Harnessing the Energy Potential of Nut Residues: A Comprehensive Environmental and Carbon Footprint Assessment

Abstract

This study provides a comprehensive thermochemical characterization of common nut residues—almonds, walnuts, hazelnuts, peanuts, and pistachios shells—as potential biomass fuels, examining their chemical composition, calorific values, and emissions profiles. Their suitability as renewable energy sources was systematically assessed by verifying compliance with ISO 17225-2 standards for pellet production. The nut residues demonstrated promising energy characteristics, with higher heating values ranging from 17.75 to 19.12 MJ/kg and most samples fulfilling ISO 17225-2 classifications A1 or A2. Specifically, the walnut residues met the highest quality classification (A1), whereas the almond, hazelnut, and pistachio residues met the A2 classification, and the peanut residues were classified as B due to higher nitrogen content. A Life Cycle Assessment (LCA) was also performed to quantify the environmental impacts, focusing on CO₂ emissions from energy recovery and transportation. The results showed significantly lower CO₂ emissions from all the nut residues compared to fossil fuels such as coal, natural gas, fuel oil (HFO), and LPG. The almond residues exhibited the lowest total CO₂ emissions at 1669.27 kg CO₂ per ton, while the peanuts had the highest at 1945.93 kg CO₂ per ton. Even the highest-emitting nut residues produced substantially lower emissions compared to coal, which emitted approximately 4581.12 kg CO₂ per ton. These findings highlight the potential of nut residues as low-carbon, renewable energy sources, providing both environmental advantages and opportunities to support local agricultural economies.

1. Introduction

In the ongoing global pursuit of sustainable energy sources, biomass has emerged as a prominent contender. Biomass offers a renewable and environmentally friendly alternative to conventional fossil fuels. Among the diverse selection of biomass resources, nut residues, derived from peanuts, hazelnuts, walnuts, almonds, and pistachios shells, stand out as promising candidates for energetic valorization [1,2]. Their widespread abundance in various regions across the globe and structural similarities to other biomass sources, such as coconut residues, emphasize their potential for sustainable energy production [3].

Dried fruit residues represent a valuable yet often overlooked byproduct of the fruit processing industry. While these residues are typically discarded as waste, their potential as renewable energy sources remain largely untapped. Agricultural residues are excellent alternative materials as substitutes for wood because they are plentiful, widespread, and easily accessible [4,5]. By harnessing the energy-rich properties inherent in these biomasses, researchers aim to not only address waste management concerns [6] but also to contribute significantly to the transition towards a more sustainable energy landscape [7].

Thermochemical characterization forms the cornerstone of understanding the energy potential of nut residues [8]. This comprehensive analysis examines their chemical composition, heating value, combustion characteristics, and emissions profile [9,10]. Advanced techniques like pyrolysis, gasification, and combustion enable researchers to unlock the latent energy stored within these organic materials, offering profound insights into their suitability for various energy conversion processes [11,12].

Recent studies have explored innovative approaches for the energetic valorization of agricultural residues. For instance, the granulation and pyrolysis of such residues have shown strong potential to support a more circular and sustainable bioeconomy by enhancing fuel properties and facilitating handling and transport [13]. Furthermore, process conditions—particularly temperature and pressure—significantly influence the physicochemical characteristics of pyrolyzed biomass, impacting its energy efficiency and final application [14]. In addition, biochar derived from nut and fruit shells has found widespread applications across various industrial sectors, including use as a sorbent, reducing agent, and bioenergy carrier, highlighting its versatility and added value [15].

Each variety of nut residue possesses unique characteristics that influence its behavior during thermochemical conversion. Factors such as lignocellulosic content, moisture content, and elemental composition play essential roles in determining energy potential and combustion efficiency [16,17,18]. Understanding these nuances is crucial for optimizing bioenergy conversion technologies and maximizing energy yield [19,20].

