A Route for Bioenergy in the Sahara Region: Date Palm Waste Valorization through Updraft Gasification

Abstract

The Adrar region (Algeria) has a total of 397,800 date palm trees (Phoenix dactylifera L.). Due to annual palm cleaning, large quantities of lignocellulosic biomass are produced. Depending on the variety, an average of 65 kg of biowaste is obtained per palm tree. Since the value of this biowaste is underrated, most of the palms are burned outdoors, causing air and visual pollution. This work explores the gasification potential of lignocellulosic waste from date palms (Phoenix dactylifera L. Takarbouche variety) into useful energy. The technology investigated is air updraft fixed-bed gasification, thanks to an originally designed and built reactor, with the capability to process 1 kg of feedstock. Four types of palm waste—namely, palms, petioles, bunch, and bunch peduncles—are first characterized (bulk density, proximate analysis, fixed carbon, elemental composition, and calorific value) and then used as feedstock for two gasification tests each. The syngas produced for the four date palm wastes is combustible, with an outlet temperature between 200 and 400 °C. The operating temperature inside the gasifier varies according to the feature of the biomass cuts (from 174 °C for the peduncles to 557 °C for palms). The experimental setup is also equipped with a cyclone, allowing for the recovery of some of the tar produced during the tests. Finally, the results show that the residence time has a positive effect on the conversion rate of date palm waste, which can significantly increase it to values of around 95%.

1. Introduction

The world energy demand is accelerating due to industrialization and population growth. Coal, oil, and natural gas are currently the main sources of energy used to meet global demand. However, these energy sources are anticipated to exhaust in the next 4–5 decades. These fossil fuels are associated with the problems of climate change, acid rain, and urban smog [1,2]. Biomass is a safe alternative to avoiding carbon dioxide (CO₂) emissions and is increasingly used in many countries; nowadays, it provides more than 10% of the world’s energy. Biomass has the advantages of being renewable, abundant, reliable, clean, stable, and uniformly distributed around the world [3]. Moreover, it is also beneficial to struggling economies as it creates an industry and more jobs and solves the problems related to industrialization, fossil fuel use, and waste management [4,5]. The energy from biomass can be used in different ways; it can be converted to heat, electricity, or biofuels. Moreover, for small, isolated communities, the design of circular bioenergy supply chains for the valorization of local biowaste is tremendously attractive, from the point of view of energy and water resources, as well from that of land and waste management.

1.1. Biomass from the Sahara Region: The Case of Adrar (Algeria)

Today, more than 40% of Africa’s population has no access to electricity. Recent policy efforts have focused on increasing investment in renewable energy resources to reach more than 80% of new electricity generation capacity by 2030 [6]. Algeria, like other African countries, has embarked on the path of renewable energy to provide global and sustainable solutions to environmental challenges and the problems of preserving fossil energy resources. To this end, a new strategy has been launched to exploit renewable energies and produce around 22,000 MW of electricity from renewable resources by 2030, including 1000 MW from biomass [7].

The Adrar region has a great biomass potential, made up mainly of date palms (Phoenix dactylifera L.), covering about 27,804 hectares, with a total of 3,978,800 palms [8]. Every year, at the beginning of the new agricultural season, the palms are subjected to a maintenance and cleaning process that produces huge quantities of lignocellulosic biomass, between 10 kg/palm and 65 kg/palm, depending on the species [9,10,11,12]. Unfortunately, this biomass is undervalued. Most of it is burned outdoors, resulting in a significant loss of energy potential and air and visual pollution. Therefore, there is a need for better management of the huge amounts of biowaste and the exploitation of its energy potential.

