Co-Gasification of Pistachio Shells with Wood Pellets in a Semi-Industrial Hybrid Cross/Updraft Reactor for Producer Gas and Biochar Production

Abstract

The possibilities of pistachio shell biochar production on laboratory-scale gasification and pyrolysis devices have been described by several previous studies. Nevertheless, the broader results of the pistachio shell co-gasification process on pilot-scale units have not yet been properly investigated or reported, especially regarding the detailed description of the biochar acquired during the routine operation. The biochar was analysed using several analytical techniques, such as ultimate and proximate analysis (62%wt of C), acid–base properties analysis (pH 9.52), Fourier-transform infrared spectroscopy (the presence of –OH bonds and identification of cellulose, hemicellulose and lignin), Raman spectroscopy (no determination of Id/Ig ratio due to high fluorescence), and nitrogen physisorption (specific surface 50.895 m2·g⁻¹). X-ray fluorescence analysis exhibited the composition of the main compounds in the biochar ash (32.5%wt of Cl and 40.02%wt of Na₂O). From the energy generation point of view, the lower heating value of the producer gas achieved 6.53 MJ·m⁻³ during the co-gasification. The relatively high lower heating value of the producer gas was mainly due to the significant volume fractions of CO (6.5%_vol.), CH4 (14.2%_vol.), and H₂ (4.8 %_vol.), while hot gas efficiency accomplished 89.6%.

1. Introduction

Gasification is one of the most feasible thermochemical processes, besides pyrolysis and torrefaction, for producing high-quality, sustainable, multi-purpose materials that can widely replace fossil fuel-based materials in many applications [1,2,3]. The equivalence ratio (ER) usually ranges from 0.1 to 0.5, while process temperatures are held between 650 and 1200 °C [4,5] depending on the gasification principle (fixed bed [6], fluidised bed [7], or entrained flow [8]). The gasification media includes air, steam, CO₂, or their mixtures [9,10,11,12]. The gasification process results in two major products: combustible gas (known as producer gas) and char (in case of biomass gasification biochar). Producer gas can be used as a fuel, e.g., in an Otto engine with consequent heat end power generation [13]. A more advanced method of producer gas application for production of added-value goods is its upgrading to syngas, along with subsequent application in chemical synthesis processes, especially in the Fischer–Tropsch process for the production of liquid hydrocarbons [14]. However, an appropriate composition rate of H₂/CO must comply sufficiently, which is quite difficult without additional H₂ supply [15]. The resulting hydrocarbon chains may consequently be utilised as carbon-neutral fuel (in the case of biomass usage as the input feedstock to the gasification process), or can be employed as CCU/CCS medium [16]. Moreover, recent research studies proposed BECCS technologies with integrated gasification [17]. Hydrogen-rich gas is also attractive currently due to policies aiming to promote hydrogen use [18,19,20].

Biochar could be implemented in circular economy systems in many different techniques [21]. The most straightforward use involves the agricultural application of this nutrition-rich substance to the soil as a soil amendment procedure [22]. Other applications include mercury capture from flue gas [23], air purification [24], drinking or wastewater treatment [25], adsorption chiller filling [26], and capture of radioactive substances [27].

The Pistacia vera is a small tree which produces pistachio nuts. Global pistachio production has been on an upward trend for more than a decade, with 1.3 million tonnes of pistachios produced in 2021, according to the latest verified data [28]. The largest producers are Iran, the USA, Turkey, and China. In terms of weight, the shell weighs about 35–45 %wt of the whole nut, so 455–585 thousand tonnes of pistachio shells are produced annually. The pistachio nuts are usually sold in a shell directly to the final customers or for further processing by the companies (production of ice cream, flour, or flavourings).

The granulometry of the pistachio shells after the standard shelling process is very homogenous, and it could be mixed and directly used in appliances designed for pellets, as shown by Ryšavý et al. [29], which is important due to the energy required for the densification of biomass [30,31,32]. In the study of Hu et al. [33], pistachio shells were gasified under specific laboratory conditions, while the KOH-activated char was described in detail and tested for its capacitive performance in aqueous electrolytes in electric double-layer capacitors. The experimental results of Lua and Yang [34] showed that it is feasible to prepare activated carbons with a high specific surface (BET) area (896 m²·g⁻¹) and micropore volume (0.237 cm²·g⁻¹) from pistachio shells by means of vacuum pyrolysis and CO₂ activation. The impacts of pH, adsorbent dosage, and temperature on Acid Violet 17 dye removal was studied by Vijayalakshmi et al. [35] while using chemically prepared adsorbents from a pistachio nut shell. The effects of different parameters on pistachio shell char were investigated during pyrolysis (at temperature 400–700 °C) and consequent activation (at temperature 800–950 °C) from the specific surface areas point of view in the study of Faramazi et al. [36].

The majority of previous studies aimed at the thermal treatment of pistachio shells have used laboratory or small/bench-scale units, indicating the great potential of pistachio shells as input feedstock and the resulting biochar as a widely usable material. Most of the previously published research articles describing the quality of the solid residue after its thermal processing were mainly focused on the optimisation of the process to achieve the highest possible parameters in terms of downstream application without evaluating the energy parameters of the process. The potential of co-gasification of the pistachio shell on a semi-industrial scale with a cross/updraft gasification reactor has not been evaluated yet as far as the research knowledge of the authors is concerned. Having a comprehensive understanding of both the energy parameters of the process, as well as the detailed characterisation of the solid by-product, is necessary for the assessment of its suitability for various applications (use of the biochar as a soil amendment or as feedstock for subsequent activation). The novelty of the study presented herein lies in its pioneering exploration of pistachio shell co-gasification on a semi-industrial unit, the detailed characterization of producer gas and resulting biochar, and its potential applications within the context of the circular economy.

