Combustion Experiments On A Solid Fuel With Low Sulphur Content

Research was carried out at the experimental installation defined by the 2 MW furnace pilot from the Politehnica University of Bucharest and ICEMENRG institute who was involved for measurements. The purpose of the experimental study is to confirm that the use of a coal with low sulphur content S<0.08% is possible without the use of sulphurpurification installations for industrial purposes. But, the high moisture content of the coal has imposed the problem of its compatibility with a certain milling technology. The pilot plant is equipped with fan mill and pre-drying tower. Under these conditions, this installation allows the grinding of fuels with high moisture content. The experiments focused on the process of ignition, combustion and pollutant emissions, with the exemplification, in particular, of the emission of sulfur dioxide. Also, the outbreak temperatures and the slugging temperature were monitored. For this purpose, the outbreak of the pilot boiler was equipped with all the equipment necessary to follow these parameters. The experiments have shown positive aspects to the burning of this fuel, thus laying the foundations of future tests at industrial installations (first of all, the power plant from Deva is considered).


Introduction
The purpose of the experimental study is to confirm that the use of a coal with low sulphur content S<0.08% is possible without the use of sulphur-purification installations for industrial purposes. The industrial use of this fuel can be implemented after the positive response obtained by the research on the pilot furnace [1]. The energy characteristics of the coal made by ICEMENERG-SA, indicate the following values: A high humidity, high content, also for oxygen and a very low value for the sulfur in the fuel are noted. The ash is very small. The volatile matter content is value: = 32.42%. The lower heating value an element of technical analysis has the value: The ignition capacity is defined by the criterion 1.09. This higher than one value indicates a h grinding. This is a characteristic of coal with high humidity. combustion (stoichiometric) resulted:  the theoretical air volume: = 4  the theoretical volume of gases: = The experiment carried out on the pilot boiler at UPB following elements of similitude with the industrial propose operation expected for this coal [2]:  milling with fan mill, with a peripheral speed of 85 grinding is adjusted by means of an inertial separator. The fan type mill allows a degree of recirculation of the flue gase an individual pulverization system furnace; A high humidity, high content, also for oxygen and a very low value for the sulfur in the fuel are noted. The ash is very small. The volatile matter content is high, having the The lower heating value is: = 15,860 ⁄ . The fixed carbon, value: The ignition capacity is defined by the criterion = , having the value = value indicates a high ignition capacity, allowing a less fine characteristic of coal with high humidity. From the calculation of The experiment carried out on the pilot boiler at UPB (Figure 1) has realized the following elements of similitude with the industrial propose operation expected for this milling with fan mill, with a peripheral speed of 85 m/s. The fineness of the grinding is adjusted by means of an inertial separator. The fan type mill allows a degree of recirculation of the flue gases before the end of the furnace of r=0.1. It's ation system with direct blowing of pulverized fuel into  the preheating of the combustion air is carried out in the range 250°C÷300°C (in industrial installations the range is between 280°C ÷300°C);  the flow of primary air introduced into the mill can be adjusted according to the volatile matter content of the fuel. Usually, it is a dependence between the proportion of air in the form of primary air and the content of volatile fuel material (ideally, these proportions should be equal);  the temperature of the flue gases recirculated by the mill is in the temperature range above 840°C;  there is thermal support for the sustaining of solid fuel combustion, achieved by combustion of natural gas;  the thermal loading of the furnace volume is within the limits imposed by the combustion stability.

The study of grinding
For the drying of coal, a primary air flow, = 120 3 ℎ (0.033 3⁄ ) ⁄ at 275°C ÷300°C was used and a flow of flue gas recirculated at the end of the furnace = 0.0166 3⁄ (corresponding to a degree of recirculation of r = 0.1). The calculation of the drying was performed on the basis of the simplified balance presented in Figure 2. The physical sizes of the thermal balance represent: . Under these conditions we have a functioning at the limit of avoiding the danger of explosion [3]. The fineness of the coal dust was determined on a sample taken at the exit of the mill, from the path of the burner mill. The fineness of the grinding is shown in the particle size distribution curve of Figure 3. With R x was noted the total sieve residues for a sieve with the mesh size x, expressed in µm. The grinding performed a residue on the standard sieve of 90µm by 80%. It results in a semi-rough grinding, but sufficient considering that the volatile content is high, which ensures an easy ignition. The milling coefficient had the value for an adsorbed humidity of 30%. This value indicates the existence of low energy consumption for coal milling.  , where P t is thermal power, in volume, m 3 . For the solid fuel consumption fuel) the thermal power was: = • + lower heating value of solid fuel and heating value of natural gas, respectively ( 35,700 3 ⁄

The study of the combustion process
). Thermal power results: P t =620 kW. For a volume of the furnace of 10 volume q v = 62 kW/m 3 .
The calorific participation of the supporting fuel had the value: The temperature at the end of the furnace had Figure 4 shows the technical operating data for the furnace, recorded by the control panel.
The temperature at the end of the furnace had an average value of about 980°C. Figure 4 shows the technical operating data for the furnace, recorded by the control panel. The through incomplete combustion, determined by measuring the slag and ash unburned value indicates a properly combustion process, validating the chosen combustion technology. The chemical analysis of the ash resulting = 7.35%; = 28.74%; = 5.89% The rest of the elements had been participated under 1%. This ash chemical composition indicates average erosion for the milling elements, because the content of SiO 2 it is relatively low. An analysis of the tendency to form adherent slag in the furnace was also imposed. The temperature behavior of the ash depends on its chemical composition, the acid oxides, SiO 2 and Al 2 O 3 having a higher temperature resistance then that of the basic ones as Fe 2 O 3 , CaO, MgO. Fusibility index of the ash has the value: This value indicates a melting temperature of less than 1200 °C. Laboratory tests have shown a softening temperature of over 1000 °C. Further research in this field is required.

Emission of SO 2
The numerical applications for an analytical calculation model [5] led to the determination of a SO 2 maximum concentration of 205.6mg/Nm3 .Calculations were performed for the real operating mode of the boiler furnace. This analytically determined size represents a maximum possible value for a given operating regime. In the parallel, experimental measurements were made with a device TESTO 350. The emission was in the field: = 137 ÷ 163 3 ⁄ . Thus, there is a possible classification in the current pollution norms (200 3 ⁄ maxim), at the industrial operation of the boiler, with this fuel.

Conclusions
The complex tests performed on the combustion of a solid fuel with low sulfur content on a pilot boiler, demonstrated the possibility of passing the researches on an industrial scale.
This conclusion resulted from:  classifying the emission of SO 2 (the main purpose of the tests) within the accepted environmental limits;  proper grinding with the help of a fast mill, with a good fineness of dust grinding, a drying accentuated with normal energy consumption;  obtaining a high temperature field and a low value of through incomplete combustion indicates an efficient combustion of this fuel in the experimental furnace.