1.2. Biomass Gasification

Several technologies have been developed to recover biomass as energy or material. Among these different technologies, biomass gasification stands out as a potential and efficient technology for producing valuable and renewable syngas [13]. This technology, first used as a source of public lighting in the XVIII century, has long been seen as an alternative to burning solid or liquid fuels [14]. Nowadays, gasification contributes to offset fossil fuel consumption and energy costs while reducing solid waste disposal costs [3,15,16]. Syngas has a variety of applications, including power, heat, and valuable chemical production [3,17]; namely, it shows favorable properties to be burned in internal combustion engines (ICEs) or supplied to fuel cells (FCs) [18] for high-efficiency heat and power generation.

The gasification working principle is based on the thermal conversion of biomass into a combustible gas mixture by partial oxidation at high temperatures in the presence of a gasifying media. This gas mixture (producer gas or, as already mentioned, syngas) consists of carbon monoxide, hydrogen, methane, carbon dioxide, and nitrogen [19,20]. Regarding the feedstock under treatment, gasification is regarded as the most appealing technology for biomass conversion due to its capacity to process a wide range of biomass and waste-derived feedstocks. Additionally, it is one of the main short-term options that allows for the almost final disposal of organic waste and the production of clean and fast energy [21].

1.3. Scope of the Paper

The availability at the local level of such an amount of biowaste under controlled and scheduled harvest poses questions about the most suitable energy conversion technology to exploit the biowaste potential. First of all, it is necessary to deeply characterize the feedstock. This paper provides an extensive characterization of the chemical and physical properties of four different parts of the ate palm biowaste. This contributes to enriching the literature with plenty of experimental data, as well as depicting a clear framework of a local variety of biomass, namely Algerian date palms (Phoenix dactylifera L.) variety Takarbouche in the region of Adrar, Algeria. From this point, it will also be possible to estimate the potential of different energy conversion processes. Second, this paper shows the advancements in the design and construction of an experimental facility built in the Research Unit on Renewable Energy in the Sahara region (URERMS Adrar). The design is novel, and the experimental tests allow for the identification of further improvements.

This paper is organized into three sections: Section 2, “Materials and Methods” (reporting on the characterization methodology used and the design modifications needed for the gasification reactor), Section 3, “Results” (concerning first the biomass sample proximate and ultimate and energy analysis, and secondly the gasification test results and temperature profile evolution along the reactor), and Section 4, “Conclusions”.

2. Materials and Methods

2.1. Biomass Preparation and Characterization

The samples of lignocellulosic waste from the date palm (Phoenix dactylifera L.) variety Takarbouche were taken in October, for 2 consecutive years (2017–2018), during the palm tree cleaning operations of the National Research Institute in Agronomy of Algeria in Adrar (27°52′00″ N, 0°17′00″ W at 264 m altitude). Four types of date palm waste—namely palm leaves (briefly referred to as “palms” in the followings), petioles, bunch, and bunch peduncles—were collected. All samples were first dried in open air and then coarsely grounded. Washing was also necessary to remove all impurities and contaminants, such as sand particles, bugs, and animal excrement, that might alter the characterization of the substrate. The washed waste was dried and crushed a second time using a micro-crusher to achieve a particle size ≤2 mm (Figure 1). Sawdust collected from a local furniture manufacturer was used as a reference substrate. Similarly to date palm waste, sawdust samples for analytic characterization were first pre-treated (washed to remove impurities, dried in the open air and ground to a granulometry ≤2 mm) and characterized. Sawdust destined for the gasification test was used in raw condition. The sample preparation is done according to ISO14780 [22].

The bulk density was determined according to the method described by [23], while the proximate analysis (fixed carbon (FC), volatile matter (VM), and ashes) of all the material powders is determined according to the American Society for Testing and Materials (ASTM) standards methods (ASTM-E-871 [24], ASTM-E-872 [25], ASTEM-D1102-84 [26]). The experimental characterization is repeated at least three times on each type of sample in order to express the results in terms of the average value and standard deviation. Therefore, the FC was determined according to the equation given by [27], as reported in Equation (1):

FC (%) = 100 − (VM + Ash)

(1)

The ultimate analysis for detecting the carbon (C), hydrogen (H), and oxygen (O) contents in all the samples was carried out using a mathematical model, suggested by [28], as in Equations (2)–(4).

