Figure 2: Diagram flow of biomass gasification plant. Click here to view Figure

Table 1 outlines the operational conditions associated with gasification and provides details regarding the sampling locations for ash.

Table 1: Experimental conditions

For each biomass, the tests were conducted in triplicate. Ash samples are taken during maintenance after the entire system has cooled down. Each sample taken is labeled, combining the code assigned to each biomass and the ash recovery point code (Figure 3).

Figure 3: Ash sample and labeled. Click here to view Figure

Gasification feedstock and preparation for the tests

The raw materials used for the gasification tests in this study are corn stalks and cobs, cotton stalks, and Acacia auriculiformis wood (Figure 4).

The agricultural sector is a significant contributor to the national economy of Benin, accounting for 27% of the country's GDP in 2021. A study of ten agricultural seasons revealed that corn and cotton are the leading crops in terms of production ¹⁹, which justifies the selection of these materials. For the 2017-2018 crop year, the quantities of maize and cotton residues produced are presented in Table 2.

Table 2: Quantity of maize and cotton residues produced for the 2017-2018 crop year

Acacia auriculiformis is a dedicated crop with high energy potential that is produced and used on the experimental site.

The physicochemical properties of the biomasses studied vary depending on whether it is the content of cellulose, hemicellulose, lignin, or the level of ash, volatile matter and fixed carbon.

Agricultural biomass was collected from fields in the commune of Parakou in northern Benin after harvest and transported to the gasification station. All samples underwent pre-treatment, which involved removing bulky leaves and branches^20-21-22.

They were then cut into particles approximately 3cm x 3cm x 3cm in size and dried to achieve a humidity level below 15%. Drying was carried out using both open-air and solar drying methods (Figure 5). The humidity level was checked with a meter before each gasification test (Figure 6).

Figure 4: Feedstock used in this work, (a) Corn stalks, (b) Cotton stalks, (c) Acacia auriculiformis Wood, (d) Corn cobs Click here to view Figure

Figure 5: Drying of feedstock: (a) Drying in a solar dryer, (b) Open-air drying Click here to view Figure

Figure 6: Checking the moisture content Click here to view Figure

Ash characterization method

The analyses were carried out at the Laboratory to Support the Improvement of Soil Health, Water Quality, and Environmental Protection (2A2S2E) of the Agricultural Research Institute of Benin (INRAB).

Ash content

It is obtained by taking the ratio between the mass of ash obtained after incineration and the mass of the incinerated sample.

Carbon content

The quantity of organic matter is determined by making the difference between the mass of the sample incinerated and the mass of the ashes obtained after incineration. The carbon content is determined by considering the empirical coefficient of 2 which assumes that the (quasi-crude) organic matter contains 50% carbon ²³.

Silica and metals content

The ash is dissolved in 5 cc of HCl, 6 N which is evaporated on a hot plate at 125 °C. The viscous residue obtained is recovered in 0.1 M HNO­­₃ and then filtered on ash-free paper in a 100 cc flask. The insoluble fraction is incinerated in the oven then reprocessed as before. The remainder recovered using the ashless paper is quantified and roughly corresponds to the siliceous fraction insoluble in acids. The solution thus obtained by successive filtrations is used to determine the metals by Atomic Absorption Spectrophotometry (AAS) or by Molecular Absorption Spectrophotometry (colorimetry). To minimize the effect of ionic interference during the absorption process, it is added to assay extracts of specific reagents. Thus, for the dosage of Ca and/or Mg, Lanthanum at 10 g L^-1 is used. For the dosage of sodium and/or potassium, the extracts are added with CsCl at 2 g L^-1.

Total phosphorus

The extract obtained above is subjected to the hot action of ammonium molybdate and ascorbic acid. This action develops a blue-colored complex around P, which can be measured by colorimetry to determine the concentration of this metallic element.

Results and discussion

The chemical analyses of the ash samples from Acacia auriculiformis wood, corn cobs, corn stalks, and cotton stalks have yielded the following results (Table 3). The values shown for each parameter are averages calculated from the data from the three trials carried out.

Table 3: Nutrient and heavy metal concentrations in Gasifier Reactor and cyclone bottom ashes produced from four biomasses.

Overall, ash from corn stalks is richer in Si, Fe, and Mn (microelements). Cotton stalk ash is richer in Ca, Mg, K, and Zn (macroelements and TMEs). Those from Acacia auriculiformis wood are richer in Na, P, and Cu. They reported that the composition of the ash depends on the nature of the woody biomass, the growth conditions and the nature of the process that led to the production of the ash.

We observe that the mineral element and heavy metal contents of the ashes collected at the cyclone level and at the reactor level are similar for corn stalks and ears. This composition is different from that presented by Adamon¹⁹, who instead carried out gasification of corn cob pyrolysis cokes. This result can be explained by the fact that the corn cobs would have lost some of their constituents during pyrolysis before being gasified and confirms that the ash content depends on the nature of the biomass used.