Although several studies have evaluated the energetic potential of agricultural biomasses, such as walnut, almond, and hazelnut residues [1,2,3], a systematic analysis of nut residues’ compliance with ISO 17225-2 standards [21] for pellet production combined with a Life Cycle Assessment (LCA) is still limited. This study addresses this gap by conducting a comprehensive and integrated thermochemical, energetic, and environmental analysis of common nut residues in Portugal. Additionally, a preliminary LCA focusing specifically on CO₂ emissions is presented, offering an initial yet realistic perspective on the potential climate benefits of using these residues as renewable energy sources.

Beyond their technical merits, the utilization of nut residues for energy production holds significant environmental and socioeconomic benefits [22]. By diverting these agricultural residues from landfills and incinerators [23], the associated greenhouse gas emissions can be mitigated, contributing substantially to climate change mitigation efforts [24,25]. Integrating nut residues into energy production systems can create new revenue streams for farmers and processors, fostering rural development and job creation while enhancing energy security and resilience [26]. Additionally, biomass plays a crucial role in current European Union (EU) strategies to mitigate climate change [27], as outlined in the green paper adopted by the Commission in 1996 [28].

Despite their underutilization, nut residues represent an accessible and homogenous biomass resource with potential for integration into existing energy systems. Their high energy content and low ash production make them suitable candidates for combustion or co-firing in small-to-medium-scale applications. However, effective integration requires addressing several challenges, including feedstock collection logistics, seasonal variability, and compatibility with current combustion technologies. The exploration of such integration pathways is particularly relevant in the context of decentralized renewable energy systems and rural bioeconomy initiatives [29].

In Portugal, the annual production of almond, walnut, hazelnut, peanut, and pistachio residues is estimated at approximately 58,000 tonnes, based on agricultural and industrial data [30,31,32,33,34,35,36]. Considering their lower heating values, this biomass corresponds to an energy potential of approximately 935 TJ per year. When compared with Portugal’s total primary energy consumption—around 863 PJ in 2023 [37]—this represents about 0.1%, entirely derived from renewable agricultural byproducts. Although modest at the national scale, this fraction could meaningfully contribute to the renewable share of the energy mix while also providing a sustainable outlet for agri-food waste. These figures underscore the potential of nut shells as a viable component in decentralized bioenergy strategies. Moreover, such residues may play a significant role in small-scale or localized applications. For example, agro-industrial operations such as nut processing and roasting facilities could reuse shell residues on-site to meet thermal demands, reducing external energy dependence and operational costs. Similarly, rural households and cooperatives could benefit from pelletized or briquetted forms of this biomass for heating purposes, particularly during the colder months. In these contexts, even a small contribution can translate into measurable energy savings and improved sustainability, aligning with circular economy principles and energy autonomy goals.

In summary, the exploration of nut residues as biomasses for energetic valorization and thermochemical characterization represents a promising possibility for advancing sustainable energy solutions and waste management practices. By examining their chemical composition, energy content, and suitability for thermochemical conversion processes, researchers can unlock new opportunities for sustainable energy production and contribute significantly to a greener, more sustainable future [38].

2. Materials and Methods

2.1. Sampling

The samples utilized in this study were sourced from the residues of the most prevalent nuts found in Portugal, including almonds, hazelnuts, walnuts, pistachios, and peanuts. These particular fruits were selected due to their widespread availability and significance within the Portuguese agricultural landscape.

Upon collection, the residues were thoroughly cleaned and dried to remove any extraneous matter and ensure uniformity across samples. Care was taken to eliminate any contaminants that could potentially affect subsequent analyses.

2.2. Proximate Analysis

Proximate analyses were conducted on all samples to determine four key properties important for biomass characterization: moisture (M), volatile matter (VM), ash (A), and fixed carbon (FC). These analyses adhered to the standards outlined in ISO 18134-1 [39], ISO 18122 [40], and ISO 18123 [41]. Each biomass yielded a representative sample, and their initial masses were recorded. These samples were subjected to a Protherm PLF 100/6 furnace set at 105 °C until reaching a stabilized weight, indicating complete moisture removal. The final mass post-drying was recorded, and moisture content in percentage was calculated based on weight variance (Equation (1)).

$$M={\displaystyle \frac{m_2-m_3}{m_2-m_1}}\times 100,$$

(1)

where $m_1$ is the mass, in grams, of the empty container; $m_2$ is the mass, in grams, of the container containing the sample before drying; and $m_3$ is the mass, in grams, of the container containing the sample after drying.

