Development of an innovative 3-stage steady bed gasifier for

municipal solid waste and biomass

RAVI KUMAR

2009JE0618

CONTENTS

• Introduction• Materials and methods Normal operation Reversed operation• Strategic analysis• Conclusions• References

INTRODUCTION

• Gasification of biomass, MSW, waste-derived fuels and residues is used as an thermal treatment method to produce power and heat.

• Gasification is a thermo-chemical process, classified between combustion and pyrolysis, for producing energy.

• Presented in this paper is a new, recently patented, 3-stage gasification scheme, designed for all aforementioned types of fuels and for producing a synthesis gas free of tar and dioxins.

• The proposed 3-stage gasification scheme comprises of three stages:

i) pyrolysis, ii) combustion and iii) gasification.• It is valid for municipal solid waste and any type of

biomass despite differences in chemical composition.• The innovation of this 3-stage gasification scheme is

based on the fact that the transition between normal and reverse operation and vice versa is achieved only by the proper rotation settings of four air blowers, thus creating a new model of gaseous flow management between the three aforementioned stages.

• It can achieve a safe industrial-scale operation

while producing a gas free of harmful components.

• It is suitable for small- to-medium scale capacities.

• Overall electrical efficiency of 30% .• Minimum environmental impacts well below

all existing thresholds.

MATERIALS AND METHODS

• Past trials for designing and constructing a multi-stage gasification scheme were performed by placing

(i) pyrolysis of fuel as first stage, (Ii)combustion of Pyrolysis Gases (PG) as second stage and, finally (iii)charcoal gasification as the third stage of the overall process.

Normal operation1. Pyrolysis zone2. Combustion zone3. Gasification zone4. Material feeding system5. Grate6. BPA blower7. Burning hearth8. Reduction layer9. Distillation layer10. Drying layer11. Pyrolysis gas duct12. Torch13. Up-draught stream14. Down-draught stream15. Gasification bed16. Flue gas17. Discharge system18. Gas output19. Gas cooling and cleaning20. Buffer zone

• Burning hearth: Partial combustion of the charcoal is

performed, which provides the energy for the reactions at the overlaying material layers. The remaining charcoal and the ash are detached by moving the grate; then, they fall downwards, pass through the buffer and combustion zones to, finally, create the fixed bed of the gasification zone.

• Reduction layer: Part of the hot carbon dioxide, which is produced from the hearth, is reduced via the charcoal into carbon monoxide.

• Distillation layer: The volatiles of the fuel are separated through a complicated sequence of pyrolytic reactions, The solid fragment (charcoal, ash) falls into the reduction zone, while the hot gases rise through the new incoming fuel and they dry it.

• Drying layer: Fuel's water content is converted into vapor, which departs together with the remaining gases.

• Most of the tars and dioxins which still exist

into the PG, when they cross the flame of the torch, are burned and/or cracked.

• The produced synthesis gas is rich in H2 and CO, has low contents of tars and dioxins and is suitable for supplying internal combustion engines.

REVERSED OPERATION1. Pyrolysis zone2. Combustion zone3. Gasification zone4. Material feeding system5. Grate6. BPA blower7. Burning hearth8. Reduction layer9. Distillation layer10. Drying layer11. Pyrolysis gas duct12. Torch13. Up-draught stream14. Down-draught stream15. Gasification bed16. Flue gas17. Discharge system18. Gasification zone19. Gas cooling and cleaning20. Buffer zone21. Additional supply of natural gas or

propane22. Up-draught stream flue gas stream23. Down-draught stream flue gas

stream24. Up-draught stream

• Presented in figure is the reversed operation

of the buffer zone, which is achieved by a modification of the normal operation, in order to gasify fuels with high water contents and/or low calorific values.

• During the reversed operation of the buffer zone, which is achieved by proper operation settings of the air blowers, the flue gas from the torch is divided into two streams; the up- draught stream passes through the buffer zone and enforces additional heat to the pyrolysis bed, whereas the down-draught stream feeds the gasification bed

• When feeding the gasifier with high-humidity

fuels, it becomes necessary to provide the combustion torch with additional external supply of gas (natural gas or propane) fuel, in order to have enough energy for heating the high vapor content which is now present in the PG.

RESIDUE PROCESSING

• AWT- Ash Water Tank

• ECA- Exchanger of Combustion

Air

• EPA-Exchanger of Pyrolysis Air

• ESA-Exchanger of Additional

Steam

• GST-Gas Scrubber Tower

• BGO-Blow of Output Gas

• VSC-Venturi Scrubber

• Bottom ash holding - The bottom ash is separated

from the produced gas due to the gravity and finally dips into a water tank (AWT) and onto a conveyor belt.

• Heat reallocation - The preheaters of the flue gas (ECA) and of the PG (EPA) are the devices (heat exchanger) which implement the heat reallocation in the system.

• Gas cooling and separation: The necessary installations and their attributes for cleaning the produced gas and making it suitable for feeding an internal combustion engine are summarized below:

• Gas Scrubber Tower (GST): a) Fly ash (>8 μm) is

trapped in this device. b) Gas is cooled down to near-atmospheric temperature. • First Baffle Scrubber (BS1): Separation of

different types of gases which exist in the produced gas, (e.g. hydrochloric acid (HCl), ammonia (NH3)), which are water-soluble.

• Venturi Scrubber (VSC): Gas is sprayed with a potassium-hydroxide (KOH) solution. Small particles (> 1 μm) are held and sulfur di- oxide is converted to potassium sulfate (K2SO4).

• Second Baffle Scrubber (BS2): Second stage of sulfur dioxide conversion.

• Droplets holding: Rasching ring columns are

foreseen at the end of the aforementioned devices (GST, BS1 and BS2) in order to divert the existing droplets out of the gas flow.

• Fans: At the upper side of each tower of BS1 and BS2, the centrifugal fans BGO1 and BGO2 are respectively located; they balance the existing pressure drop in the gas cleaning devices.

STRATEGIC ANALYSISSWOT analysis for the three-stage gasification scheme

• StrengthsPotentially adequate to replace fossil fuelled energy

conversion.Low volume of produced solid residues.Low weight of produced liquid residues.Appropriate gaseous flow management in the direction

of low tars, dioxins and emissions.Electrical efficiency ~30Decentralized technologyHigh efficiency rate of the reactor (~76%)

• WeaknessesReliability not yet provenModerate investment cost (~1900€/kWe)Bureaucratic requirementsFinancing issues

• OpportunitiesApplication in small-scale industries which

have available biomass residuesApplication in small-medium municipalities-

CHP plant: Generated electricity to the grid Generated heat to appropriate heat customersA developing market Moving into new market segments that offer

improved profits

• ThreatsCompetitors have superior access to channels

or distributionChange of the existing charging policiesDevelopment of other competitive newWaste-to-Energy advanced conversion

technologies

CONCLUSION

• The innovation of the presented three-stage gasifier is characterized by the fact that the transition from normal to reversed operation is implemented via the existence of a buffer zone, which is thus creating a new model of gaseous flow management.

• This model can handle a wide range of variations in water content and/or composition of the inserted fuel.

REFERENCES• Antonopoulos,I.S.,Karagiannidis,A.,Elefsiniotis,L.,Perkoulidis,G. and Gkouletsos,A. 2011.

Development of an innovative 3-stage steady-bed gasifier for municipal solid waste and biomass. Fuel Processing Technology 92 2389–2396.

• http://www.gasification.org/page_1.asp?a=87 as viewed on 20/01/2013• http://en.wikipedia.org/wiki/Gasification as viewed on 20/01/2013