Company for environmental technology

Transcription

1 Adsorption Chiller Energy Systems Waste Treatment Systems Research Company for environmental technology

2 PREFACE 1 PREFACE Nature does not produce any wastes. All by-products and final products of natural processes are used in a continuous cycle of the composition and mineralization of or-ganic substances. The biosphere has a high buffer potential giving it a wide tolerance range for all natural products and processes. Only with the growth of human population and its economical activities, waste became a serious danger to the steady-state of the natural metabolic processes. With the continuous growth of the cities and the concentration of an increasing part of the population in municipal areas, a solution of the waste problem becomes more and more inevitable. While a big part of the inorganic wastes - like glass, plastics, metals, etc. - meanwhile are being recycled, the biggest part of the organic waste fraction is still simply put on waste disposal sites. The uncontrolled decomposition of these materials adds another stress factor to our endangered environment. The partially anaerobic conditions cause gaseous emissions of carbondioxyd, ammonia and methane to the atmosphere, while the products of the mineralization processes contaminate the ground water with phosphates, nitrates, and other mineral salts, thus poisoning the basic resources of human life. On the other hand, organic wastes contain significant energy potentials as well as valuable plant nutrients and the capability of improving and conserving agricultural soils. For these reasons, efforts have been taken during the last decades on the development of waste processing technologies which are ecologically safe and, at the same time, make use of the valuable components and characteristics of the materials. Technologies which have been developed for the large-scale-processing of organic wastes are the composting and the anaerobic fermentation. Both methods have their specific advantages and disadvantages. The decision for one of these technologies can only be taken with regard to infrastructural, technical, and environmental conditions of the particular area GBU mbh Waste Treatment System Page 1

3 VOLUME AND COMPOSITION OF MUNICIPAL WASTES 2 VOLUME AND COMPOSITION OF MUNICIPAL WASTES The most important fraction of organic municipal wastes is the household waste. In Germany the yearly household waste production ranges from 55 to 170 kg per inhabitant, depending on the settling structure of the appropriate area. Inhabitants of larger cities produce household wastes of about 80 kg per year, while the output of households in rural areas amounts to about 50 kg per year and person in average. The share of the organic fraction on the total waste volume out of households is about 32 % of the unsorted wastes and 57% of the presorted wastes.. The total production of organic household wastes in Germany is estimated to reach 1.5 to 2 million tons per year. Besides the household wastes, there are other sources of wastes which are suited for biological treatment and have to be evaluated as basic information for further planning: parks and gardens (grass and wood cut) restaurants and hotels food processing plants breweries dairy plants slaughter houses waste water treatment plants (sludges and fats) In addition, there are wastes and by-products of the agricultural production. The suit-ability for biological treatment of these materials mainly depends on their dry matter content GBU mbh Waste Treatment System Page 2

4 VOLUME AND COMPOSITION OF MUNICIPAL WASTES Only the exact determination and quantification of all these waste-flows leads to reliable figures for the planning and dimensioning of biological treatment plants. In Germany, comprehensive statistics about each individual region allow the determination of the total amount of biogenious wastes per capita. Capacity of Biological Treatment Plants biogenious wastes in t/a bio waste sludges from waste water treatment green cuts Inhabitants 1999 GBU mbh Waste Treatment System Page 3

5 GENERAL 3 GENERAL The collection of the raw material for biological treatment is the mandatory separa-tion at the originator (citizen) or, where this is not possible, the semiautomatic sepa-ration of the garbage before it is delivered to the plant. However, only the separation done by each citizen, i.e. collection of household wastes in different garbage containments, does prevent an intensive contact with other, sometimes highly polluted waste materials, which could raise the content of nocuous matters in the organic garbage. Composition of pre-sorted Organic Waste Plastics 3% Paper 9% Wood 3% Mineral. Fraction 23% Glass 3% Metals 2% Organic Fraction 57 % The pre-separated material is delivered to the plant. The high quality required, how-ever, makes a second-stage selection an obligation GBU mbh Waste Treatment System Page 4