2. Materials and Methods

2.1. Feedstock

Pistachio shells were obtained from the supplier of Californian Pistachios with cores included (verified quality for consumption by a human). After the separation of the cores from the shells, they were stored in the laboratory for several months before the experiments, allowing the moisture content to reach equilibrium with ambient air in the building. For the gasification test, a mixture of pistachio shells with certified wood pellets of EN plus A1 quality was used in a weight ratio of 15/85 (pistachio shells/wood pellets).

2.2. Gasification—Pilot-Scale Rig and Gas Analysis

The technology utilised for the purpose of this research was based on a semi-industrial scale sliding bed gasification reactor equipped with a circular grate. The reactor scheme is presented in Figure 1. The fuel and oxidising media flows defined this reactor as a hybrid cross/updraft with a tangential oxidising media inlet. The reactor worked in an autothermal regime, which required no additional energy input from an external source. A more detailed description of the performance of this technology on wood biomass was included in the study of Čespiva et al. [37], on torrefied biomass in the study of Čespiva et al. [38], and on solid recovered fuel in another work of Čespiva et al. [39].

The experimental gasification process was held under a low load regime, with ER reaching only 0.205 (stoichiometric air need was 5.55 kg per kilogram of fuel), using air as the oxidising medium, while the fuel flow was equal to 7.2 kg·h⁻¹. The gasification temperature was maintained between 625.4 and 704.7 °C during the entire duration of the experimental measurement, with an average value equal to 649.7 °C. The relative pressure within the reactor was –0.1 kPa. The producer gas composition, as well as its LHV, were determined by the Gas 3000p (Pollutek Gas Analysis, Belgium) gas analyser with NDIR, TCD, and ECD sensors for CO, H₂, CO₂, CH₄, C_nH_m, and O₂ detection. All measured values were determined for a dry gas, which passed through a series of 6 impinger bottles immersed in an ice bath before going through the analyser, allowing for the condensation of tars and moisture present in the gas.

Efficiency determination is a tool for the direct comparison of gasification processes regardless of reactor design type, fuel characteristics, and operating conditions. The hot gas efficiency (HGE) value presents the efficiency of converting the input chemical energy from feedstock material to the energy of the producer gas, including its sensible heat, while the cold gas efficiency (CGE) does not include this value. These efficiencies were determined according to the following Equations (1) and (2) [40,41,42]:

$${\eta }_{hg}=\frac{{\dot{m}}_g\times c_p\times \Delta T+{\dot{m}}_g\times {LHV}_g}{{\dot{m}}_f\times {LHV}_f}$$

(1)

$${\eta }_{cg}=\frac{{\dot{m}}_g\times {LHV}_g}{{\dot{m}}_f\times {LHV}_f}$$

(2)

• LHV_f—lower heating value of the feedstock [J·kg⁻¹];
• LHV_g—lower heating value of the producer gas [J·kg⁻¹];
• ΔT—temperature diff. between ambient temperature (293.15 K) and the actual temperature of the producer gas, measure at the outlet of the gasifier [K];
• c_p—specific heat capacity at a constant pressure of the gas [J·(kg∙K)⁻¹];
• ṁ_f—mass flow rate of the feedstock [kg·h⁻¹];
• ṁ_g—mass flow rate of the gas [kg·h⁻¹].

The mass flow of the feedstock was determined via calibration of the feeding auger for different rotations per minute (RPM), which could be set by the control system of the gasification installation. During the calibration, the auger was unplugged from the gasifier and fed the material to an empty container for 30 min. The mass of the container was checked before and after the feeding period, and the mass flow rate of the feedstock was subsequently calculated. The producer gas flow rate was measured using a measuring orifice. Pressure on both sides of the orifice was measured using Siemens Sitrans P pressure sensors with a 7MF4433 pressure transformer (Siemens AG Bereich Automatisierungs und Antriebstechnik Geschäftsgebiet Process Instrumentation, D-76181 Karlsruhe, Germany). The volumetric flow rate of the producer gas was recalculated to the mass flow rate based on the measured composition of the gas and the densities of each of the measured compounds. The air flow rate was measured using a Testo 6444 flow meter, operating according to the thermal (calorimetric) measuring principle and using a glass-coated ceramic sensor, with the response time lower than 0.1 s.

2.3. Analytical Methods for the Analysis of Feedstock and Obtained Biochars

The solid residue was collected from a drop zone below the reactor, subsequently separated (gasified pistachio shells, gasified wood pellets), and analysed. The contents of C, H, N, and S were determined using CHN628 and CHNS628 (both Leco, St. Joseph, MI, USA). The amount of O was calculated following the EN 16993 standard. The content of W was determined as a gravimetric difference in the VF110 electric furnace (Memmert, Germany) (ISO 181234-2). The ash content was determined in the LEO 5/11 furnace (LAC, Zidlochovice, Czech Republic) (ISO 18122). The lower heating value (LHV) was calculated according to the Equation (1) [43], using the HHV value obtained with the calorimeter AC600 (LECO, USA).

$$LHV=HHV-(k\times 8.94\times {\omega }_{\mathrm{H}}+0.8\times ({\omega }_{\mathrm{N}}+{\omega }_{\mathrm{O}})+k_1\times w$$

(3)

• k: heat of vaporization considering the volumetric work done by the water formed from the hydrogen during combustion at 25 °C, 2.37 MJ·kg⁻¹;
• k₁: specific heat of water evaporation at constant pressure at 25 ° C, 2.44 MJ·kg⁻¹;
• $\omega$_H: mass fraction of hydrogen in the fuel, [kg·kg⁻¹];
• $\omega$_N: mass fraction of the nitrogen in the fuel, [kg·kg⁻¹];
• $\omega$_O: mass fraction of the oxygen in the fuel, [kg·kg⁻¹];
• ω_w: mass fraction of the water in the fuel, [kg·kg⁻¹].

