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8/5/2016
1
Waste to energy
• process of generating energy in the form of electricity
and/or heat from the primary treatmen...
8/5/2016
2
A PARADIGMATIC SHIFT IS REQUIRED
WtE is part of the “Urban Mining Process”
8/5/2016
3
..which is part of “Circular Economy”
Circular economy
generic term for an urban/industrial economy that is pro...
8/5/2016
4
Circular economy - Basic principles
Waste is (someone’s/something’s) food
Waste does not exist… the biological ...
8/5/2016
5
WASTEWATER
PROCESS
RESIDUES
HAVE A PLACE
IN CIRCULAR
ECONOMY AND
WtE / WtR
COMBUSTION
Combustion of organic mat...
8/5/2016
6
COMBUSTION (2)
A critic is that incinerators destroy valuable resources and they
reduce incentives for recyclin...
8/5/2016
7
COMBUSTION (4)
CURRENT TECHNOLOGY - MOVING GRATE INCINERATORS
COMBUSTION (5)
CURRENT TECHNOLOGY - FLUIDIZED BED...
8/5/2016
8
COMBUSTION (6)
CURRENT TECHNOLOGY - ROTARY-KILN
Used by municipalities and by large industrial plants. This des...
8/5/2016
9
GASIFICATION (2)
Waste gasification has the following advantages over incineration:
• flue gas cleaning may be ...
8/5/2016
10
GASIFICATION (4)
In addition, reversible gas-phase water-gas shift reaction reaches equilibrium
very fast at t...
8/5/2016
11
GASSIFICATION (6)
CURRENT TECHNOLOGY – NIPPON STEEL
A patented spurious process derived from the blast furnace...
8/5/2016
12
PYROLYSIS
Thermochemical decomposition of organic material at elevated
temperatures in the absence of oxygen (...
8/5/2016
13
PYROLYSIS (3)
Yields grater than 10% oil to sludge (in volume) where obtained
using pre-digested sludge.
Based...
8/5/2016
14
CO-DIGESTION (2)
Merits of Co-digestion
Ecological, technological and economical advantages:
Improved nutrient...
8/5/2016
15
CO-DIGESTION (4)
Feed waste’s C:N ratio
is the main factor in
determining final biogas
yield.
CO-DIGESTION (5)...
8/5/2016
16
COMPOSTING
mmmmmm
COMPOSTING (2)
composting consists of making a heap of wetted organic matter
(green waste: l...
8/5/2016
17
COMPOSTING (3)
Compost is rich in nutrients and thus it is used in gardens,
landscaping, horticulture, and agr...
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Lectures Capodaglio - Session 4

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Sustainable Water - Energy - Centric Communities school
May 9 - 13, 2016 – Lake Como School of Advanced Studies

Published in: Science
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Lectures Capodaglio - Session 4

  1. 1. 8/5/2016 1 Waste to energy • process of generating energy in the form of electricity and/or heat from the primary treatment of waste. • form of energy recovery. WtE processes can produce electricity and/or heat directly (combustion), or produce a combustible fuel commodity, such as methane, methanol, ethanol, synthetic (gas/liquid) fuels. • various technologies applicable to all types of waste (urban, industrial, etc…) • some processes allow the additional recovery of secondary raw materials that can be reused in industrial production or other environmental applications
  2. 2. 8/5/2016 2 A PARADIGMATIC SHIFT IS REQUIRED WtE is part of the “Urban Mining Process”
  3. 3. 8/5/2016 3 ..which is part of “Circular Economy” Circular economy generic term for an urban/industrial economy that is producing no waste and pollution, by design or intention, and in which material flows are of two types, biological nutrients, designed to re-enter the biosphere safely, and technical nutrients, which are designed to circulate in the production system without entering the biosphere. It is restorative and regenerative by design. HOWEVER Circular economy doesn’t just “happen”. Proper economic, regulatory and public awareness frameworks must exists and be locally and globally consolidated (i.e. competition with traditional economy, raw materials and product standards, public acceptance of waste-derived products).
