*3.1. Proximate and Ultimate Analysis of Napier Grass Feedstock*

The results of the ultimate and proximate analysis of NG and other biomasses reported in the literature are presented in Table 2. TGA was conducted at three different heating rates (5, 10 and 20 ◦C/min) and almost similar results were produced. Therefore, the result acquired at 10 ◦C/min was considered for further discussion. As shown in Figure 2, NG in air atmosphere recorded a higher moisture content (8.78%) compared to the N2 atmosphere (7.73%). The same pattern was observed for volatile matter where a high content was recorded in air (81.49%) compared to nitrogen (62.70%). This is due to the presence of oxygen in the air that promotes biomass devolatilization. In contrast, lower fixed carbon content was recorded in air atmosphere (3.54%) compared to the N2 atmosphere (9.69%). Ash content was three times higher under N2 as compared to air atmospheres. According to

Ptasinski [17] substantial variation in characteristics of biomass might have a direct effect on syngas composition. In addition, due to the environmental conditions and harvesting techniques, the biomass composition may vary remarkably [18].

**Figure 2.** Proximate analysis of Napier grass in (**a**) air and (**b**) nitrogen at a constant flowrate of 25 mL/min.

NG contains a high fraction of volatile matter (>80%) and moisture (30%) but low ash content (<7%). The high content of volatile matter in the Napier grass is desirable since this feature enhances the gasification process and reduces the amount of char produced [7]. High moisture content (>40 wt%) of biomass tends to degrade the gasification performance as more energy is required in the drying process [19]. Besides, moderate fixed carbon compared to other biomasses indicates shorter burning time is required as the solid-gas combustion reactions are slower than gas–gas reactions [7].

Ultimate analysis of NG found that NG contained 48.52% oxygen, 45.10% carbon, 5.94% hydrogen, 0.45% nitrogen and 0% sulphur. These values were comparable to NG, *Miscanthus* and coconut shell reported in literature. However, in comparison to all biomasses except for the NG in literature, NG had higher oxygen but lower carbon content, indicating a lower calorific value. Higher oxygen composition (carbon–oxygen bonds) would have a significant negative effect on the heating value of biomass [20]. NG is considered as an environmentally friendly feedstock as it contains a low amount of nitrogen and

no sulphur, which may improve fuel quality and reduce toxic gas emission. The heating value of NG (16.73 MJ/kg) was comparable to coconut shell but slightly lower than other biomasses in literature because of its high level of moisture and oxygen. These desirable characteristics indicate that NG is a good solid biofuel candidate for energy generation via gasification.

**Table 2.** Comparison of Napier grass (NG) feedstock characteristics with various types of biomass in literature.


#### *3.2. Inorganic Compounds in Napier Grass Ash*

Table 3 compares inorganic compounds in ash of NG and other biomasses reported in literature. More than half of inorganic compounds detected in NG ash were potassium (54.39%), with some iron (15.53%), silicon (9.81%), chlorine (8.84%), calcium (8.20%) and sulphur (2.30%). The ash composition of NG in literature is rich in silicon (43.00%), potassium (30.5%), magnesium (9.90%) and phosphorus (7.20%). The differences of NG ash properties in the current study as compared to the literature were due to the fact that the trees were dependent on growth conditions and other environmental factors. The ash composition has a negative effect on the gasification performance reactor when subjected to high temperature combustion [24].

According to Obernberger and Thek [25], a high potassium content in ash causes agglomeration at high temperature, which may bring severe damage to the gasifier system. Arvelakis, et al. [26] reported that potassium content in ash could react with bed material such as silica sand and break Si–O–Si bonds to form silicates that deposit on the reactor wall and on the bed particle surface, causing agglomeration. According to Arvelakis, Gehrmann, Beckmann and Koukios [26], high proportion of SiO2 found in the ash of *Miscanthus* (62.21%) and agricultural residues (89.57%) would cause severe agglomeration when the potassium reacts with SiO2.