The pH value was determined following the ISO 10523:2008 standard by using a pH meter, CyberScan pH 110 (EUTECH INSTRUMENTS, Singapore), with a detection range from −2.00 to 16.00 pH. The material was dissolved in deionised water for 24 h prior to the measurement.

The ash from pistachio shell biochar was prepared by ashing the biochar at 550 °C for 4 h in an oven under an oxidation atmosphere for the purpose of XRF spectrometry analysis on a Xepos (SPECTRO, Kleve, Germany) device. Nicolet 6700 FT-IR (Thermo Scientific, Waltham, MA, USA) with ATR diamond crystal was used for FTIR spectra collection. The number of scans was set to 32. Raman spectra were measured on a Smart Raman System XploRA™ (Horiba, Kyoto, Japan). A laser of 532 nm (100 mW) was reduced to 1% of the initial intensity. Grating with 600 grooves/mm, acquisition for 10 s, and 10 repetitions were customised. Raman spectra of each sample were measured at 10 different points. The textural properties of the biochar were evaluated via nitrogen physisorption. The nitrogen adsorption−desorption measurements at −196 °C were performed using the Autosorb iQ Station 3 (Quantachrome Instruments, Odelzhausen, Germany). Prior to the nitrogen physisorption measurement, the biochar (0.315 mm particle size) was degassed at 100 °C for 16.9 h under vacuum. The specific surface area, S_BET, was calculated according to the Brunauer–Emmett–Teller (BET) theory. The total pore volume was determined from the nitrogen adsorption isotherm at p/p₀ (~0.99). The bulk densities of both the feedstock and biochar fraction were determined using a container with a volume of 2 dm³ (79 mm diameter, 470 mm height) and a laboratory-scale XS Balance BL 6001 (XS Instruments, Carpi, Italy) with a maximum permissible error of ± 0.03 g.

3. Results and Discussions

3.1. Parameters of the Producer Gas

The setup of the gasification technology sufficed for the stabilised production of the producer gas parameters, as summarised in

Table 1. Parameters of the sampled producer gas (dry and clean), including standard deviation.

CO [%vol.]	CH4 [%vol.]	CO2 [%vol.]	H2 [%vol.]	O2 [%vol.]	N2 [%vol.]	LHV [MJ·m−3]	Q [l·min−1]	Gas Sampling Time [min]
6.5 ± 0.3	14.2 ± 0.1	4.7 ± 0.18	4.8 ± 0.12	0 ± 0.02	69.8 ± 1.28	6.33 ± 0.09	2.2	60

. The producer gas was rich in CH₄ (14.2%_vol.), and despite the relatively high content of N₂ (69.8%_vol.) due to the usage of air as the oxidising medium, the LHV = 6.33 MJ·m⁻³ was sufficient and very promising, compared to similar studies on fixed bed gasification reactors. On the other hand, the high CH4 was compensated by much lower contents of CO and H₂, which were only 6.5 and 4.8%_vol., respectively. The presence of CO₂ is a natural consequence of the partial direct combustion of the fuel in the autothermal operation of the reactor. All presented values are defined for STP conditions (T = 273.15 K, p = 101,325 Pa).

The producer gas parameters can be compared with previous studies, such as the study of Cerone et al. [44], where almond shells were used as the feedstock. The obtained LHV = 6.2 MJ·m⁻³ resulted from the gasification process in the medium-scale device (0.5 × 2.4 m), with ER = 0.21. However, the gas composition was significantly dissimilar. The CH₄ content was lower (1.7–2.3%_vol.), while the H₂ and CO contents were significantly higher (14.3–21.6%_vol.; 26.1–30.8%_vol.) according to the process conditions, indicating suggestively different management of the gasification process resulting in producer gas appropriate for different utilisations [45]. The differences could be attributed towards differences in the feedstock as well as the design and operation of the gasifier. Cerone et al. [42] performed the gasification process in an updraft gasifier, using almond shells as the feedstock. Moreover, Cerone et al. [42] obtained significant amounts of C_nH_m (ethane and propane alone exceeding 0.5%_vol.) and tars (70 g·m⁻³). In the sliding bed reactor developed by the Energy Research Centre in Ostrava and used for the investigation performed within the scope of this work, the gas flows through the bed in a different way, as the bed is relatively flat in comparison to the updraft gasifiers. Therefore, the gas from the pyrolysis zone is not subject to significant cooling down, as takes place for a gas in an updraft gasifier when flowing through the bed of material [42]. On the other hand, ceramic refractory in the freeboard zone of the sliding bed gasifier allows for a higher residence time of the gas without such a detrimental effect on the gas temperature. It seems plausible to suspect that the methane comes from the thermal cracking (C_nH_m → nC + mH₂/2) [42] of heavier hydrocarbons and the subsequent methanation of C (C + 2H₂ → CH₄) [42]. On the other hand, temperatures in the sliding bed gasifier were not favourable for reforming the obtained methane.

