(63b) Shredded Waste Downdraft Gasification

Downdraft co-current moving bed gasifiers are most suitable to convert high volatility fuels, such as municipal solid waste (MSW) and biomass, to low tar syngas for use in generating electricity.  Downdraft gasifiers have been limited in their ability to use unprocessed fuels, such as fluffy, low density shredded materials because of a high pressure drop across the gasifier.  As a result, the solid fuel is pelletized or briquetted before use in the gasifier.  Pelletizers are expensive and require pretreatment, e.g. metals separation, to prevent equipment breakdown and maintenance.

The use of shredded waste, instead of pellets, in a packed bed will reduce the footprint and costs of the feedstock conditioning pre-processing system and will result in faster reaction kinetics, increasing the conversion efficiency because of the high surface area to volume ratio.  The development of a downdraft gasification system that uses shredded waste is not without its problems.  Gasification of shredded waste has only been done successfully in fluidized bed reactors which are bulky and used primarily for much higher shredded waste flows and energy production.  Shredded waste has very high wall friction for typical gasifier geometries, retarding the solids flow along the walls of the reactor and resulting in non-uniform flow across the cross-sectional area (also known as funnel flow) and isolated regions of low permeability and excessively high temperatures.

The feasibility of developing a downdraft packed bed gasifier to process MSW and other biomass was determined by testing a prototype in the laboratory.  A diverging tapered gasifier with a rectangular cross section area was fabricated and extensively instrumented with thermocouples, pressure gauges and flow meters.  The diverging tapered gasifier reduces the friction between the shredded waste and the reactor surface, reducing the potential for bulk solids flow problems, such as arching and rat-holing.  The rectangle design reduces the distance of secondary air coverage maximizing its reach without extending nozzles into the reactor.  A manual operating grate was installed at the exit of the gasifier to control the waste flow through the gasifier and to remove char and ash.  A heat exchanger to cool the syngas and a filter system to remove tars and other particulates were placed downstream of the grate.  The syngas was sampled downstream of the filters in Mylar bags and its composition determined by gas chromatography.

A vacuum pump (blower) located downstream of the filters is the primary mover for the system.  Five rings of secondary air injection ports were installed along the length of the gasifier, with the top three rings placed in the pyrolysis zone.   Eight secondary air injection ports were installed in each ring; three ports each in the front and back face of the gasifier and one port each on the side faces.  Secondary air was injected through the ports normal to the walls of the gasifier.  The air flow rate at each ring was independently controlled, as was the syngas flow through the vacuum pump.  The differential between total gas being moved by the blower from the amount of secondary air injected as well as the syngas produced would result in the amount of primary air added to the reactor through the top opening

A simulated MSW with a composition of 44.5% food, 42.2% paper and cardboard and 13.3% plastic was used as the feed solid waste.  The moisture content of the MSW was varied from 10 to 30% and the shred size varied from 15-25 mm.  The gasifier was designed to handle a shredded waste flow of 7.6 kg/hr (16.7 lb/hr) and a syngas flow rate of 0.40 m³/min (13.2 ft³/min).  Operating test data was also obtained with pellets of the same composition and compared to those of the shredded waste.

The following data and analyses will be presented in the paper:

• Design of the internal gasifier and grate configurations based on the following material flow/reactor surface properties test data:  cohesive strength, particle interlocking, compressibility, wall friction and permeability.  Surface properties test data were obtained on shredded MSW with shred sizes of 15 mm and 20 mm and moisture contents of 10% and 20%.  Gasifier designs were made for diverging tapered rectangular and conical configurations.  The rectangular configuration was selected because of the ease in installing secondary air injection ports and thermocouples.
• For high conversion efficiency, secondary air should penetrate to the center of the gasifier, reacting with as much of the waste in the pyrolysis zone as possible.  Shredded waste has a low permeability (high resistance) to gas flow compared to pellets and the extent of the secondary air gas penetration into the gasifier can be limiting.  Solid Works simulation analyses were carried out to determine the penetration distance as a function of the following operating conditions:  secondary air flow as a percentage of the total syngas flow; waste permeability (resistance); nozzle diameter (flow velocity); mass flow per ring; and number of secondary air flow rings.  The penetration distance was determined by the distance of the limiting streamline from the wall of the gasifier.  Test data was compared to the simulation analyses using the composition of the syngas as a marker.
• The effect of secondary air flow rate and distribution and syngas flow rate on the temperatures in the pyrolysis and reduction zones was determined experimentally for short run times and the results compared to an equilibrium model.  The temperatures showed a marked increase primarily with secondary air flow and less with total syngas flow.
• An extensive test program was conducted to determine the effect of shred waste size (fixed MSW composition), MSW flow rate, moisture content, and syngas flow rate on the syngas composition and energy and tar content.  A secondary air flow rate and distribution from the previous set of tests were used.  Gasifier temperature distributions and pressure drop data were also obtained.  The reactor temperatures and syngas composition test data were compared to an equilibrium model.

Test data obtained to date show that the diverging tapered gasifier configuration produced bulk solids mass flow without any regions of stagnant flow in the gasifier, such as problems of arching and rat-holing.  Although the low permeability of the shredded waste flow resulted in higher pressure drops, it did not inhibit the mixing of the secondary air with the solid waste.