**Table 3.** Inorganic contents in ash of Napier grass and other types of biomass.

Mohammed, Salmiaton, WanAzlina and Mohamad Amran [16] stated that CaO acts as a CO2 adsorbent where its presence might accelerate the secondary reaction and therefore improve hydrogen content in the syngas. In addition, Mahishi and Goswami [30] supported that the presence of CaO has a significant effect on hydrogen production during the gasification process at an elevated temperature. In relation to iron content, NG in the current study contains a high amount of iron, which was 15.53%. According to Lahijani and Zainal [28], magnesium, iron and calcium are good agents for reducing agglomeration. Low silica content (9.81%) was found in Napier grass in this current study and this amount was comparable to olive residues (11.84%) and cardoon (8.34%). Mohammed, Salmiaton, Wan Azlina and Mohamad Amran [16] mentioned that the combination of low silica content with reasonable amount of MgO, Fe2O and Al2O3 is effective in reducing agglomeration. Furthermore, NG in the current study contained a significant amount of Cl (8.84%), which could react with potassium to form potassium chloride and subsequently promote potassium devolatilization [26].

#### *3.3. Components of Bio-Liquid*

Gasification of biomass generates bio-liquid as one of the by-products. The liquid is visually dark brown, usually comprised of water, oxygenated hydrocarbons and other hydrocarbons. Bio-liquid generated from pyrolysis and gasification can be used for fuel in direct combustion or as chemical products after further treatment processes. Table 4 presents the fraction of individual compounds detected in the bio-liquid produced from gasification of Napier grass. The liquid contains a mixture of hydrocarbon, oxygenated and nitrogenated compounds. Phosphonic acid, (p-hydroxyphenyl) was found to be the major constituent (31.94%), followed by pentane, 2,2-dimethyl (16.10%). The oxygenated compounds in bio-liquid were corrosive in nature with low pH value. Bio-liquid with high amount of oxygenated compounds should undergo further treatment such as hydrothermal processing to produce biofuels or value-added chemicals [13].


**Table 4.** Chemical compounds detected in bio-liquid obtained from gasification of Napier grass at 850 ◦C and equivalence ratio (ER) of 0.2.

A = carboxylic acid; Al = alkane; E = ester; K = ketal; N = nitrogenated; P = phenols.

#### *3.4. E*ff*ects of ER on the Product Yield and Composition of Producer Gas*

Equivalence ratio (ER) is defined as the ratio of the amount of air supplied during the thermochemical process to the amount of air required for stoichiometric combustion of the fuel (Equation (1)). The concept of ER is applicable in gasification when air or oxygen is injected for partial combustion of the biomass feedstock. In comparison to the combustion process, which requires excess air and ER to be greater than 1 for complete fuel combustion, the range of ER for gasification is usually limited to a value below 0.4.

ER = (Actual weight air/weight of dry biomass)/(Stoichiometric air/biomass ratio). (1)

The formula for stoichiometric combustion with oxygen is:

$$\rm{CH}\_{1.56}\rm{O}\_{0.81} + 0.985\rm{O}\_2 \to 0.78\rm{H}\_2\rm{O} + \rm{CO}\_2\tag{2}$$

where:

$$\rm{AC}\_{\rm{stq}} = (\rm{M}\_{\rm{O2}} + 3.76 \rm{M}\_{\rm{N2}}) / 100 \times (\rm{C}/\rm{M}\_{\rm{c}} + \rm{H}/2 \rm{M}\_{\rm{H2}} + \rm{S}/\rm{M}\_{\rm{S}} - \rm{O/M}\_{\rm{O2}}) = 5.12. \tag{3}$$

Taking the information below into consideration.