Feedstock could also have a significant influence on the results, as Karatas and Akgun [46] achieved CH₄ concentrations of approx. 7% during the gasification of pistachio shells in a laboratory-scale bubbling fluidized bed reactor, with air as a gasification medium, an ER of 0.19, and the gasification temperature maintained at 770°C [46]. The study reported that the LHV of the producer gas attained 9.9 MJ·m⁻³ (while using steam as a gasification agent) at STP conditions [46]. The experiments with air as a gasifying medium resulted in the LHV of the producer gas accomplishing 5.5 MJ·m⁻³ [46]. In a former study of Čespiva et al. [37], where pure A1 class pellets were used as the input feedstock, the value of LHV reached 4.39 MJ·m⁻³ in the same reactor as presented in this study. Also, the ratio between the three mentioned carriers of chemically-bound energy was different (CH₄/H₂/CO − 5.1/5.8/15.4%_vol.), probably caused by differences in ER, temperature, and fuel mass flow (efficient use of reactor space). In the study by Calì et al. [47], where pine and eucalyptus wood were gasified in an updraft demo gasifier, the LHVs reached 4.58 MJ·m⁻³ in the case of pine and 4.29 MJ·m⁻³ in eucalyptus (ER ranging between 0.26–0.33; CH₄/H₂/CO − 2/12/5–6%_vol.). A similarly constructed updraft reactor was used by James et al. [48], achieving LHV = 5.59 MJ·m⁻³ with ER 0.25. Hosseinzaei et al. [49] performed an investigation on the pyrolysis of pistachio shells at temperatures ranging between 350 °C and 550 °C in an investigation of the pyrolysis gas composition. The LVH of the gas was amplified from 4 MJ·m⁻³ up to 10 MJ·m⁻³ with the increasing temperature of pyrolysis, with methane and other light hydrocarbons contributing significantly to the escalation in the LHV of the pyrolysis gas. Efficiency determination is a tool for the direct comparison of gasification processes regardless of reactor design type, fuel characteristics, and operating conditions. The hot gas efficiency reached 89.6% during the described measurement, while the cold gas efficiency attained its maximum at 73.9%. Both values were attained with respect to an amount of producer gas flow equal to 15.3 m³·h⁻¹. The average gas production rate was then around 2.1 m³·kg⁻¹.

3.2. Characterization of Carbonaceous Material

The separated carbonaceous material (gasified pistachio shells) derived from the gasification process was solely used for analysis in accordance with the aforementioned procedure. A photographic image of the gasified pistachio shells is presented in Figure 2.

Table 2. Raw and gasified PS characteristics (wherever not marked, values are on a dry basis; ar—values on an as-received basis; oxygen content calculated by difference; n.d.—not determined; SRF—solid recovered fuel).

	C [%wt]	H [%wt]	N [%wt]	S [%wt]	O [%wt]	Moisture Content [%wt]	Ash [%wt]	LHV [MJ·kg−1]	pH [–]	Bulk Density [kg·m−3]
Raw pistachio shells ar (this study)	43.85 ± 2.20 ar	5.38 ± 0.19 ar	<0.20 ± 0.10 ar	<0.02 ± 0.10 ar	40.94	8.64 ± 0.24 ar	0.97 ± 0.56 ar	16.00 ± 2.40 ar	4.70 ± 0.01 ar	286.90 ± 1.20 ar
Gasified pistachio ar shells (this study)	62.08 ± 2.50 ar	4.71 ± 0.17 ar	0.21 ± 0.10 ar	0.02 ± 0.10 ar	27.69	2.96 ± 0.11 ar	2.33± 0.78 ar	22.51 ± 2.40 ar	9.52 ± 0.01 ar	226.40 ± 1.30 ar
Gasified olive pomace pellets d [41]	69.5 ±0.20 ar	0.80 ±0.10 ar	1.54 ±0.05 ar	0.34 ±0.03 ar	5.90	10.50 ±0.10 ar	21.90 ±0.10 ar	n.d.	n.d.	363 ar
Gasified softwood pellets d [23]	81.1 ar	2.6 ar	0.2 ar	0.0 ar	10.8 ar	4.1 ar	1.2	n.d.	n.d.	333 − 351 * ar
Gasified, steam-activated, softwood pellets ar [23]	93.4 ar	0.8 ar	0.5 ar	0.0 ar	1.3 ar	2.0 ar	2.0	n.d.	n.d.	284 − 308 * ar
Gasified SRF ar [23]	73.7 ar	1.4 ar	0.9 ar	0.1 ar	5.3 ar	7.7 ar	11.0	n.d.	n.d.	250 − 272 * ar
Gasified, steam-activated, SRF ar [23]	80.0	0.7	0.8	0.1	1.3	2.7 ar	13.9	n.d.	n.d.	217 − 238 * ar
Pistachio shells torrefied at 200 °C d [50]	48.1	5.38	46.3 **	<0.01	46.3 **	7.32 ar	0.19	n.d.	n.d.	n.d.
Pistachio shells torrefied at 300 °C d [50]	62.3	3.85	32.9 **	<0.01	32.9 **	5.78 ar	0.90	n.d.	n.d.	n.d.
Pistachio shells after pyrolysis at 400 °C d [50]	76.2	3.5	19.3 **	<0.01	19.3 **	3.73 ar	0.96	n.d.	n.d.	n.d.
Pistachio shells after pyrolysis at 600 °C d [50]	87.2	2.23	9.5 **	<0.01	9.5 **	1.82 ar	1.09	n.d.	n.d.	n.d.
Pistachio shells after pyrolysis at 850 °C d [50]	88.0	1.18	9.5 **	<0.01	9.5 **	0.44 ar	1.37	n.d.	n.d.	n.d.
Bamboo biochar d [51]	77.63	2.81	1.07	0.17	18.32	4.68 ar	3.90	n.d.	10.1	n.d.

