J. Nig. Soc. Phys. Sci. 4 (2022) 954 Journal of the Nigerian Society of Physical Sciences Theoretical Air Requirement and Combustion Flue Gases Analysis for Indigenous Biomass Combustion M. A. Lalaa,∗, O. A. Adesinaa, O. J. Odejobib, J.A. Sonibareb aDepartment of Chemical and Petroleum Engineering, Afe Babalola University, Ado-Ekiti, Nigeria bDepartment of Chemical Engineering Obafemi Awolowo University, Nigeria Abstract Rice-husk is one of the most abundant and commonly utilized biomass. It can be used directly to generate primary heating agent in combustion plants. However, emission characteristic of biomass usually vary depending on factors such as the variety of the parent plant, region of cultivation and climatic condition. Moreover, gases emission from biomass combustion could be detrimental to health; hence there is a need for information on the theoretical volume of air and flue gases required for optimal operation of combustion equipment. Thus, this work analyzed the theoretical combustion air requirement and the resulting flue gas volume of the abundant Nerica-rice-husk variety. Rice-husk samples were collected from southwestern part of Nigeria. The samples were characterised for their ultimate and proximate content. Afterward, the combustion analysis was carried out using the elemental composition obtained from the ultimate analysis to determine the theoretical combustion air requirement and the resulting flue gas volume. The proximate analysis result showed the moisture content level, fixed carbon content and ash content value of 10.93%, 12.59% and 18.26% respectively for sample A and 9.80%, 20.76% and 14.81% for sample B. This reveals that Nerica-rice-husk is advisable for use as an alternative energy source in a combustion process. The calculated theoretical volume of air for sample A was 3.69596 (N m3/kg fuel), while 3.53947 (N m3/kg fuel) was obtained for sample B. This signifies a slight different between the combustion properties of rice-husk of same variety. It also showed that more quantity of theoretical air will be required to burn sample A compared to sample B, and that sample A will generate little more emissions compared to sample B. Hence, this study reveals the volume of air required for low emission combustion of Nerica-rice-husk in combustion plants. DOI:10.46481/jnsps.2022.954 Keywords: Rice-husk, Biomass combustion, Theoretical combustion air, Ultimate analysis, Proximate analysis Article History : Received: 25 July 2022 Received in revised form: 16 August 2022 Accepted for publication: 17 August 2022 Published: 19 September 2022 c© 2022 The Author(s). Published by the Nigerian Society of Physical Sciences under the terms of the Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Communicated by: E. E. Anand 1. Introduction National Ambient Air Quality Standards (NAAQS) by gov- ernment regulatory agencies are available for the allowable con- centration of pollution in the air. These standards apply to ∗Corresponding author tel. no: +234 7031549936; +234 8163280469 Email address: lalam@abuad.edu.ng (M. A. Lala) emissions from combustion reactors such as fireplace, stoves, furnaces, woodchip and pellet furnace, furnaces with boilers, industrial combustion systems and many others [1]. Biomass such as rice-husk is a good alternative source of energy which is now mostly used in combustion plants to generate primary heat- ing agent in boiler [2]. Emission of more toxic pollutants from the combustion of fossil fuels together with the abundant na- ture of biomass has switched people attention to burning cleaner 1 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 2 and renewable biomass [3]. Nevertheless, analysis of flue gases from biomass-fired combustion reactors is highly important in ensuring proper compliance of combustion operation with stan- dards set by the environmental pollution regulatory agencies [2]. Operations where the volume of the emitting flue gases is resulting in high pollutant concentration can be turn off or mod- ified since it could be detrimental to people within the proximity of the emission. Excessive flue gas can also indicate a need for maintenance on the combustion equipment to reduce the emis- sion of toxic gases. The presence of gaseous emission such as carbon monoxide (CO) signifies incomplete combustion and the process can be modified to operate more effectively [1]. Combustion analysis which involves the determination and monitoring of flue gases from solid fuels combustion system is extremely significant in designing and operating combus- tion equipment. It also ensures safe operating procedure and maximum combustion efficiency [4]. Therefore, this work ana- lyzed the theoretical combustion air requirement and the result- ing flue gas volume for the abundant and indigenous rice-husk as an alternative energy source. The rice-husk samples were collected from two different geographical locations in south- western Nigeria. The samples were characterised for their ele- mental composition, energy content and proximate content (mois- ture, volatile, ash, fixed carbon and total carbon content) prior to the combustion analysis. Characteristic properties