Research Repository :: Browsing by Author "Mhilu, Cuthbert F."

Browsing by Author "Mhilu, Cuthbert F."

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Advances in the HTAG Technology and Process of Biomass
(Academic Journals, 2008) John, Geoffrey R.; Wilson, Lugano; Mhilu, Cuthbert F.
High Temperature Air/Steam Gasification (HTAG) is a process in which a highly preheated air/steam is utilized as the oxidizer. The HTAG process follows the developments in the High Temperature Air Combustion (HiTAC), which has shown to be superior in energy saving and pollution reduction compared to the conventional combustion technology. The preheated oxidizer provides additional energy into the gasification process that enhances thermal decomposition of the gasified solid feedstock. Consequently, the HTAG increases both the calorific value of the producer gas, and the cold gasification efficiency. In this work, the advantages of the HTAG processes is presented by considering performance influencing parameters that include materials quality, oxidizer type, equivalence ratio (ER), gasification temperature, and bed additives.
Analysis of Pyrolysis Kinetics and Energy Content of Agricultural and Forest Waste
(2014-03) Said, Mahir M.; John, Geoffrey R.; Mhilu, Cuthbert F.; Manyele, Samwel
Selected agricultural and forest wastes included rice husk, coconut husk, cashewnut shell, eucalyptus, pine and mangrove were used for thermal characterization. The studied materials have heating value between 13 and 23 kJ/kg, such that the biomass material can be used as a fuel by directly burning, but their energy content is lower as compared to coal and other fossil fuels. The energy content of the biomass material can be improved through pyrolysis process for the mentioned materials, the cashew nut shell had higher energy content than other biomass material due to its high hydrogen to carbon ratio and low oxygen to carbon ratio. Thermochemical characteristic depicted high degradation at a heating rate of 10 K/min. All studied materials except mangrove and pine, maximum degradation occurred at 5 K/min. The reactivity of coconut husk was highest whilst cashew nut shell had the least reactivity. The activation energy for cashew nut shell obtained was 336.41 kJ/mole and the activation energy of the other biomass ranged between 220 and 130 kJ/mol. The coconut husk, pine, eucalyptus and rice husk are more reactive than mangrove and cashewnut shell.
Characterization of Pyrolysis Kinetics for the Use of Tropical Biomass as Renewable Energy Sources
(2013-06-05) Ndalila, P.; John, Geoffrey R.; Mhilu, Cuthbert F.
Tropical biomass such as rice husks, sugar bagasse, coffee husks and sisal waste are among typical biomass wastes abundant in most of the tropical countries. However, despite their enormous potential as energy sources, they are hardly studied and their thermal characteristics are still not well known. The purpose of this work is to determine the thermochemical characteristics and pyrolysis behavior of these selected biomasses. Proximate, ultimate and heating value analyses were carried out on the samples. Results show that all biomass have a range of, volatile contents (50–80 % w/w), fixed carbon (10–20 % w/w), ash content (<3 % w/w), carbon (50–56 % dry basis) low nitrogen (0.7–1.3 % dry basis) and sulphur (<0.1 wt % dry basis) contents with heating value (HHV 14–18 MJ/kg). The biomasses were thermally degraded through thermogravimetry analysis and their characteristics such as devolatilisation profiles and kinetics parameters (activation energy E, and frequency factor A) were determined, in an inert atmosphere. It is found that the kinetic parameters obtained can predict not only global devolatilization of biomass pyrolysis but also can predict the pyrolysis pathway of cellulose in the target biomass.
