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August 2010, Volume 4, No.8 (Serial No.33) Journal of Energy and Power Engineering, ISSN 1934-8975, USA Design and Development of a Laboratory Scale Biomass Gasifier S.J. Ojolo1 and J.I. Orisaleye2 1. Mechanical Engineering Department, University of Lagos, Lagos 101017 Nigeria 2. Mechanical Engineering Department, Lagos State University, Lagos 101017 Nigeria Received: February 10, 2010 / Accepted: March 24, 2010 / Published: August 31, 2010. Abstract: A laboratory scale downdraft biomass gasif
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  August 2010, Volume 4, No.8 (Serial No.33) Journal of Energy and Power Engineering, ISSN 1934-8975, USA Design and Development of a Laboratory Scale Biomass Gasifier    S.J. Ojolo 1  and J.I. Orisaleye 2 1. Mechanical Engineering Department, University  o   f Lagos, Lagos 101017 Nigeria 2. Mechanical Engineering Department, Lagos State University, Lagos 101017 Nigeria Received: February 10, 2010 / Accepted: March 24, 2010 / Published: August 31, 2010. Abstract:  A laboratory scale downdraft biomass gasifier was designed to deliver a mechanical power of 4 kW and thermal power of about 15 kW. The gasifier was manufactured as a single piece having a water seal and cover. The gasifier was tested in natural downdraft and forced downdraft mode. Ignition of the fuel beneath the grate, during natural downdraft mode, using wood shavings as fuel, produced gas which burned with a blue flame for 15 minutes. Ignition at the throat, using either palm kernel shells or wood shavings, during the natural downdraft mode, the gasifier did not produce syngas. During the forced downdraft mode, fuel was ignited at the throat. Gasification was successful with the palm kernel shells, during forced downdraft, which produced gas which burned steadily with luminous flame for 15 minutes per kilogram of biomass fed. However, wood shavings experienced some bridging  problems during the forced downdraft mode of operation. The fuel conversion rate of the gasifier, when using palm kernel shells as fuel in forced downdraft mode, was 4 kg/h. Forced downdraft mode of operation yielded better results and is the preferred operation of the gasifier. Key words:  Biomass, gasifier, design, downdraft, energy. 1. Introduction   Agriculture and energy have always been tied by close links, but the nature and strength of the relation- ship have changed over time [1, 2]. The linkages  between agriculture and energy output markets weak- ened in the twentieth century as fossil fuels gained  prominence in the transport sector. The use of rene- wable resources would contribute to a country’s economic growth, especially in developing countries, many of which have abundant biomass and agricultural resources that provide the potential for achieving self-sufficiency in materials [3].   In most African countries, biomass continues to be the main energy source for subsistence activities such as cooking, heating and lighting. Solid biofuels such as fuel wood, charcoal and animal dung constitute by far the largest Corresponding author: S.J. Ojolo, senior lecturer, research fields: design and manufacturing, renewable energy. E-mail:ojolosunday@yahoo.com. segment of the bioenergy sector, representing a full 99  percent of biofuels [4, 5]. Gasification means the transformation of solid fuels into combustible gases in presence of an oxygen carrier (air, O 2 , H 2 O, CO 2 ) at high temperatures. It is a process for converting carbonaceous materials to a combustible or synthetic gas like biomethane or producer gas [6]. Biomethane can be used like any other fuel, such as natural gas, which is not renewable [7]. The gasification process occurs at temperatures between 600-1,000 degrees Celsius and decomposes the complex hydrocarbons of wood [8]. The gasification  process, with high temperature, produces ash and char, tars, methane, charcoal and other hydrocarbons. The corrosive ash elements such as chloride and potassium are removed, allowing clean gas production from otherwise problematic fuels [9]. Conversion of solid  biomass into combustible gas has all the advantages associated with using gaseous and liquid fuels. Such  Design and Development of a Laboratory Scale Biomass Gasifier 17 advantages include clean combustion, compact burning equipment, high thermal efficiency and a good degree of control. Biomass is also economic in places where  biomass is already available at reasonable low prices or industries using fuel wood [6].   Biomass gasifiers are reactors that heat biomass to  produce a fuel gas that contains from one-fifth to one half (depending on the process conditions) the heat content of natural gas. A biomass gasifier converts solid fuel such as wood waste, saw-dust briquettes and agro-residues converted into briquettes into a gaseous fuel through a thermo-chemical process and the resul- tant gas can be used for heat and power generation applications [10, 11]. Biomass gasifiers have been classified based on their operation principles such as gasification and product temperature, oxygen require- ments, product gas composition amongst others. The major types of gasifiers are updraught or counter current gasifier, downdraft or co-current gasifiers, cross-draft gasifier and fluidized bed gasifier [12]. The throated downdraft gasifier is suitable for biomass gasification, has a low tar yield, high carbon conver- sion, low ash carry over and simple construction and operation. However, it has a high gas exit temperature, requires uniformly sized feed stock and limited moisture content of feed.   This work presents the design of a laboratory scale  biomass downdraft gasifier.   