of biomass usually vary depending on factors such as the variety of the parent plant, region of cul- tivation, climatic condition and postharvest processing method. The proximate analysis gives the moisture, ash, volatile and fixed carbon content; ultimate analysis provide information (kg component/ kg fuel) on carbon (%C), hydrogen (%H), nitrogen (%N), sulphur (%S), oxygen (%O) and other elementary chem- ical composition, and; calorimetry presents the energy content or the heating value of the fuel. The heating value measured in (J/g) is the heat released by the complete combustion of a unit mass of fuel under normal physical conditions, P0 = 1.013 bar and T = 00C. Heating value is either high heating value (HHV) or low heating value (LHV) depending on the amount of water vapour available in the combustion products. The water vapour is a result of hydrogen oxidation, fuel moisture content and combustion air moisture content. All these characteristic properties determine the combustion characteristic of biomass. Similarly, factors such as the origin of biomass, variety of the parent plant, region of cultivation and climatic condition also affect biomass compositions [5-7]. The use of agricultural waste biomass as fuel has been broadly studied by many researchers [5]. Rice-husk is one of the most abundant and commonly utilized biomass and can be classified as combustible solid waste [5, 6]. During combustion process, variations in the composition of biomass will influence the vol- ume of air required for burning and the volume of the emitted flue gases. Hence, it is important to know the optimum quantity of air that will be needed for maximum combustion efficiency, thus permitting appropriate control of the combustion process. This will culminate in optimum functioning of the equipment, reduce emissions and reduce heat losses [4]. Combustion is a form of chemical reaction process during which combustible elements in fuel react with oxygen in the air to produce large quantity of heat energy. Heat has various ap- plications in industrial processes such as; expansion of gases in cylinder and piston push, environmental heating, boilers, fur- naces, power plants and many other systems [4, 7, 8]. Fuel can be fossil or organic and must be able to produce a reasonable amount of heat during combustion with oxygen in the air. A good fuel, under normal condition, will react exothermically with oxygen at high speed and temperature to produce environ- mentally friendly and non-corrosive flue gases. Also, such fuel must be cheap and abundant in nature. Biomasses are good al- ternative source of energy which can burn readily in combustion plants to generate primary heating agent in boiler [2]. Burning of biomass and determination of the theoretical combustion re- quirements necessitate analysis of the biomass to establish: • the combustible mass, • ballast, that does not participate in combustion, and • the energy content of the fuel. In industrial boiler and many other industrial and house- hold applications, the combustion process involves the reac- tion between the two major combustion reactants (fuel and air) in a combustion chamber to produce new chemical substances called combustion gases [9]. In most combustion processes, air is the most common and readily available oxidizing agent be- cause it contains free and cheap oxygen while metallurgy and some other specialized system use high-purity oxygen. The oxy-fuel combustion technology is suitable for CO2 emission reduction [4, 10-12]. Air ratio or excess air coefficient (γ) indi- cates the nature of the combustion and this determine the com- position of the combustion flue gases [4]: • γ < 1, indicates incomplete combustion and the products may contain CO, CO2, SO2, H2O and N2. • γ = 1, indicates theoretical or stoichiometric combustion and the products may contain CO2, SO2, H2O and N2. • γ > 1, indicates excess air combustion and the products may contain CO2, SO2, H2O, N2 and O2. 1.1. Theoretical combustion Theoretical or stoichiometric air is the minimum amount of air necessary for the complete combustion of a fuel. This result in no uncombined oxygen in the flue gas and no combustible chemical element such as soot or carbon particles especially for solid fuel. Complete combustion is otherwise called theoret- ical or stoichiometric combustion. Excess combustion occurs when the amount of air supplied for combustion is more than the theoretical amount needed. A combustion process is termed incomplete when the amount of air or oxygen supplied is less than the theoretical requirement, or when there is an inadequate degree of mixing of sufficient or even excess oxygen. Incom- plete combustion can either be mechanical or chemical. Me- chanical incomplete combustion is identified by the presence of 2 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 3 combustible elements such as soot or carbon in the combustion gases while chemical incomplete combustion contains only the flue gases [4]. Combustion process calculation is mainly aimed at determining: • the thermal energy resulting from the combustion pro- cess, • the volume of air required for the chemical reactions