Coffee Husks Gasification Using High Temperature Air/Steam Agent
(Elsevier, 2010) Wilson, Lugano; John, Geoffrey R.; Mhilu, Cuthbert F.; Yang, Weihong; Blasiak, Wlodzimierz
Analyses made on the world's biomass energy potential show that biomass energy is the most abundant sustainable renewable energy. The available technical biomass energy potential surpasses the total world's consumption levels of petroleum oils, coal and natural gas. In order to achieve a sustainable harnessing of the biomass energy potential and to increase its contribution to the world's primary energy consumption, there is therefore a need to develop and sustain contemporary technologies that increase the biomass-to-energy conversion. One such technology is the high temperature air/steam gasification (HTAG) of biomass. In this paper we present findings of gasification experimental studies that were conducted using coffee husks under high temperature conditions. The experiments were performed using a batch facility, which was maintained at three different gasification temperatures of 900 °C, 800 °C, and 700 °C. The study findings exhibited the positive influence of high temperature on increasing the gasification process. Chars left while gasifying at 800 °C and 700 °C were respectively 1.5 and 2.4 times that for the case of 900 °C. Furthermore, increased gasification temperature led to a linear increment of CO concentration in the syngas for all gasification conditions. The effect was more pronounced for the generally poorly performing gasification conditions of N2 and 2% oxygen concentration. When gasification temperature was increased from 700 °C to 900 °C the CO yield for the 2% O2 concentration increased by 6 times and that of N2 condition by 2.5 times. The respective increment for the 3% and 4% O2 conditions were only twofold. This study estimated the kinetic parameters for the coffee husks thermal degradation that exhibited a reaction mechanism of zero order with apparent activation energy of 161 kJ/mol and frequency factor of 3.89 × 104/min.
The Combustion Characteristics of Biomass Syngas from High Temperature Air, Entrained Flow and Circulating Fluidized Bed Gasifiers
(SSRN Electronic Journal, 2014-02-21) Said, Mahir M.; Chaula, Zephania; John, Geoffrey R.; Mhilu, Cuthbert F.
The study has been performed to determine fundamental combustion characteristics of syngas. Three technologies were selected to produce the syngas; High Temperature Agent Gasifier (HTAG), Entrained Flow Gasifier (EFG) and Circulating Fluidized Bed Gasifier (CFBG). Although the material used for production of syngas was the same, wood biomass, the compositions of syngas obtained were different. The adiabatic flame temperatures were determined at different air to fuel ratio. The maximum adiabatic temperature for HTAG, EFG and CFBG syngas at stoichiometric condition were 1846 K, 2250 K and 2234 K respectively. It has been observed that combustion of CFBG syngas produces more nitrogen oxide (NOx) than when using syngas of EFG. The high NOx in CFBG is caused by the high methane content, which increases the adiabatic flame temperature to 2200 K at stoicheometric condition. The lowest NOx emission was observed in HTAG syngas. The adiabatic temperature increased linearly with the preheating temperature, whilst oxygen enrichment increased the adiabatic temperature. It has been concluded that syngas produced from EFG and CFBG are better candidate as gaseous fuel in combustion chamber than HTAG syngas.
Exergy Analysis of High Temperature Biomass Gasification
(2012) Kasembe, Ethel D.; John, Geoffrey R.; Mhilu, Cuthbert F.
Biomass gasification is considered as one of the most promising thermo-chemical technologies but the gasifier unit renders itself to internal inefficiencies. This paper addresses the gasifier performance analysis using the exergy analysis modeling which utilizes both the first and second laws of thermodynamics. An exergy model incorporating a chemical equilibrium model is developed. Gasification is envisaged to be carried out at atmospheric pressure of 1 bar with the typical biomass feed, sugarcane bagasse, represented by the formula CH1.42 O0.65 N0.0026 at the temperature range of 800-1400K. In the model, the exergy contained in the biomass was converted into chemical exergy of the product gas, physical exergy, the rest was the unavailable energy due to process of irreversibilities (losses). The model evaluated the product gas molar concentrations and efficiency. The results from the model showed that the mole concentration of H2 increased from 9.8% to 23.7% and the formation of CO2 ranges from 5.6% to 12.1%. While this is the case for H2 and CO2, CO mole concentration is reduced from 26.9% to 17.4%. The maximum efficiencies value obtained based on chemical energy and physical exergy was lower than the efficiency value based on chemical exergy (84.64% vs. 76.94%). This is because the sensible or physical heat (used for drying biomass) is less beneficial for the efficiency based on total exergy. Hence, the gasification efficiency can be improved by increasing the temperature with the change of equivalence ratio (ER) and with the addition of heat in the process.