Note: The oxygen content was calculated; ar—as-received (wet) basis; d—dry basis; *—depending on the particle size; **—reported as O + N; n.d.—not determined.

depicts the results of the ultimate and proximate analyses, where data from the analyses of the raw material are included for comparison. The bulk density of raw pistachio shells was estimated to be 286.9 kg·m⁻³, while the bulk density of gasified pistachio shells was assessed to be 226.4 kg·m⁻³.

The pH values of the raw and gasified pistachio shells were determined to be 4.7 and 9.52 on the pH scale, respectively. The upsurge in pH value could have been caused by the transformation of inorganic materials during the gasification process, which might have influenced the pH of the output products. For instance, the release of acidic compounds was capable of increasing the pH of solid residues while reducing the pH of producer gas [52].

Nowicki et al. [53] described the acid–base properties of the activated carbon produced through the pyrolysis technique and the consequent activation of the pistachio shells. The acidity was primarily dependent on the process of activation. Char activated by H₃PO₄ had a pH of 2.22, while char activated by CO₂ had a pH of 10.6 (non-activated char was not measured), although in this present study, it was also alkaline. El-Bassi et al. [54] described the acid–base properties of the biochar produced from exhausted grapes as a function of the pyrolysis temperature (300 and 500 °C). It was determined that with increasing temperature, the pH also increased from 7.2 to 9.9, which was in agreement with other published works. A similar trend was observed by Komnitsas et al. [55] during the pyrolysis of pistachio shells, with a resulting value of pH fluctuating between 4.68 and 8.81 in the temperature range of 250–650 °C.

The variability in the pH value was observed by other authors dealing primarily with soil amendment applications. The biochars prepared from corn straw and oak sawdust by pyrolysis were used as a soil amendment material in the study of Li et al. [56], with the resulting pH ranging between 6.63 and 8.50. Their application into the soil had a limited effect on its properties. However, calcium modification after the treatment significantly increased the influence of their application. A study by Shaoqing et al. [57] described the properties of pyrolytically prepared biochars (at 500 °C for 2 h) from halophyte Salicornia europaea and glycophyte Zea mays, with pH values reaching 10.39 ± 1.37 and 9.09 ± 1.32.

Moreover, the advantage of the described material usage was the carbon mass fraction, which optimistically impacted the soil properties from the carbon sequestration and water retention points of view [58]. Nonetheless, a detailed analysis of the trace elements should be performed before the application of biochar as a fertilizing substitute to avoid the hazard of contamination by persistent and environmentally harmful substances. A detailed analysis of the pistachio shell biochar ash is tabulated in

Table 3. Detailed substance analysis of the pistachio shell ash.

Al2O3 [%wt]	CaO [%wt]	Cl [%wt]	Cr2O3 [%wt]	CuO [%wt]	Fe2O3 [%wt]	K2O [%wt]
0.58	2.40	32.50	0.0003	0.0005	0.05	7.44
MgO [%wt]	MnO [%wt]	Na2O[%wt]	NiO [%wt]	P2O5 [%wt]	PbO [%wt]	SiO2 [%wt]
0.15	0.0009	40.08	0.0006	0.94	0.0014	0.34
SO3 [%wt]	SrO [%wt]	TiO2 [%wt]	ZnO [%wt]
1.13	0.0017	0.0011	0.0014

Note: SUM = 85.62%_wt, ash by TGA = 85.73% _wt, weight loss by annealing in TGA (950 °C) = 14.27%_wt, total SUM 99.89%_wt. Compounds with a mass fraction below the limit of detection (limit of detection): Ag (0.0002%_wt), As (0.000007%_wt), Ba (0.00002%_wt), Cd (0.00002%_wt), Cr (0.000001%_wt), Na (0.002%_wt), Se (0.000001%_wt), V (0.00002%_wt).

.

In comparison with biochar originating from different feedstocks (kenaf stems [59] and rice husk [60]) treated by pyrolysis, the most significant differences were in Cl and Na₂O mass fractions. High mass fractions of these two compounds may be caused by pretreatment of the pistachio before selling—the addition of NaCl for their flavouring. However, the combination of Na and Cl might positively affect the soil salinity and, consequently, influence the yield of some crops, such as Raphanus sativus [61].

It should not be overlooked that the contents of heavy metals and polycyclic aromatic hydrocarbons (PAH) are limited by national and international legislation documents [41,62,63]. One of the most complex legislation documents aimed at soil quality is the Government Decree on the Assessment of Soil Contamination and Remediation Needs of Finnland [64], including the limit values for antimony (Sb), arsenic (As), mercury (Hg), cadmium (Cd), cobalt (Co), chrome (Cr), copper (Cu), lead (Pb), nickel (Ni), zinc (Zn), and vanadium (V). Similarly, obtaining certification of biochar (European Biochar Certificate) requires, among others, the determination of the content of heavy metals and PAH [65]. Another aspect that is worth considering is the fact that the biochar prepared using thermochemical pistachio shells has been proven to be an effective option for heavy metal adsorbing [55]. However, the suitability of using biochar as an adsorbent is determined by its physicochemical properties, including physical parameters describing pores (porosity, pore size distribution, surface area, volume) as well as the presence of particular functional groups at the particles’ surface, which vary according to the method of preparation and subsequent activation.