From the dried biomass sample used in moisture analysis, a portion was extracted and weighed. This sample was then subjected to thermal decomposition under inert atmosphere in a covered crucible in a muffle furnace at 550 °C, in accordance with ISO 18123 [40]. During this thermal treatment, volatile matter is released as a combination of gases and vapors formed by the thermal degradation of organic compounds. The percentage of volatile matter was determined by the mass loss during this step, as calculated in Equation (2).

$$VM={\displaystyle \frac{m_5-m_6}{m_5-m_4}}\times 100,$$

(2)

where $m_4$ is the mass, in grams, of the empty container plus the lid; $m_5$ is the mass, in grams, of the container plus the lid containing the dried sample before thermal decomposition under inert atmosphere; and $m_6$ is the mass, in grams, of the container plus the lid containing the sample after thermal decomposition under inert atmosphere.

The residue from volatilization underwent ashing in a muffle furnace set at 550 °C until reaching a consistent weight, indicative of complete organic matter combustion. The ash content in percentage was then calculated using (Equation (3)).

$$A={\displaystyle \frac{m_8-m_9}{m_8-m_7}}\times 100,$$

(3)

where $m_7$ is the mass, in grams, of the empty container; $m_8$ is the mass, in grams, of the container with the sample after thermal decomposition under inert atmosphere; and $m_9$ is the mass, in grams, of the container with the ash.

Fixed carbon content was subsequently calculated by deducting moisture, volatile matter, and ash content from 100% (Equation (4)).

$$FC=100\mathrm{\%}-\mathrm{M}\mathrm{\%}-\mathrm{V}\mathrm{M}\mathrm{\%}-\mathrm{A}\mathrm{\%},$$

(4)

2.3. Elemental Analysis

The biomass samples underwent weighing in a tin container, and their masses were recorded. Subsequently, elemental analysis of carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents was performed on all samples in accordance with the guidelines outlined in ISO 16948 [42], utilizing a ThermoScientific Flashsmart CHNS/O elemental analyzer. The oxygen content was determined by subtracting the contents of C, H, N, S, and ash from 100%, as expressed in Equation (5).

$$\%O=100\%-\%C-\%H-\%N-\%S-A\%$$

(5)

2.4. Calorimetry

The sample combustion was carried out using an isoperibolic calorimeter (mod. 6300, Parr Instruments Co., Moline, IL, USA) in accordance with the procedures outlined in ISO 18125 [43]. The calorimeter underwent calibration utilizing a benzoic acid standard (Benzoic Acid Parr No. 3415). The net heat value (NHV) was determined according to ISO 18125.

2.5. Biomass Evaluation System

To assess the suitability of biomass samples for energy valorization, the classification scale provided in ISO 17225-2 was used. This scale categorizes potential biomasses into three groups based on specific parameters, including ash content (A), nitrogen content (N), and moisture content (M) (

Table 1. ISO 17225-2 standards.

Parameter	A1	A2	B
Moisture (%)	≤10	≤10	≤10
Ashes (%)	≤0.7	≤1.5	≤3.0
N (%)	≤0.3	≤0.5	≤1.0

). The ISO 17225 series provides precise guidelines for classifying solid biofuels, which helps facilitate efficient biofuel trading and promotes mutual understanding between buyers and sellers. Additionally, it serves as a communication tool for equipment manufacturers.

2.6. Life Cycle Assessment

A Life Cycle Assessment (LCA) methodology was employed to quantify the environmental impacts associated with using nut residues (almond, walnut, hazelnut, peanut, and pistachio) as renewable energy sources. The LCA accounts for both energy recovery and transportation emissions to provide a comprehensive understanding of the CO₂ emissions associated with each shell type.

In the energy recovery phase, the total energy output (in MJ) for each shell type was determined based on their respective heating values. The CO₂ emissions from the samples were calculated using the results from elemental analysis, which provided data on the carbon content of each shell. By applying stoichiometric combustion equations, the total CO₂ emissions per ton of shell were calculated based on the amount of carbon present in each biomass type.

