ABE 482 Environmental Engineering in Biosystems. September 29 Lecture 11

Transcription

1 ABE 482 Environmental Engineering in Biosystems September 29 Lecture 11

2 Today Gasification & Pyrolysis Waste disposal balance

3 Solid Waste Systems Solid Waste Air Limited air No air Combustion Gasification Pyrolysis Flue ash Gas + Ash + Tar C C Gas + Char + Tar O 2 supplied by air, steam or pure O 2

4 Gasification Stages Drying Grinding Size of material depends on feeding system < 5 mm for fluidized bed < 50 mm for updraft/downdraft Pyrolysis Oxidation Gasification Gas scrubbing By-product utilization

5 Pyrolysis/Gasification Pyrolysis: thermal degradation of organic waste in the absence of O 2 to produce a carbonaceous char, oil and combustible gases Gases: CO, CO 2, H 2, CH 4 CO and CO 2 derived from oxygenated structures of original material (cellulose, hemicellulose and lignins) Quantity of each product depends on process conditions (temp and heating rate)

6 Pyrolysis/Gasification Gasification: thermal process that uses high temperature to break down any carbonaceous materials Decomposes waste to produce ash or char and synthetic gas (syngas) using a controlled amount of oxygen (less O 2 than incineration) Syngas: N 2, CO, H 2 and some CO 2

7 Nexterra Gasification Technology 1: fuel in-feed system (updraft) 2: gasifier 3: ash removal system 4: syngas

8 Pyrolysis Outputs Oils derived from pyrolysis an be used in fuel applications or chemical feedstock Char can be used as a solid fuel or upgraded to activated carbon Gases have medium to high calorific values and may contain sufficient energy to supply the energy requirements of a pyrolysis plant

9 Controlling Outputs of Pyrolysis 800 Max Gas Temperature ( C) 600 Max Char Even Distribution Max Liquid Heating Rate ( C/min)

10 Calorific Values Chars MSW = 10 MJ/kg Tires = 29 MJ/kg Wood waste = 33 MJ/kg Coal = 30 MJ/kg Oils MSW = 25 MJ/kg Tires = 42 MJ/kg Petroleum-derived oil = 46 MJ/kg Gases MSW = 18 MJ/m 3 Wood waste = 16 MJ/m 3 Tires = 40 MJ/m 3 Natural gas = 37 MJ/m 3

11 Gasification O 2 reacts at high temperature with available carbon in waste to produce a gas product, ash, and a tar product Temps are higher than pyrolysis (800 to 1400 C) Introduce certain gases to encourage formation of gaseous products and release heat C+O 2 CO 2 C+H 2 O CO+H 2 C+1/2O 2 CO C+2H 2 CH 4

12 Gasification Higher temps (higher than incineration) alter the physical and chemical character of MSW Calorific values for gas 4-6 MJ/m 3 air gasification MJ/m 3 O 2 gasification Char or ash can be used as plant nutrients or for industrial purposes

13 Feedstock supply Updraft/downdraft Fluidized bed Entrained flow

14 Benefits of Gasification Gasification complements anaerobic digestion in that it is conducive to dry materials, wood by-products, tires, plastics, etc. Normally converts 90% of the organic material it receives One plant can produce 350,000 excess MW-h of electricity annually Net reduction of 500,000 tonnes of CO 2 emissions Trading around $6-$25 per tonne

15 Limitations of gasification Raw gas will contain tar, char and hydrocarbon gases so boiler or furnace system must not be susceptible to clogging Heterogenous material (MSW) produces gas with varying composition and calorific values Direct combustion: gas does not need to be cleaned or cooled Turbines or internal combustion: gas must be cleaned and cooled

16 Energy/Cost Balance Drying (energy demand) Grinding (energy demand) Gas scrubbing (energy demand) Heat/energy input to run system (provided by syngases) Oils (fuel or chemical value) Chars (fuel value) Gases (heat or electricity surplus) Carbon credits (trading or $$ surplus)

17 Waste Disposal Balance Gasification alone, takes care of all materials with an energy surplus, but eliminates nutrient rich solid that may be land applied Digestion alone, cannot handle dry or wood waste, or plastics or non-organic materials, but provides energy surplus and a nutrient rich solid that can be land applied Gasification and digestion combination takes care of all materials and provides nutrient rich solid material

18 Co-digestion and co-generation Co-digestion is the simultaneous digestion of two or more substrates Ex: hog manure and cull potatoes Benefits Improved nutrient balance, rheological properties, increased biogas production, disposal of another waste stream, etc. Drawbacks Increased mixing requirements, pre-treatments, larger digester required, etc. Co-generation refers to the generation of both heat and useful electricity from syngas

19 Gasification output calculations We are able to estimate the $ value of the anaerobic digester outputs by calculating the volume of gas produced from a specific feedstock What information do we need to make the same estimate from a gasification system?

20 Next Day Waste to energy calculations Online calculator nkins/cbc/calculator/index.html