PILOT COMBUSTION EXPERIMENT OF THE COAL AND Alternative Fuels ON THE FBC UNIT
D. JUCHELKOVA*, P. KOLAT*, H. RACLAVSKA* and K. KOPPE**
* VSB-Technical University of Ostrava, Energy Department, 17. listopadu 15, 708 33 Ostrava, Czech Republic, dagmar.juchelkova@vsb.cz
** Technical University of Dresden, Germany
SUMMARY: Not only in the Czech Republic is a strong interest to utilize alternative fuels in the conventional boilers. The Energy Department made a lot of test on the combustion units, especially at the FBC. The article discusses the questions of thermal usage of mechanically drained stabilized sludge from the sewage plants in the boiler with circulated fluid layer. The paper describes the thermal analysis of coal, sewage sludge and its mixtures, mud transport to the fluid boiler, effects on efficiency, operational reliability of the combustion equipment, emissions and solid combustion residues.
1. INTRODUCTION
The goal of research project carried out at the Department of Power Engineering at VŠB-Technical University Ostrava is work on the various possibilities to use alternative fuels and sorbents at the conventional condition.
Alternative sorbents such as waste substances, by-products from the paper production industry (caustic sludge) and from the sugar production industry (saturation sludge) can be used to replace the most frequently used sorbent – natural limestone. The reuse of such waste products will reduce the use of naturally occurring sorbents and hence limit the depletion of valuable raw materials. There is a lot experiences on this field.
One of the research is to verify if the sludge from waste water treatment plants may be used as alternative energy source in respect of the EU legislation, and/or its other modifications (with additives, decontamination technologies) for suitable fuel, which would comply with emission limits or the proposal of energy process optimizing the preparation of coal/sludge mixture for combustion in the existing power engineering equipment. Co-combustion, for example of biomass, with coal is by far the most cost-effective possibility, as existing installations can be used with minor adaptations and preservation of the regular high efficiencies of modern coal-fired power stations.
For this paper was the information about sludge’s selected. The limiting factor for sludge utilization from WWTP in agriculture is the increased content of risk elements and also the occurrence of organic pollutants – primarily polyaromatic hydrocarbons, PCB and AOX. The limiting factor for sludge combustion at incineration plants is water content. The sewage sludge is a heterogeneous mixture of organic elements (both live and lifeless microorganism cells) and an organic element. The organic part of the sewage sludge is mainly represented by the proteins, sugars and lipids. The organic part sustains mainly of the compounds of silicon, ferrum, calcium and phosphorus. Moreover, the sludge consists of a wide range of harmful substances as well – heavy metals, persistent organic elements PCB, PCDD/F, PAU etc. and other organic harmful elements. The table 1 illustrates the summary of the organic pollutants in the sewage sludge dry residues taken from the Central Sewage Plant of Ostrava (CSPO) and it is evident that almost all limits of the monitored pollutants are exceeded. Such high values prevent from using the sewage sludge for agricultural purposes and land reclamation – meaning the usage of both the underground and exterior storage. The biggest problem is in this case the high content of the polyaromatic hydrocarbons that is ten times higher than the limit. It is probably because of the industrial waste-water disposal. The value of TOC (total organic carbon) that does not fit can be considered rather useful than limiting factor.
Table 1.1. Organic pollutants in sewage sludge
The energy content of the sewage sludge is based on the chemical energy of the organic components that are able of oxidation. Being allowed to call the sewage sludge the fuel – the energy material that being combusted the primary energy transforms into the thermic energy - the condition of being flammable must be fulfilled.
To make the combustion process balanced it is necessary to make the sludge dry residues fuel efficiency and other heat distributed to the furnace to cover the water vaporization heat contained in fuel, the heat needed for the superheating of the water vapours in the waste gases and the heat needed for the waste gases heating. The important criterion of keeping the combustion process balanced is thus the water ratio in the sludge. Thus a problem arrives because water ratio of the mechanically drained sludge is high (ca. 60 – 80 %) for the relatively low fuel efficiency and there wise the sludge cannot be combusted singly. The most important energy characteristic of each single fuel is its efficiency. The dry residue efficiency of the anaerobic stabilized sewage sludge is in range 7 – 10 MJ.kg-1. The figure 1 shows the sewage sludge structure.
2. THE COMBUSTION TEST DESCRIPTION
The combustion test with the mechanically drained digested sewage sludge (the water ratio in the sludge ca. 63 %) was done at the boiler with the circulating fluid layer of its output 130 MWt. The mixture of hard energy coal and the coal shed of average efficiency Qir = 19 MJ.kg-1, water ratio wr = 7,5 % a ash content Ar = 30 % is generally combusted at the fluid boiler.
During the combustion test the fuel was distributed to the boiler having the ratio: 11 %weight. Sewage sludge from the waste-water treatment plant of Ostrava, 28 %weight. energy coal and 61 % weight. coal shed. During the additional combustion of the sludge the mixture characteristics changed this way: efficiency Qir = 17 MJ.kg-1, water ratio wr = 14,5 % a ash content Ar = 28 %. Based on the fact that the total efficiency of the fuel mixture thus dropped by ca. 2 MJ.kg-1 during the additional combustion the volume of the mixture must be increased ca. by 0,65 kg.s-1 to maintan the constant boiler output. However, the total coal consumption does not raise and this fact is important. The description of the combusted fuel is illustrated at the table 2.1, 2.2, 2.3, 2.4.