Additionally, the transportation phase considered the CO₂ emissions associated with transporting the residues from nut production areas to biomass energy plants. A transport distance of 400 km was assumed, which reflects the typical distances between agricultural regions and energy facilities in the country. The CO₂ emissions for transportation were calculated using an emission factor of 62.1 g of CO₂ per ton of biomass per kilometer, in accordance with transportation emissions data provided by the European Environment Agency [44].

To provide a comparative analysis, the emissions from fossil fuels (coal, natural gas, fuel oil, and LPG) required to generate the same energy output were calculated using standard CO₂ emission factors and lower heating values (LHVs) for each fuel. For coal, the LHV was considered to be 24 MJ/kg with an associated CO₂ emission factor of 0.278 kg CO₂/MJ [45]. Natural gas was assigned an LHV of 50 MJ/kg with an emission factor of 0.198 kg CO₂/MJ [46]. Fuel oil (HFO) was calculated using an LHV of 42.5 MJ/kg and an emission factor of 0.271 kg CO₂/MJ, while LPG had an LHV of 46 MJ/kg and a CO₂ emission factor of 0.228 kg CO₂/MJ [45].

3. Results and Discussion

3.1. Proximate Analysis Results

The moisture content of biomass residue plays a crucial role in its suitability as fuel [47]. Higher moisture levels diminish the heating value, consequently influencing how biomass behaves during pyrolysis and the quality of the resulting products. For effective biofuel application, biomass residue should ideally maintain a moisture content below 10% [47]. Excessive moisture in biomass residue necessitates greater energy input for moisture removal, making it less efficient for pyrolysis. Consequently, the low moisture content observed in these nut residues renders them well suited for pyrolysis applications, around 7.3 to 9.3% (

Table 2. Results of proximate analysis.

Shell	Moisture (%)	Volatile Matter (%)	Ash (%)	Fixed Carbon (%)	VM/FC
Almonds	8.681 ± 0.209	69.583 ± 0.447	0.762 ± 0.003	20.974 ± 0.263	3.31
Walnuts	9.097 ± 0.190	66.700 ± 0.145	0.438 ± 0.001	23.765 ± 0.215	2.80
Hazelnuts	9.248 ± 0.127	63.175 ± 0.230	0.767 ± 0.003	26.810 ± 0.127	2.35
Peanuts	7.800 ± 0.109	61.400 ± 0.091	1.410 ± 0.844	29.390 ± 0.813	2.08
Pistachio	7.677 ± 0.307	67.198 ± 0.121	0.108 ± 0.001	25.017 ± 0.304	2.68

).

VM and FC contents are crucial factors for assessing the ignition and subsequent gasification or oxidation characteristics of biomass, depending on its intended use as an energy source [48,49]. Biomass with higher VM content tends to be more reactive, readily volatilizing and producing less char. This makes the studied residues particularly suitable for pyrolysis processes, facilitating biofuel production [48,49]. The FC contents of the residues ranged from 20% to 29%, showing somewhat high values compared to the other biomass values [50].

Elevated ash levels in biomass can significantly diminish the fuel’s energy content. Moreover, during thermochemical conversion processes, the chemical composition of the ash can lead to operational challenges, such as slag formation during combustion at high temperatures [51]. The observed ash content levels in these samples fell comfortably within acceptable ranges, making them highly suitable for combustion and pyrolysis conversion processes [52].

The almond residues exhibited lower percentages of FC and VM compared to those reported by [18]. The results for the walnut residues indicate lower volatile matter and ash content, but a higher fixed carbon percentage, compared to the values reported in [53], which were 70%, 1.26%, and 15.895%, respectively.

The hazelnut residues, in comparison to the findings of [38], differed only in moisture and ash content, with values of 3.96% and 8.70%, respectively. The peanut residues exhibited substantial differences across all parameters in contrast with the study by [54]. Particularly noteworthy were the high VM (84.90%) and FC (13.40%) percentages on a dry basis.

