Cement is considered the world’s second most consumed material after water [1]. The cement manufacturing process consumes high energy with exhaustion of non-renewable fuel sources and significant contributions to carbon emissions and degradation [2][3]. Cement manufacture contributes about 8% of the global anthropogenic CO₂ emission [4][5]. About 80% - 90% of these emissions take place during calcination and burning processes [6].

One possible means to reduce fuel cost is to introduce alternative fuels [7]. Use of different types of alternative fuels in the cement industry is a practice that is in existence worldwide and expected to grow as the cement kiln is a good incinerator due to its alkaline nature and as it operates at high temperature and counter-current flow of gas with raw meal [8].

Disposal of agricultural wastes constitute difficulties for farmers as they are usually in large sizes and on the increase [9][10] due to increasing growth in population and are poorly managed and underutilized as sources of energy in Nigeria [11] as about 56.9 ± 3.3 metric tons of agricultural food waste are wasted annually which could meet a third of Nigeria’s bioenergy potential [12]. Agricultural waste is rich in organic matter making it an essential as good sources of energy recovery [13][14]. If not properly managed, agricultural wastes can cause a noteworthy challenge and increase environmental pollution [15][16].

Changing alternative fuels presents some challenges as they have diverse physico-chemical and emission characteristics compared to the conventional fuels [3] which are critical to determining their workability and environmental benefits. A readily available group of alternative fuels are biomass [17] changing or co-processing of biomass offers the potential of reducing greenhouse gas emissions and promoting sustainable cement production [1]. This study investigates and compares the proximate and thermal properties of selected biomass, composite biomass blends, and conventional fossil fuels to evaluate their suitability for use in cement kilns.

The results of the proximate analysis for the selected biomass fuels, composite biomass blend and conventional fossil fuels are presented in Table 1. The data reveal clear variations in the proximate properties analyzed. Among the fossil fuels, LPFO exhibited the highest Gross Calorific Value (GCV), while coal had considerably higher ash content compared to the biomass fuels. Notably, all biomass fuels basically showed higher volatile matter content compared to coal.

Table 2 summarizes the thermogravimetric Analysis (TGA) results showing the percentage mass loss of each fuel sample across critical temperature ranges that correspond to different stages of thermal decomposition (loss in moisture, active pyrolysis of volatiles and combustion of fixed carbon). The data indicate that biomass fuels like coconut husk undergo the majority of their mass loss (80.9%) in the volatile release phase (110˚C - 400˚C), a confirmation of their high reactivity. However, fine coal fuel showed different mass loss profiles and having the highest proportion of residual char.

The thermal decomposition behavior is further illustrated in Figure 2Figure 2, which presents the Derivative thermogravimetry (DTG) and Differential Thermal Analysis (DTA) curves for all fuels. The DTG peaks indicate the rate of mass loss, with distinct profiles for different fuel types. For instance, the similar DTG peak shapes for the composite blend and sawdust suggest comparable chemical compositions, particularly as regards cellulose and hemicellulose content. All DTA curves confirm an exothermic combustion reaction for each fuel sample.

Table 1. Proximate analysis of biomass, composite blend and fossil fuels including gross calorific value, ash content, volatile matter and moisture content.

Table 2. TGA Mass Loss of biomass, composite and fossil fuels across major thermal decomposition stages.

(a) (b)

(c) (d)

(e) (f)

(g) (h)

(g)

Figure 2. DTA and TDG profiles of LPFO, Fine coal, coconut husk, composite biomass, corncobs, groundnut shell, palm kernel shell, sugarcane bagasse and sawdust fuels illustrating their pyrolysis and combustion behavior.

As shown in Table 1, the fuels exhibited substantial variation in gross calorific values, ash content, volatile matter and moisture content. According to Table 1, the coal sample had the highest moisture content of 12.07%, the least being LPFO with 0.00% followed by coconut husk, corncobs, palm kernel shell, composite biomass, groundnut shell, sugarcane bagasse, and saw dust respectively as revealed in table. Statistical analysis using analysis of variance (ANOVA) reveals a significant difference in moisture content of all fuels while, least significant difference (LSD) test indicates palm kernel and corn cobs are statically similar. The residual moisture values are low or moderately low due to varying harvest dates, weather conditions (in this case, dry season), genetics and soil composition [20]. The moisture content has the ability to influence the energy value and combustion performance [20]. Lower moisture improves combustion efficiency, as buttressed by [21]. Moisture content can also influence how the fuel is handled and its storage. All the samples except coal were sun dried to remove some moisture according to [22] while coal was air dried according to ASTM D 5373-02. as, the main controlling parameter of the combustion process after drying is the ratio between the amount of air added and the amount of air (oxygen) necessary for a complete combustion of the combustible parts of the fuel [23].

The gross calorific value (GCV) is a key fuel property of any fuel, as it represents the energy released during combustion [24]. From Table 1, the LPFO sample recorded the highest GCV of (10992 Kcal/Kg) and it is used here as benchmark because LPFO is a conventional high grade liquid fuel, widely used in cement plants. Its high energy density reflects its effectiveness in sustaining the high temperature requirement of clinker formation, making it a suitable upper limit comparator for evaluating alternative fuels. Relative to LPFO, all biomass exhibited lower GCVs, with coconut husk (4585 Kcal/Kg) and palm kernel shell (4188 Kcal/Kg) showed the highest values among the agricultural biomass options. Coal, traditionally used in cement production has a GCV of 4496 Kcal/kg, placing it between the higher quality biomass fuels and the lower calorific options, these results align with reported ranges of 3500 - 4500 Kcal/kg for low rank-coal coals [25]. Groundnut shell and sawdust showed considerably lower GCVs (3800 and 3733 Kcal/kg. Statistical analysis using ANOVA shows a significant difference in GCV across most fuels while LSD test indicates no significant difference in gross calorific values between corncob and groundnut husk. The comparative high GCV of coconut husk and palm kernel shell suggests as that these fuels could serve as feasible partial substitute for fossil fuels in cement kilns. However, their operational performance must still consider biomass moisture content, which can influence thermal efficiency in the kiln.