C (%) = −35.9972 + (0.7698 VM) + (1.3269 FC) + (0.3250 Ash)

(2)

H (%) = 55.3678 − (0.4832 VM) − (0.5319 FC) − (0.5600 Ash)

(3)

O (%) = 223.6805 − (1.7226 VM) − (2.2296 FC) − (2.2463 Ash)

(4)

The higher heating value (HHV) of the lignocellulosic waste of the date palm was determined according to the mathematical model by [29] (see Equation (5)).

HHV (MJ/kg) = 19.2880 − (0.2135 VM/FC) + (0.023 FC/(Ash − (1.9584 Ash/VM)))

(5)

2.2. Gasifier

The basic updraft fixed-bed gasifier prototype used in this study was built in the URERMS Adrar laboratory, and it has been previously described in [30]. To improve the performance of the gasifier, those 5 main modifications were made to the preliminary design:

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Feedstock inlet: The substrate supply is maintained in the upper part of the gasifier body, with the addition of a feeder made of a 41 mm diameter pipe controlled by a valve and a 10 × 10 × 10 cm tank. The feeder facilitates the substrate supply and limits substrate losses (Figure 2).

-

Syngas outlet and gas cleaning system: The syngas outlet is moved from the top to the upper side of the gasification body with an increase in the outlet diameter from 21 mm to 40 mm to avoid risks linked to the pressure increase. To clean the synthesis gas, a cyclone-type cleaning system is connected to the gasification body to remove undesired compounds (mainly tars). The syngas outlet and the cleaning cyclone are shown in Figure 3.

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Feedstock bed: The grid used as a substrate bed is replaced by a steel circle (Figure 4, left), approximately 27 cm in diameter, with 3.5 mm diameter holes. The grid is installed 9 cm above the gasifier agent inlet.

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Gasifying agent inlets: In the preliminary design, the gasification agent distribution system consisted of a 29 cm long single copper tube. This is replaced by two half circles of 26 cm diameter connected with a straight pipe all in copper equipped with 3 mm diameter holes over their entire surface, as shown in Figure 4 (right), to provide better air distribution during the gasification process.

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Ignition point: A small hole with a plug is placed at the level of the gasification bed, as shown in Figure 5. The plug allows for the biomass to be ignited in order to better initiate the process.

The modified reactor is shown in Figure 6.

2.3. Gasification Tests

Sawdust is used to test the correct functioning of the gasification prototype after modification in the same way as the date palm waste. To facilitate the ignition of the substrate inside the gasifier, 100% renewable bioethanol (without any petroleum derivatives or chemical additives) is used as the combustible agent. Air is used as the gasifying agent and is supplied by a medium-sized air compressor, where the air flow can be controlled using an integrated valve. Two gasification tests are carried out separately for each type of date palm waste (palms, petioles, bunch, and bunch peduncles), feeding 1 kg of waste to the gasifier. Temperature monitoring is carried out, every 5 s, using high-temperature (+700 °C) K-type probe thermocouples through 5 points starting 10 cm above the substrate bed (T1) and distributed along the gasifier height at 10 cm intervals. The thermocouples are connected to a Fluck Hydra Series II data acquisition system (Figure 7).

2.4. Biomass Conversion Rate

The biomass conversion rate “t” is calculated directly by Equation (6), making w₀ the weight of the raw material, and w the weight of the solid residue after the reaction [31]

t (%) = (w₀ − w)/w₀

(6)

3. Results

This section reports on the results related to the two main objectives of this research: (i) a detailed characterization of the biomass samples combining experimental techniques for the proximate and calorific analysis to numerical algorithms to retrieve the elemental composition (Section 3.1); (ii) the measurements performed during the gasification tests, in order to correlate the time and space temperature evolution inside the gasifier with the feedstock properties (Section 3.2). In addition to that, the calculation of the biomass conversion rate is reported at the end of section (Section 3.3).

