http://ago.helion.pl ISSN 1733-4381, Vol. 1 (2005), p-73-82
Co-combustion of coal with sludge in laboratory scale
Nadziakiewic J., Czekalski R.
Silesian University of Technology, Department of Technology and Installations for Waste Management, Konarskiego 18, 44-100 Gliwice, POLAND
ph.: +48 32 237 2104, fax: +48 32 237 1116 e-mail: czekalski@waste.ise.polsl.gliwice.pl
Streszczenie
Wsp€łspalanie węgla z osadami ściekowymi w skali laboratoryjnej
Artykuł przedstawia wyniki badań procesu wsp‚łspalania węgla z osadem ściekowym przeprowadzanym w specjalnie do tego celu zaprojektowanej instalacji laboratoryjnej. Badaniom poddano sześć mieszanek paliwowych zawierających od 5 do 30% osadu ściekowego o wilgotności 40%, a także czysty węgiel. Celem badań było wykazanie wpływu udziału osadu w mieszance paliwowej na emisję gazowych substancji szkodliwych takich jak CO, NOx i SO2. Jak wykazała analiza wynik‚w, poza r‚żnicą w składzie
elementarnym mieszanek, wpływ na wielkość emisji wspomnianych mieszanek ma r‚wnież zwartość wilgoci.
Abstract
The paper presents results of research of co-combustion process of coal with sludge carried out in laboratory installation. Six mixtures were investigated containing 5 up to 30% of sludge (containing 40% of moisture) as well as clean coal. The research was conceived to investigate the influence of sludge in fuel mixture on emission of gaseous substances in exhaust gas, like CO, NOx and SO2. The results show that influence on the emission of mentioned substances has, besides changes in composition, also moisture content of the fuel mixture.
1. Introduction.
Improvement of life standard of society is connected with the development of many infra-structures that supply media (e.g. electricity, heat, water) as well as the ones which take care of the environment, such as sewage treatment systems. The final product of the sewage treatment process carried out in such installations is sludge. There are many
groups of methods of sludge utilization, one of the thermal methods is co-combustion with coal in existing installations [1] (small boiler houses, power plants, etc.), so there is no need to design and invest in a new installation. Many research are carried out regarding co-combustion process of coal with biomass as well [7, 8, 9]. Before feeding the fuel mixture to boiler, which was designed for coal, research should be carried out. Those research should investigate the influence of new fuel parameters (e.g. composition, density, moisture content) on process run and emission of harmful substances. The second issue, which was the aim of the described research, might be simulated and investigated in a laboratory installation.
2. Laboratory installation and characteristics of research.
The essential element of the laboratory installation was a small grate furnace [3,6]. To simulate the combustion process that is carried out in a real boiler, some modifications were made (Fig. 2.1.). The furnace’s walls were insulated with bricks (4) inside and glass wool (5) on the outside. To monitor temperature during the process, four thermocouples were installed: in the upper part of the combustion chamber (t1), over the grate (t2) and on parallel walls (t3 between the furnace wall and external insulation and t4 between the internal insulation and furnace wall). In the ash chamber door a hole was drilled for connector pipe to distribute air under the grate.
Fig. 2.1. Diagram of a furnace used in laboratory: 1 – ash chamber, 2 – grate, 3 –
combustion chamber, 4 – additional bricks, 5 – additional glass wool, 6 – hole in ash chamber door, 7- combustion chamber door, t1, t2, t3, t4 – localization of thermocouples.
The whole installation is presented on Fig. 2.2. Air was supplied by a compressor (9) and its stream was regulated by a set of rotameters (8). Levels of temperatures were registered during measurement as well as gas composition by MADUR GA 40 T plus gas analyzer (3). Average levels of CO2, O2, CO, NO and SO2in each minute of measurements were
recorded by a computer (5). Before each measurement the furnace was heated up using a gas burner, to determined temperature levels, then a prepared portion of fuel was loaded to the combustion chamber and a pipe supplying air was installed. Ignition of the fuel mixture was a consequence of hot walls. To simulate the travel of fuel through different air-flow zones in a real combustion chamber, the air stream that was distributed under the grate, was modified several times during each measurement. Each experiment lasted 50 minutes and the air stream was changed every 10 minutes.