of the combustible elements in fuel, and • the volume of flue gas resulting from the combustion pro- cess. Monitoring the volume of air supplied for combustion is highly important. It helps in ensuring that the process is operating with a sufficient quantity of air. Insufficient air will lead to in- complete combustion and unregulated excess air will decrease combustion temperature. This will increase the resulting flue gas volume and then culminates into heat loss [13]. Moreover, monitoring the volume of flue gas will assist in the [4]: • design and appropriate sizing of the exhaust pipes, • design and appropriate sizing of the chimney, and • heat recovery system. In this work, the combustion air volume and the resulting flue gas volume for a unit mass of rice-husk obtained from two different locations in southwestern Nigeria were determined. This was done by assuming complete combustion and with- out taking the complex exothermic oxidation of the fuel into consideration. Combustion analysis of the rice-husk samples was achieved using the formulations described under theoreti- cal calculations. These formulations are developed into a web application by Paraschiva et al. [4, 14]. The method is sim- ple and straightforward and also avoids cumbersome calcula- tion that may bring errors. 1.2. Rice-husk Rice-husk also known as rice hull, can be described as the coating on a grain of rice. It protects the rice grain or seed during the growing season and is formed from materials, such as silica and lignin. Rice-husk is an agricultural by-product of rice-milling industry and belongs to the category of biomass/bio- resource material. Each kilogram of milled rice paddy results in roughly 0.28 kg of rice-husk as a by-product. This is about 20-25% of the rice paddy. Singh et al., and Muhammad et al., reported rice to husk ratio to be 0.20 [15, 16]. Therefore, abun- dance of rice-husk is a function of rice production and the pro- portion of husk in a paddy. Husk generation rate in Nigeria is high because rice (Oryza Sativa) is one of the most important staple foods in the country [17]. About 70% of Nigerians feed directly on rice and about 30% feeds on cereal-based foods which are derived from rice [18]. In West Africa, Nigeria is ranked highest as both the pro- ducer and consumer of rice [19, 20]. Statistic shows that there is a continuous increase in rice production in Nigeria, especially from the year 2011 due to the government agricultural reform program put in place. This includes Agricultural Transforma- tion Agenda (ATA) and Agriculture Promotion Policy (APP) [21]. According to GEMS4/Coffey International Development LTD report [21], rice is produced in almost all the 36 states of Nigeria and also in the federal capital territory (FCT), Abuja. But majorly, only 18 states represent cumulatively about 80% of the aggregate domestic paddy output in the country [21]. Among these 18 states, preference is given to Ekiti and Ogun state because of their local rice varieties such as Ofada rice. Fig- ure 1 shows the continuous increase in the production of rice in Nigeria from the year 2011. Rice-husk has numerous uses and among many others, it is a lignocellulosics biomass. It can be used directly in its raw form or converted either into briquette, liquid fuel, tar, and ash before use [22]. Utilization of rice-husk as a feedstock in a biomass combustion system requires prior ultimate, prox- imate and energy content analysis. That is, in other to opti- mize the combustion process in adequate reactors, a compre- hensive study of the characterization of biomass fuel properties is needed because it remains one of the factors that heavily in- fluence the decomposition of biomass under oxidative condi- tion. 2. Materials and methods 2.1. Materials and instrumentations Two (2) rice-husk samples of the same paddy variety but from different geographical locations were considered for this research work. Sample A was collected from a local rice mill in Igbemo town, Ekiti state and sample B was obtained from another local rice mill in Ofada town, Ogun state. Both states located in southwestern Nigeria. The rice-husks considered are from Nerica variety. The test materials and instrumentations include; bomb calorimeter, oven; muffle furnace; gas desicca- tor; digital weighing balance; Cal 2k-Eco calorimeter assem- bly, pulveriser with Shaker, and atomic absorption spectrometer (AAS). 2.2. Proximate analysis of rice-husk The rice-husk samples were analyzed for their proximate moisture, volatile, ash, fixed carbon and total carbon content, using American Standard Testing Methods (ASTM) D 3173-87 [1, 20, 21]. The percentage of total carbon in the samples was determined directly by adding the volatile matter and the fixed carbon content. 