Creating energy from tannery waste

12 September 2004




Solid waste - the issues The solid waste generated during leather making is significant. Whilst the leather industry is making use of a byproduct from the meat industry, only 20% of the raw material ends up in the finished leather. This creates a huge environmental challenge for the modern tannery as the traditional options for the disposal of this waste may become cost prohibitive in the future. Within the leather industry, it is possible to view waste as a valuable resource. This can be either as a substrate for the production of value added products or as a fuel. Waste is energy In terms of thermal processing, BLC has been engaged in an extensive programme of research and development aimed at developing a gasification process that can convert organic waste from the leather making process into energy. Whilst gasification technology has been in existence for a number of years, its application for the treatment of industrial biomass is relatively new and many substrates remain untested for this application. Significant research and engineering development over five years has resulted in a system that will convert leather waste within a robust, safe and economically attractive process to a source of energy that can be converted to heat and/or power. What is gasification? The gasification process converts any carbon-containing material into a combustible gas comprised primarily of carbon monoxide, hydrogen and methane, which can be used as a fuel to generate electricity and heat. Gasification adds value to low or negative value feedstocks by converting them to marketable fuels and products. Gasification technologies differ in many aspects but share certain general basic characteristics. Typical raw materials used in gasification processes are coal, biomass, agricultural wastes, sewage sludge etc. The feedstock is prepared and fed to the gasifier in a dry briquetted form. The feedstock reacts in the gasifier at a high temperature in a reducing (oxygen starved) atmosphere. This produces a combustible gas (fuelgas) made up primarily of carbon monoxide, hydrogen, methane (more than 30% by volume) and quantities of carbon dioxide, nitrogen and higher hydrocarbons. Gas treatment facilities refine the fuel gas, and the trace elements or other impurities that are removed from the gas are either re-circulated or recovered. If the fuelgas is to be used to produce electricity, it is typically used as a fuel in an integrated gasification combined cycle (IGCC) power generation configuration. The fuelgas can also be processed using commercially available technologies to produce a wide range of products - fuel, chemicals, fertiliser or industrial gases. Some facilities have the capability to produce both power and products from fuelgas, depending on the plant's configuration. The most effective tannery application may be the direct use of fuelgas in boiler applications or regenerative thermal oxidisers. This application avoids electricity generation but does lead to the creation of heat (steam) required for leather production, avoiding a large proportion of the primary energy currently consumed from other sources. Chemistry of gasification During the gasification process, the fuel undergoes a complex physical and chemical change starting with the drying or removal of water contained as moisture. The dried fuel is then pyrolised or thermally decomposed. Finally, the pyrolysis products, condensable vapours and char undergo gasification where they are concurrently oxidised or reduced to permanent gases. Each of these stages is described below: 1. Pyrolysis process in a downdraft gasifier The dry fuel from the drying zone is pyrolised or thermally degraded at a temperature above 250°C. The following result from this process: Solid residue char - This is the solid residue of the feed consisting mainly of carbon together with hydrogen, oxygen, ash and some volatiles from incomplete pyrolysis. Char can be recycled into the reactor after separation. Ash - This is the final residue of fuel and does not have a high calorific value; however, it can be used as an input material for cement manufacture, carbon filters or for metal recovery. Condensate - These are liquid pyrolysis products consisting of a complex mixture of more than 200 compounds. The condensate contains two phases, an organic and an aqueous phase. The organic phase consists mainly of insoluble matter and hydrocarbons, while the aqueous phase contains up to 99% water. All these products are vapours at the pyrolysis temperatures of 250-450°C. Gases - This is a mixture of non-condensable gases such as CO, CO2, H2 and light hydrocarbon such as CH4, C2H4, C2H6, C3H8 and C3H6. 2. Gasification stage In this stage, the pyrolysis products undergo a series of oxidation and reduction processes to produce a final fuelgas which is mainly CO, CO2, H2, CH4 and H2O. Gasification generally takes place at temperatures above 850°C. 3. Chemical reaction Various chemical reactions take place during the gasification process at different zones within the gasifier. These different zones are illustrated in Figure 1. Drying zone - The solid waste descends in the downdraft gasifier as a result of the consumption of the waste material in the reaction zones below. This is the process whereby moisture contained in the waste is volatilised and removed by evaporation. The heat required for this process is provided by the heat transfer from the hotter lower zones of the reactor by recirculation of hot gas produced from the reduction zone. Various factors influence the rate of drying. Pyrolysis zone (carbonisation zone) - Pyrolysis is the irreversible thermal degradation of the organic matter. This takes place in the downdraft gasifier using the thermal energy released by the partial oxidation of the pyrolysis products. Oxidation (throat) zone - The volatile products of pyrolysis are partially oxidised in highly exothermic reactions resulting in a rapid rise in temperature up to 1,200°C in the throat zone. The heat generated is used to drive the drying and pyrolysis of the feed and gasification reactions. The oxidation reactions of the volatiles are very rapid and the oxygen is consumed before it can diffuse to the surface of the char. No combustion of the solid char can, therefore, take place. Oxidation of the condensable organic fraction to form lower molecular weight products is important in reducing the amount of tar produced by a gasifier. The pyrolysis and oxidation processes within a downdraft gasifier are typically described together as flaming pyrolysis. The products of CO2, CO, H2 