University of Groningen

Design and modeling of a novel continuous open core downdraft rice husk gasifierManurung, Robert

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Publication date:1994

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Citation for published version (APA):Manurung, R. (1994). Design and modeling of a novel continuous open core downdraft rice husk gasifier.s.n.

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Summary

Rice husk is one of the most widely available agricultural wastes in many riceproducing countries around the world. Conversion of the abundant supply of rice husk intoclean energy is an opportunity to expand the energy resource base while reducing the en-vironmental burden now associated with the disposal of the husk.

One prospective option is the conversion of rice husk into gaseous fuel bygasihcation. Until recently however, a number of problems, all related to the demandingproperties of rice husk, prevented the application of downdraft gasihers for rice huskgasihcation at the 10-50 kg/h scale which is the relevant capacity for the major part ofattractive rural applications in developing countries (rice mill powering, water pumping,rural electrification, etc.). These problems are:- poor flow due to low density and swelling in the pyrolysis zone,- poor oxygen distribution due to small particle size,- sintering arising from oxygen distribution,- lack of a well designed continuous ash removal system,- relatively low Lower Heating Value due to a relatively large amount of natural ashes in

the rice husks.Based on the analysis of the problems as presented above and an analysis of

the gasification process structure the principle weakness of existing downdraftgasifiers operating on rice husk have been identified and solutions to get at a properfunctioning design have been developed, constructed and used in this study.

The features of the new gasifier are:- no throat or other obstacles that may hamper solids flow,- a low bed height without creating channeling of air,- open air suction over the cross section to avoid hot spots,- continuous ash removal by a rotating grid,- proper thermal insulation of the wall.

Such a rice husk gasifier with an internal diameter of 0.45 m was tested inthe range of 50-300 kg/(h.m2) corresponding to an air velocity ranging from 50 to500 Nm3/(h.m2). The product gas had a lower heating value in the range of 27C0 -

4700 kJlNm3.In order to obtain general design rules for the open core rice husk gasifier, a

mathematical model was developed by which the behaviour and performance of thesystem can be predicted and evaluated quantitatively.

The performance of the gasifier firstly is analyzed using a Schlapfer type ofthermodynamic model. This model is based on a combination of mass and energybalances assuming that one or more reactions are in chemical equilibrium atempirically determined temperatures. The experimentally observed value of thecomposition of the producer gas, the volumetric gas production, the heating valueof the gas, the carbon conversion and the rice husk to producer gas conversionefhciency are compared to the calculated values from the model.

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Lral wastes in many ricet supply of rice husk into;e while reducing the en-

k into gaseous fuel byrelated to the demandingt gasifiers for rice huskity for the major part ofrwering, water pumping,

rount of natural ashes in

rove and an analysis ofof existing downdraft

tions to get at a properlhis study.

' 0.45 m was tested inity ranging from 50 toin the range of 2700 -

rre rice husk gasifier, amd performance of the

ing a Schlapfer type ofrn of mass and energytemical equilibrium atbserved value of thelion, the heating value'oducer gas conversion

For improved simulation purposes the gasifier is divided into two separatezones where different processes occur. In the top zone: the drying, pyrolysis, andoxidation processes occur which Íre described by a Schlapfer type ofthermodynamic model. The bottom zone (the reduction zone) is described by a nonequilibrium chemical reaction engineering model. That model is based on mass andenergy balances, reaction kinetics, transport rates, and the fundamentalthermodynamic relations.

Gasification kinetics of rice husk char was investigated experimentally. Theresults could be modeled with an isothermal Modihed Volume Reaction (MVR)model as developed by Kasaoka et. al.

The simulated temperature, gas composition and carbon conversion profileshave been compared with those obtained from pilot plant experimental runs. Themodel can be used to predict the effects of changing operating parameters and feedproperties on reactor performance data such as carbon conversion efhciency,producer gas yield and heating value. It also can predict the temperature, thecarbon conversion, the gas composition and the producer gas lower heating valueprofiles within the reactor as a function of the distance from the grate.

Based on the rice husk gasiher design described above, two other rice huskgasihcation units have been constructed. The hrst unit was sent to a state metal workshopas a model for further engineering development and production. The second unit wasdirectly implemented in the field to generate electricity to the local community of a villageat Java, Indonesia.

The field plant set-up, the operating performance, and the economicfeasibility of implementing the developed rice husk gasification technology forenergy generation in rice producing rural aÍeas are described.

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