2.3. Chemical analysis of rice-husk The Chemical analysis considered for this research work is the ultimate analysis. This determines the elemental composi- tion of C (Carbon), H (Hydrogen), N (Nitrogen), S (Sulphur) and O (Oxygen) in the rice-husk samples (CHNSO test). The information obtained was sufficient to determine the combus- tion air volume and the resulting flue gas volume for a unit mass of rice-husk. 3 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 4 Figure 1. Rice Production Trends in Nigeria from 1960-2018 1. Ultimate analysis: Ultimate analysis was carried out to determine the per- centage of C (Carbon), H (Hydrogen), O (Oxygen), N (Nitrogen), and S (Sulphur) in the rice-husk samples. i Determination of carbon and hydrogen: The per- centage composition of carbon and hydrogen in the rice-husk samples were determined using ASTM D: 5868-10a procedure [23, 24]. Here, a known quantity of rice-husk sample was burnt in the pres- ence of dry oxygen to ensure that carbon and hy- drogen content in the biomass was converted into carbon (iv) oxide (CO2) and water (H2O) respec- tively. Combustion gas from the combustion pro- cess was passed over a weighed tubes of anhydrous calcium chloride (CaCl2) and potassium hydroxide (KOH), which absorb the H2O and CO2 present in the combustion gas respectively. To this effect, the weight of water (H2O) and carbon (iv) oxide (CO2) present in the combustion gas was determined as the increase in weight of CaCl2 and KOH in the tube re- spectively. ii Determination of nitrogen: The percentage compo- sition of nitrogen in the rice-husk samples was de- termined using Kjeldahl’s method [23, 24]. Here, a known quantity of pulverized rice-husk sample was heated together with concentrated tetraoxosulphate (vi) acid (H2SO4) in a Kjeldahl’s flask, in the pres- ence of potassium tetraoxosulphate (vi) salt solu- tion (K2SO4) and copper (ii) tetraoxosulphate (vi) salt solution (CuSO4). The reaction converted the nitrogen present in the rice-husk sample to ammo- nium sulphate and the solution became clear after the entire nitrogen was converted into ammonium sulphate. This clear solution was then treated with 50% NaOH solution to form ammonia which was distilled and absorbed over a given quantity of stan- dard H2SO4 solution. Unused volume of H2SO4 acid was determined by titrating against standard NaOH solution. This gives the amount of acid neu- tralized by liberated ammonia in the rice-husk sam- ple. Thus the percentage of nitrogen was deter- mined. iii Determination of sulphur: The percentage compo- sition of sulphur in the rice-husk samples was de- termined using ASTM D: 5868-10a procedure [23]. Here, a known quantity of rice-husk sample was heated with Eschka mixture, a combination of 2 parts of Magnesium oxide (MgO) and 1 part of an- hydrous Sodium trioxocarbonate (iv) (Na2CO3) at 8000C. This formed a sulphate solution and later precipitates Barium tetraoxosulphate (vi) salt (BaSO4) after treating with BaCl2. iv Determination of oxygen: The percentage of oxy- gen in the sample was calculated indirectly by sub- tracting the sum of the percentage content of car- bon, hydrogen, nitrogen and sulphur and from 100%. This is calculated without considering the percent- age composition of ash and is similar to the proce- dure used by Ghetti’s [7, 25]. 2. High heating value of rice-husk Energy content of each sample of rice hush was deter- mined by Bomb Calorimeter (Cal 2k-Eco Calorimeter), 4 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 5 in Afe Babalola University, Ado-Ekiti Laboratory (ABUAD) as described by Rominiyi and Adaramola, 2020. Here, 0.5g of dry pulverized rice-husk sample was measured into a clean crucible whose weight has been corrected to zero on the digital weighing balance. And just before the determination of the energy content by the calorimeter, the identification and corresponding weight of the sam- ple were entered into the calorimeter through the con- nected keyboard. To determine the energy content, a pre- cut of firing cotton was looped over the firing wire of the setup in such a way that the firing wire touches the weighed crucible (containing the sample) placed in the crucible holder. The lid assembly was inserted into the vessel body and the cap down screwed until it touches the top of the lid. This vessel was then placed in the ves- sel holder in an upright position together with the filling station and filled with oxygen to 3000kPa. After this, the entire vessel was inserted into the measuring cham- ber and then the lid was closed. This was then followed by the temperature stabilization phase which was carried out for about 10 minutes until the vessel fires automati- cally at the initial condition of 220 V. The heating value was calculated automatically every 6 seconds taking into account the calibration curve, heating value corrections and sample mass. This also takes place for 10 minutes. Finally, the heating value was displayed on the screen of the calorimeter and taken. 