Lastly, in the case of the pistachio residues, these results diverged from the analysis conducted by [55], revealing higher VM values but lower FC and moisture levels.

3.2. Elemental Analysis Results

Within the chemical composition of fuel, carbon stands as a fundamental element [56]. The quality of fuel is significantly influenced by its carbon content, as higher levels typically denote superior quality [56]. Hydrogen, the second most significant component in fuel, contributes greatly to its thermal value, manifesting in a visible flame upon combustion. Oxygen, on the other hand, diminishes the heating value of fuel [56]. From an environmental perspective, nitrogen’s presence can contribute to the elevation of greenhouse gases, rendering it an undesirable element within biomass [57,58]. Sulfur is typically found in trace amounts within biomass [59].

Comparing the composition of these almond residues to those studied by [18], only insignificant differences were observed, except for the oxygen content, which was higher at 41.6% (

Table 3. Elemental analysis results.

Shell	N (%)	C (%)	H (%)	S (%)	O (%)
Almond	0.219 ± 0.002	44.848 ± 1.427	5.713 ± 0.157	n.d.	48.458 ± 1.585
Walnut	0.273 ± 0.047	47.072 ± 0.907	5.915 ± 0.020	n.d.	46.301 ± 0.972
Hazelnuts	0.417 ± 0.015	45.543 ± 0.218	5.334 ± 0.070	n.d.	47.939 ± 0.141
Peanuts	0.923 ± 0.041	52.393 ± 0.529	5.230 ± 0.193	0.029 ± 0.001	40.013 ± 1.131
Pistachio	0.470 ± 0.018	48.721 ± 0.322	5.715 ± 0.039	n.d.	44.986 ± 0.350

n.d. Not detected.

). Similarly, the walnut residue results showed minimal difference when compared with those of [53], with the results of this study being 5% higher.

Comparing the results of hazelnut residues to the values reported by [38], only a small difference was found. However, when contrasted with [18], more significant variations emerged, particularly in carbon content at 51.2% and oxygen content at 40%.

In the case of the peanut residues, these results closely resembled those of the study by [54], with the most notable difference being the carbon content, which stood at 46%.

Lastly, when comparing the pistachio residues to the values reported by [55], minimal differences were noted, with the primary distinction being a 4% lower carbon content and a 4% higher oxygen content.

3.3. Calorimetry Results

The higher heating values (HHVs) of the residues ranged from 13.8 to 18.45 MJ/kg, falling closely within the range typical of biomass with substantial heating values and high-quality lignite coals [60] (

Table 4. Calorimetry results.

Shell	HHV (MJ/kg)	LHV (MJ/kg)
Almonds	17.753 ± 0.030	16.479 ± 0.007
Walnuts	18.153 ± 0.140	16.836 ± 0.141
Hazelnuts	18.408 ± 0.160	17.198 ± 0.165
Peanuts	19.115 ± 0.064	17.942 ± 0.027
Pistachio	17.879 ± 0.019	16.616 ± 0.028

).

Comparing these findings with those from the literature, several differences emerge. The almond residues presented a lower HHV than that reported by [18], which was 20.58 MJ/kg. Conversely, the results from the walnut residues were similar, although slightly higher, than those of [53], who reported an HHV of 17.93 MJ/kg. The HHV of the hazelnut residues exhibited minimal deviation from the results of [18], which stood at 19.09 MJ/kg.

For residues that presented higher HHV values, the peanuts demonstrated a slightly elevated HHV and LHV compared to those reported by [54], which were 18.547 MJ/kg and 17.111 MJ/kg, respectively. The results for the pistachio residues were very similar in comparison to the research by [55], with an HHV of 17.522 MJ/kg.

Table 5. Proximate analysis and calorimetry results of various biomasses.

