from the air is removed and not released back into the atmosphere. In other words, the carbon cycle of the global food chain is broken. A significant benefit of this process compared to traditional CO₂ capture methods like combustion equipped with CCS is that carbon remains in solid form instead of gas, which can be stored much more easily than carbon dioxide in gaseous or liquid form. On the other side, the energy content of food waste is not fully utilized, as it partially remains in the char. It is worth noting that if the food waste is only landfilled, then its carbon will sooner or later appear in the atmosphere in gaseous form (CO₂); therefore, either from a carbon footprint point of view or from energetic utilization, it is not beneficial.

The utilization of pyrolysis gas is desirable as it has significant heating value, although the hydrogen and carbon monoxide concentrations are also relatively high alluring the possibility of fuel upgrading. On the other side, pyrolysis gas combustion can supply the heat necessary for the pyrolysis process; the simplified flowchart of this scenario is depicted in Figure 3. The combustion air is preheated in a flue gas heat exchanger in this case, which lowers the fuel need and enhances the overall efficiency. Additionally, the flue gas temperature is cooled down for the subsequent flue gas treatment step. Typically, the drawback of food waste utilized in this study is the relatively high moisture content, which needs to be evaporated during the pyrolysis process. However, based on the energy content of the total pyrolysis gas produced, moisture evaporation itself consumes only 26.7 ± 7% portion of thermal energy from the combustion process of dry pyrolysis gases on average. This means that it is not necessary to dry feed the food waste, which makes the process more robust and attractive.

Based on the measured pyrolysis gas composition, flue gas components and their concentrations can be calculated. SO₂ concentration can go up to 0.12 Vol% in flue gas; therefore, gas treatment might be necessary. The typical CO₂ concentration of flue gas lies between 16.0 Vol% and 23.1 Vol% under stoichiometric conditions, which is emitted back into the atmosphere. Note that the carbon dioxide concentration in flue gas during methane combustion with air under stoichiometric conditions is 9.53 Vol%, meaning that the CO₂ stream from pyrolysis gas is more concentrated compared to that of natural gas. Therefore, a CCS unit following the post-combustion capture pathway might be more efficient. More investigation is needed to evaluate the impact of capture technologies on such elevated CO₂ concentrations in flue gas.

Figure 4 depicts the carbon flow of the pyrolysis process in various cases. The weight highlighted in the middle of the circle plots represents the total carbon input, while the portion of carbon appearing in pyrolysis gas and solid char is also presented. Basically, the carbon in pyrolysis gas is emitted in the form of CO₂ due to the combustion of gases, and, on the other side, another portion of carbon remains in char in solid form. The char from the pyrolysis of rice and French fries resulted in the highest captured amount of carbon with 146.3 g_C/kg_sample, which equals to 536 g_CO2/kg_sample removed at 650 °C pyrolysis temperature. The blend of roast pork and breaded chicken resulted in a significantly lower capture potential being 348 g_CO2/kg_sample at 650 °C pyrolysis temperature. The pyrolysis temperature increase from 650 °C to 900 °C in the cases of roast pork and breaded chicken, rice and French fries, and rice and roast pork resulted in 34.6%, 50.4%, and 29.6% lower captured carbon, respectively.

The elemental composition and ash content of an additional 34 individual food waste samples were analyzed to investigate the quantitative variation between the different food wastes. The samples were collected from the canteen of the Univerity of Miskolc, Hungary. The composition of each sample is summarized in Tables S1 and S2. Additionally, the samples were categorized into two groups based on edibility; Figure 5 summarizes the average, minimum, and maximum values regarding the composition of these groups. The food waste blends used in this study show similar elemental composition compared to the average values, and therefore, it can be stated that the pyrolysis of other food waste might give similar results. In this study, 227 g_CO2/kg_sample capture potential was the worst-case scenario measured in the case of roast pork blended with breaded chicken pyrolyzed at 900 °C. Additionally, as stated in the introduction, 1.3 billion tons of food is wasted on a global scale annually. By calculating with these inputs, the global carbon capture potential can be estimated at roughly 1.1 Gt/year. In 2022, the global CO₂ emission reached 36.8 Gt [29,30], meaning that about 3.0% of the emitted CO₂ can be eliminated by the proposed route if all the food waste generated globally is treated in this way.

4. Conclusions

The negative carbon emission potential of the chars generated by pyrolysis of three different food waste blends was investigated in a laboratory tube furnace at 650 °C, 725 °C, and 900 °C temperatures. The blend of rice and French fries resulted in the highest char yield, being 212 g/kg at 650 °C pyrolysis temperature and resulting in 536 g_CO2/kg_sample removed from the atmosphere. Conversely, the blend of roast pork and breaded chicken showed significantly less carbon removal potential of 348 g_CO2/kg_sample measured at 650 °C pyrolysis temperature, compared to rice and French fries. The pyrolysis gas contained mainly CO, CO₂, H₂, and hydrocarbons with a heating value of 12.0–16.6 MJ/Nm³ during the heat-up period and 10.3–12.3 MJ/Nm³ during the constant temperature period. The carbon dioxide coming from the combustion of pyrolysis gas is considered as emitted carbon, and the generated heat contributes to maintaining the pyrolysis process itself. The global carbon capture potential can be estimated at roughly 1.1 Gt/year, meaning that about 3.0% of the emitted CO₂ can be eliminated by the proposed route if all the global food waste is treated this way. It is important to note that the solid char with high carbon content should be stored underground or utilized as a soil additive (or treated by other equivalent methods) to avoid carbon release in the form of CO₂ into the atmosphere.