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Project Profile #5

2002-2004: Effective Cr(III) recovery by gasification with heat production, Pittards PLC and Biomass Engineering Ltd., UK

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50 kg/h leather wastes containerised gasifier coupled with gas cleaning system

Project Objective

The combustion of leather wastes cause the Chrome(III) used in the tanning process to be converted to the toxic form Chrome (VI) and this has significant disposal costs associated with it. Gasification operates in a reducing atmosphere, therefore the Cr(III) should remain as Cr(III) after gasification. To this end, working with Biomass Engineering Ltd., the British Leather Corporation (BLC) and Pittards PLC, C.A.R.E. Ltd. worked on the retrofit of a existing gasification technology with a new gasifier with the objective of gasifying a range of leather wastes produced by Pittards with the primary aim of recovering Cr(III) from buffing dust and sludge cake in the residual ash from the process. The off gases would be water scrubbed to remove acid gases and other contaminants and then combusted for heat. C.A.R.E. Ltd. contributed to:

Project Summary

The containerised gasifier was operated on Pittards site in Leeds, converting a range of leather wastes (buffing dust, wet blue and sludges). The unit was installed and commissioned on site in February 2004 after initial trials proved successful in late 2003 at Biomass Engineering Ltd.

Approximately 300 hours operation were obtained from February 2004 to May 2004. During this time the solids, liquids and gases were extensively analysed and characterised to assess the operability of the unit and environmental implications for larger units. Biomass Engineering Ltd. supplied training to Pittards staff to operate the unit and also provided support when required. Most of the work flared the process gases and only on a few occasions were the producer gases used for the drying of material for the briquettor. Analyses of the producer gases and char and ash are given in Table 1 and Table 2. The unit was typically operated for 6-8 hours, 3-5 days/week over the 3-month period, processing 40-50 kg/h of materials.

Observations made during the processing of the fuels were:

Table 1. Average dry gas analysis (vol%) Feedstock wet blue

Gas Jan 04 July 03#
Carbon Monoxide [CO] 15.06 10.52
Hydrogen [H2] 11.1 13.31
Methane [CH4] 1.1 1.44
Carbon Dioxide [CO2] 8.6 12.50
Ethylene [C2H4] 0.3 1.31
Ethane [C2H6] 0.01 0.16
Oxygen [O2] 1.07 ND
Nitrogen + Argon [by difference] 58.65 60.49
HHV dry gas [MJ/Nm3] 3.9 4.7
LHV dry gas [MJ/Nm3] 3.7 4.3


Note: # work carried out by Biomass Eng. Ltd. at Newton-le-Willows

Table 2. Analysis of the char/ash samples in May 2004 (wt%, as received)

Material C H N S Cr Ash
Fly ash from sludge cake 13.46 0.98 1.18 4.71 1.96 77.7
Sludge cake char/ash 7.22 0.05 0.25 4.35 2.84 85.3
Buffing dust char/ash 15.43 0.22 0.81 0.27 3.50 79.8

Project Status

The testing of the leather wastes proved successful in recovering Cr(III) in the residual char and gas. An assessment of the gas cleaning costs to reduce contaminants to a manageable level for a gas engine showed that it would be uneconomic to do so. After the trials, no further work was carried out.