2.4. Theoretical calculations Stoichiometry combustion air volume and the resulting flue gas volume for a unit mass of rice-husk were calculated ac- cording to the procedures given by Paraschiva et al. [4]. These procedures have been developed into a user-friendly web appli- cation for easy analysis of the combustion fuel [14]. Utilization of this web application requires knowledge of the analyzed fuel elemental composition, excess air coefficient, absolute humid- ity of air and the solid fuel flow rate. 1. Stoichiometric combustion air i Stoichiometric volume of oxygen: Stoichiometric vol- ume of oxygen required for combustion when the minimum quantity of air required for combustion is used (γ = 1) is given as Equation 1: V sO2 = 22.41 100 ( %C 12 + %H 4 + %S − %O 32 ) ( Nm3O2 kg fuel ) (1) ii Stoichiometric oxygen flow rate: Equation 2 describes the stoichiometric flow rate of oxygen required for combustion QsO2 = • mRH · V s O2 ( Nm3O2 h ) (2) iii Stoichiometric volume of dry air: Stoichiometric volume of dry air required to burn one kilogram of fuel with air having 21% oxygen volume participa- tion is given as Equation 3: V sair = V sO2 0.21 ( Nm3air kg fuel ) (3) iv Dry combustion air flow rate: Equation 4 presents the flow rate of dry combustion air. Qsair = • mRH · V s air ( Nm3air h ) (4) The correlation given in Equation 1 to 4 is appli- cable only to dry air. The stoichiometric air vol- ume for wet air is greater than the stoichiometric air volume for dry air due to water vapour con- tent [1]. Therefore, the correlation for wet air in- volves taking into consideration, absolute humidity, x (kg H2O/kg dry air) of the air; air temperature, tair (oC) and the relative humidity of air, φair (%). The absolute humidity, x is usually giving as 0.01 kg H2O/kg for combustion calculation and this cor- responds to air at tair =25 oC and φair =50% accord- ing to Paraschiva et al.. But for more accuracy, the absolute humidity can be obtained at known valves of tair andφair , using Molliere diagram for moist air. The more accurate absolute humidity can be cal- culated using Equation 5 and the volume of water vapour in wet air can be determined using Equation 6. x = 0.622φair Ps (Pb −φair Ps) ( kg H2O kg dry air ) (5) V sH2 O = xρair V sair ρH2 O ( Nm3 H2O kg fuel ) (6) v Stoichiometric volume of wet air: Equation 7 gives the stoichiometric volume of wet air required for combustion. V sairW = V sair + xρair Vair ρH2 O = (1 + xρair ) ρH2 O · V sair = (1 + 1.61x) · V sair ( Nm3 kg fuel ) (7) vi Wet combustion air flow rate: The required wet com- bustion air flow rate for complete combustion can be calculated using Equation 8. QsairW = • mRH · V s airW ( Nm3airW h ) (8) where • mRH = Rice-husk (RH) mass flow rate ( kg fuel h ) %C, %H, %S and %O= Elemental composition of RH (ultimate analysis). ρair = Density of air, usually 1.2925 kg/m3 at nor- mal state. ρH2 O = Density of water vapour, usually 0.804 kg/m 3 at normal state. 5 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 6 2. Stoichiometric combustion products The following minimum volume of combustion gases will be obtained when the minimum quantity of air required for combustion ′γ = 1′is used. i Volume of carbon dioxide in flue gas: The volume of carbon dioxide obtainable when minimum qual- ity of dry air requirement is used is given by Equa- tion 9. VCO2 = 22.41 12 · %C 100 ( Nm3CO2 kg fuel ) (9) ii Volume of sulphur dioxide in flue gas: The volume of sulphur dioxide obtainable when minimum qual- ity of dry air requirement is used is given by Equa- tion 10. VS O2 = 22.41 32 · %S 100 ( Nm3S O2 kg fuel ) (10) iii Volume of water vapour: The volume of water vapour in the flue gases is the summation of the volume of water vapour from combustion hydrogen and vol- ume of water vapour due to combustion air humid- ity. The dew point temperature of cooled flue gases depends on the volume of water vapour present in the flue gases. • Volume of water vapour from combustion hy- drogen: V ′ H2 O = 22.41 100 · ( %H 2 + %W 18 ) ( Nm3 H2O kg fuel ) (11) • Volume of water vapour due to combustion air humidity: V ′′ H2 O = 1.61 · x · V s air ( Nm3 H2O kg fuel ) (12) Therefore, the total volume of water vapour in flue gas: V sH2O = V ′ H2 O + V ′′ H2 O = 22.41 100 · ( %H 2 + %W 18 ) ( Nm3 H2O kg fuel ) (13) iv Volume of nitrogen in flue gas: The volume of ni- trogen obtainable from minimum quality of dry air and wet air requirement is given by Equation 14 and Equation 15 respectively. V sN2 = 22.41 28 · %N 100 + 0.79 · V sair ( Nm3 N2 dry kg fuel ) (14) V sN2 Wet = 22.41 28 · %N 100 + 0.79 · V sairW ( Nm3 N2 wet kg fuel ) (15) Nitrogen in flue gas is a result of the elemental flue composition of nitrogen and also the 79% nitrogen composition in the combustion air. v Total stoichiometric volume of flue gas: Equation 16 presents the total stoichiometric volume of flue gas obtainable from dry stoichiometric combustion air while Equation 17 present total stoichiometric volume of flue gas obtainable from wet stoichio- metric combustion air. V sf g = VCO2 + VS O2 + V s N2 ( Nm3dry fuel gas kg fuel ) V sf g = 22.41 100 · ( %C 12 + %S 32 + %N 28 ) + 0.79 · V sair ( Nm3dry fuel gas kg fuel ) (16) V sf gW = VCO2 + VS O2 + V ′ H2 O + V sN2 Wet ( Nm3wet fuel gas kg fuel ) V sf gW = V s f luegas + V s H2 O ( Nm3fuel gas kg fuel ) (17) vi Flue gas flow rate: The total wet flue gas flow rate is given by Equation 18. Q f gW = • mRH · V f gW ( Nm3fuel gas h ) (18) This research work used Paraschiva et al web application [14] to calculate the theoretical volume of air required for the chemical reactions of the combustible elements in the sampled rice-husk. Also, the application was used to evaluate the volume of flue gas that will result from the