Table 2.1. The fuel characteristics in crude form
Table 2.2. The waterless fuel sample composition
Table 2.3. The fuel combustible composition
Table 2.4. The silicate analysis of the energy coal, sludge and coal shed (RTG-fluorescence method)
Figure 1. The structure of the sewage sludge from the waste water treatment plant of Ostrava
3. THE BOILER EFFICIENCY AND ITS OPERATIONAL RELIABILITY
Based on the combusted coal and the sewage sludge of given ratio the ca. 0.3 % drop in efficiency of the boiler was monitored (see the table 3.1). The content of the combustible carbon in the products of the combustion corresponds with the fine hard coal combustion.
Figure 2. The scheme of the fluid bed boiler 160 t.h-1
Figure 3. The scheme of the distribution and the fuel ratio to the boiler.
Table 3.1. The silicate analysis of the energy coal, sludge and coal shed (RTG-fluorescence method)
where CLP indicates the combustible matter content in the bedding ash and CUP indicates the combustible matter content in the ash.
If we focus on the operational efficiency of the boiler under the condition of the additional combustion of the sludge it menas monitoring possible unwanted states given by the high and low-temperature rust, silting the heat transfer surfaces and abrasion. Concerning the boilers with the fluid furnace and the additive desulphurisation the marks of the chlorine rust pop up even if the chlorine content in the fuel is low. The ratio Cl/SO2 has an impact on the high-temperature chlorine rust intensity. The chlorine content in entire fuels and it is evident that the chlorine content in the sewage sludge does not exceed the volume that was found out concerning the hard coal. The HCl concentration in waste gases influences the low-temperature rust intensity. In our case the rust is to be taken into consideration considering the recuperative air heater.
4. THE EMISSIONS
During the combustion test the continual measurements of the harmful gases CO, NOx, SO2 and relative oxygen behind the boiler. Furthermore, the single measurements of the emissions of cadmium, mercury, lead, arsen and their compounds, polychlorinated dibenzodioxines PCDD, polychlorinated dibenzofurans PCDF, polychlorinated bifenyls PCB, polycyclic aromatic hydrocarbons PAU, gaseous anorganic chlorine and fluor compounds, hard pollutants TZL. The measurements were done by the company TESO Ostrava.
Table 4.1. The chosen emissions of the pollutants (6 % O2, 101.32 kPa, 0 °C)
*) … limit values given by the public notice No. 352/2002, 354/2002 of The Codes of Law.
5. CONCLUSIONS
The combustion test proved the possibilities of the opportunities of the additional fuel combustion in the fluid bed boilers. The advantages of such usage of the sewage sludge are mainly the reliable decomposition and the oxidation of the organic harmful elements and the significant sludge volume reduction. Another possibility of using the sludge is to lower its humidity and thus to improve the fuel efficiency, transport and manipulation. The disadvantage of the thermal usage of the sewage sludge is higher concentration of the heavy metals and the microelements entering the combustion equipment.
ACKNOWLEDGEMENTS (please list without numbers)
The Authors wish to thank MSM 6198910019 DeCOx Processes.
REFERENCES (please list without numbers)
ČECH, B.: The co-firing of the sewage sludge from The Central Sewage Plant of Ostrava, the coal shed and the hard coal at the fluid boiler K12 at the ENERGETIKA TŘINEC,a.s. – Technical Report.
TILLMAN, D. A., E. HUGHES AND B. A. GOLD (1994), Cofiring of biofuels in coal fired boilers: Results of case study analysis., 1st Biomass Conference of the Americas, Burlingtion, VT
KOPPEJAN, J., 2004: Overview of experiences with cofiring biomass in coal power plants, IEA Bioenergy Task 32: Biomass Combustion and Cofiring, (in draft, 2004)
KOPPEJAN., VAN LOO, S et al (2002) Handbook of Biomass Combustion and Cofiring, IEA Bioenergy Task 32: Biomass Combustion and Cofiring, Twente University Press, 2002
ZYGARLICKE C. J. I INNI: Investigating the Impacts of Cofiring Biomasswith Fossil Fuels. EERC Univ. of North Dakota (Internet).
BENSON S. A.: Ash Formation and Behavior in Utility Boilers. Microbeam Technologies – quarterly newsletter (Internet).
THORWARTH H.; SCHEFFKNECHT G.: The Effect of Conventional Air Pollution Control Devices and Biomass Co-Combustion on Mercury Behaviour, Proceedings of the 31st International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, USA, 2006.
OBERNBERGER I.: Aschen aus Biomassefeuerungen– Zusammensetzung und Verwertung. VDI Bericht 1319, pp. 199–222. VDI Verlag GmbH, Düsseldorf 1997.
SALMENOIA K.: Chlorine–Induced Superheater Corrosion in Boilers Fired with Solid Biofuels. Power Lines 2000/1.
MORY A., TAUSCHITZ J.: Mitverbrennung von Biomasse in Kohlekraftwerken. VGB Kraftwerkstechnik 1/1999.
BENSON S., SONDREAL E., HURLEY J.: Fuel Processing Technology, 44, 1995, 1.
SUGIYAMA H., KAGAWA S., KAMIYA H., HORIO M.: Env. Eng. Sci., 15, No.1, 1998, 97.
SEEKER W., LANIER W., HEAP M.: Report EPA/530–SW–87–021C, 1987.
ŠOOŠ L.: Environmental technology.STU Bratislava, ISBN 978-80-227-2627-6Blakey N.C., Cossu R., Maris P.J. & Mosey F.E. (1992) Anaerobic lagoons and UASB reactors: Laboratory experiments. In Landfilling of waste: Leachate, Christensen, Cossu, Stegmann (Eds), Elsevier Applied Science Publisher, Amsterdam, pp. 245-263.
Quelle : Vortrag Second international Symposium on Energy from Biomass and Waste. Venedig 2008
Copyright © Koppe 2009