The presence of different functional groups at the surface of the produced biochars could be determined based on the obtained FTIR spectra presented in Figure 3. The FTIR spectra of ground, non-gasified pistachio shells corresponded to spectra reported in the literature [66]. Infrared spectroscopy revealed the occurrence of alkane (C–C), alkene (C=C), hydroxy (–OH), alkyl (C–H), ester (C=O), alkoxy (C–O), and ether (C–O–C) functional groups of untreated and gasified pistachio shells. The most intensive bands in both spectra at 3336 − 3365 cm⁻¹ were connected to –OH bonds. This broadband was assigned to O–H stretching of H₂O, the intensity of which decreased with drying [35]. Bands in lower wavenumbers were connected to the different stretching and deformation vibrations of cellulose, hemicellulose, and lignin. Although both samples contained bands connected to similar compounds, several differences did persist. Variances between untreated pistachio shells and gasified pistachio shells were prominently observable. The bands in the region of 1700 − 500 cm⁻¹ were broader, and were connected to a more amorphous structure in gasified pistachio shells. Correspondingly, the band at 1731 cm⁻¹ (C=O) disappeared (due to the decomposition of the lignin), several bands lost their intensities (e.g., 1034 cm⁻¹ in pistachio shells), and their positions were shifted (e.g., 1020 cm⁻¹ in gasified pistachio shells), which is again connected to the decomposition of the cellulose/hemicellulose/lignin and other organic compounds due to thermochemical treatment, as previously described in [55]. Band positions are summarised in Table 4 and Table 5.

FTIR spectra of untreated pistachio shells and activated carbons from pistachio shells were described previously, e.g., by da Silva et al. [67], who studied the combustion of pistachio shells and exhibited similar functional groups of untreated pistachio shells to the presented material. Other groups, such as alkyne stretching vibration of C≡C (2139 cm⁻¹), carboxylic acid groups C–O–H (1427 cm⁻¹), and =C–H bending in alkynes (604 cm⁻¹) were found in untreated pistachio shells. Açikalin et al. [66] studied the pyrolysis of pistachio shells and presented the difference between raw pistachio shells and solid products obtained at different pyrolysis temperatures. Some of the functional groups present in the pistachio shells were removed partially or totally from the structure with increasing temperature. At the same time, a decrease in the aliphaticity and an upsurge in the aromaticity of the solid product were observed. With higher temperatures (500 °C and higher), the decrease in aromaticity of the solid product was indicative.

Table 4. Band assignments of FTIR spectra for raw and gasified pistachio shells.

Raw PS	Gasified PS	Band Assignment	Ref.
[cm−1]	[cm−1]
3335	3336	O–H stretching vibration in hydroxyl groups	[55,66,68]
2922, 2857	2914, 2853	aliphatic CH asymmetric and symmetric stretching vibration	[66,68]
1731		C=O stretching vibration in carbonyl or carboxylic bonds	[55,66,68]
	1691	conjunction of the carbonyl group with the aromatic ring	[66]
1637, 1594	1594	aromatic C=O ring or C=C aromatic in lignin, skeletal vibration	[55,66]
1503, 1458, 1421	1513, 1449, 1425	aromatic C=O and C=C ring stretching	[55,66]
1373	1371	CH deformation vibration in alkanes and alkyl groups	[55,66]
1238	1237	C=C stretching	[55]
1156, 1034	1157, 1020	aliphatic ether, alcohol C–O or aromatic stretching, O–H deformation, b-glycosidic bond in cellulose and hemicellulose	[55,66]
900–500	900–500	CH wagging vibrations	[55,66]

Table 4. Band assignments of FTIR spectra for raw and gasified pistachio shells.

Raw PS	Gasified PS	Band Assignment	Ref.
[cm−1]	[cm−1]
3335	3336	O–H stretching vibration in hydroxyl groups	[55,66,68]
2922, 2857	2914, 2853	aliphatic CH asymmetric and symmetric stretching vibration	[66,68]
1731		C=O stretching vibration in carbonyl or carboxylic bonds	[55,66,68]
	1691	conjunction of the carbonyl group with the aromatic ring	[66]
1637, 1594	1594	aromatic C=O ring or C=C aromatic in lignin, skeletal vibration	[55,66]
1503, 1458, 1421	1513, 1449, 1425	aromatic C=O and C=C ring stretching	[55,66]
1373	1371	CH deformation vibration in alkanes and alkyl groups	[55,66]
1238	1237	C=C stretching	[55]
1156, 1034	1157, 1020	aliphatic ether, alcohol C–O or aromatic stretching, O–H deformation, b-glycosidic bond in cellulose and hemicellulose	[55,66]
900–500	900–500	CH wagging vibrations	[55,66]

Table 5. Band assignments of FTIR spectra for hemicellulose, cellulose, and lignin in raw and gasified pistachio shells.

