and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 6 (2007), p-33-40
Thermal conversion of municipal organic waste
and sewage sludge to hydrogen
Leszczyński S.
Akademia Górniczo-Hutnicza, Katedra Techniki Cieplnej i Ochrony Środowiska - Kraków
Abstract
Annually in Poland comes into being about 4500 thousand Mg of organic wastes and over 350 thousand tons dry masses of municipal sewage sludge. In most cases they are deposited on municipal landfills determining serious hazard for natural environment. This paper presents possibilities of hydrogen production from municipal organic waste and sewage sludge in the thermal two-stage process (outgassing and gasification). Experiments were carried out in low and high temperature under atmospheric pressure. The gas product during utilization was quantitatively and qualitatively analyzed, hydrogen production ratios was over 20%. Hydrogen as a final product after utilization organic waste can be used directly for powering Otto engines or fuel cells achieving high thermodynamic efficiencies and relatively low environmental impacts.
Streszczenie
Termiczne przekształcenie komunalnych odpadów organicznych oraz osadów
ściekowych w wodór
Rocznie w Polsce powstaje około 4500 tys. Mg odpadów organicznych i ponad 350 tys. ton suchej masy komunalnych osadów ściekowych. W przeważającej liczbie przypadków trafiają one bez specjalnej obróbki na komunalne wysypisko odpadów stanowiąc poważne zagrożenie dla środowiska naturalnego. W niniejszym artykule zaprezentowano możliwość produkcji wodoru z organicznych odpadów komunalnych i osadów ściekowych w dwuetapowym termicznym procesie odgazowania i zgazowania wsadu. Eksperymenty prowadzono jako proces nisko i wysokotemperaturowy przy ciśnieniu atmosferycznym. Poprocesowe produkty gazowe poddano analizie ilościowej i jakościowej osiągając ponad 20% wskaźnik produkcji wodoru. Wodór jako czyste paliwo może być efektywnie wykorzystywany w silnika i ogniwach paliwowych bez szkodliwego wpływu na środowisko naturalne.
1. Introduction
Sewage sludge is a by-product of the municipal and industrial wastewater treatment plants. It is the residual slurry of settleable solids. In recent decades, environmental issues have increasingly focused on sewage sludge treatment because wastewater treatment standards have become more stringent. Annually, in Poland produces over 350 thousand tons dry masses of municipal sewage sludge and 1 million tons dry masses of industrial sewage sludge [1]. Additional hazard problem for natural environment concern organic waste originated from municipal waste as a secondary stream of biomass. In Poland 98.5% of municipal waste deposited on landfills but volume of landfilling biodegradable waste should decrease in next years. In relation to mass of waste produced in 1995 year (4400 thousand Mg) quantity should reduce about 25% to 2010 year, about 50% to 2013 and about 65% to 2020 [1]. It is possible to achieve if biodegradable organic waste will utilization on biological and thermal methods about 3000 thousand Mg per year [2].
2. Characteristic of biodegradable waste management
Tekst rozdziału 2. [1]. In Poland produced 359819 Mg d.m. of sewage sludge in communal wastewater treatment plants. Part of this waste used on industrial aims 28274 Mg d.m., 50628 Mg d.m. on agricultural aims, 25528 Mg d.m. was composting, 5904 Mg d.m. on thermal methods utilized, and 151618 Mg d.m. deposited on landfills, the rest one neutralized otherwise 97867 Mg d.m. (Fig. 2.1). On the end 2000 year in 2200 municipal wastewater treatment plants accumulated 675011 Mg d.m. of sewage sludge. Rough estimated, that in the next ten years quantity of sewage sludge will grow up to 420000-450000 Mg per year. Whereas, nowadays in Poland produces over 12.5 million Mg of municipal wastes per year and about 41-55% biodegradable wastes.