Samples	N (%)	C (%)	H (%)	S (%)	O (%)	H/C	O/C	HHV	References
Studied Residues
Almond S.	0.219	44.848	5.713	n.d.	48.458	1.51	0.81	17.753	Present Work
Walnut S.	0.273	47.072	5.915	n.d.	46.301	1.49	0.73	18.153	Present Work
Hazelnut S.	0.417	45.543	5.334	n.d.	47.939	1.39	0.79	18.408	Present Work
Peanut S.	0.923	52.393	5.230	n.d.	40.013	1.18	0.57	19.115	Present Work
Pistachio S.	0.470	48.721	5.715	n.d.	44.986	1.39	0.69	17.879	Present Work
Woods, barks, and derivatives
Softwood	0.20	52.10	6.10	0	41	1.39	0.59	20	[61]
Wood Bark	1.40	53.42	6.12	0	39.06	1.36	0.54	-	[62]
Eucalyptus bark	6.90	45.20	5.70	0	42.20	1.50	0.70	17.31	[63]
Pine Needles	0.60	70.20	3.77	1.43	15.21	0.63	0.16	-	[64]
Straws
Wheat Straw	1.80	45.50	5.10	0	34.10	1.33	0.56	17.30	[61]
Barley Straw	2.04	39.50	6.48	0	20.75	1.95	0.39	-	[65]
Rice Straw	0.28	35.68	4.62	0	39.14	1.54	0.82	14.85	[66]
Agricultural Wastes
Sugarcane Bagasse	0.15	45.48	5.96	0	45.21	1.56	0.74	18.73	[66]
Olive Husk	1.60	50	6.20	0	42.20	1.47	0.63	19	[61]

n.d. Not detected.

and

Table 6. Elemental analysis results of various biomasses.

Samples	Moisture (%)	Volatile Matter (%)	Ashes (%)	Fixed Carbon (%)	VM/FC	References
Studied Residues
Almond S.	8.681	69.583	0.762	20.974	3.31	Present Work
Walnut S.	9.097	66.700	0.438	23.765	2.80	Present Work
Hazelnut S.	9.248	63.175	0.767	26.810	2.35	Present Work
Peanut S.	7.800	61.400	1.410	29.390	2.08	Present Work
Pistachio S.	7.677	67.198	0.108	25.017	2.68	Present Work
Woods, barks, and derivatives
Wood	8.80	70	1.70	28.10	2.49	[61]
Wood Bark	10	68.90	4.90	16.20	4.25	[62]
Eucalyptus bark	10.20	79.90	6.20	13.90	5.74	[63]
Pine Needles	7.34	24.68	8.79	59.19	0.41	[64]
Straws
Wheat Straw	8.50	63	13.50	23.50	2.68	[61]
Barley Straw	8.65	69.54	9.560	12.25	5.67	[65]
Rice Straw	8.10	65.70	20.38	13.91	4.72	[66]
Agricultural Wastes
Sugarcane Bagasse	8.10	83.66	3.20	13.15	6.36	[66]
Olive Husk	9.20	70.30	3.60	26.10	2.69	[61]

show the compilation of the thermo-chemical composition of the studied residues and of the more commonly utilized biomasses for energy production.

The sample values closely align with those of common biomasses in terms of composition and characteristics, often displaying comparable or even superior results. When compared to wood, one of the oldest and most prevalent biomasses, the primary differences are higher carbon content and lower oxygen content, resulting in a noticeable variance in the O/C ratio, while the H/C ratio remains consistent. Although the higher heating value (HHV) results are slightly lower than those of wood, they remain close.

In elemental analysis, the samples exhibit satisfactory results. Compared to wood, they demonstrate lower ash percentages, indicating favorable biomass quality. However, they display lower fixed carbon percentages, consequently resulting in a reduced volatile matter to fixed carbon (VM/FC) ratio.

Assessing the suitability of the nut residue samples for energy valorization, using the classification scale provided in the ISO 17225-2 [31] standards for the studied characteristics, the samples were categorized accordingly (

Table 7. ISO 17225-2 evaluation.

	Moisture (%)	Ashes (%)	N (%)
Almond S.	8.681	0.762	0.219
Walnut S.	9.097	0.438	0.273
Hazelnut S.	9.248	0.767	0.417
Peanut S.	7.800	1.410	0.923
Pistachio S.	7.677	0.108	0.470

Note: Cell background colors indicate compliance with ISO 17225-2 classes for each parameter (moisture, ash, nitrogen): Light green: complies with class A1; Medium green: complies with class A2; Dark green: complies with class B only.