3.1. Biomass Characterization

Table 1. Chemical properties of date palm waste. Abbreviations: C: carbon; H: hydrogen; O: oxygen; VM: volatile matter; FC: fixed carbon; HHV: high heating value.

Feedstock	Bulk Density (kg/m3)	Ultimate Analysis (wt% Dry Basis)			Proximate Analysis (wt% Dry Basis)				HHV (MJ/kg)
		C	H	O	Moisture	VM	Ash	FC
Palms	271.57 ± 1.45	37.15	6.03	44.03	3.32 ± 0.55	85.86 ± 0.55	11.70 ± 0.35	2.45 ± 0.55	11.81 ± 1.99
Petioles	124.48 ± 0.56	46.34	3.62	32.78	5.81 ± 0.07	82.47 ± 0.93	10.10 ± 0.23	11.73 ± 0.93	17.81 ± 0.13
Bunch	102.81 ± 0.87	41.80	6.69	48.36	4.99 ± 0.10	94.05 ± 0.47	2.49 ± 0.40	3.46 ± 0.47	13.52 ± 0.81
Peduncles bunch	111.42 ± 0.77	39.21	8.07	53.40	5.61 ± 0.50	91.41 ± 0.63	2.76 ± 0.20	2.97 ± 0.63	12.74 ± 1.72
Sawdust	100.77 ± 0.76	46.91	6.48	45.59	5.87 ± 0.13	88.52 ± 0.401	0.47 ± 0.12	11.01 ± 0.88	18.12 ± 1.05

shows the proximate, ultimate, and HHV analyses of date palm waste. The bulk density values for the different substrates vary from 102.81 kg/m³ for bunches to 271.57 kg/m³ for palms, while for peduncles and petioles, they are 111.42 kg/m³ and 124.48 kg/m³, respectively. For our reference substrate, sawdust, the bulk density is very similar to the bunch and lower than the bulk density of the other substrates. These values correspond to the literature that gives the interval of the apparent density of biomass fuel between 100 kg/m³ and 700 kg/m³ [32].

The amounts of ash and VM are very important, especially during the thermochemical conversion of the feedstock. The presence of large amounts of ash can create many issues, including the production of slag, which can make gasifier operation difficult, and in extreme circumstances, they can clog the air pipes, posing a danger of explosion. A lower ash percentage is preferable in biomass feedstocks; hence, determining the ash concentration for biofuels is important [33,34,35].

The VM concentration affects the combustion properties and facilitates ignition. In general, a low volatile matter concentration indicates low reactivity, which creates challenges during gasification, resulting in inefficient conversion and high coal output [35,36]. In addition, the biomass is compared to similar and other types of date palm residues, as comprehensively shown in Figure 8. Figure 8a reports the analysis of the final moisture of the sample analyzed, which is on average drier compared to the other samples to whom the literature refers to. However, a good agreement is found with palms from [37]. Considering the samples on a dry basis, Figure 8b shows that the obtained VM values in this study for date palm waste are higher than those of our reference substrate and those obtained in previous studies [13,37,38,39], ranging from 82.47% to 94.05%. This high percentage of VM is favorable for thermal conversion, as mentioned above [35].

The low VM content implies low reactivity and therefore creates difficulties during gasification, resulting in inefficient conversion and high Char production. As we can see in Figure 8c, the ash content of lignocellulosic date palm waste is higher than the ash content of sawdust (0.47%) and varies very significantly between 2.49 and 11.7%, but these values are within the range of ash contents obtained for different biomasses, such as date palm waste, whose ash content varies between 8.02% and 18.79% [37], date palm waste leaves (DPWLs) (15.03%) [39], as well as date palm waste (DPW) and date palm fond (DPF) (6.99% and 6.71%) [13,38].