In this research, six fuel mixtures were investigated, containing 5, 10, 15, 20, 15 and 30% of sludge, as well as coal itself. An ultimate analysis of coal and sludge is given in Table 2.1. To obtain an assumed level of moisture in the sludge (40%), water was added to each fuel mixture. Table 2.2 contains compositions of all fuel mixtures used in the research as well as the air stream supplied during each measurement. Values of air streams supplied in different time periods under the grate are based on an air distribution curve along the grate.
Fig. 2.2. Diagram of laboratory installation: 1 – furnace, 2 – chimney, 3 – gas analyzer, 4
– probe, 5 – computer, 6 – temperatures register, 7 – air pipe, 8 – rotameters, 9 – air compressor
Table 2.1. Coal and sludge used in measurements, composition by mass %.
c h n o s cl ash moisture LHV, kJ/kg
Coal 73 4,57 1,53 9,66 0,37 0,512 6,06 4,785 26787
To learn about the factors in the curve equation, some conditions must be determined, such as the value of total air volume and the distance of the point of maximum air stream from the beginning of the grate. The excess air factor here was = 1,4 and the maximum air stream was at 1/6 grate length.
Table 2.2 Composition of fuel mixtures and air streams
Composition, kg Air streams, m3
/h Sludge
% coal sludge water 1-10
min 11-20 min 21-30 min 31-40 min 41-50 min 5 3,135 0,195 0,08 73,2 75,7 35,9 12 2,1 10 2,875 0,375 0,155 68,8 71 33,6 11,2 2 15 2,615 0,545 0,225 60,6 62,6 29,7 9,9 1,8 20 2,335 0,685 0,285 58,3 60,3 28,6 9,5 1,7 25 2,115 0,835 0,345 54,5 56,4 26,9 8,7 1,6 30 1,92 0,97 0,405 50,6 52,4 24,8 8,3 1,5 0 3,4 0 0 82,2 85 40,3 13,4 2,4 3. Results.
The combustion process of each fuel mixture was carried out three times, and in case of each experiment concentrations of NO, CO, SO2, CO2and O2were measured. The other
steps of the analysis of results were calculation of emission of mentioned substances and then its summation to obtain a total emission from each process.
3.1. Changes of concentration of CO, NO, SO2in time.
Diagrams below show a variation of concentration as a function of time. Those relationships are results of co-combustion of the fuel mixture containing 20% of sludge [1,3]. At the beginning, during the ignition period, the process is very unstable, concentrations of NO and SO2change rapidly (Fig 3.1. and Fig. 3.2.). Poor mixing of fuel
with air and local lacks of oxygen cause very high levels of CO in flue gas (Fig. 3.1.). After ten minutes, when the temperature over the grate reaches maximum, the process stabilizes and concentrations decrease slowly with time and modification of the air stream.
3.2. Total emission as a function of sludge part in fuel mixture.
Fig. 3.3. presents a total emission of CO, NO and SO2related to the heating value of the
fuel mixture as a relation between sludge part in the fuel mixture [1, 3]. With the addition of sludge, SO2 emission grows (it is caused by greater contents of sulfur in sludge than in
coal) – in all cases, where the fuel mixtures were combusted, total emission of SO2 is
greater than in the one with coal only.
However, emission does not rise with the growth of sludge part in the fuel mixture – the tendency is rather opposite. Probably it is caused by the growth of moisture of the fuel
mixture and passing of sulfur from fuel to slag; a similar tendency was obtained in research carried out in real boiler [3, 4, 5, 6], likewise it was also ascertained in other research [2]. As regards CO emission, a similar trend is noticeable. A very high value obtained in the measurement of the fuel mixture containing 20% of sludge is caused by a measuring error. In case of one of the three experiments, very high levels, and different from the other two, were registered by the analyzer. Emission of NOxdoes not change
significantly with the addition of sludge, but it slowly increases.
0 100 200 300 400 500 600 700 800 0 10 20 30 40 50 time, min c o n c e tr a ti o n , m g /m 3 n 0 100 200 300 400 500 600 700 800 900 1000 te m p e ra tu re , oC
NO concetration CO concetration temperature
Fig. 3.1. Concentration of NO, CO and temperature over the grate as a function of time for fuel mixture containing 20 % of sludge.