6 GENERAL 3.1 ANAEROBIC FERMENTATION During fermentation the disintegration of the organic substances is performed under exclusion of oxygen inside of a completely capsulated digester. Fermentation requires a liquid media with a dry-matter-content from 8-12%. Since bio-wastes have a dry-matter-content of 20-25% in average, it has to be suspended in water prior to fermen-tation. The process is divided into two principal phases: The 1st phase is a hydrolisation process where facultatively anaerobic bacteria starts the hydrolisation and disintegration of the volatile organic substances. Complex or-ganic compounds are split under production of organic acids, CO2, H2S and alcohol. Dissolved oxygen is consumed by the bacteria, nitrates and sulphates are reduced. The ph value is lowered. In the 2nd phase, methane bacteria digest the products of the metabolism of phase 1. Final products of the process are methane gas, CO2, and mineral salts. Due to the re-ducing athmospere inside the fermenter and the production of ammonium out of the anaerobic degradation of proteins, the ph-value is raising continuously. The ph-value of the fermented substrate ranges from 7.5 to 8.5. Fermentation plants can be designed as single stage fermenters, where both phases of the process are performed in one reaction chamber, or as dual-stage fermenters, where the acidification and the methane production are done in separate tanks GBU mbh Waste Treatment System Page 5

7 GENERAL Anaerobic Fermentation Process Polymere Substrates (Bio-waste suspension) Proteines, Carbonhydrates, Fats Hydrolyses Acidification Amino-acids Sugar Fatty acids Fermenting Bacteria Fermenting Bacteria Organic acids Alcohols Acidification Acetogene Bacteria Acetic acid Hydrogen Carbondioxyd Methanisation Methanogene Bacteria Methane Carbondioxyd = BIOGAS Anaerobic Fermentation Process In contrary to the fast growing aerobic bacteria, methane bacteria are not present in the initial substrate, but have to be bred inside the fermenter. Degradation rates of organic substances are very high in between the temperature ranges where methane bacteria have their maximum activity. Mesophilic methane bacteria have a tempera-ture optimum of about 32 C to 35 C, while thermophilic stems require substrate tem-peratures of 50 C to 55 C. Under present conditions, this capacity will last for 15 to 20 years. Since 35% of the capacity is already used, the remaining area will be filled up within 10 years under static conditions. Beside the temperature, degradation rates depend mainly on volatility of the organic substances, the specific organic load of the fermenter (kg of organic dry matter per m3 of fermenter volume) and the retention time of the substrate inside the fermenter. Gas yields are directly correlated with the decomposition rates. Each substrate has a specific gas yield expressed in litres of biogas produced per kg of decomposed organic matter. In case of pre-sorted bio-wastes, average gas yields can be calculated as follows: 1999 GBU mbh Waste Treatment System Page 6

8 GENERAL Dry-matter- content of the bio-wastes (DM) 25% Organic dry matter content (ODM) 75% of DM Retention time inside the digester 20 d Specific load of the digester max. 4 kg ODM/m 3 Expected decomposition rate of ODM 70% Specific gas yield 900 l/kg ODM average gas yield from 1 kg of bio-waste (fresh material): = 0.25 x 0.75 x 0.7 x 900 = 118 l average gas yield per citizen and year (80 kg bio waste): = 80 x 0.25 x 0.75 x 0.7 x 900 / 1000 = 9.45 Nm 3 /y Calculation of Biogas Yield "Biogas" is a water saturated gas mixture containing about 65-75% methane, 20-30% carbondioxyd and small quantities of hydrogen sulphide and ammonia. A typical analysis of biogas is shown in the following table: CH 4 CO 2 70 vol% 29 vol% Total Cl 0,9 mg/m 3 Total F 0,5 mg/m 3 H 2 S 419 mg/m 3 Biogas Analysis The average energy content of the biogas is about 7 kwh/nm3. In larger plants the gas is used as fuel for cogenerating systems producing electricity and hot water. Mod-ern cogenerators operating with an overall efficiency of about 90% produce 2.24 kwh electrical energy (= 32%) and about 4 kwh of thermal energy (= 58%) out of 1 Nm3 of biogas. The thermal energy produced is partly used for the heating of the bio-waste suspen-sion up to 35 C. The biggest part of the electrical energy can be fed into the public electricity network. The effluent of the fermenter is a homogeneous, nearly odourless liquid with a dry matter content of about 5-8% and a temperature of 35 C. There is only little reduction in volume (approx. 2%). As a result of the mineralization, plant nutrients are con-verted in mineral salts, which are well available to the plants. The substrate might be either used directly as a liquid fertilizer or composted after mixing with paper, straw, rice shells or other residues from agricultural plant prodution GBU mbh Waste Treatment System Page 7