$$\text{Density of air, } \rho = \text{m/V.}\tag{4}$$

$$1.18 = 0.032/\text{V}.\tag{5}$$

$$V = 0.027 \text{ m}^3 = 27 \text{ liters.}\tag{6}$$

$$\text{Superficial velocity} = \text{V/A.}\tag{7}$$

$$\text{Diameter of reactor, D} = 0.054 \text{ m.} \tag{8}$$

$$\text{Surface area of reactor, A} = \pi \times \text{(D/2)}^2 = 2.29 \times 10^{-3} \text{ m}^2. \tag{9}$$

$$\text{Superficial velocity} = 3.0 \text{ L/min} \times 1 \text{ m}^3/1000 \text{ L} \times 1 / 2.29 \times 10^{-3} \text{ m}^2 \times 1 \text{ min} / 60 \text{ s} = 0.0218 \text{ m/s}. \tag{10}$$

In this study, the ER was manipulated by varying the air flowrate while the biomass feeding rate remained unchanged. The effects of ER on product yield are shown in Figure 3. Bio-gas and bio-liquid yields recorded an upward trend with increasing ER while the bio-char demonstrated the opposite. The ER is an indicator of the quantity of oxygen supplied to the reactor and gasification temperature under autothermal operation [29]. Higher ER leads to higher gasification temperature, accelerates oxidation reactions and leads to enhanced product quality. Conversely, lower ER limits the amount of oxygen available for gasification reactions and therefore is not a favorable condition [31].

**Figure 3.** Effect of ER on the NG gasification product yield.

Figure 4 shows the gas composition at different ER. As ER increased from 0.20 to 0.40, H2 and CH4 contents in the producer gas decreased from 11.54 mol% to 8.29 mol% and 9.85 mol% to 7.09 mol% respectively. The effect of ER on gas composition is attributed to the oxidation reactions. Higher ER implies that more air (oxygen) is injected into the reactor, which in turn promotes combustion of CH4 with O2 while the CH4 formation by methanation reaction is inhibited at high temperature [32]. Therefore, the volume fraction of CH4 decreases as ER increases. Furthermore, Kuo, et al. [33] also reported that the fraction of H2, CO and CH4 decreases as ER increases. As the amount of oxygen supplied decreases with decreasing ER, the carbon converts to CO instead of CO2 through oxidation and partial combustion reactions [30]. As the amount of CO increased, more CH4 and H2 are formed through methanation and water-gas shift reactions. Hence, higher concentration of CH4 and H2 are detected in producer gas at lower ER.

+ &2 &+ &2

**Figure 4.** Effect of ER on the composition of producer gas produced from the gasification of Napier grass at a temperature of 850 ◦C.

At ER of 0.20, the concentration of CO and CO2 were 26.36 mol% and 52.26 mol%, respectively. As ER increased to 0.30, the concentration of CO increased to 30.32 mol% and decreased to 26.94 mol%

as ER was further increased to 0.40. The observation is in good agreement with finding reported by Ghassemi and Shahsavan–Markadeh [34]. Meanwhile, the concentration of CO2 decreased slightly to 50.04 mol% as ER increased to 0.3, and increased to 57.68 mol% as ER was further increased to 0.40. The phenomena could be deciphered by reversible water-gas shift and dry reforming reactions. Excess air would promote oxidation of bio-char and other combustible species and consequently leads to elevated CO2 production.

#### *3.5. E*ff*ects of Temperature on Product Yield and Quality of Producer Gas*

In air gasification of biomass, gasification temperature is one of the crucial operating parameters, which is usually manipulated to investigate thermodynamic behavior of the reactions. Production of syngas consists of multi-step chemical reactions where temperature has significant impacts on the kinetics of the reactions involved. The influence of temperature on syngas production from the gasification of Napier grass was investigated at five different reactor temperatures (650 ◦C, 700 ◦C, 750 ◦C, 800 ◦C and 850 ◦C) while ER was fixed at 0.25.