  4. 4. 8/5/2016 4 Circular economy - Basic principles Waste is (someone’s/something’s) food Waste does not exist… the biological and technical components (nutrients) of a product are designed by intention to fit within a materials cycle, designed for disassembly and re-purposing. Non- toxic biological nutrients can be simply returned to the biosphere. Technical nutrients – polymers, alloys and other man-made materials are designed to be used again with minimal energy input. Diversity is strength Modularity, versatility and adaptiveness are to be prioritised in an uncertain and fast evolving world. In working toward the circular economy, we should focus on longer-lasting products, developed for upgrade, ageing and repair, considering strategies like emotionally durable design. Circular economy - Basic principles (2) Energy must come from renewable sources As in life, any system should ultimately aim to run on ‘current sunshine’ and generate energy through renewable sources. Systems thinking The ability to understand how things influence one another within a whole. Systems thinking usually refers to nonlinear systems: systems where through feedback and imprecise starting conditions the outcome is not necessarily proportional to the input and where evolution of the system is possible, like in are all living systems. Prices or other feedback mechanisms should reflect real costs Prices are “messages”, and therefore need to reflect full costs in order to be effective, including those of negative externalities.
  5. 5. 8/5/2016 5 WASTEWATER PROCESS RESIDUES HAVE A PLACE IN CIRCULAR ECONOMY AND WtE / WtR COMBUSTION Combustion of organic material with energy recovery, is the most common and old WtE technique (first plant in Nottingham, 1874). Modern incinerators reduce the volume of the original waste by 95-96 percent, depending upon composition and degree of recovery of materials (e.g. metals) from ash. • They are commonly used for MSW, including dried WWTP sludge, disposal. • May emit fine particulate, heavy metals, trace dioxin and acid gas, even though these emissions are relatively low in modern facilities.
  6. 6. 8/5/2016 6 COMBUSTION (2) A critic is that incinerators destroy valuable resources and they reduce incentives for recycling. The issue, however, is open, since EU countries recycling the most (up to 70%) also incinerate all their residual waste to avoid landfilling. Incinerators have electric efficiencies of 14-28%. The rest of the energy, (thermal cogeneration) can be used for district heating or industrial uses. Total efficiencies of cogenerating plants are typically higher than 80% (based on the lower heating value –LHV- of waste). Proper flue gas treatment is necessary to comply with air emission regulations. Incinerators are among the least accepted environmental installations by the public. COMBUSTION (3) CURRENT TECHNOLOGY - MOVING GRATE INCINERATORS typical for MSW. A moving grate enables movement of waste through the combustion chamber, optimised to allow more efficient and complete combustion. A moving grate boiler can handle up to 35 tons of waste per hour, operate 8,000 hours/yr. Must be designed to ensure that flue gases reach a temperature of at least 850 °C for 2 seconds in order to ensure proper breakdown of toxic organic substances (dioxins). It is therefore required to install backup auxiliary burners into the boiler in case the waste LHV becomes too low to reach this temperature. Flue gases are then cooled in superheaters, where heat is transferred to steam, for the electricity generation in a turbine. After, the flue gas at a temperature of around 200 °C is passed to the cleaning system.
  7. 7. 8/5/2016 7 COMBUSTION (4) CURRENT TECHNOLOGY - MOVING GRATE INCINERATORS COMBUSTION (5) CURRENT TECHNOLOGY - FLUIDIZED BED The fluidized bed is created by a strong airflow forced through a sandbed, seeping through the sand until the point where sand particles separate to let the air through and mixing and churning occurs. Fuel and waste are then introduced. Sand and pre-treated waste and/or fuel are kept suspended on pumped air currents and take on a fluid-like character. The bed is violently mixed and agitated keeping small inert particles and air in a fluid-like state. This allows all of the mass of waste, fuel and sand to be circulated through the furnace.
  8. 8. 8/5/2016 8 COMBUSTION (6) CURRENT TECHNOLOGY - ROTARY-KILN Used by municipalities and by large industrial plants. This design of incinerator has 2 chambers. The primary chamber in a rotary kiln incinerator consist of an inclined refractory lined cylindrical tube. Movement of the cylinder on its axis facilitates movement of waste. In the primary chamber, there is conversion of solid fraction to gases, through volatilization, destructive distillation and partial combustion reactions. The secondary chamber is necessary to complete gas phase combustion reactions. Clinkers spill out at the end of the cylinder. A tall flue-gas stack, fan, or steam jet supplies the needed draft. Ash drops through the grate, but many particles are carried along with the hot gases. These particles and any combustible gases may be combusted in an "afterburner". GASIFICATION Process that converts organic carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide, by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas (synthesis gas) is itself a fuel. The power derived from gasification and combustion of the syngas is considered to be renewable energy if the gasified compounds were obtained from waste or renewable biomass. In the XIX century, so called “city gas” was obtained with gasification of coal.
