Abstract
The ash formation behaviour and associated compositional characteristics of the combustion of shoe manufacturing waste (SMW) in a 2.5 MWth pilot-scale circulating fluidized bed combustor (CFBC) were investigated to better understand the combustion behaviour and to find effective management strategy for the disposal of the ash streams produced. The compositional characterisations for the ashes produced from the pilot demonstrations showed the presence of a variety of trace heavy metals both in the fly ash and bottom ash. A pronounced uneven partitioning behaviour was observed on the distribution of these heavy metals between the fly ash and bottom ash, and it was found that all the heavy metals except chromium were preferentially enriched in the fly ash, with the contents of lead and cadmium in the fly ash being over 11 and 6 times higher than in the bottom ash. Leaching tests demonstrated that the concentrations of most of the aforementioned metals present in the leachates from the fly ash and bottom ash could meet the permissible limits for landfill disposal but with lead as an exception with its concentration in the fly ash leachates being over 2 times higher than the limit. The total PCDD and PCDF contents both in the fly ash and bottom ash were also much below the legal limit. To further understand the ash behaviour, the slagging and fouling tendency during SMW combustion in the CFBC was examined by use of the characterisation of the ash mineralogy and the results indicated that the SMW ash likely had low tendencies for slagging and fouling. The best two valorization routes for the SMW bottom ash and fly ash were recommended, one being to use them to produce colloidal silica medium to obtain safe inert filler and the other being to use them as the raw materials in the cement industry.
Highlights
The production of shoe manufacturing waste has emerged over recent years as an ever growing new class of solid industrial waste stream produced in large quantities, with the global market for footwear projected to reach US$430 billion by 2024 and the number of shoes produced to reach 40 billion pairs by 2025 led mainly by Asia-Pacific countries, including China in particular where the growth in footwear industry increased by 10 times in the past 10 years [1, 2, 3]
The contents of K2O and Na2O in both ashes were lower than 1.5 wt%, which indicates that the chemical composition of the shoe manufacturing waste ashes was more similar to that of municipal solids waste (MSW) than to that of biomass waste
These two ashes were considered unsuitable for being used as fertilizer due to the low contents of K, Na and P, which are the main nutrients needed for plant growing
Summary
The production of shoe manufacturing waste has emerged over recent years as an ever growing new class of solid industrial waste stream produced in large quantities, with the global market for footwear projected to reach US$430 billion by 2024 (http://www.strategyr.com/MarketResearch/Footwear_Market_Trends.asp) and the number of shoes produced to reach 40 billion pairs by 2025 led mainly by Asia-Pacific countries (http://www.indexbox.co.uk/store/world-footwear-market-report-analysis-and-forecast-to2020), including China in particular where the growth in footwear industry increased by 10 times in the past 10 years [1, 2, 3]. The disposal of shoe manufacturing waste (SMW) is of increasing environmental concern. SMW, which usually consists of hard-to-separate rubbers, fabrics, and cellulose-based and wood materials at varying quantities [4,5,6,7], is usually classified as low-value organic waste with variable high ash and moisture content [5, 6] and is often disposed of in landfill [1, 8]. Given the increasing environmental concerns of landfill and severe shortage of suitable landfill sites, finding alternative solid waste management strategies with potential socioeconomic benefits has received ever growing attention. Circulating Fluidized bed (CFB) combustion is considered to be one of the best available technologies for waste incineration due to its great inherent fuel flexibility and high combustion efficiency [13] in addition to other advantages, such as the relatively low combustion temperatures and lower emissions levels of toxic particulate matter and gaseous pollutants of low NOx and CO etc. [14,15,16]
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