According to Table 1, ash analyzed for each fuel is in the order; 1.76 < 2.06 < 2.77 < 3.01 < 3.59 < 3.67 < 6.2 < 28.85. Ash is the noncombustible residual or mineral matter produced from fuel combustion [26]. The coal sample had the highest ash of 28.85% which indicates that it is a lignite formation, followed by palm kernel shell, sugar cane bagasse, saw dust, composite biomass, coconut and groundnut shell respectively with the least being corn cobs with 1.76%. Statistical analysis using ANOVA shows a significant difference in gross calorific value across almost all fuels while LSD shows no significant difference in ash values between corncob and groundnut husk. During clinker production process, the organic part of waste fuel is totally destroyed while the inorganic compounds and heavy metals in the ash are assimilated in the clinker phases or substitute calcium or silicon in the silicate phases of the clinker. However, due to their origin these wastes introduced into the cement making process, introduce several elements that may affect the process itself as well as the composition and quality of clinker [27]. Higher ash invariably leads to increased silica and other inorganic components in resultant clinker which may affect the quality. Studies by [10] and [28] confirm that biomass with high volatile matter and low ash increases flame stability and reduces fouling risks.

Volatile matter analyzed for each fuel in Table 1 is in the order; 34.10 < 62.52 < 72.62 < 75.63 < 79.30 < 79.33 < 82.00 < 82.57. The coal sample had the least volatile matter of 34.10% followed by coconut, palm kernel shell, composite biomass, groundnut shell and sugar cane bagasse with equal volatile matter (VM) and saw dust respectively with the highest being corncobs with volatile matter of 82.6%. These findings are consistent with biomass combustion behavior documented by [29] and [30]. The highest volatile matter observed in corncobs followed by sawdust, is an indicator of combustibility. This suggests rapid ignition and flame propagation, qualities favorable for kiln firing. Statistical analysis using ANOVA indicates a significant difference in volatile matter across almost all fuels while LSD indicates no significant difference in volatile matter of sugarcane bagasse and ground nut husk. High volatile matter (VM) observed in biomass can boost combustion efficiency. The volatile matter content of fuel affects storage behavior (oxidation, danger of spontaneous combustion, and loss of heating value), required fineness for pulverization, burner settings, combustion behavior, and efficiency [26].

From Table 2, mass loss between 27˚C - 110˚C corresponds to moisture loss, dominated by coconut husk (4.11%). All DTA peaks in Figure 2Figure 2 indicate their combustion reaction was exothermic. Biomass fuels displayed higher mass loss in the volatile release zone (110˚C - 400˚C) as revealed in Table 2, consistent with their high cellulose and hemicellulose content. The DTG peak of composite fuel and sawdust are similar, DTG peak of palm kernel shell and corncobs are also similar, suggesting similarity in their major composition (cellulose/hemicellulose). Mass loss at 27˚C - 110˚C can be attributed to moisture loss, the moisture is within the range 0.231% - 4.107%, with the least moisture from sawdust while higher moisture is from coconut husk fuel, the values for moisture content for all fuel are reasonably low, high moisture content of fuel in the kiln can cool the flame zone affecting clinker formation. Active pyrolysis occurs at temperature between 110˚C - 400˚C with high volatilization. At this stage, hemicellulose and cellulose break down in biomass. Percentage mass loss for both fossil and agricultural biomass are within the range of 37.91% - 80.91% with the highest mass loss observed in coconut husk fuel while least mass loss observed in sawdust, indicating coconut husk contains more hemicellulose and cellulose compared to other biomass and sawdust contains least among the biomass samples. Mass loss at temperatures above 400˚C are within the range of 6.83% - 55.77%, with the highest mass loss observed in sawdust while coconut husk in contrast is observed to have the least mass loss, indicating saw dust contains more lignin compared to other biomass and coconut shell contains least. The observed chars are within the range of 3.73% - 23.46% with the highest char observed in fine coal while least observed in palm kernel shell and corncobs. Residue left after pyrolysis otherwise known as char is mostly fixed carbon and inorganic minerals in form of ash. The highest char in fine coal is consistent with proximate analysis of ash observed in the same sample.

A limitation of this study is that, it focuses primarily on proximate and thermal analyses without incorporation other fuel characteristics such as elemental composition, ash mineralogy and trace metal content.

Adoption of these biomass from agricultural waste and its composite blend fuel offer practical benefits as sustainable alternatives to fossil fuels for cement production in Nigeria, providing acceptable calorific values, thermal properties and combustion qualities. Their incorporation in cement pyro processing can support decarbonization goals within the industry while reducing dependence on non-renewable fossil fuels.

I sincerely appreciate the Joseph Sarwuan University, Makurdi, Benue State and Ahmadu Bello University, Zaria, Nigeria, for giving us the opportunity and necessary supports that enabled us to carry out analysis of this work using their laboratory equipment.