combustion process. The elemental composition of the solid fuel samples was obtained from the ultimate analy- sis, excess air coefficient ‘γ = 1’ was considered for the- oretical or stoichiometric combustion and rice-husk flow rate of 1 kg/hr was used as the calculation basis. The list of all parameters is given in Appendix A. 3. Results and discussion 3.1. Proximate analysis Moisture content of biomass is expected to be below 10% for a pre-dried sample and about 50% if not dried [7, 14]. More- over, the volatile matter is expected to be between 65% and 85%, fixed carbon from 7% to 20% and ash content from be- low 5% to 20% [7, 26]. The details in Table 1 shows that the moisture content of the studied samples is almost 10% which is considered suitable for biomass combustion. Sample B has 9.8% moisture content while sample B has a moisture level that is slightly above 10%. This potential biofuel can be considered 6 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 7 suitable for use in biomass boilers because of the low moisture levels. Furthermore, from the analysis result shown in Table 1, sample A and sample B has 18.26% and 14.81% ash content which are within the recommended range. These low values in- dicated that at normal combustion conditions the level of partic- ulate matter emission will be low or negligible and the emission from sample B is expected to be slightly below that of sample A [7]. The values obtained for volatile matter, 58.22% and 54.63% for sample A and B respectively are slightly below the range given by so researchers, therefore their reactivity may not be as high as biomass with a volatile matter in the range of 65% to 85% [7, 26]. The higher the volatile matter content the higher the reactivity of biomass [7, 27]. The fixed carbon content of the samples was determined empirically and not experimentally mainly because this study focused more on the elemental composition for the combus- tion gases analysis. The values obtained for sample A and B as shown in Table 1 is 12.59% and 20.76%. Biomass with such fixed carbon value is advisable for use in combustion process [4]. The moisture content values and the low fixed carbon val- ues obtained indicate that the level of CxHy emission will be low and will be generated at a trivial level during theoretical combustion, under proper conditions [7]. 3.2. Ultimate analysis Results obtained for ultimate analysis of the rice-husk sam- ples are shown in Table 2. The major components in solid fuels combustion are carbon, nitrogen and oxygen. Carbon and oxy- gen react exothermically to generate CO2 and H2O, and they also contribute positively to the HHV of fuel [7]. According to studies [7, 26], carbon, hydrogen, oxygen and nitrogen con- tent is expected to range from 47% to 54%, 5.6% to 7%, 40% to 44% and 0.1% to 0.5% respectively. Sulphur is expected to have a value of about 0.1%. As shown in Table 2, carbon content is 45.08% and 43.99% for sample A and B respectively. These values are approach- ing the expected value of between 45% and 48% given in the literature. Also, the value obtained for oxygen is 49.27% and 50.64% for sample A and B respectively. These are slightly above the expected value of 40% to 44%. The values of carbon and oxygen obtained for the charac- terised rice-husk samples are in contrast with the values ob- tained by Garcı́a et al [7]. Garcı́a et al have carbon and oxy- gen content to be 26.69% and 70.05% which indicate poor en- ergy density, thus showing that biomass characteristic proper- ties vary depending on factors such as the variety of the parent plant, region of cultivation, climatic condition and postharvest processing method. Nitrogen content (0.84%) in sample A presented in Table 2 is slightly higher than the recommended value of nitrogen in biomass [26], while sample B is within the recommended value of 0.1% to 0.5%. But as the case may be, both analyzed samples are expected to produce a very little or negligible amount of N2 and NOx emissions during the combustion process. The majority of the NOx in waste gases can be attributed to nitrogen present in the combustion air [7]. A low level of sulphur content is highly recommended for biomass combustion because they generate SO2 which leads to sulphates. Sulphates constitute great problem in heat exchang- ers. The result presented in Table 2 showed little and negligible amount of sulphur in the samples, thus making the samples suit- able as biofuel. Hydrogen content of both samples as presented in Table 2 slightly agrees with literature. Reports from studies show that hydrogen content of biomass is expected to be between 5.6% and 7% [7, 26]. The energy content of the two samples consid- ered is also similar to those in literature [7]. HHV of sample A, 14.112 MJ/kg is slightly higher than HHV of sample B, 13.326 MJ/kg. The low hydrogen content of sample A (4.76%) and sample B (4.93%) compared with the higher carbon content of the samples (45.08 for sample A and 43.99 for sample B) sug- gested that carbon contributes more to rice-husk HHV. 3.3. Theoretical calculations The theoretical volume of air needed for the chemical re- actions of the combustible elements in