The experimental results are shown in Figures 5 and 6. As observed in Figure 5, the bio-gas yield increased from 56.92 wt% to 67.56 wt% while the bio-char yield decreased from 27.40 wt% to 17.88 wt% with the rise of temperature, showing a divergent trend. The yield of bio-liquid showed an upward trend with increasing temperature and recorded a peak of 20.12 wt% at 800 ◦C. The yield subsequently declined as the temperature was further increased. A high operating temperature provides a conducive condition and supplies sufficient thermal energy for Boudouard, water-gas and methanation reactions, consuming more solid carbon to produce combustible gases. Furthermore, as the temperature increased from 650 ◦C to 800 ◦C, more H2 was produced and reacted with O2 to form water, and thus increased the yield of bio-liquid. As the temperature further increased to 850 ◦C, secondary reactions such as tar-cracking consume water and therefore reduce the yield of bio-liquid.

**Figure 5.** Effects of temperature on Napier grass gasification yield.

Figure 6 illustrates the effects of temperature on the composition of H2, CO, CH4 and CO2 in the gases produced from gasification of NG at ER of 0.25. H2 gas concentration increased remarkably from 7.31 mol% to 11.47 mol% as the operating temperature increased from 650 ◦C to 800 ◦C and dropped slightly to 10.27 mol% at 850 ◦C. The high operating temperature provides favorable conditions for endothermic reversible steam methane reforming, water-gas and dry reforming reactions. The production of hydrogen is enhanced as the heat absorbing reactions shift the equilibrium to the right [28].

,Ϯ K ,ϰ KϮ

**Figure 6.** Effects of temperature on gas composition at ER of 0.25.

No distinctive trend can be observed for CO and CO2 production. CO concentration increased from 26.23 mol% to 29.93 mol% as the temperature was increased from 650 ◦C to 700 ◦C. Subsequently the CO content decreased to 26.69 mol% at 750 ◦C and dropped further to 25.16 mol% at 850 ◦C. Within the gasifier operating temperature range of 650–850 ◦C, the recorded CO2 concentration was within the range of 48.48–59.07 mol%. The pattern in CO2 production at varying operating temperatures was opposite to CO. The high bed temperature stimulated complete combustion while deterring incomplete combustion processes, and accelerated solid carbon burning to produce CO2 instead of CO.

## *3.6. E*ff*ects of ER on the Higher Heating Value (HHV) of Syngas and Carbon Conversion E*ffi*ciency (CCE)*

The calculation of the gross calorific value or higher heating value (HHV) of Napier grass can be performed by using Equation (11) [35]. Carbon conversion efficiency (CCE) is one of the key indicators of gasification performance, which provides the information pertaining to the degree of reaction completion. CCE can be determined from carbon element content in biomass feedstock and syngas composition, and can be calculated by using Equation (12) [36].

$$\text{HHV (M)/Nm}^{3}\text{)}=(\text{CO}\% \times \text{3018} + \text{H}\_{2}\% \times \text{3052} + \text{CH}\_{4}\% \times \text{9500})(0.01 \times 4.1868), \tag{11}$$

where CO%, H2% and CH4% are molar fraction of syngas components.

$$\text{CCE (\%)}=(1 - \text{M}\_{\text{a}}/\text{M}\_{\text{0}}) \times 100,\tag{12}$$

where Ma and M0 are final and initial total mass of biomass respectively.

