Method to convert refuse derived fuel into a combustible gas

Abstract: Method to convert refuse derived fuel (RDF) into a combustible gas, whereby the refuse derived fuel is gasified and the derived gas undergoes a process of catalytic cracking in which at least one alkaline additive or an equivalent product is used, the gas being passed through a cyclone separator to collect the particulate solids and thereafter undergoing a cooling process with recovery of hot air before being passed through sleeve filters, the ashes of the gasification and the particulate solids being sent to a dump for ashes, the ashes of gasification being riddled beforehand to gather the fines, which are mixed with fines coming from the sleeve filters and are employed as further catalysts in the catalytic cracking process.

Description: This invention concerns a method to convert solid-refuse derived fuel, whether of an urban or industrial origin, into a combustible gas, as set forth in the main claim. The method of the invention is applied advantageously, but not only, to the processes of gasification and/or pyrolysis of conventional solid fuels or fuels derived from the screening of solid urban refuse or from the processing of biomasses. The gasification technology which has been developed for some time now for the production of gas from coal or wood has been used recently with more difficult fuels, such as the residues of the processing of biomasses, solid urban refuse, and solid industrial refuse or the combustible fraction derived therefrom and conventionally called refuse derived fuel. This technology is worthwhile since it enables the ashes to be separated from the gaseous products, which have a smaller volume than the products resulting from simple incineration, and therefore makes possible a reduction of the volumes of the equipment required to treat the products and a more efficient technique for the combustion. Moreover, the various available technologies give birth to a gaseous mixture consisting essentially of light gases such as CH₄, CO, H₂, H₂O, N₂ and CO₂, but also of a moderate quantity of vapours of high-boiling hydrocarbons (tars) and of ashes containing volatile metals. The energy content of the tars amounts to about 20% of the calorific power of the gas produced and is therefore an important fraction for the yield of heat of the process. The chemical-physical properties of these products make problematical the employment of the gas produced in traditional usage equipment such as boilers or gas-powered engines since the gas produces harmful emissions and also deposits along the pipes and in the equipment. It is therefore worthwhile to recover and convert these products into a usable form. One of the most promising ways has been found to be catalytic cracking. In the state of the art the gasification process and the successive treatment to scrub the gas thus produced comprise according to the state of the art the following steps: the feeding of refuse derived fuel into a gasification furnace with a controlled quantity of oxidizing gas; discharge of the residual ashes from the bottom of the furnace; extraction of the gas thus produced from the top of the gasification furnace, the gas consisting mainly of CH₄, CO, H₂, H₂O, N₂, CO₂ and of hydrocarbons with a wide boiling-point spectrum (tars); catalytic cracking of the tars and ammonia by the addition of a controlled quantity of a high-temperature oxidising gas (air, for instance) and of a catalytic support (dolomite, for instance); neutralisation of the hydrogen-halogen acids, mainly hydrochloric acid and hydrogen sulphide, contained in the gas by means of adsorption on beds of dolomite; separation of part of the entrained particulate solid after the cracking step, in a cyclone chamber or another suitable separator; cooling the gas by mixture with low temperature (quench) recirculated gas or by atomised water; cooling the gas in heat exchangers with a simultaneous pre-heating of process air to be used for the gasification and catalytic cracking; final scrubbing of the gas by filtration through sleeves of fabric; combustion of the gaseous mixture containing only light fractions, such as CH₄, CO, H₂, H₂O, N₂ and CO₂, and therefore suitable for subsequent employment in technical usage apparatus devoid of final treatment of fumes, such as endothermic engines, boilers or heating furnaces; the ashes from the the gasification furnace, catalytic cracking reactor and systems to filter the gas thus produced are sent to appropriate controlled dumps. The combustion of the gas obtained by this treatment produces fumes with a reduced corrosive action and a minimum content of harmful products such as dioxin, NO₂ and acid products derived from the heating of hydrogen-halogen products present in the refuse derived fuel. The method of cracking the condensable fraction, commonly called "tars", in the gases of pyrolysis is disclosed in patent WO-A-8.809.364, which describes how the gases produced by the pyrolysis of straw are partly burnt with the addition of pre-heated secondary air and are then conveyed to a reactor containing a fixed bed of granular dolomite, where the reactions of cracking of the tars take place at a temperature of at least 800°C. The ashes produced in the process of pyrolysis and combustion are deposited on the layer of dolomite, which therefore has to be renewed continuously or at a pre-set frequency by the application of fresh dolomite. Patent FR-A-2.385.037, which considers the employment of the pyrolysis process for the conversion by heat of refuses of various kinds into gases, oils of pyrolysis and other re-usable products, describes among other things the possibility of proceeding to the cracking of the tars instead of their condensation. In particular, the process, which is generically described, takes place in the presence or absence of a cracking catalyst, and the application of heat required to produce the cracking is provided electrically from without at temperatures of about 1100°C. Lastly, patent EP-A-0152912 describes a process of pyrolysis of solid refuse, whereby the tars drawn in the gaseous phase undergo cracking on a catalytic support of granular coke at temperatures between 800° and 1200°C. In this process the gases of pyrolysis, after separation of the solid carbonous phase produced by pyrolysis of the original fuel, are partly burnt with the addition of pre-heated air and then pass through a layer of red-hot coke, which acts as a catalytic support for the cracking reaction. The necessary coke is renewed periodically with fresh coke to maintain the required catalytic activity. The present applicant has studied, tested and brought about this invention so as to obviate the shortcomings of the state of the art and to achieve further advantages. The invention is set forth and characterised in the main claim, while the dependent claims describe variants of the idea of the main solution. The attached figure shows as an example a block diagram of the cycle according to the invention. In the process of gasification of the refuse derived fuel according to the invention a part of the ashes gathered from the bottom of the gasification furnace and the ashes separated by the system of filter sleeves installed on the line of the gas downstream of the heat exchangers are recycled and not sent directly to the disposal dump. The ashes from the bottom of the gasification furnace are rich in residual alkalinity and, according to the invention, are riddled and divided into a fine fraction (smaller than 2-3 mm.) and a coarse fraction intended for dumping. The fine fraction of the ashes from the bottom of the gasification furnace and the ashes gathered by the sleeve filter system are mixed with a suitable alkaline additive so as to generate the catalyst mixture to be sent to the cracking step. This mixture, which acts as a catalyst for the reactions that take place during catalytic cracking, can be sent to the cracking reactor, for instance by a pneumatic conveyor. In this way the quantity of catalyst to be fed to the cracking reactor is reduced considerably, with a resulting financial advantage for the process of treatment of the gas itself. According to the invention a suitable quantity of lime or of an equivalent product to abate the chlorine content is introduced into the cycle together with the ashes. According to the invention the ashes and the lime are introduced into the cycle in the gas line upstream of the catalytic cracking reactor so as to accomplish a reduction of unburnt material and a partial recovery of energy. In this way, besides the financial advantage of a greater yield of the gasification process, there is also an environmental advantage resulting from the reduction of the solid effluent to be sent for dumping. Moreover, during the gasification process according to the invention the gas leaving the catalytic cracking reactor undergoes a step of separation, by means of a cyclone separator for instance, before being cooled. The particulate solid thus gathered undergoes a postcombustion treatment with preheated air at a high temperature. The heat generated by this postcombustion is used directly to complete the catalytic cracking reactions. The mass of recirculated ashes consists, for the most part, of activated carbon, which in the established cracking conditions is converted efficiently, by means of reaction with the water vapour present, into oxides of carbon and hydrogen, thus increasing the yield of combustible gases. Moreover, in the method according to the invention the recirculated ashes contribute towards keeping the cracking conditions stable and homogeneous and increasing the heat exchange during the subsequent treatment steps. The quantity of recirculated ashes can be regulated suitably, and also automatically, so as to keep the operational cracking conditions stable and thus to lessen the problems due to fluctuations, even sudden fluctuations, in the characteristics of the gas produced in the previous gasification step. The operational conditions of the postcombustion enable residual ashes to be obtained with a minimum content of unburnt material and with an efficient thermal destruction of harmful organic carbonous products. This postcombustion of unburnt ashes takes place advantageously, but not necessarily, in an appropriate chamber located below the discharge of the cyclone separator. In this way the residual solids of the postcombustion step consist of inert materials and represent the only solid effluent of the treatment cycle together with the coarse ashes from the bottom of the gasification furnace. The quantity of this solid effluent of the cycle amounts to 8% to 10% by weight of the burnt refuse derived fuel. In this way, besides the reduction of the unburnt material, there is also a partial recovery of energy, which increases the yield of the gasification process and makes the process still more worthwhile financially.