	Raw Pistachio Shells	Gasified Pistachio Shells	Ref.
	[cm−1]	[cm−1]
Hemicellulose	1458, 1238, 1156, 1034	1449, 1237, 1157, 1020	[68]
Cellulose	1421, 1373, 1034	1425, 1371, 1020	[68]
Lignin	1594, 1503	1594, 1511	[68]

Table 5. Band assignments of FTIR spectra for hemicellulose, cellulose, and lignin in raw and gasified pistachio shells.

	Raw Pistachio Shells	Gasified Pistachio Shells	Ref.
	[cm−1]	[cm−1]
Hemicellulose	1458, 1238, 1156, 1034	1449, 1237, 1157, 1020	[68]
Cellulose	1421, 1373, 1034	1425, 1371, 1020	[68]
Lignin	1594, 1503	1594, 1511	[68]

In the case of gasified pistachio shells, the intensities of the D and G bands (characteristic bands of carbonaceous materials) were relatively high at only a few points to be distinguishable in the fluorescence background. This outcome served as a clear suggestion that the remaining compounds in the sample could have been the source of the fluorescence. Likewise, at certain points, bands were measured in the lower wave numbers, which could be connected to the presence of TiO₂ in the rutile or anatase phase (according to the spectral library). Due to the high fluorescence, the ratio between D and G bands was complicated to determine. FTIR and Raman spectra of raw pistachio shells and their activated carbon were described in the study of Foroushani et al. [69].

Specific surface area, pore volume, and pore diameter are considered as important values for various possible applications of the material, including its use as a feedstock for the production of activated carbon. The study of Niksiar and Nasernejad [70] was aimed at optimising the production process of activated carbon with the highest possible specific surface area, using pyrolysis in a spouted bed reactor. The chars produced by pyrolysis reached a specific surface area of 430 m²·g⁻¹ [70], while after the steam activation (under temperature 750–850 °C), they achieved a specific surface area that was increased up to 2596 m²·g⁻¹ [70].

Wu et al. [71] focused their study on the preparation of sorbents from pistachio shells, while the carbonaceous process consisted of calcination (550 °C) of the input material in a ceramic oven. Two strategies of char activation were employed, namely, steam activation and KOH activation. After the processes, the specific surfaces reached 1009 m²·g⁻¹ (steam activation) and 1096 m²·g⁻¹ (KOH activation). The specific surface areas of char without activation were not listed. The pore volume for both techniques ranged between 0.61 and 0.67 cm³·g⁻¹, and the pore diameter ranged between 2.2 and 2.6 nm.

The second study by Wu et al. [72] was also focussed strictly on laboratory preparation of adsorbents from pistachio shells following the aforementioned techniques, with an annealing temperature of 450 °C, under different KOH/char ratios. The best KOH/char ratio from the highest accomplished specific surface area point of view proved to be ratio 3, in which it achieved a specific surface area of 1,687 m²·g⁻¹ (pore diameter: 2.6 nm, pore volume: 1.08 cm³·g⁻¹). A. C. Lua and Yang [73] performed pyrolysis of pistachio shells, along with subsequent CO₂ activation. The resulting specific surface area ranged between 333 and 778 m²·g⁻¹ according to the pyrolysis temperature. The pore volume ranged, according to the same parameter, between 0.078 and 0.466. The highest specific surface and pore volume were reached at a pyrolysis temperature of 500 °C and a hold time of 2 h. The quality of the producer gas was not determined. The same temperature proved to be the best in the case of vacuum pyrolysis during the study of A. C. Lua and Yang [34], where a higher specific surface (896 m²·g⁻¹) and pore volume (0.237 cm³·g⁻¹) were reported. The average pore diameter under the mentioned conditions was 2.38 nm. In the study of Faramarzi et al. [36], the crushed and sieved pistachio shells underwent a pyrolysis process in the laboratory-scale tubular reactor. A pyrolysis temperature of 600 °C, a heating rate of 20 °C·min⁻¹, a 1 h hold time, and an N₂ flow rate of 100 ml·min⁻¹ were the best conditions to produce char with a specific surface area of 350 m²·g⁻¹, which was approximately seven times higher than the results obtained in this study. The activation process for increasing the specific surface area was only mathematically modelled. Foo and Hameed [74] used a laboratory pyrolysis reactor heated at 700 °C under a pure nitrogen atmosphere. Activation of the char by KOH occurred under a ratio of 1:1.75 (char:KOH) in a modified microwave oven. The specific surface area was listed for non-activated (115 m²·g⁻¹) and activated pistachio shells (700 m²·g⁻¹). The total pore volume also significantly differed between the non-activated char (0.069 cm³·g⁻¹) and activated char (0.375 cm³·g⁻¹). The composition of the producer gas was not provided in this study. The biochar made via the pyrolysis of bamboo was presented in the study of Deng et al. [51], where similar surface area and pore size (S_bet = 46.93 m²·g⁻¹, V_p = 0.04 cm³·g⁻¹) were accomplished, while the biochar application proved to be an efficient method for limiting the translocation of Cr into aboveground plant parts, which enhanced the antioxidant activities and reduced the harmful effects.