The use of air as a gasifying agent is cheap and widely practiced. However, the presence of abundant nitrogen in air dilutes the concentration of syngas and consequently reduces the syngas heating value. The values of producer gas HHV and biomass CCE at varying ER are summarized in Table 5. The calculation of HHV is performed for both with and without taking nitrogen content in the air into consideration. With the presence of nitrogen, the highest HHV was found at ER of 0.20, and the value decreased as ER was further increased. According to Lv, et al. [37], ER is more than just a measurement of oxygen supply. ER represents the border-line between combustion and gasification reactions in the gasification system. Jayathilake and Rudra [38] reported that lower ER values resulted in higher CH4 and H2 concentration. The higher amount of air supplied at higher ER promotes combustion of H2 and CH4 components in the syngas. In addition, CH4 formation by methanation reaction is retarded at higher temperature that comes with higher ER. Consequently, the reduction of H2 and CH4 concentration in syngas at higher ER will directly reduce the HHV of the producer gas. The findings are in good agreement with research done by Sheth and Babu [39]. Syngas HHV under N2 free condition increased slightly from 8.72 MJ/m3 to 8.78 MJ/m3 as ER increased from 0.20 to 0.30, and decreased to 7.28 MJ/m<sup>3</sup> with a further increase in the ER to 0.4. On the other hand, as shown in Table 5, higher CCE is achieved at higher ER. The increased amount of air supplied during gasification improves the combustion process and contact with solid carbon, thus enhances the carbon conversion rate.

The effects of varying gasification temperature on producer gas HHV and biomass CCE are presented in Table 6. The HHV appeared to be increasing with increasing temperature. A peak was recorded at a gasification temperature of 800 ◦C where HHV subsequently decreased with a further temperature rise. Gasification temperature had a significant effect on syngas composition, which directly influenced the HHV of producer gas. As reported by Wu, et al. [40], elevated CO2 concentration from high temperature biomass combustion process dilutes the concentration of H2 and CH4, results in a lower calorific value. As depicted in Table 6, CCE demonstrates a positive correlation with gasification temperature. The highest CCE is recorded to be 82.12% at 850 ◦C. The findings are in good agreement with work reported by Lv, Xiong, Chang, Wu, Chen and Zhu [37]. As shown in Tables 5 and 6, the dilution of the syngas by nitrogen can degrade the HHV of the syngas by at least twofold. The syngas produced from gasification thermochemical conversion of biomass can be used to produce heat and electricity in the combined heat and power (CHP) system, internal combustion engines or other applications.

**Table 5.** Effects of ER on higher heating value (HHV) and carbon conversion efficiency (CCE) at a gasification temperature of 850 ◦C.



**Table 6.** Effects of temperature on HHV and CCE.

#### **4. Conclusions**

Napier grass energy crop demonstrates a good potential as a renewable solid biofuel. Its calorific value of 16.73 MJ/kg is comparable to other biomasses reported in literature. The high content of volatile matter in Napier grass is highly desirable as this feature promotes the gasification process. Thermochemical gasification of Napier grass produces syngas and value-added by-products such as bio-char and bio-liquid. The presence of a high amount of potassium in Napier grass ash might impose a further problem for the long-term operation. The dynamics of the reactions involved were observed as a significant variation in product yield and biogas components were recorded at varying ER and gasifier operating temperatures. There was a positive correlation between ER and gasification temperature with CCE. Enhancement of combustion process at elevated temperature and air supply produced CO2 that degraded the syngas quality and resulted in low HHV. The highest HHV was recorded at a gasification temperature of 800 ◦C with ER of 0.2 and thus these conditions were determined as the optimum operational conditions for bench-scale gasification of Napier grass. The findings from this study were encompassed within the limited explored range and further research (scale-up) will be carried out as future development.

**Author Contributions:** Conceptualization, T.H.B. and W.A.W.A.K.G.; methodology, W.A.W.A.K.G. and D.K.S.N.; formal analysis, T.H.B.; investigation, T.H.B.; resources, D.K.S.N.; writing—original draft preparation, M.S.M.S. and T.H.B.; writing—review and editing, M.S.M.S. and W.A.W.A.K.G.; supervision, W.A.W.A.K.G. and S.A.H.; funding acquisition, W.A.W.A.K.G.

**Funding:** This research was funded by Ministry of Education Malaysia, grant number LRGS/2013/UKM/KPT.

**Acknowledgments:** Feedstocks provided by Crops for the Future Research Centre (CFFRC) are gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