samples of rice-husk, and also the volume of flue gas that will be produced from the combustion process were calculated using Paraschiva et al web application [14]. Excess air coefficient of ′γ′ = 1 and 1 kg/hr biofuel flow rate were considered for the calculation. The results obtained are shown in Table 3. The stoichiomet- ric volume of oxygen required for sample A is slightly greater than sample B. Sample A requires 0.76385 (N m3/kg fuel) while sample B requires 0.74329 (N m3/kg fuel). The contrast is due to the slight variation in the elemental composition of the two samples, and this variation may be as a result of the difference in the area of cultivation and postharvest processing method. The contrast is slight because both samples are of the same va- riety and are both from southwestern part of Nigeria. The trend observed in the stoichiometric volume of oxygen required for the samples is also noticed for the stoichiometric combustion air required and wet combustion air flow rate. This is because stoichiometric combustion air required and wet combustion air flow rate both depend on the stoichiometric volume of oxy- gen. As presented in Table 3, stoichiometric volume of dry air, stoichiometric volume of wet air and wet combustion air flow rate are 3.63740 (N m3/kg fuel), 3.69596 (N m3/kg fuel) and 3.69596 (N m3/h) respectively for sample A, and 3.53947 (N m3/kg fuel), 3.59645 (N m3/kg fuel) and 3.59645 (N m3/h) for sample B. The difference between the stoichiometric volume of dry air and the stoichiometric volume of wet air accounts for the water vapour present in the combustion air. The combustion air flow rate is for a unit mass of fuel per hour. Analyzed results of the major stoichiometric combustion products are given in Table 3. Carbon dioxide is majorly from the fuel carbon, and the volumes evaluated are 0.84187 (N m3/kg fuel) and 0.82151 (N m3/kg fuel) for sample A and B respec- tively. The volume of sulphur dioxide in the flue gas estimated for both samples are very small, 0.00035 (N m3/kg fuel) for sample A and 0.00021 (N m3/kg fuel) for sample B. These quantities are proportional to the level of sulphur in the fuel. 7 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 8 Table 1. Proximate analysis of rice-husk (dry weight basis %) Rice-husk Moisture content Volatile matter Ash content Fixed carbon Total carbon Sample A 10.93 58.22 18.26 12.59 70.81 Sample B 9.80 54.63 14.81 20.76 75.39 Table 2. Ultimate and calorific value of rice-husk (dry weight basis %) Rice-husk C H N S Oa HHV (MJ/kg) Sample A 45.08 4.76 0.84 0.05 49.27 14.112 Sample B 43.99 4.93 0.41 0.03 50.64 13.326 a % of O calculated from the difference of C, H, N and S. Table 3. Rice-husk combustion analysis Rice Husk Stoichiometric combustion air a,b, c (N m3/kg fuel) Stoichiometric combustion products a,b, c (N m3/kg fuel) QsO2 V s air V s airW Q s airW (N m3/h) VCO2 VS O2 V s H2O V sN2 V s f g V s f gW Q f gW (N m3/h) Sample A 0.76385 3.63740 3.69596 3.69596 0.84187 0.00035 0.59192 2.88027 4.31441 4.31441 4.31441 Sample B 0.74329 3.53947 3.59645 3.59645 0.82151 0.00021 0.60939 2.79946 4.23058 4.23058 4.23058 aγ = 1, b • mRH = 1 kg/hr, c x = 0.01 Water vapour and nitrogen are inert gas. Water vapour volume is 0.59192 (N m3/kg fuel) and 0.60939 (N m3/kg fuel) for sam- ple A and B respectively. Nitrogen gas is 2.88027 (N m3/kg fuel) and 2.79946 (N m3/kg fuel) for sample A and B respec- tively. It is noticed that the volume of nitrogen in the flue gas is greatly influenced by the 79% composition of nitrogen in the combustion air. Finally, the stoichiometry volume of flue gas and the flue gas flow rate is proportional to the stoichiometry volume of wet air and the wet combustion air flow rate. There- fore as presented in Table 3, the stoichiometry volume of flue gas for sample A is 4.31441 (N m3/kg fuel) and its flue gas flow rate is 4.31441 (N m3/h), while and sample B has stoichiometry volume of flue gas of (N m3/kg fuel) and flue gas flow rate of 23058 (N m3/h). 4. Conclusion Rice-husks of one of the most commonly planted and abun- dant rice variety in Nigeria (Nerica-rice-husk) were characterised and evaluated for theoretical combustion air requirement and potential combustion flue gases. The sampled husks were ob- tained from two different geographical locations within the south- western part of Nigeria. The characterization gives insight into the elemental composition and the energy content of the sam- ples considered. Also, the proximate moisture, volatile, ash, fixed carbon and total carbon content of the samples were de- termined using standard methods. It was revealed that the dif- ference in the region of cultivation slightly influenced the char- acteristic properties of the rice-husk. The low proximate moisture content level of the studied sam- ples which is almost 10% indicated that Nerica-rice-husk is suitable for use in biomass combustion chambers. Ash content value of 18.26% and 14.81% obtained for sample A and sample B respectively showed that at proper combustion condition the level of particulate emission will be low or negligible. The low fixed carbon content of 12.59% and 20.76% obtained for sam- ple A and B