9 GENERAL Advantages of the fermentation process: use of a source of renewable energy reduction of CO2 emission high degree of automatisation and comparitively low running costs stable and easy to control process no odour annoyance little space requirements immobilising heavy metals through precipitation in the reactor Following items have to be taken under consideration: needs of process water no decomposition of lignin (wood, paper, etc.) no reduction of volume 1999 GBU mbh Waste Treatment System Page 8

10 4 TECHNIQUES 4.1 FERMENTATION PLANTS In connection with increasing air pollution, especially the discussion about carbondi-oxyd- and methane emissions, the use of renewable energy sources became more and more important. However, bio-wastes as a source for energy have been discovered just some years ago, and only a few plants are operating at present. The plants differ mainly in the ways of preconditioning the bio-waste for the ferment-ing process, in the type of methane reactors, and in the processing of the fermented substrate. Principal procedures are as follows: Take-over Station Pre-Crushing (Screw Mill) Homogenisation Suspension Metals, Glass Dump Site Hygienisation Separation Plastics Process Wood, cuts etc Composting Water Acidification Methanisation Biogas Dewatering Cogenerator Hot Water Electricity Fertilizer Public Network Compost Anaerobic Process 1999 GBU mbh Waste Treatment System Page 9

11 The following picture shows a modern fermentation plant for municipal wastes from GBU, Germany. The well fermented substrate is directly delivered to the farms as a substitute for mineral fertilizers or - as an option - might be mixed with structured material as straw, paper, rice shells etc. and be composted. The gas is gathered inside a pressure-less gas holder and used for the production of electricity and hot water by means of a cogenerator. The hot water is partly used to heat up the biogas suspension and to maintain the re-actor temperature. Surplus heat might be used for heating purposes ore - in connec-tion with an adsorption chiller - for cooling and climatization. Since no dewatering station is foreseen, the need of electrical energy by the process is very low. Thus most of the electricity produced can be sold to the public network GBU mbh Waste Treatment System Page 10

12 1999 GBU mbh Waste Treatment System Page 11

13 Take-over Station Take over procedure is the same as described before. In addition, there is the possibil-ity to take over liquid wastes like fats, restaurant garbage and residues from indus-trial processes and slaughter houses which are stored in separate tanks. Pre-Crushing Pre-crushing is necessary only to open plastic bags and other packing. It is done by a slow running screw mill producing a particle size of approx. 40 mm. The final milling and homogen-isation takes place in the next step. Since pre-separated garbage is delivered, the separation is done automatically. The screw mill is placed on top of the suspending unit and is fed with the bio waste by a crane. Homogenisation, Suspension, and Separation This is done in one single stage by means of a special suspending unit consisting of a steel tank with a built in mixer. The pre-crushed material is fed from the top and di-luted with the process water up to a dry matter content of about 10 %. During the di-luting process the pre-crushed material is intensively mixed and suspended. In order to achieve better solu-bility of fats and proteins and for hygienisation purposes, the suspension is heated up to about 70 C. Due to the intensive movement of the liquid, small plastic particles, wood, and paper are separated from adhesive fats etc. and float. The floating fraction is removed from the surface by an especially designed screen and conveyed to a container. Glass and metals are separated from the liquid by sedimentation. They are removed from the bottom of the tank and gahered in a separated container. As a result, a homogeneous suspension of water and organic material is created which is free of foreign matters. This suspension with a dry-matter-content of about 10% is pumped to a buffer tank to allow a constant feeding of the rector over 24 h/d. From the buffer tank the suspension can be either pumped directly into the methane reactor or mixed with other liquid wastes prior to fermentation GBU mbh Waste Treatment System Page 12