H2O, hydrocarbons gases, residual tars and char then pass on into the gasification zone below. Reduction zone - In the reduction zone, the char is converted into product gas by reaction with the hot gases from the upper zones. The gases are reduced to form a greater proportion of H2 and CO. The temperature of the gas reaches 1,000-1,200°C. Gas cooling and cleaning - The gas leaves the gasifier at a temperature of between 250-300°C and is loaded with dust, pyrolytic products and water vapours. It is necessary to cool and clean the gas, via cyclones, scrubbers or ceramic filters, in order to remove as much water vapour, dust and pyrolytic products as possible from the gas before it is usable in an engine (or alternative application). Cost benefits With the incentive of controlling rising waste disposal costs and the constant threat of tightening environmental legislation, there has been a considerable amount of research into alternative disposal routes. With heat and electricity requirements contributing to a large proportion of the operational expenditure in most industries, methods that utilise the energy within waste to satisfy this demand are seen as having significant additional benefits. Of the thermal technologies capable of treating biomass/waste few are applicable to waste within the leather sector because of the waste's peculiarities. The majority of the biomass is collagen based, and it has been shown that only pyrolytic liquefaction or autothermic gasification can accommodate such a waste stream. For this specific application, gasification is preferred over pyrolysis; one reason being the potential to recover chromium from the inert product ash. Chromium recovery would be a further contribution to sustainability and provides additional cost benefits for the industry. The need to meet more stringent environmental requirements has not only increased the amount of waste that a tannery has to deal with, but also overall production costs. It is seen as a major factor in making the European leather industry less competitive. However, in the case of gasification, this environmental need promotes a technology that could help reduce production costs and enable more competitive pricing. This is why BLC is committed to implementing the process and has devoted much time and expertise to researching and developing a plant suitable for application within the sector. Depending on the waste stream and the blend of materials, sludge and buffing dust will produce approximately 1kw/h of energy per 1kg of waste. Industrial trials The process has now been evaluated commercially in conjunction with a new industrial partner, Biomass Engineering (www.biomass.uk.com). Biomass Engineering were established in 1996 to research and manufacture biomass-fuelled Combined Heat and Power (CHP) systems. The company focuses on the relatively small-scale one megawatt and sub-megawatt systems for de-centralised as well as national grid power generation. Recently they won the United Utilities Low Carbon Award, distinguishing them as a company that has already been successful in reducing carbon dioxide emissions through renewable energy generation. The result of the relationship between BLC and Biomass Engineering has been a gasification plant specifically designed for the leather industry, though still adaptable to other sectors. The pilot plant (illustrated in Figure 2) is capable of treating around 75kg/h of waste depending upon the characteristics of the waste. The scale of a full size plant would vary up to a one megawatt unit that would treat around 24 tonnes per day, so the capabilities of the system are very flexible. Interest in the new gasification pilot plant has been growing following several hundred hours of operation on leather waste to demonstrate its capabilities. One forward thinking tannery has taken this interest one step further and have been conducting on-site trials. Initial results have been very encouraging. Figure 2 shows the plant in operation. Figure 3 shows a temperature profile of the flare stack that is consistent at between 400ºC and 500ºC. Figure 4 demonstrates the profile of some of the principal product gas constituents. In particular, the methane content is notably high at between 6% and 10%. This helps increase the calorific value and, hence, the heating or electricity production value of the gas. Both of these sets of data were gathered during runs on buffing dust. However, although wastewater effluent sludge is more difficult to treat, results have shown a performance that is comparable to that on buffing dust demonstrated here. Full or pilot scale plants BLC and Biomass Engineering are in the position to offer full or pilot scale gasification plants to the leather sector. For pilot scale operation, there are options to rent and trial a unit for a charge or for those organisations that require their own unit, these can be purchased and made to order. Full and pilots scale plants include driers, compactors, emission management and fuelgas powered generators as required. The full-scale plants can be built to any size required using a modular format. For further information please contact BLC. Conclusions Legislative enforcement and fiscal drivers are urging the acceptance of this technology in the short term. The drive to take-up renewable energy from sources, including biomass waste, stems from governmental targets to reduce fossil fuel dependency, carbon dioxide emissions (from fossil fuels) and the corresponding pressure upon existing disposal routes and local waste management policies. It has been demonstrated that a wide range of tannery wastes can be macerated, flash dried, densified and gasified to generate a clean syngas for reuse in boilers or other Combined Heat and Power systems. As a result, up to 70% of the intrinsic energy value of the waste can be recovered as syngas, with up to 60% of this being surplus to process drying requirements so can be recovered for on-site boiler or thermal energy recovery uses. The work described above illustrates the vast potential for application of gasification to the treatment of leather waste. There are still some barriers responsible for impeding application of the technology, however, these are becoming less significant. The impact of cost is reducing with time as the environmental cost of tanning is becoming a larger part of operating costs. Contact details For more information on the BLC/Biomass gasifiers, contact Victoria Addy on +44 1604 679953 or [email protected] For an overview of BLC services, visit [http://www.blcleathertech.com]



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