Fast Pyrolysis and Kinetics of Sugarcane Bagasse in Energy Recovery
(Springer Berlin Heidelberg, 2013-06-05) Said, Mahir M.; John, Geoffrey R.; Mhilu, Cuthbert F.; Manyele, Samwel
The trend for material and energy recovery from biomass-waste along with the need to reduce green house gases has led to an increased interest in the thermal processes applied to biomass. The thermal process applied to biomass produces either liquid fuel (bio-oil) or gaseous fuel. Liquid fuel is more preferred because it is easier to transport from one point to another and also it can be used for production of chemicals. One of the biomass obtained in Tanzania is sugarcane bagasse. The sugarcane bagasse is the fibrous materials that remain after sugarcane is crushed to extract juice. Currently, it is burnt directly in the boilers for production of steam, but it can be used for production of bio-oil. The bio-oil can be optimally obtained by fast pyrolysis, which is a fast thermal decomposition of biomass material at temperature range 523–800 K in the absence of an oxidizing agent. In order to undertake a parametric study on the fast pyrolysis of sugarcane bagasse, it is imperative to establish its thermal characteristics. The paper reports the proximate and ultimate analysis, and thermal degradation of sugarcane bagasse in nitrogen as heating agent. The thermal degradation was conducted in a thermo-gravimetric analyzer from room temperature to 1,000 K at different heating rates of 5, 10, 20 and 40 K min−1. The thermo-gravimetric analyzer was used to study the effect of heating rate on the thermal degradation characteristics and to determine mass loss kinetics. The sugarcane bagasse was observed to be suitable for use in pyrolysis since it contains high volatile level of 80.5 % and fixed carbon of 8.2 %. The peak temperature was observed at 573 K at 10 K min−1 and corresponding activation energy was 387.457 kJ/mol.
Modelling the Suitability of Pine Sawdust for Energy Production via Biomass Steam Explosion
(Scientific Research, 2014) Chaula, Zephania; Said, Mahir M.; John, Geoffrey R.; Mhilu, Cuthbert F.
Biomass material as a source of fuel is difficult to handle, transport, store, and utilize in its original form. To overcome these challenges and make it suitable for energy prodution, the material must be pre-treated. Biomass steam explosion is one of the promising pretreatment methods where moisture and hemicellulose are removed in order to improve biomass storage and fuel properties. This paper is aimed to model the suitability of pine saw dust for energy production through steam explosion process. The peak property method was used to determine the kinetic parameters. The model has shown that suitable operating conditions for steam explosion process to remove moisture and hemicellulose from pine sawdust. The temperature and pressure ranges attained in the current study are 260 -317 ℃ (533 -590 K), 4.7 -10.8 MPa, respectively.
Performance Prediction of a Pressurized Entrained Flow Ultra-Fine Coal Gasifier
(Scientific Research, 2014-01) Mashingo, P. P.; John, Geoffrey R.; Mhilu, Cuthbert F.
Gasification is an efficient method of producing clean synthetic gas which can be used as fuel for electric generation and chemical for industries use. Gasification process simulation of coal inside a generic two-stage entrained flow gasifier to produce syngas was undertaken. Numerical simulation of the oxygen blown coal gasification process inside a two-stage entrained coal gasifier is studied with the commercial CFD solver ANSYS FLUENT. The purpose of this study is to use CFD simulation to improve understanding of the gasification processes in the state of art two-stage entrained flow coal gasifier. Three dimensions, Navier-Stokes equations and species transport equations are solved with the eddy-breakup reaction model to predict gasification processes. The influences of coal/water slurry concentration and O2/coal ratio on the gasification process are investigated. The coal-to-water slurry concentrations in this study were 0.74 and O2/coal ratio is 0.91. Coal slurry fed the predicted concentration of 47.7% and CO was 25% with higher syngas heating value of 27.65 MJ/kg. The flow behavior in the gasifier, especially the single fuel injection design on the second stage, is examined and validated against available data in the literature.
The Study of Kinetic Properties and Analytical Pyrolysis of Coconut Shells
(Hindawi Publishing Corporation, 2015) Said, Mahir M.; John, Geoffrey R.; Mhilu, Cuthbert F.; Manyele, Samwel V.