14 Pre-treatment - Station GBU-System 1999 GBU mbh Waste Treatment System Page 13

15 Acidification and Methanisation The technology of biogas production is a complex one, since biological processes need to be optimised taking individual structural and hydraulic requirements into account. Perfect thermostatization, continuous blending, homogenisation, reduction and injec-tion of the substrate are all vital preconditions. Construction Principle of the Digester Gasdome 2 Gas Outlet 3 Substrate Outlet 4 Heating Pipes 5 Main Fermentation Chamber 6 Post Fermentation Chamber 7 Main Fermentation Chamber 8 Sludge Withdrawel 9 Mixers 10 Man whole 11 Insulation 12 Mixing Shafts 13 Feeding Pipe 14 Gas Shut-Off Walve 15 Pressure-Equalizing Pipe Biogas Reactor The GBU reactor performs all these functions, featuring optimal substrate management and blending without moving parts or additional energy. The gas produced causes the pressure in the main fermenting chamber to rise, which in turn leads to a drop in the fluid level combined with a rise in the level in the secondary fermenting chamber GBU mbh Waste Treatment System Page 14

16 Once the two chambers have reached a certain predetermined level, the gas mixing flap opens, causing instantaneous pressure equalisation. The re-turning substrate is guided in such a way that it destroys both surface scum and sediment layers and ensures that the mixture is reliably blended. Thus, feeding and blending of the fermentation material is achieved automatically without any use of ex-ternal energy. Due to the structural principle of 2 separated fermentation chambers a controlled flow of material is achieved and mixing of freshly fed substrate with already fermented substrate is prevented. Therefore, the effluent of the reactor does not contain any fresh material causing odour annoyance. The reactor operates fully automatical and nearly maintenance free GBU mbh Waste Treatment System Page 15

17 Interconnection of Multiple Digesters 1999 GBU mbh Waste Treatment System Page 16

18 Gas Utilisation The biogas obtained is gathered in a pressureless gas holder consisting of a standard silo which contains a PE foil bag. The PE bag is filled by the gas overflow from the re-actor and emptied by special gas blowers which transport the gas to the consumers. The gas holder is equipped with an automatically working control and security device GBU mbh Waste Treatment System Page 17

19 For security reasons, a gas flare is foreseen, in case that the produced biogas cannot be used temporaryly. The gas is finally burned in a heat-recoverygenerator set (cogenerator) consisting of a adapted diesel engine and a synchron generator. The engine heat and the heat of the exhaust gases are recovered by heat exchangers and converted into hot water GBU mbh Waste Treatment System Page 18

20 Modern cogenerator sets work with an overall efficiency of approx. 90% i.e. 1 Nm3 of biogas - containing 7 kwh - will be converted in 2.24 kwh of electricity and 4 kwh of thermal energy in form of hot water. The generator works fully automatical under control of a computer according to the gas level in the gas holder. The electricity obtained normally is fed into the public network, but also can be used locally. The biogas obtained is gathered in a pressureless gas holder consisting of a standard silo which contains a PE foil bag. The PE bag is filled by the gas overflow from the re-actor and emptied by special gas blowers which transport the gas to the consumers. The gas holder is equipped with an automatically working control and security device. The hot water produced is partly used for the heating of the reactor and for the hygienisation of the bio wastes. However, this requires not more than 30-40% of the thermal energy produced. The surplus heat therefore might be used for other heating or drying purposes or, where this is not possible, might be converted into cold water for cooling and/or climatization by an adsorption chilling machine GBU mbh Waste Treatment System Page 19