Considering the studies describing the similar thermal treatment of biomass-based material as in the presented study, the resulting values can be compared with the research study of Čespiva et al. [23], where the certified A1 pellet was gasified in the same reactor as that mentioned in this study. The specific surface of non-activated char ranged between 36.1 and 37.7 m²·g⁻¹ (according to the fraction granulometry), its pore diameter ranged between 3.01 and 3.5 nm, and its pore volume was 0.03 cm³·g⁻¹, which are very analogous values to the newly presented ones. All three mentioned parameters increased after the steam activation: the specific area ranged between 661 and 737.2 cm³·g⁻¹, the pore diameter ranged between 3.33 and 3.6 nm, and the pore volume ranged between 0.19 and 0.33 cm³·g⁻¹. The significant increase in the mentioned parameters enabled a remarkable capture of mercury from flue gas produced during coal combustion. The results obtained from the nitrogen physisorption of the gasified pistachio shells are tabularized in

Table 6. Textural physisorption—results of this study in comparison to other works.

Sample	Specific Surface Area [m2·g−1]	Average Pore Diameter [nm]	Total Pore Volsume [cm3·g−1]	Reference
Pistachio shells after gasification in cross-updraft hybrid gasifier	50.89	3.55	0.035	This study
Pistachio shells after vacuum pyrolysis at 350 °C and subsequent CO2 activation at 900 °C	611	2.57	0.393	[34]
Pistachio shells after vacuum pyrolysis at 400 °C and subsequent CO2 activation at 900 °C	874	2.33	0.509	[34]
Pistachio shells after vacuum pyrolysis at 500 °C and subsequent CO2 activation at 900 °C	896	2.38	0.532	[34]
Pistachio shells after vacuum pyrolysis at 600 °C and subsequent CO2 activation at 900 °C	645	2.63	0.424	[34]
Pistachio shells after vacuum pyrolysis at 700 °C and subsequent CO2 activation at 900 °C	690	2.63	0.454	[34]
Pistachio shells after vacuum pyrolysis at 800 °C and subsequent CO2 activation at 900 °C	724	2.51	0.454	[34]
Pistachio shells after vacuum pyrolysis at 900°C and subsequent CO2 activation at 900 °C	724	2.07	0.374	[34]
Pistachio shells after vacuum pyrolysis at 1000 °C and subsequent CO2 activation at 900 °C	418	1.97	0.206	[34]
Pistachio shells after pyrolysis at 700 °C	115.5	2.339	0.069	[74]
Pistachio shells after pyrolysis at 700 °C and subsequent KOH activation using microwaves (7 min)	700.5	2.144	0.375	[74]

, along with a comparison of the literature data.

As can be understood from Table 6, other processes, such as pyrolysis, are proficient at producing char with a higher specific surface area, lower average pore diameter, and an advanced total pore volume. The pyrolysis of pistachio shells at 700 °C, performed by Foo and Hameed [74], resulted in a specific surface area that was more than two times higher than the char produced within the scope of this study. However, the literature studies shown in Table 6 clearly indicated the fact that activation was indeed essential after pyrolysis in order to produce activated carbon with a surface area sufficient to make such activated carbon beneficial. In pyrolysis, the heat supply for the process is a crucial parameter to be solved, along with the energy requirements needed to create vacuum conditions, implying additional consumption of energy. On the other hand, gasification, performed within the scope of this study, was autothermal. Therefore, no external energy source was required apart from the fuel for startup. Moreover, gasification produced combustible gas, which can be used for the generation of electricity and heat in a CHP unit. Furthermore, heat recovered by the CHP unit could be used to generate the steam necessary for activation. Thus, the advantage of gasification lies in the possibility of supplying the needs of the installation, along with the production of surplus energy, which would be the key for a self-sufficient polygeneration installation producing electricity, heat, activated carbon, and biochar (solid residues from wood pellets). Further work is recommended in order to quantify the benefits of using gasification as the first step before activation of the solid residues from the gasification process, including the comprehensive mass and energy balances of such an installation. Consistently, sustainability analysis appears to be important in context of such novel technologies in bioenergy systems [75]. The results presented in this work provide a good starting point for such an extensive analysis.

4. Conclusions and Future Perspectives

The pistachio shells proved to be an appropriate material for co-gasification with wood pellets in a pilot-scale cross/updraft type reactor, resulting in technologically applicable producer gas and valuable biochar. The gasification process caused a change in many chemical properties of the pistachio shells, which significantly facilitated its applicability in various industries. Analysis of the pistachio shells’ biochar unveiled noteworthy transformations. The obtained biochar exhibited a high LHV = 22.51 MJ·kg⁻¹, alkaline pH (9.52), and altered chemical composition. The pH shift from 4.7 in raw pistachio shells to 9.52 in the gasified material suggests transformations in inorganic materials during gasification, impacting the product’s pH.

Co-gasification, performed within the scope of this study, was autothermal. Thus, no external energy source was needed aside from the fuel needed for startup. The process produced combustible gas with an LHV of 6.53 MJ·m⁻³, which could be used for the generation of electricity and heat in a CHP unit.

These aforementioned outcomes may be especially interesting for pistachio nut producers, mostly situated in locations where intermittent but intense sources of renewable energy, namely, solar energy through direct photovoltaics or photothermal harvest principle, are available. The intermittence would not be harmful in this case, as the accumulated material could be utilised solely in inappropriate energy supply conditions.

Further in-depth research analyses, with detailed mass and energy balances of such novel installations, as well as comprehensive analyses of economic feasibility and environmental impact, are recommended for quantification of the benefits of using gasification as the first step before activation of the solid residues from the gasification process. Moreover, future research endeavours should determine whether the char from the gasification process could be applied to soil as biochar, which would require a comprehensive investigation of the deposition of PAH on the surface of biochar during gasification in sliding bed conditions.