  9. 9. 8/5/2016 9 GASIFICATION (2) Waste gasification has the following advantages over incineration: • flue gas cleaning may be performed on the syngas instead of the much larger volume of combustion flue gas. • electric power may be generated in engines and gas turbines, which are much cheaper and more efficient than the steam cycle used in incineration. Fuel cells could potentially be used, but they have stringent requirements regarding the purity of the syngas. • Chemical processing (Gas to liquid) of syngas may produce other synthetic (liquid) fuels instead of electricity. Major challenge for gasification technologies is to reach a positive gross electric efficiency. The high efficiency of converting syngas to electric power is counteracted by significant power consumption in waste pre-processing, consumption of large amounts of pure oxygen (which is often used as gasification agent), and gas cleaning. Another challenge apparent in real life is to obtain long service intervals in the plants, so that it is not necessary to shut down the plant frequently for cleaning the reactor. Some people consider gasification an "incineration in disguise“, and arguing that the technology is still dangerous to air quality and public health. GASIFICATION (3) THE PROCESS In a gasifier, carbonaceous material undergoes several different reactions: Dehydration at around 100 °C. The resulting steam is mixed into the gas flow and may be involved with subsequent reactions Pyrolysis of carbonaceous matter. Pyrolysis (or devolatilization) process occurs at around 200-300 °C. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. Combustion occurs as volatile products and some of the char react with oxygen to primarily form carbon dioxide and small amounts of carbon monoxide, which provides heat for the subsequent gasification reactions. Basic reaction C + O2 → CO2 Gasification occurs as the char reacts with steam to produce carbon monoxide and hydrogen, C + H20 → H2 + CO
  10. 10. 8/5/2016 10 GASIFICATION (4) In addition, reversible gas-phase water-gas shift reaction reaches equilibrium very fast at the temperatures in the gasifier. This balances concentrations of carbon monoxide, steam, carbon dioxide and hydrogen CO + H2O ↔ CO2 + H2 In practice, a limited amount of oxygen (air) is introduced into the reactor to allow some of the organic material to be "burned" to produce carbon dioxide and energy, which drives a second reaction that converts further organic material to hydrogen and additional carbon dioxide. Further reactions occur to form methane and excess carbon dioxide 4 CO + 2 H2O → CH4 + 3 CO2 in reactors with high residence time of gases and organic materials, heat and pressure. Catalysts are used in sophisticated reactors to improve reaction rates, moving the system closer to equilibrium. GASIFICATION (5) CURRENT TECHNOLOGY
  11. 11. 8/5/2016 11 GASSIFICATION (6) CURRENT TECHNOLOGY – NIPPON STEEL A patented spurious process derived from the blast furnace iron making technology, 33 operating plants in Japan and 2 in S. Korea. It is a fixed bed system, where coal (about 5% in mass) is added to the waste . Temperature at the bottom of the furnace reaches in excess of 1300oC, yielding molten metals and vitrified (inert) slag as final residues. Organic waste content is decomposed in the mid section of the furnace into Syngas, that is burned in a subsequent plant section. Overall energetic efficiency around 23%. GASSIFICATION (7) CURRENT TECHNOLOGY – FLUIDIZED BED MELTING FURNACE
  12. 12. 8/5/2016 12 PYROLYSIS Thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen (or any halogen). Involves the simultaneous change of chemical composition and physical phase, and is irreversible. In general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization. • has been used since ancient times for turning wood into charcoal on industrial scale, and into coke for steelmaking • carbon fiber items are often produced from fibers of a suitable polymer, pyrolyzed at high temperature (1500–3000 °C) • basis of methods developed for producing fuel from biomass • anhydrous pyrolysis used to produce diesel-like fuel from plastic waste • pyrolysis of waste tires allows the high energy content of the tire to be recovered as fuel: 38-56% oil (benzene, diesel, kerosene, fuel oil and heavy fuel oil), 10-30% gas (natural-gas like) and 14-56% char. PYROLYSIS (2) Microwave-assisted pyrolysis can be used for further valorization of urban waste sludge, otherwise destined to final disposal, with high yields of a biodiesel-like product. Max Temp. oC Time at TMAX [min] Total process Time [min] % oil to total sludge % oil to sludge org. fraction TQ 60 = = 3.57* 7* TEST 1 270 20 55 9.68 19 TEST 2 180 28 50 3.30 7 TEST 3 400 5 55 8.64 17 TEST 4 490 1 54 10.25 21 TEST 5 600 3 56 8.71 17 TEST 6 400 6 46 11.79 24 TEST 7 500 9 51 7.63 15 TEST 8 650 - 60 7.38 15 TEST 9 280 2 8 12.52 25 TEST 10 400 2 18 10.77 22