Claims: Method to convert refuse derived fuel (RDF) into a combustible gas, whereby the refuse derived fuel is gasified and the derived gas undergoes a process of catalytic cracking in which at least one alkaline additive is used, the resulting gas being passed through a cyclone separator to collect the particulate solids and thereafter undergoing a cooling process with recovery of hot air before being passed through sleeve filters, the ashes of the gasification and the particulate solids being sent to a dump for ashes, the method being characterized in that the ashes of gasification are riddled beforehand to gather the fines, which are mixed with fines coming from the sleeve filters and are employed as further catalysts in the catalytic cracking process. Method as claimed in Claim 1, in which the alkaline additive or an equivalent product is added during the step of mixing the fines. Method as claimed in Claim 1 or 2, in which lime or an equivalent product is added during the step of mixing the fines. Method as claimed in any claim hereinbefore, in which the particulate solids leaving the cyclone separator undergo a postcombustion step. Method as claimed in any claim hereinbefore, in which the hot air recovered by the cooling is employed for the operation of postcombustion of the particulate solids leaving the cyclone separator. Method as claimed in any claim hereinbefore, in which the postcombustion of the ashes produces heat used directly to complete the catalytic cracking reactions. Method as claimed in any claim hereinbefore, in which the mass of recirculated ashes consists, for the most part, of activated carbon which in the established cracking conditions is converted efficiently, by reaction with the water vapour present, into oxides of carbon and hydrogen, thus increasing the yield of the combustible gases. Method as claimed in any claim hereinbefore, in which the recirculated ashes contribute towards keeping the cracking conditions stable and homogeneous and towards increasing the heat exchange in the subsequent treatment steps. Method as claimed in any claim hereinbefore, in which the quantity of recirculated ashes can be regulated suitably, and also automatically, in such a way as to keep the operational cracking conditions stable, thus lessening the problems due to fluctuations, even sudden fluctuations, in the characteristics of the gas produced in the previous gasification step. Method as claimed in any claim hereinbefore, in which the operational conditions of the postcombustion enable residual ashes to be obtained with a minimum content of unburnt material and with an efficient thermal destruction of harmful organic carbonous products.

Inventors: VELCICH GIANFRANCO, VELCICH, GIANFRANCO
Applicants: DANECO DANIELI ECOLOGIA SPA
Application year: 1992
Main classification: C10B53
IPC codes: C10J3, C10B53