). Notably, the walnut shell meets all the criteria for Class A1 classification. Three of the samples (almond, hazelnut, and pistachio residues) meet all the requirements for Class A2 classification. However, the peanut shell only qualifies for Class B classification due to the high nitrogen content.

3.4. Life Cycle Assessment Results

The Life Cycle Assessment (LCA) presented in this study primarily aims to quantify the carbon dioxide (CO₂) emissions associated with the use of nut residues as a bioenergy source. The assessment is structured around two main stages: (i) emissions resulting from the energy conversion of biomass (direct combustion) and (ii) emissions associated with the transportation of biomass to the point of use. To allow a scientifically valid comparison with fossil fuels, all the results are expressed in terms of specific emissions, i.e., kilograms of CO₂ per megajoule (kg CO₂/MJ) of useful energy produced. Transport emissions were estimated assuming a 400 km average distance by heavy-duty diesel truck, reflecting typical national logistics in Portugal, where biomass residues are transported from production regions in the north and south to central processing facilities. This yields an emission factor of 24.84 kg CO₂ per tonne of biomass transported. Although transport emissions represent a smaller proportion of the total emissions compared to production, they still constitute an important factor, particularly for products that require long-distance transportation.

Table 8. Carbon footprint associated with the energy recovery and transportation of the various shell types.

Shell	CO2 Emissions from Combustion (kg/ton)	Total Energy Output (MJ/ton)	Transport CO2 Emissions (kg/ton)	Total CO2 Emissions (kg/ton)
Almond	1644.43	16,478.86	24.84	1669.27
Walnut	1725.98	16,835.83		1750.82
Hazelnuts	1669.90	17,198.43		1694.74
Peanuts	1921.09	17,941.86		1945.93
Pistachio	1786.42	16,615.51		1811.26

highlights the carbon footprint associated with both the energy recovery and transportation phases for various shell types. Peanuts exhibit the highest CO₂ emissions at 1945.93 kg, followed by walnuts (1750.82 kg) and pistachios (1811.26 kg). Almonds and hazelnuts have slightly lower production emissions, at 1669.27 kg and 1694.74 kg, respectively.

Figure 1 illustrates the specific emissions (kg CO₂/MJ) of the different nut residues—almond, walnut, hazelnut, peanut, and pistachio—compared to the four most common fossil fuels: coal, natural gas, heavy fuel oil (HFO), and liquefied petroleum gas (LPG). To ensure methodological consistency, the fossil fuel values include both direct combustion emissions and transport-related emissions, based on average distribution distances reported in the literature (100 km for natural gas and HFO, 150 km for coal and LPG) [67,68,69].

The results show significant differences between fossil fuels and nut residues. On average, the nut residues exhibit specific emissions ranging from 0.099 to 0.109 kg CO₂/MJ, with hazelnut as the lowest and pistachio as the highest emitter. In contrast, coal and HFO exceed 0.27 kg CO₂/MJ, underscoring the high environmental impact of conventional fossil fuels. Natural gas is the least carbon-intensive fossil fuel (0.202 kg CO₂/MJ), yet it is still notably higher than the average emissions of nut-based biomass.

Previous research supports the energy potential of nut shell residues. For instance, pyrolysis studies on pistachio, almond, and walnut shells demonstrated favorable thermal degradation behavior, with activation energies between 102 and 121 kJ/mol [70]. Other studies highlight positive carbon balances—e.g., biochar production from walnut shells offset 107.7 kg CO₂-eq per tonne of feedstock [71], while almond byproduct energy systems achieved net emissions of just 0.9 kg CO₂-eq/kg through energy displacement effects [72].

The findings from this study complement the existing research, reinforcing the viability of using agricultural byproducts in bioenergy systems to achieve carbon reductions and promote sustainable energy practices.