Total volume 359819 t d.m./year 42,1 14,1 7,9 7,1 1,6 27,2 landfills agriculture industry composting thermal method other
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 6(2007) 35 The majority of treatments aim to decrease odour, kill pathogens and reduce volume of the sewage sludge. The traditional disposal routes for sewage sludge have been also: aerobic or anaerobic digestion; composting; heat treatment; organic or inorganic chemical conditioning; or dewatering using drying beds, filter press, vacuum filtration or centrifugation [3]. Biodegradable municipal wastes are in most cases landfilling [4]. In the present moment there are three well-known main directions of thermal utilization of sewage sludge and biodegradable waste: combustion, gasification, degassing [5].
3. Pyrolytical utilization of sewage sludge and municipal organic waste
Pyrolysis is one of the methods of thermal utilization of wastes it is a distillation process affected by the application of heat in an insufficiency of air. Pyrolysis gases, untreated oils, and solid matter in a form of char are the main products of the process (Fig. 3.1). These products may be utilized as an ecological fuel during next step of the waste neutralization. The low-temperature pyrolysis runs at the temperatures between 350oC-400oC, and above 600oC runs high-temperature pyrolysis. Pyrolytical utilization can be favorably used not only for gas and also for oil recovery from biodegradable wastes.
biomass wood waste stack gas clean-up preheater combustion chamber solid rest combustion air auxiliary fuel flue gas pyrolitical gas gas oil mixture feed pyrolytical reactor mixer and loader municipal
organic waste stack
flue gas flue gas flue gas dewatered sewage sludge
Figure 3.1. Schematic flow for biodegradable waste pyrolytical utilization.
4. Experimental arrangement
The pyrolysis process was carried out using an experimental arrangement showed in Fig.4.1. Pyrolytical reactor, preheater with electric heaters and thermocouple were the main parts of the system. The design of the reactor allowed controlling the temperature
inside reactor in a wide range to 1000oC. It was achieved by the use of the type K thermocouple connected to the temperature control system.
Automatic control system Cyclone-Thermometer R o ta m e te r Combustion gas analyser Combustion gases to chromatographic analysis Pyrolysis reactor C o m b u s ti o n c h a m b e r Preheater Tank of oil condensate Burner Air Demister Pyrolysis gas Tank of oil condensate Pyrolysis gas Natural gas
Figure 4.1. Schematic diagram of experimental apparatus.
Cyclone and demister were the main components of the purification system for pyrolysis gas and then burnout in combustion chamber equipped with burner system. Liquid products were separated by the condensation and they were collected in the cyclone and in the demister.
5. Experimental procedure
Experiments were conducted in the pyrolyser with possibility of collecting the pyrolysis products. The minimal time of the proper pyrolysis was about two hours for one feed and maximal time was three hours. Charge for pyrolytical reactor was mixture 50% dewatered sewage sludge and 50% municipal organic waste. Low temperature pyrolysis was conducted at 450oC, whereas high temperature process was performed at 750oC. Pyrolysis experiments were performed to find optimal process parameters assuring high decomposition of organic material (low content of carbon in solid residues), as well as good quality of pyrolytical gas towards hydrogen production. Control measurements of temperature distribution inside reactor, the quantity of pyrolytical gases, the amount of solid residues, and the amount of the process liquids were conducted during all experiments. Laboratory experiments were continued until the evolution of gases was completed.
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 6(2007) 37
6. Results and discussion
The final results of low and high-temperature pyrolysis experiments for sewage sludge and biodegradable waste originated from municipal wastes are presented below in two series. In table 6.1 are presented physical, chemical and thermal parameters for dewatered sewage sludge feed and for municipal organic waste. Table 6.2 shows gaseous product for low and high-temperature pyrolysis with hydrogen evolution rate. Whereas, in table 6.3 and 6.4 are presented main parameters for flue gases and heavy metals analysis for solid rest after pyrolysis process.
On the basis of the obtained data it can be noticed that municipal biodegradable waste has higher calorific value than sewage sludge, pyrolytical gas from high temperature process is better than low temperature process, production of hydrogen after pyrolysis is above 20%, burn out process is effective and harmless for natural environment, solid rest after pyrolysis could be hazard waste because of heavy metals content.