2. Theory of Gasification The substance of a solid fuel is usually composed of the elements carbon, hydrogen and oxygen. In addition, there may be nitrogen and sulphur present in small quantities. Biomass gasification in air can be expressed in three stages which are oxidation or combustion,  pyrolysis and reduction or gasification [12].   Combustion/oxidation reactions provide the heat energy required to drive the pyrolysis and char gasification reactions:  –111 MJ/kmol  –283 MJ/kmol  –242 MJ/kmol The pyrolysis reactions involve the cracking of the heavier biomass molecules into lighter organic molecules and carbon monoxide: The reduction/gasification reactions involve, mainly, the gasification of tar, depending on the technology used. They include: The Boudouard reaction: The water gas reactions: Methane synthesis reactions: 3. Design of the Gasifier/Reactor A laboratory scale biomass gasifier is for a micro scale application which is to produce mechanical  power of about 1 to 7 kW. A mechanical power of 4 kW is assumed and the design of the gasifier is based on this. The design of the reactor is basically empirical, that is, implied from charts based on past experiences. 3.1 Power Consumption of the Gasifier For an engine with a compression ratio of 9.5:1, the effeciency has been estimated to be 28 per cent [12]. Therefore, the thermal power in the gas can be estimated as If the thermal effeciency of the gasifier is taken at 70  per cent, the thermal power consumption at full load can be estimated as  Design and Development of a Laboratory Scale Biomass Gasifier 18 3.2 Biomass Consumption of the Gasifier A heating value of biomass with 14% moisture content is taken to be 17,000 kJ/kg, according to Ref. [12]. The biomass consumption of a gasifier can be estimated as [12, 13]. 3.3 The Hearth Load: Specific Gasification Rate and Specific Solid Flow Rate The hearth load, B g , is defined as the amount of  producer gas reduced to normal (p, T) conditions, divided by the surface area of the throat at the smallest circumference and is expressed in m 3 /(cm 2  h) [12]. This may be referred to as the specific gasification rate (SGR), which is the volumetric flow rate of gas per unit area based on throat diameter, the gas volume being measured at the standard conditions [14, 15]. The hearth load can also be expressed as the amount of dry fuel consumed divided by the surface area of the narrowest constriction, B s , and is expressed in kg/(cm 2  h) [12]. This may also be referred to as the specific solid flow rate (SSR) which is the mass flow of fuel measured at throat [14, 15]. One kilogramme of dry fuel under normal circumstances produces about 2.5 m 3  of producer gas [12, 14, 16]. Thus [12, 14] The recommended value for B g  falls in the range of 0.30 to 1.0 [12, 14, 16]. Taking the value of B g  as 0.3, 3.4 Throat Sizing The cross sectional area of the throat is thus The diameter of throat, d  t , can be calculated using Sivakumar et al. [17] discovered from their model that for throat angles of about 4 5 °, the cumulative conversion effeciency is increased while larger angles of about 90° decrease the cumulative conversion effeciency because of a decreased temperature for larger throat angles due to the divergent effect and the reaction rate. Venselaar [18] also recommended, after comparison of the design characteristics of a number of gasifiers, that the throat inclination should be around 45° to 60°. A throat angle of 60° is used. 3.5 Sizing of the Fire Box or Hearth Diameter of the fire box or hearth, d  h  is a function of throat diameter and can be estimated from Fig. 1a using 3.6 Nozzle Design and Air Blast Velocity The height of nozzle plane above the smallest cross section of the throat is a function of the throat diameter and can be evaluated from Fig. 1b, The ratio between the nozzle flow area and the throat area is a function of the throat diameter and is given from Fig. 1c as Where, A n  is the total nozzle area. It is assumed that the gasifier will be equipped with 5 nozzles as recommended by Shrinivasa and Mukunda [19] for operating slow two-cycle engines. Hence, Sivakumar et al. [14] suggested optimum results are obtained when the angle of inclination of the nozzles is  between 10° and 25°. An inclination of 15° is used. The nozzle tip ring diameter d  nt  is also a function of the throat diameter as seen in Fig. 1d. The ratio  between the nozzle tip ring diameter and the throat diameter is  Design and Development of a Laboratory Scale Biomass Gasifier 19   a. b. c. d. Fig. 1 a. Diameter of the fire box, d r , as a function of the throat diameter, d t  ; b. Height of the nozzle plane above the throat, h nt , as a function of the throat diameter; c. Ratio between nozzle flow area, A n , and throat area, A t , as a function of the throat diameter; d. Nozzle tip ring diameter, d nt , as a function of the throat diameter, d t [1]. The air blast velocity (V  b ) can be estimated by equating the volumetric flow rate of the producer gas through the throat to the volumetric flow rate of air through the nozzle. The volumetric flow rate of producer gas through the throat is estimated using Using this flow rate as the flow of air through the nozzle, 3.7 Air Inlet and Outlet The general range for air inlet velocity is 6 m/s to 10 m/s [17]. The dimensions for the air inlet can be obtained using the continuity equation. By taking the velocity of air to be 6 m/s, For a circular opening, the diameter is . The gas inlet is taken to be 25 mm. The gas outlet is taken to be 20 mm. 3.8 Length of Reduction Zone Sivakumar et al. [17] proposed that for a throat diameter of about 100 mm and for a throat angle of  between 45 and 90 degrees, the reduction zone with a
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