The results obtained for Acacia auriculiformis wood ash corroborate those of Pan and Eberhardt³, who focused on the characterization of fly ash generated during the gasification of pine wood chips. They noted a high concentration in the order Ca-K-Mg-P for mineral matter and in the order Zn-Cu for TMEs. This suggests that wood in general is mainly rich in Ca and Zn.

It is also noted that with the exception of Ca, Na and Zn, the ashes collected in the reactor hopper contain higher levels of nutrients and Cu than those coming from the cyclone hopper. This could be justified by the presence of chemical components such as dolomite (CaMg(CO₃)₂) and limestone (CaCO₃), which are frequently used in gasification.

This would be the reason why ash from the cyclone hopper in our case is highly alkaline. In addition, volatile ash constituents such as Zn evaporate at high combustion chamber temperatures ^24-26. Consequently, volatile constituents can be extracted from the heated ash bed and concentrated in the cyclone ash through condensation. There is an enhanced concentration of these elements in the cyclone ash compared to the bottom ash. This is also confirmed by Yao and al.⁵ who explain that during gasification, certain alkali metals in combination with other chemical elements present in the gasifier, cause partial condensation thus leading to retention of residual ashes in the form viscous. These results corroborate those of Bachmaier and al.²⁷ who showed that Cu, and the main nutrients Mg, P, Ca, and K are less volatile and are therefore, predominantly remaining in the residual ash.  For Li et al.²⁸, gasification conditions play an important role in ash deposition. Indeed, Lanzerstorfer²⁵ observed that at combustor temperatures between 830 and 920 °C, Zn accumulates in the fly ash, while most nutrients (Ca, Mg) remain in the residual ash.

It is noted that the potassium contents in the bottom ash of all biomasses, except for corn cobs, are higher than those in the cyclone ash. Lanzerstorfer ²⁵, attributes the results to the high reaction temperatures, which are sufficient to remove volatile heavy metals while remaining below the threshold for high potassium losses. Bachmaier et al.²⁷, found that if high concentrations of highly volatile elements are present in bottom ash intended for use as a fertilizer, increasing the temperature of the combustion bed may reduce these elements in the bottom ash and lead to an increase in cyclone ash.

The bottom ash analyzed in this study could therefore be used in the field of agriculture in the form of organic fertilizer^{3, 9, 29}.

However, the composition of mineral substances is not sufficient to conclude. Another key point is the concentration of trace elements in the ash obtained. Based on the data available in the literature, Table 4 shows the maximum authorized thresholds of TMEs in biomass ash for soil application.

Table 4:  Maximum authorized limit for TMEs in biomass ash

Authorized limits vary from region to region. As the threshold values for Switzerland and the United Kingdom are close for both elements (Cu and Zn), we will use them for comparison with our results (Table 5).

Zn concentrations in the ash collected from the cyclone hopper are well above the threshold values of the two references (around 5 to 13 times). The ash is therefore less suitable for agricultural use. We will therefore focus the remainder of our analysis on the ash collected in the reactor hopper.

Comparison with the threshold values authorized in Switzerland shows that corn stalk ash and cotton stalk ash both comply with the 400 mg kg^-1 limit for Zn. However, the Cu concentration of cotton stalk ash is high compared with the threshold (100 mg kg^-1), whereas for corn stalk ash, it is relatively close to the threshold (123.122 mg kg^-1).

With the values presented for the United Kingdom, the Cu concentration in corn stalk ash complies with the threshold limit, as does the Zn concentration in cotton stalk ash. However, the Cu concentration in cotton stalk ash is well above the threshold, while the Zn concentration in corn stalk ash is slightly above the threshold.

From the above, it can be deduced that corn stalk ash recovered from the reactor hopper is more suitable for agricultural use.

Table 5: Comparison of ETM concentrations with authorized limits

Conclusion

This study analyzed the composition of gasification ash from four types of biomasses: corn stalks and cobs, cotton stalks, and wood residues, based on their mineral matter content, but especially their TME content, it emerged that only ash from maize stalks collected at the reactor hopper could be used in agriculture to fertilize soil. However, N should be added to boost the fertilizing effect. Similarly, appropriate dosing is required to dilute the ETM content and comply with the admissible limits.

Acknowledgements

The authors sincerely thank the heads of Songhaï Center of Benin for their acceptation to data collection in their gasification plant.

Funding Sources

There is no funding sources.

Conflict of interest

The author(s) declares no conflict of interest.

Data Availability Statement

The manuscript incorporates all datasets produced or examined throughout this research study

Ethics Approval Statement

No experiments on humans or animals