The kinetic properties of coconut shells during pyrolysis were studied to determine its reactivity in ground form. The kinetic parameterswere determined by using thermogravimetric analyser.The activation energywas 122.780 kJ/mol.The pyrolysis products were analyzed using pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The effects of pyrolysis temperature on the distribution of the pyrolytic products were assessed in a temperature range between 673K and 1073K.The set time for pyrolysis was 2 s. Several compoundswere observed; theywere grouped into alkanes, acids, ethers and alcohols, esters, aldehydes and ketones, furans and pyrans, aromatic compounds, and nitrogen containing compounds.The product compositions varied with temperature in that range. The highest gas proportion was observed at high temperature while the acid proportion was observed to be highest in coconut shells, thus lowering the quality of bio-oil. It has been concluded that higher pyrolysis temperature increases the amount of pyrolysis products to a maximumvalue. It has been recommended to use coconut shell for production of gas, instead of production of bio-oil due to its high proportion of acetic acid.
Syngas Production and Losses Encountered in Gasification of Rice Husks
(Scientific Research, 2015-01) Kasembe, Ethel D.; Mganilwa, Zacharia M.; John, Geoffrey R.; Mhilu, Cuthbert F.
This paper addresses the syngas production and evaluation losses in high temperature gasification process using coffee husks. A fast and inexpensive way to evaluate the losses in gasification processes is by the application mathematical models which allow to predict the values needed in full scale. Hence, the quantification of gasifier’s losses at temperatures ranges of 800 K - 1400 K at an equivalence ratios of 0.3, 0.35 and 0.4 at 1 bar are revealed by using exergy model incorporating a chemical equilibrium model. The model evaluated the product syngas compositions, syngas heating values and degree of irreversibility values (losses). The results from the model showed that the production of H2 increased from 9.9% to 18.9% and the formation of CO2 ranges from 7.2% to 12.3%. CO production is from 21.8% to 17.2%. The irreversibility values obtained were less than 27%. Hence, reduction of losses protracts biomass resources to be used in energy generation.
Thermal Characteristics and Kinetics of Rice Husk for Pyrolysis Process
(International Journal of Renewable Energy Research, 2014-03) Said, Mahir M.; John, Geoffrey R.; Mhilu, Cuthbert F.
The trend for material and energy recovery from biomass-waste along with the need to reduce green house gases has led to an increased interest in the thermal processes applied to biomass. The thermal process applied to biomass produces either liquid fuel (bio-oil) or gaseous fuel. One of the biomass wastes that are produced in large quantities in Tanzania is rice husk. The behaviour of this waste is important to any designing of thermal handling equipment when subjected thermal environment such as burning or thermal degradation. Due to this it is imperative to establish thermal characteristics of the rice husk pursued in a laboratory to understand its thermal degradation behaviour. The thermal degradation was conducted in a thermo-gravimetric analyzer from room temperature to 1273 K at different heating rates. The activation energy was 180.075 kJ/mol and suitable heating rate for high degradation of rice husk is 10 K/min, and gives 77.20 wt% of volatile release which is the suitable heating rate for pyrolysis and energy released was -4437 J/kg, although it has been recommended that the rice to be used for gasification since it contains high amount of char.
Thermal Degradation and Kinetics of Rice Husk
(International Journal of Renewable Energy Research (IJRER), 2014-06-20) Said, Mahir M.; John, Geoffrey R.; Mhilu, Cuthbert F.
The trend for material and energy recovery from biomass-waste along with the need to reduce green house gases has led to an increased interest in the thermal processes applied to biomass. The thermal process applied to biomass produces either liquid fuel (bio-oil) or gaseous fuel. One of the biomass wastes that are produced in Tanzania is rice husk. It can be used either in pyrolysis and gasification process. In order to undertake a parametric study on the fast pyrolysis of rice husk, it is imperative to establish its thermal characteristics. The paper reports thermal degradation and kinetics of rice husk. The thermal degradation was conducted in a thermo-gravimetric analyzer from room temperature to 1273 K at different heating rates. The rice husk was observed to be suitable for gasification since it contains high ash content of 22.21% and fixed carbon of 12.60%. The suitable heating rate for high degradation of rice husk is 10 K/min, which has peak temperature at 633 K. The resulting activation energy of 180.075 kJ/mol and pre-exponential factor of 2.401E+27 s-1 was determined.

Browsing by Author "Mhilu, Cuthbert F."

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