respectively also indicated that Nerica-rice-husk is advisable for use in the combustion process. The proximate volatile matter of the two Nerica-rice-husk samples slightly fall below the recommended range of 65% to 85%, therefore the re- activity of the biomass may not be very high during combustion process. The characterization revealed low level of nitrogen and sulphur in both samples, thus at normal combustion conditions the level of NOx and SO2 will be low. Moreover, there will be a trivial level of CxHy emission during proper theoretical com- bustion of Nerica-rice-husk. Therefore, effective utilization of the abundant Nerica-rice-husk as biofuel will help reduce our dependence on fossil fuels. Results obtained for the volume of air required for the chem- ical reactions of the combustible elements in the samples are slightly different. Sample A requires 0.76385 (N m3/kg fuel) and 3.69596 (N m3/kg fuel) stoichiometric oxygen and air re- spectively while sample B requires 0.74329 (N m3/kg) and 3.53947 (N m3/kg fuel). This showed that more quantity and higher flow rate of theoretical air will be required to burn sam- ple A in combustion reactors such as in steam boiler compared to sample B. The result of the flue gas analysis shows that com- bustion of sample B will generate less emission compared to sample A. Also, at theoretical combustion of Nerica-rice-husk, the ratio and the amount of emitted sulphur dioxide in the flue gas will be negligible due to low sulphur content of the husk. Also, the ratio and the amount of nitrogen dioxide in the flue gas will be high. The high nature of nitrogen dioxide, 2.88027 (N m3/kg fuel) and 2.79946 (N m3/kg fuel) for sample A and 8 Lala et al. / J. Nig. Soc. Phys. Sci. 4 (2022) 954 9 B are due to the inert nitrogen present in the combustion air. Moreover, the combustion of sample A will emit more nitrogen dioxide due to the 0.84% nitrogen content which is more than the recommended nitrogen content for biomass. This research work indicates a better way of managing the abundant Nerica-rice-husk as an alternative energy source. It can be burn readily in combustion plants to generate primary heating agent. The discrepancies noticed in the results of the two samples considered in this work simply illustrate potential intrinsic risk that is associated with using generally assumed values for the design and sizing of combustion reactors. Such can lead to high calculation errors, thus creating financial and infrastructural problems. Also, the results obtained for the volume of air required for the chemical reactions of the combustible elements in Nerica- rice-husk together with the volume of flue gas resulting from the combustion process will assist in investigating the oper- ating conditions for minimizing air emissions from rice-husk fired combustion reactors. This will ensure proper compliance of the combustion operation with the standards set by the en- vironmental pollution regulatory agencies. It will also assist in reducing thermal losses from any rice-husk fired combustion system, as theoretical combustion provides the perfect fuel to air ratio, which lower losses and extracts all the energy from a given fuel. In this work, stoichiometry combustion air volume and the resulting flue gas volume for a unit mass of Nerica rice-husk va- riety were determined by assuming complete combustion and without taking the complex exothermic oxidation of the fuel into consideration. This can be improved upon in future works by considering the exothermic oxidation of the fuel. Also, a study on excess air combustion analysis of rice-husk which is highly important in attaining better combustion efficiency is recommended. Acknowledgment We thank the referees for the positive enlightening com- ments and suggestions, which have greatly helped us in making improvements to this paper. 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Sci. 4 (2022) 954 10 x = Absolute humidity tair = Air temperature φair = Relative humidity of air Ps= Water vapour pressure in saturated wet air Pb= Water vapour partial pressure ρair = Density of air ρH2 O = Density of water vapour • mRH = Rice-husk mass flow rate QsO2 = Stoichiometric oxygen flow rate in dry air QsairW = Wet combustion air flow rate Q f gW = Wet flue gas flow rate V sO2 = Stoichiometric volume of oxygen in dry air V sH2 O = Volume of water vapour in wet air V sair = Stoichiometric volume of dry air V sairW = Stoichiometric volume of wet air VCO2 = Volume of carbon dioxide in dry flue gas VS O2 = Volume of sulphur dioxide in dry flue gas V ′ H2 O = Volume of water vapour from combustion hydrogen V ′′ H2 O = Volume of water vapour due to combustion air humidity V sH2O = Total volume of water vapour in stoichiometric dry flue gas V sN2 = Volume of nitrogen in stoichiometric dry flue gas V sN2 Wet = Volume of nitrogen in stoichiometric wet flue gas V sf g = Total stoichiometric volume of dry flue gas V sf gW = Total stoichiometric volume of wet flue gas %C, %H, %S and %O= Elemental composition of RH (ulti- mate analysis) Nm3 = Normal cubic meter (Amount of gas present in a volume of 1m3 at a temperature of 0 0C and pressure of 1013 mbar) 10