21 GBU Gas Utilisation 1999 GBU mbh Waste Treatment System Page 20

22 4.2 Treatment of the Fermented Substrate The further treatment of the fermented substrate mainly depends on local conditions, especially on the market for the final products. Therefore two alternatives are presented. Evaporation After decantation the solid fraction of the effluent is composted. The liquids are stored in a buffer tank where they are acidificated. After adjustment of the ph to about 6 they are concentrated by a specially designed fluidized-bedevaporation plant up to a dry matter content of 30%. The concentrate is composted together with the solid fraction and the sorted out organics. There is no waste water from this process. Evaporation-Granulation After the treatment as described under "Evaporation" all of the material is dried and granulated in a combined granulation and drying plant. The final product is a dry granulate with a dry matter content of more than 90% which can be stored, handled and applied like a mineral fertilizer GBU mbh Waste Treatment System Page 21

23 Sketch of a Plant with Waste Water Treatment 1999 GBU mbh Waste Treatment System Page 22

24 Evaporation The integration of an evaporation plant offers the advantage, that no waste water will be released to the environment. The evaporation plant uses the surplus heat out of the energy production for the concentration of the digester effluent up to a dry matter content where it is suited for composting (about 30%). Major components of the system are: Decanter Station Buffer- and Pre-acidification Tank Evaporation Plant From the decanter station the liquid fraction is pumped into a capsulated buffer and acidification tank. The fermentation process generates substantial amounts of CO 2 which partly is dissolved in the liquid phase. During the concentration process in the evaporators this CO 2 is set free rapidly causing foam problems inside the evaporators. It has to be removed anyway by adjusting the ph - value of the liquid to a value of about 6 in order to expel the diluted CO 2. This is done by adding specific amounts of sulphuric acid into the pre-acidification tank. The liquid inside the tank is agitated and the automatic dosage is controlled by a ph sensor. The fermented substrate shows a reasonable concentration of ammonium (2-4%) as all nitrogen, originally contained in the biowaste, is converted into this form by the fermentation process. The low ph is also needed to avoid gaseous emission of ammonia during evaporation. Since ammonium/ammoniac is a strong ph-buffer system, the amount of sulphuric acid needed to lower the ph is directly determined by the ammonium content of the substrate. The consumption of acid equals - as a rough estimation - three times the weight of the ammonium in the effluent GBU mbh Waste Treatment System Page 23

25 The tank is a corrosion safe construction of glass enamelled steel. From the buffer tank the liquid is pumped to the evaporation plant, where it is concentrated by distillation in a 4-stage process under reduced pressure. The final dry matter content is about 30-35%. After concentration it can be mixed with wood, paper, straw, etc. and composted. The evaporation process requires hot steam supply (180 C) from a steam boiler in addition to the surplus thermal energy from the gas engine or turbine. The water out of the evaporators is regained by condensation and can be released into the environment without any risk or may be used as process water inside the plant. The evaporation plant is installed inside a building with a size of 26 m x 9 m and a height of 12 m. The operation principle of the 4-stage evaporation plant is shown in the following scheme. The liquid passes through a preheating unit before it enters the heat exchanger of the first evaporation stage (W3) at a temperature of 85 C. From here it is subsequently pumped to W4, W2 and W1. From the heat exchanger of the last stage (W4) the concentrate and the condensed water is withdrawn GBU mbh Waste Treatment System Page 24

26 Scheme of the Evaporation Plant The evaporators of all of the 4 stages are designed as fluidized bed evaporators. Fluidized bed evaporators are suitable for the concentration of liquids which exhibit severe fouling. They operate on the forced circulation principle and have vertical calandrias and are characterised by the fluidized bed calandria. In its tubes, particles, e.g. glass pellets, ceramic particles or steel cylinder particles, are put into motion by the liquid flowing upwards thus continuously cleaning the heat exchanger surface. The fluidized material is recirculated to the inlet chamber via suitable fluidic devices so that it remains in the calandria. The motion of the particles results in the avoidance of critical over-concentrations and temperatures and the heating surfaces are cleaned from encrustations, as they are in permanent contact with these particles. In contrast to tube bundle apparatuses, standard fluidized bed calandrias have the following main construction features: 1999 GBU mbh Waste Treatment System Page 25