  13. 13. 8/5/2016 13 PYROLYSIS (3) Yields grater than 10% oil to sludge (in volume) where obtained using pre-digested sludge. Based on the oil energy content, the process –once optimized- can yield energy recoveries greater than 100%. Test no./ Temp. oC Emitted Energy [kJ] Reflected energy [kJ] Absorbed Energy [kJ] Oil yield [g] Recoverable Energy [kJ] Process Efficiency Gross [%] Process Efficiency Net [%] 1/ 270 49.36 13.96 35.40 1.7164 56.6 114.67 159.89 2/ 180 65.94 23.82 42.11 1.7894 59.1 89.63 140.35 3/ 400 47.22 25.08 22.14 1.5621 51.5 109.06 232.61 4/ 490 76.07 31.85 44.22 1.0775 35.6 46.80 82.54 5/ 600 44.09 21.53 22.56 1.1365 37.5 85.05 166.22 6/ 400 90.64 62.7 27.94 1.8413 60.8 67.09 217.61 7/ 500 60.19 18.79 41.40 1.022 33.7 55.99 81.40 8/ 650 94.51 33.31 61.20 0.86221 28.4 30.05 46.40 9/ 280 92.99 19.17 73.82 1.4137 46.7 50.22 63.26 10/ 400 134.62 33.34 101.28 1.4077 46.6 34.61 46.01 CO-DIGESTION Simultaneous digestion of a homogenous mixture of two or more substrates, e.g. manure or sewage sludge, mixed and digested together with amounts of a single, or a variety of additional substrates. The expression is applied independently of the types and ratio of the respective substrates used simultaneously.
  14. 14. 8/5/2016 14 CO-DIGESTION (2) Merits of Co-digestion Ecological, technological and economical advantages: Improved nutrient balance. Digestion of a variety of substrates instead of a single waste type improves the nutrient ratio of TOC:N:P (optimally should be around 300:5:1), and maintains a good mix of minerals (Na, K, Mg, Mn, ...) and balanced composition of trace metals, helping maintaining stable and reliable digestion performance and good fertilizer quality of the digestate. Optimisation of rheological qualities. Wastes with poor fluid dynamics, aggregating, particulate or bulking, and floating wastes can be much easier digested after homogenisation with dilute substrate such as sewage sludge or liquid manure. Mixing of different substrates allows flexibility to compensate for seasonal mass waste fluctuations, thus underloading/overloading of digesters can be avoided and digestion process can be maintained at constant rate. CO-DIGESTION (3) Merits of Co-digestion (cont’d) Gate fees and biogas recovery. Application of co-substrates can considerably improve the economics (payback time) of a plant. Gate fees are significantly lower at farm-scale digestion plants than at an incineration or composting facility (usually by a factor of 3-4). The operator takes credit of the increased biogas production and the income from the gate fee. Provided there is sufficient farmland available, digestate can be directly recycled as a fertilizer at reasonable cost. Energy crops as co-substrate. In a limiting situation of other wastes, energy crops might become an interesting alternative, especially when plants are grown on fallow or set-aside land which attracts subsidies. A number of crops (food AND non-food) demonstrate good biogas potentials.
  15. 15. 8/5/2016 15 CO-DIGESTION (4) Feed waste’s C:N ratio is the main factor in determining final biogas yield. CO-DIGESTION (5) Nowadays, a small part of codigested organic waste consists of sewage sludge residues in the EU.
  16. 16. 8/5/2016 16 COMPOSTING mmmmmm COMPOSTING (2) composting consists of making a heap of wetted organic matter (green waste: leaves, food waste) and waiting for the materials to break down into humus after a period of weeks or months. Modern, composting is a closely monitored process with measured inputs of water, air, and carbon- and nitrogen-rich materials. The decomposition is aided by shredding the plant matter, adding water and ensuring aeration by regularly turning the mixture.
  17. 17. 8/5/2016 17 COMPOSTING (3) Compost is rich in nutrients and thus it is used in gardens, landscaping, horticulture, and agriculture. The compost itself is beneficial for the land in many ways, including as a soil conditioner, a fertilizer, addition of vital humus or humic acids, and as a natural pesticide for soil. In ecosystems, compost is useful for erosion control, land and stream reclamation, wetland construction, and as landfill cover (see compost uses). However, among the possible negative effects of compost application to cropland is the potential release of toxic heavy metals into the environment, and the transfer to these elements from the soil into the food chain. mmmmmm

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