0 200 400 600 800 1000 1200 1400 0 10 20 30 40 50 time, min c o n c e tr a ti o n , m g /m 3 n 0 100 200 300 400 500 600 700 800 900 1000 te m p e ra tu re , oC
SO2 concetration temperature
Fig. 3.2. Concentration of NO, CO and temperature over the grate as a function of time for fuel mixture containing 20 % of sludge
0 0,1 0,2 0,3 0,4 0,5 0 5 10 15 20 25 30 sludge part, % e m is s io n , m g /k J 0 0,05 0,1 0,15 0,2 0,25 0,3 e m is s io n , m g /k J
CO, left axis NOx, right axis SO2, right axis
NOx trend, right axis SO2 trend, right axis CO trend, left axis Fig. 3.3. Total emission as a function of sludge part in fuel mixture.
3.3. Energy and mass balance of the process.
Energy balance was calculated to find out how primary chemical energy supplied with fuel distribute to the products combustion process. Fig. 3.4 shows the energy balance of a co-combustion process in case of a fuel mixture containing 20% of sludge.
34,3
0,3 0,9 12,8
51,7
physical enthaply of flue gas chemical enthaply of flue gas physical enthalpy of slag chemical enthaply of slag heat loss
Fig. 3.4. Energy balance of co-combustion process in case of fuel mixture containing 20% of sludge.
Over a half of the chemical energy contained in fuel turns into a heat loss that was calculated as a closing of energy balance. A significant part in the energy balance also have a physical enthalpy of flue gas (consequence of its temperature) and a chemical enthalpy of slag (consequence of the burning fraction that left after 50 minutes). The physical enthalpy of slag (consequence of its temperature, calculation based on temperature over the grate in the last minute of measurement) as well as the chemical enthalpy of flue gas (consequence of CO emission) have no important meaning in the energy balance. Proportion of those terms were similar in other cases with different fuel mixtures.
A mass balance was calculated in each case to check the correctness of measurements and calculations. Because of lack of some data, such as sulphur content in slag, only a carbon balance might be calculated and treated as a reliable tool for analysis. Fig. 3.5. shows a distribution of coal contained in the fuel mixture (for a fuel mixture containing 20% of sludge and 80% of coal) into products of the combustion process.
79%
1% 17%
3%
CO2 CO slag measurement error
Fig. 3.5. Carbon balance in co-combustion process of mixture containing 20% of sludge. As it is seen, almost 80% of coal is emitted as a carbon dioxide, and about 1% as a carbon monoxide. Time that was set for the process run was not sufficient for combustion of all the fuel, and this results in quite high level of burning fraction in the slag. In cases of other fuel mixtures the carbon balance is similar and no changes with the sludge part are noticeable.
4. Summary.
The aim of this research was to check the influence of the sludge part in the fuel mixture on the emission of gaseous substances emitted from the co-combustion process carried out in a boiler equipped with a mechanical stoker. To do this, a laboratory installation was
designed to simulate such process. Results show that the beginning of this process is very unstable and is characterized by very rapid changes in concentration of gaseous substances in flue gas and very high CO emission, connected with poor mixing of fuel and air at the beginning. Addition of sludge in the fuel mixture influences the emission of gaseous substances. Sulphur dioxide emission decreases with the growth of the sludge part in the fuel mixture, that is probably a result of moisture content and passing of sulphur to slag, but lack of investigation of sulphur content in slag did not allow to calculate the sulphur balance. Nitrogen oxide emission slowly increases with the growth of the sludge part, which is result of higher content of nitrogen in sludge and formation of fuel NO. Influence of sludge part on carbon monoxide emission is difficult to conclude in case of this research for the sake of random and unstable changes of concentration of CO at the ignition period. Because levels of CO were incomparably higher and with a tendency to random changes at the beginning, the influence of those first changes on the total emission of CO was very significant. Nevertheless, total CO emission decreases slowly with the growth of the sludge part in the fuel mixture, but this tendency is not continuous and clear.
Similar tendencies and results were obtained in other research carried out in a real boiler, which allows to conclude that the designed installation simulated the co-combustion process to a sufficient extent. However, before the addition of sludge to fuel in a particular boiler, suitable research should be previously done on this object to investigate the influence of the sludge part on the process run and boiler parameters. Practice shows that in each case the boiler must be regulated to obtain optimal efficiency.
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