Table 6.1. Parameters of dewatered sewage sludge and municipal organic waste.
No. Denotation Sewage sludge Municipal organic waste
1. Moisture, % 38.11 27.70 2. Ash, % 12.71 12.20 3. Carbon,% 22.72 31.47 4. Hydrogen, % 3.47 5.58 5. VM (volatiles), % 78.28 87.80 6. Combustible sulphur, % 1.19 0.98 7. Oxygen, % 15.02 17.08 8. Chlorine, % 0.71 0.17 9. Nitrogen, % 6.07 4.82 10. HCV, kJ/kg 8611 13311 11. LCV, kJ/kg 7489 11816 12. Heavy metals, mg/kg d.m. 2273 481
Table 6.2. Parameters of gaseous product for low and high-temperature pyrolysis (temp.450oC and 750oC).
No. Denotation Sample I 450oC Sample II 750oC
1. H2, % 22.5 26.4 2. CH4, % 4.2 7.9 3. CO,% 21.6 25.1 4. CO2, % 10.5 11.7 5. O2, % 2.3 1.9 6. N2, % 16.9 16.0 7. H2O, % 21.5 11.0 8. HCV, (kJ/m3) 7200 9600 9. LCV, (kJ/m3) 6600 8700
Table 6.3. Parameters of exhaust gas for low and high-temperature pyrolysis.
No. Denotation Sample I 450oC Sample II 750oC
1. CO2, % 8.1 8.3 2. CO, mg/Nm3 430 390 3. NO, mg/Nm3 117.6 111.9 4. NO2, mg/Nm3 6.2 7.5 5. O2, % 12.2 9.5 6. SO2, mg/Nm3 21.3 9.2 7. Air excess, λ 2.3 2.4 8. Temperature T, oC 450 446
Table 6.4. Heavy metals in solid rest after pyrolysis.
No. Denotation Solid rest
1. Zn, mg/kg d.m. 428-1020 2. Pb, mg/kg d.m. 42-51 3. Cu, mg/kg d.m. 53-135 4. Cr, mg/kg d.m. 23-170 5. Cd, mg/kg d.m. 2-4 6. Ni, mg/kg d.m. 9-40 7. Mg, mg/kg d.m. 24-2301
7. Results and discussion
According to the experiments carried out, it was found that:
• pyrolysis can be used for destruction of sewage sludge and biomass waste with simultaneous recovery of pyrolytical gas and liquid fuel,
• the main component of gaseous products of high-temperature pyrolysis is hydrogen, • hydrogen as a final product after utilization organic waste can be used directly for
powering Otto engines or fuel cells achieving high thermodynamic efficiencies and relatively low environmental impacts,
• gases generated during the pyrolysis may be burned in situ and may be utilized after cleaning as an additional energy source,
• pyrolysis is a good process for biodegradable waste liquidation with absolutely slight pollution of atmospheric air,
• basic toxic components: CO, SO2 and NOx from process of combustion of pyrolytical gases were not large and will not cross of admissible coefficients,
• after pyrolysis waste volume reduce up to 80-92%,
• process is effective for sewage sludge after gravitational dewatering.
Acknowledgement. This work was supported by Polish Committee of Scientific Research (MNiSW), Grant No 10.10.110.650 in 2007 year.
Archiwum Gospodarki Odpadami i Ochrony Środowiska, vol. 6(2007) 39
References
[1] GUS - Statistical yearbook, Poland, 2002
[2] National Solid Waste Management Plan, Polish Government Resolution, 2002
[3] Lester J., Sewage and sewage sludge treatment. in Pollution: Causes, Effects and Control. ed: R.M. Harrison, Royal Society of Chemistry, 1990
[4] Leszczyński S., Słupek S., Energetic utilization of stabilized organic wastes, Heat Management and Industrial Furnace Operation, XII Scientific and Technology Conference, Poland, 2005
[5] Leszczynski S., Slupek S., Sewage sludge and biomass pyrolytical utilization, XIX International Symposium on Combustion Processes, Poland, 2005