Despite the relevance of these findings, it is important to acknowledge the limitations of this assessment. The present LCA focuses solely on CO₂ emissions, excluding other relevant environmental impacts such as water consumption, land use, biodiversity loss, or acidification potential. Furthermore, the assumed transport distances are average values drawn from the literature, representing a balance between geographical representativeness and methodological comparability. A more comprehensive assessment—including full environmental impact categories or multi-criteria LCA—will be essential to fully evaluate the sustainability of using these residues for bioenergy purposes.

Based on the estimated availability of 56,683 tonnes of nut residues annually and their lower heating value (LHV), the energy potential reaches approximately 259.89 GWh per year. Assuming substitution of fossil thermal fuels such as natural gas (≈41.73 EUR/MWh [73]), the avoided energy cost is estimated at ~10.85 million EUR/year, or ~191.5 EUR/tonne of biomass). The cost estimates for biomass valorization were based on typical values from the technical literature and reports: 0–20 EUR/tonne for biomass acquisition (as these are low-value agricultural residues) [74], 32 EUR/tonne for transport over 400 km, and ~64 EUR/tonne for drying and shredding operations [74]. This results in a total cost range of 50 to 116 EUR/tonne, yielding an estimated net profit margin of ~75.5 to 141.5 EUR/tonne. Overall, this simplified economic analysis supports the viability and competitiveness of using nut residues as a local renewable energy resource.

While the present study demonstrates the environmental and economic potential of using nut residues for energy production, it is important to consider the challenges associated with scaling up these practices to an industrial level. Based on national estimates, Portugal may generate approximately 56,683 tonnes of such residues annually, corresponding to an energy potential of nearly 260 GWh. Although this is a relevant contribution to the renewable energy portfolio, large-scale utilization would require the establishment of efficient logistics chains, preprocessing infrastructure (e.g., drying and grinding), and possibly the creation of regional biomass aggregation centers. These factors, combined with the seasonality and geographical dispersion of production, highlight the need for targeted policy and investment to enable scalability within the national energy system.

4. Conclusions

This study highlights the potential of common nut residues—including almonds, walnuts, hazelnuts, peanuts, and pistachios—as a renewable energy source. Through comprehensive chemical and energy analyses, the suitability of these residues for energy production was assessed. The findings reveal promising characteristics, with some samples meeting high-quality standards comparable to established biomass sources. Most residues classify well on the ISO 17225-2 scale for pellet production, reinforcing their viability for energy use. Although the energy values of these residues do not surpass those of traditional biomass sources, the results demonstrate their potential as alternative energy sources, offering both environmental benefits and the opportunity to support local agricultural economies.

The Life Cycle Assessment (LCA) further expands on this by evaluating the CO₂ emissions associated with both energy recovery and transportation of these residues. The results show that using nut residues as biomass can significantly reduce carbon emissions compared to traditional fossil fuels such as coal, natural gas, fuel oil (HFO), and LPG. Among the residues studied, peanuts exhibited the highest CO₂ emissions during energy recovery, at 1921.09 kg CO₂ per ton, while almonds showed the lowest emissions at 1644.43 kg CO₂ per ton. However, even the highest-emitting shell types produced considerably lower CO₂ emissions than coal, which emits 4581.12 kg CO₂ to produce the same energy as 1 ton of almond residues alone.

When considering both energy recovery and transport emissions, the total LCA results further demonstrate the environmental benefits of using biomass. Transport emissions, although smaller in comparison to energy recovery emissions, are still a significant factor, especially when large distances are involved in the supply chain. Almonds exhibited the lowest total LCA emissions, while peanuts and pistachios were the most carbon-intensive. Nonetheless, all shell types offer a significant reduction in carbon emissions when compared to fossil fuels, making them viable candidates for sustainable energy production.

Overall, this study underscores the potential for nut residues to serve as a renewable alternative to fossil fuels, contributing to both carbon footprint reduction and local economic support through agricultural byproducts. However, further research is needed to explore the full potential of nut biomass in larger-scale applications and to assess other environmental impacts, such as land use and water consumption. By leveraging this abundant and often underutilized resource, nut residues could play an important role in advancing sustainable energy solutions and supporting the global transition toward greener energy systems.