27 Less pollution and encrustation of heat exchanger surfaces, therefore less maintenance requirement. Suitable inlet valves so that fluidized matters cannot flow into the feed lines. Inserts that influence the flow of the fluidized material. Enlarged outlet chamber so that process liquid and fluidized matters are separated by sedimentation. Fluidized bed evaporators can also be used for high viscosity liquids, like fermented sludges, by appropriate selection of the fluidized material. Therefore they are suitable as high concentrators in multi-stage evaporation plants. Evaporation/Granulation The most sophisticated technology for the treatment of the fermented substrate is the combined drying and granulation process, offering a homogenous and dry granulate as the final product and a maximum reduction of volume. This process, however, requires rather high additional investments. It has a huge demand of process energy thus limiting the amount of energy that can be sold. Major components of the system are: Decanter Station Buffer- and Pre-acidification Tank Evaporation Plant Fluidized Bed Granulation Dryer Buffer Silo for the Granulate 1999 GBU mbh Waste Treatment System Page 26

28 The treatment of the digester effluent is the same as described under "Evaporation" but the output of the evaporation plant is not composted but dried and granulated. Prior to drying the concentrated liquids are mixed with the solids out of the decanter station. This mixture is injected into the fluidized bed drier. The ready granulate contains the organic substance and all mineral salts - which mainly are valuable plant nutrients - out of the fermented organic material. It is easy to handle and to store and can be applied to the field like a mineral fertilizer. Furthermore, it easily can be customised concerning its nutrient contents by adding mineral fertilizers during or before the drying process. The production of fertilizer granulate is about 20 tons per day. The installation requires a building of 26 x 16 m with a height of 12 m and additional storage facilities (30 m x 10 m) GBU mbh Waste Treatment System Page 27

29 INPUT Concentrate from evaporation plant LC M Mixing tank INPUT Solids from decanter INPUT Fresh air Air heater Heating steam circulation OUTPUT Condensate TC Filter Condensator Granulation dryer Product chiller OUTPUT Granulate Fluidized Bed Granulation Drier The fluidized bed drier provides high rates of heat and mass transfer as well as homogeneous conditions in the process chamber. This produces a uniform end product of high quality. The fluidized bed is generated by a suction and/or pressure fan. The fluidization medium is hot steam in a closed circuit. At the start of the drying process, the fluidization vessel is refilled with seed particles on which the fluidized bed is built up. A heat transfer system supplies the required thermal energy GBU mbh Waste Treatment System Page 28

30 Function Scheme Fluidized bed drier discharge unit. The substrate is held in an intermediate storage tank from which it is pumped to the fluidization vessel. Here, it is sprayed into the bed. At this point, granules begin to form in the fluidization vessel. When the required particle size is reached, granules are continuously discharged via a classifying Because of the nature of the process, the medium discharged from the fluidization vessel contains fine particles. This material is separated in a cyclone with a hose filter downstream. The cyclone product is returned to the fluidization vessel. The liquid entering the plant with the feed is cleaned by a cyclone and used for the heating of the evaporation unit. The exhaust air is eliminated via the dust filter downstream. Fluidized bed dryers are characterised by a number of important features: High quality of the finished product. High density of solid matter in the dried material. Avoidance of damage to the heat sensitive organic substance. Capability of generating customised products. Simple erection. Absence of parts subject to wear and tear, if designed accordingly. Good corrosion resistance. Low space requirements. The flow through the fluidized bed is continuous. The requested granulate size ranges are achieved by classified continuous discharge from the bed GBU mbh Waste Treatment System Page 29

31 GBU mbh Wiesenstrasse 5 D Bensheim GERMANY Tel.: Fax info@gbunet.de GBU Hellas Co Ltd. 47, Trias Str. GR Athens GREECE Tel.: Fax dpaschos@hellasnet.gr