Waste treatment technologies I - Mechanical treatment, waste recycling, thermal treatment - INVENT Final Meetings
Content 1. Waste recycling - basics 2. Mechanical waste treatment - Size reduction - Screening - Sorting - Compaction 3. Thermal treatment - Combustion - Pyrolysis - Hydrolysis - Drying
Waste recycling - basics relevance of recycling: - keep items out of landfills and conserve natural resources - goal of every recycling process: use or reuse materials from garbage in order to minimize the amount of solid waste recycling of waste allow: - production of potential recyclables - separation of potential recyclables out of the waste stream recycling methods: - different waste collection systems separation into different waste streams - mechanical waste treatment: size reduction, classification, sorting and compaction - thermal waste treatment: combustion, pyrolysis, hydrolysis, drying
Mechanical waste treatment Size reduction: - to conduce to increase the specific surface area - influence of the properties of the material on the aggregate (grain size, hardness, brittleness, cleavage property, application of the products, required properties of the products (e.g. grain size distribution, grain shape)) - aggregates: hammer mills, impact crusher, cutting mill, rotary shear, jaw crusher, cascade mill basic product rotary shear final product
Size reduction INVENT Integrated Waste Management modules for different courses of graduate studies principle of a hammer mill: - used for size reduction of municipal solid waste, commercial waste, waste wood and car wrecks - advantages: no requirements on the grain size of the waste, high delivery rates, all-purpose - disadvantages: high rate of abrasion, loud, danger of dustexplosions, high energy consumption
Screening Screening: - screening is split into sieving and air classifier Sieving: - divides particles of different grain sizes - separation is effected by specific length of the particles and the separation plan of the sieve - during the sieving emerge a fine grit (fall through the separation plan) and a coarse grit (remain on the separation plan) - examples: trommel screen, oscillating screen, bucket sieve, ballistic separator Air classifier: - separator divide the particles due to their decantation rate which is addicted by the grain shape and the specific weight of the particles - examples: zig-zag air classifier, cross-flow and horizontal air classifier, suction hood trommel screen principle of a counter flow ballistic separator light fraction dense fraction
Sorting Sorting: - using the differences of physical properties of the materials - aggregates: density sorter, magnetic and electric separator, optical sorter, flotation Magnetic separation: - using magnet separator for removal of ferrous components out of the waste stream (cross-flow or parallel to the conveying direction of the waste) - necessary pre-treatment: prior size reduction, gapping - often used for pre-sorting the household wastes Eddy current separation: - induction of eddy currents in metal objects in response to an electromagnetic field (e.g. to sort tin plate) principle of a magnetic separator magnetic separator
Sorting Optical sorting: - developed to sort crushed glasses into different colours (recognize the colour and use a blast of compressed air to sort out) - nowadays it can also be used to sort different kinds of plastics Flotation: - is used to remove contaminants in plastics (in Germany: used in paper mills to produce high quality graphic paper optical sorter Density Sorting: - 2 methods: float-sink method and hydrocyclones - principle: heavy fractions sink to the bottom and light fractions swim on top of the liquid medium Manual Separation: manual separation
Compaction INVENT Integrated Waste Management modules for different courses of graduate studies primary goal: reduce surface and increase bulk 2 principles are used: build-up agglomeration, compaction agglomeration important for the cost of transport and storage aggregates: bale press, compactor, pellet press, briquette press - a bale is made by pressing the waste against a movable wall - e.g. pellet presses for producing refuse derived fuels (RDF) bale press compactor for landfills pellet press
Thermal treatment INVENT Integrated Waste Management modules for different courses of graduate studies thermal treatment includes combustion, pyrolysis, hydrolysis and drying of waste goals of combustion of waste as the main method: - treatment of non-recyclable residuals - inertisation of the residuals minimization of the emissions - destruction of organic contaminants and accumulation of inorganic contaminants - decrease the volume of the waste send to landfill - energy recovery of household wastes by using the heating value - conversion residues to useable secondary raw material to prevent the resources (cycle of materials) Pyrolysis: - thermal decomposition of organic wastes under air tight to produce storable and energetic products - currently at experimental stage for treatment of household waste Hydrolysis is for treatment and utilisation of residuals of refinery processes Drying is especially used for drying sewage sludge the methods of cleaning the flue gases of the combustion processes will not be explain
Combustion INVENT Integrated Waste Management modules for different courses of graduate studies Principle: combustion of household waste (grate firing) delivery hall waste storage boiler house flue gas cleaning chimney control unit, laboratory
Combustion INVENT Integrated Waste Management modules for different courses of graduate studies Principle of the grate firing: post-combustion secondary air biomass drying degasification burnout cooling primary air ash
Combustion INVENT Integrated Waste Management modules for different courses of graduate studies Principle of a rotary kiln: - used for hazardous wastes which can be solid, liquid and pasty - rotary kiln can reach high temperatures with a long dwell period for the waste - it is necessary to install a post-combustion (1000-1200 C) behind the rotary kiln to ensure the complete combustion
Combustion flue gas Fluidized-bed combustors: - burn waste in a hot bed of granular material, such as sand - injection of air into the bed creates turbulences resembling a boiling liquid - turbulences distributes and suspends the waste - the design of a fluidized-bed reactor increases heat transfer and allows for operating temperatures below 950 C, reducing nitrogen oxide emissions - fluidized-bed combustors can handle high-ash fuels, agricultural residues and sewage sludge secondary air secondary air waste ash air combustion bed ash
Pyrolysis and gasification Pyrolysis: - conversion of the organic components of the waste with lack of O 2 and influenced by heat (150-900 C) - emergence of solid, fluid and gaseous products which can use for energy recovery - advantages: recovery of pyrolysis gas which has a high energy density and can be used to burn the emerged pyrolysis coke - currently: no established pyrolysis treatment of waste Gasification: - conversion of organic components of the waste to gaseous fuels by high temperatures and the use of a reactive gas (steam, CO 2, O 2, air) - production of lean gas or water gas for energy recovery - the produced pyrolysis coke is oxidised in the firebed (more than 800 C) with deficient of air
Hydrolysis and drying INVENT Integrated Waste Management modules for different courses of graduate studies Hydrolysis / hydrogenation: - conversion of substances (e.g. coal, chemical macromolecule, old plastics) with H 2, pressure (300 bar) and raising temperatures (440 480 C) splitting of chemical bonds with water - advantage: this method produce the best products (synthetic crude oil), produce new plastics by hydrogenation of old plastics - disadvantage: quiet expensive Drying: - high relevance for drying sludges (e.g. sewage sludge) - aggregates: fluidized-bed reactor (for stinky sludge), exhaust vapours, convective drying in an open system (for non-stinky sludge) - used mediums: hot flue gases, superheated steam, air - employed processes: - direct contact (drying medium has contact with the sludge) - indirect contact (wall between drying medium and sludge)
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