The FC contents obtained vary from 2.45 to 11.73% (Figure 8d), which are lower than those reported by [13,37,38] for different parts of the date palm. However, these values remain within the range defined for the biomass, which varies from 1 to 38% [40]. It is well known that biomass with a high VM and low FC produces gas easily when heated.

When comparing the results of the HHV with those of sawdust and other biomass (Figure 8e), they vary over a wide range (11.81–17.81 MJ/kg). However, the values obtained indicate that the date palm waste has an interesting energy potential to be exploited. In particular, the petioles show the highest value of HHV (17.81 MJ/kg) resulting from the high percentage of carbon in the volatile and fixed phases. The obtained results of the ultimate analysis were found to be satisfactory and within the range of previous studies reported for different date palm residues, as shown in Table 1. The C content of date palm waste varies from 37.15% to 46.34%, while the O content varies between 32.78% and 53.40%. In the four types of lignocellulosic waste analyzed, the C content is lower than the O content, except for petioles, where the carbon content is significantly higher than the oxygen content. These results are compatible with those of [41] for most of the lignocellulosic waste from date palms and [42] for palms and bunches. The hydrogen values obtained agree with the bibliography, which determines the range of hydrogen content between 3 and 11% for biomass [40].

3.2. Gasification Tests

All tests were performed with a feedstock load of 1 kg. The gasification test on each type of feedstock was repeated twice. Good repeatability is found between trials on the same feedstock, as may be seen from the data series reported in Figure 9 and Figure 10.

3.2.1. Time Temperature Profiles

The experimental temperature collected in all tests is reported in Figure 9 (Figure 9a reports two independent measurements conducted on the reference sample, while Figure 9b reports two independent measurements obtained for each substrate investigated). The temperature inside the gasifier is higher in the oxidation zone (marked by T1 measurement in the current setup, as indicated by the thermocouples shown in Figure 7) and generally decreases gradually as we move away from this zone towards the gas outlet (ultimately, measurement T5). Therefore, the temperature profiles measured in the gasifier allow for the main reaction zones (drying, pyrolysis, reduction, and oxidation) to be distinguished in all cases. In the first period of the gasification trial, T1 is the highest measurement for all substrates, indicating that oxidation happens in the lowest part of the reactor volume. For all the tests shown in Figure 9b, the initial temperature rise caused by oxidation is still evident in the measurement of the second thermocouple, even if T2 < T1. This effect is extended to T3 regarding the test with palm bunch. For the bunch, the change in temperature depending on the time between the different phases is smaller compared to other types of waste and cannot be notified on the graph, except for some areas near the maximum temperature peak. In the cases of palms, petioles, and peduncles, T3, T4, and T5 are almost aligned. According to the literature, the updraft fixed-bed gasifier has the lowest syngas outlet temperature compared to other reactor designs.

The maximum outlet temperature of the gas produced (T5) ranges between 375 and 409 °C for sawdust, 310 and 378 °C for palms, 311 and 327 °C for bunches, and 207 °C for petioles. Although sawdust shows higher temperatures, all these values are within the range indicated in the bibliography [43]. However, the peduncles of the bunch present a lower temperature profile compared to the other substrates; this is due to the low reaction temperature recorded inside the gasifier for this type of substrate. According to [9], this low temperature may be due to the predominance of hemicellulose, which is very thermally unstable.

3.2.2. Axial Temperature Profile

The axial profile of the temperature evolution for the sawdust substrate shows that the temperature reaches its maximum value at 10 cm above the bed (Figure 10a), which represents the oxidation zone (point T1), and then gradually decreases away from this zone. Similarly, for the tested substrates obtained from the date palms, the general pattern of temperature evolution as a function of height is very similar, as shown in Figure 10b. This approach is compatible with [44].

The maximum temperature is recorded at point T1, which is the oxidation zone. Most of the endothermic reactions take place in this zone, with the temperature gradually decreasing away from this zone. The temperature at 50 cm from the bed (point T5) represents the temperature of the synthesis gas produced; its value is significantly lower than the oxidation temperature but is within the range given in the literature for the gasification of biomass, with air as the gasification agent. The results reported in this study are based on a modified updraft gasifier prototype. Further development of the gasification setup can lead to experimental values in fine agreement with the literature, namely higher gasification bed temperatures.

3.2.3. Gas Cleaning System

The cyclone system allowed for the recovery of some of the tar produced during the gasification tests, as shown in Figure 11. On the other hand, the amount of tar recovered during the gasification of dry palm was superior to that recovered during the gasification of other substrates. This is due to the difference in reaction temperature, which is in agreement with the work of [45], which states that the concentration of tar increases with temperature, especially for lignin.

3.3. Biomass Conversion Rate

The initial amount of feedstock was 1 kg for each substrate tested. The reaction time influences the conversion rate of the date palm waste and allows for this rate to be increased considerably.

Table 2. Conversion rate based on holding time.

N°	Substrate	Holding Time	Conversion Rate (t %)
1	Palms	3 h	75 ± 1.852
		5 h	94 ± 1.452
2	Petioles	4 h	74.5 ± 4.769
		6 h	95 ± 1.053
3	Peduncle of the bunch	4 h	70 ± 2.451
		6 h	89 ± 2.645
4	Bunch	4 h	72 ± 2.783
		6 h	90.5 ± 1.307

shows the conversion rate based on holding time.

All substrates reached a conversion rate of about 90% after 6 h. For the palms, a high conversion rate (75%) was already recorded after 3 h of reaction, and this value increases to 94% after 5 h of reaction. After 4 h of gasification, a conversion rate higher than 70% was recorded for the other substrates (peduncle of the bunch, bunches, and petioles). This difference finds an explanation in the proximate analysis and from the empiric correlations for the evaluation of the elemental composition. Proximate analyses were carried out on finely ground dry samples, which differ slightly from the coarsely-ground biomass used in the tests. However, it is reasonable to assume a similar trend also for the residual moisture in all of the four tested materials. Palms exhibited the lowest moisture at the proximate analysis test; this could lead to a shorter time to obtain a higher conversion rate, which is also confirmed by the quickest and most evident temperature rise (see Figure 9b and Figure 10b). Peduncles, on the other hand, show the lowest result in terms of conversion rate after the given reaction time, which is in correlation with a higher value of oxygen content and the subsequent trend of the reaction temperature.

It is worth mentioning that the correlation between the holding time and the conversion rate strongly depends on the gasification volume case-specific design and dimension.

4. Conclusions

The results of this study are very satisfactory and encourage the orientation towards energy recovery by the gasification of lignocellulosic waste from date palms in the Adrar region. The elemental composition results show that the date palm waste has an oxygen content varying from 32.78% to 53.40%, a carbon content varying from 37.15% to 46.34%, and hydrogen content in line with the interval given in the bibliography, between 3 and 11%. The tests conducted on the different date palm waste are aligned with a reference biomass (sawdust). From these two points, it is possible to conclude that the date palm waste is suitable for the purpose and meaningful for the design of local energy supply chains. The syngas produced for the four date palm wastes is combustible, with an outlet temperature at the updraft gasifier between 200 and 400 °C for the air gasification of palms, petioles, and bunches (within the range given in the bibliography). However, the peduncle of the bunch has a lower temperature profile compared to the other substrates and, therefore, a lower conversion rate. However, this cut represents a minor fraction of the overall biowaste collected from a palm tree. The general pattern of temperature evolution as a function of height is compatible with the bibliography and the studies already carried out for all the date palm wastes tested. The residence time has a positive effect on the conversion rate of date palm waste, allowing it to reach up to around 95%.

Therefore, the annual energy potential generated by date palms is recommended for further investigation in future studies.