The performance of the firing machine plays one of the key roles in the pellet production volume and the degree of its influence on the quality characteristics of the pellets. 

      The following main factors influence the performance:
       a. technical - the condition of the main process equipment that does not allow the process to be carried out with the                         specified productivity while maintaining the quality characteristics of the pellets (for example, the lack of sufficient                        compaction of the induration machine, the technical condition of the grate field and induration trolleys), downtime -                  they can be caused both by the condition of the equipment and by various other reasons (for example, power outages,                  lack of natural gas, supply of process water for cooling the furnace elements, or an emergency failure of the                                      main equipment) associated with the shutdown of the induration machine equipment for a short or long period and                     returning to the operating mode after these stops;
       b. technological - the type of pellet produced (blast furnace or direct reduction pellet, non-fluxed or highly fluxed), the                      type  of iron ore concentrate (magnetite, hematite, limonite), inadequate quality characteristics of the initial raw                            materials (high moisture content in the concentrate), raw pellets (high moisture content in the concentrate, low                              strength  properties and inadequate particle size distribution by size classes);

        c. human factor - violation of the technological process by the plant personnel, failure to comply with the requirements                   of regulatory documentation (instructions, process maps, standard operating procedures) resulting in the shutdown of                the firing machine or a decrease in productivity, insufficient training of personnel in the technological process;
        Since the firing machine is a thermal unit, any short-term shutdown for scheduled maintenance or emergency downtime is associated with a decrease in the temperature in the furnace to 750-800 C with reduced gas consumption and the operation of some burners, usually 4-6 pieces, at a reduced consumption of natural gas "low flame". It is also common practice to turn off all gas burner devices with a decrease in the temperature in the furnace to certain values, which are associated with the duration of the shutdown period. In this case, when subsequently supplying natural gas to the firing machine and igniting the burners, it is imperative to take a sample in the furnace for the absence of an explosive mixture. During longer cold shutdowns of the thermal unit, the temperature in the furnace decreases to the ambient temperature with gas consumption turned off and burners not working. The subsequent start-up and return to the previous value in terms of productivity is associated with the time intervals required for the temperatures in the furnace zones to reach the initially set values and the heating of the furnace (an example of returning to the mode after stopping for scheduled maintenance).

The firing machine's entry into the mode with reduced productivity and the loss of part of the production volumes should be taken into account when planning production volumes. There can be two approaches: the first is to take into account the time of entering the mode during repair work, the second is to take into account part of the loss of production volumes after starting the firing machine into operation after repair. In fact, both the first and second options are used when planning production volumes. The effective productivity of the firing machine (or, more correctly, the technological line, including the firing machine and other related equipment directly involved in the technological process of pellet production) and, as a consequence, the production volumes are determined based on the total calendar operating time minus the downtime, regardless of their cause. On this basis, the equipment utilization rate EUR is calculated.
                                                                                                     
                                                                              EUR – equipment utilization rate,
                                                                              Ta      – actual time worked by equipment, hour
                                                                              Тc      – calendar operating time of equipment, hour
    There are some differences in calculating the EUR. At some enterprises, when calculating the actual EUR, only the readiness of the firing machine to produce pellets is taken into account, excluding all stops of other associated equipment, which naturally increases the values of this indicator in the reporting data. The productivity of the firing machine is defined as the ratio of the number of pellets produced (tons) for a certain amount of time (hour, day, month, year), for example - t/hour. Specific productivity is defined as the ratio of the number of pellets produced for a certain time to the useful sintering area of the firing machine, for example - t/m2hour or more often in calculations - t/m2day. The higher it is, the better the thermal unit works. The calculation formulas for determining the productivity of the firing machine are as follows:
                                                                                                   
                                                                              Qr - actual productivity of raw pellets, t/hour
                                                                               h   - average height of the layer of raw pellets, m

                                                                               b   - width of the layer of raw pellets, m
                                                                               v   - speed of firing trolleys, m/min
                                                                                    - bulk density of pellets, t/m3
                                                                                                          
                                                                              Qf - actual productivity of fired pellets, t/hour
                                                                              Cf - the yield coefficient of roasted pellets 

                                                 (calculated on the basis of the material balance, example annex 2) is 0.9-0.93units
                                                                                                          
                                                                              Qs - actual productivity of suitable pellets, t/hour
                                                                              Cs - the coefficient of yield of suitable pellets (calculated based on

                                                 the removal of dust and screenings of class 6.3mm after screening) is 0.98-0.95units
     Then the general formula for suitable pellets,

                                                                                   
     The yield of suitable pellets increases with an increase in the iron content in the charge, a decrease in the basicity modulus, a decrease in the moisture content in the raw pellets, a decrease in the screening of fines during the screening of the roasted pellets and the removal of dust into the environment. Below are the actual approximate specific productivity of the firing machine for suitable pellets depending on the type of concentrates used:

                                                                                           magnetite: - 19-32 t/m2day;
                                                                                           hematite:   - 16-26 t/m2day;
                                                                                           limonite:    - 12-18 t/m2day;
        An example of the material balance of non-fluxed blast furnace pellets is shown in figure 2.

According to their hardening ability, all pellets can be divided into materials with heavy and lightened firing conditions. Lightened conditions are those in which the firing temperature does not exceed 1250-1280 C. This occurs when firing pellets from a charge with an increased content of slag-forming agents. To obtain high mechanical strength of pellets, it is sufficient to have a short firing zone in which the required temperatures are maintained. For severe firing conditions, temperatures in the range of 1300-1350 C are typical, and at the border with the bottom bed, it is necessary to maintain temperatures of about 1220-1250 C with a time interval of 1-4 minutes. There is no layer separation by temperature in the grate-tube furnace-ring cooler (GK) type of firing units; pellets move inside the rotary kiln from loading to unloading, being simultaneously roasted at temperatures of 1250-1280 C. If we compare the characteristics of the pellets obtained using the two technologies, then the pellets roasted using the GK technology, all other things being equal, have higher values for compressive strength, impact strength, and abrasion resistance. This is due to more uniform firing by the pellet volume, minimal presence of underroasted pellets and pellets with a two-zone hematite-magnetite structure, and the absence of re-firing of bed pellets. The main advantages of the GK technology are the flexibility of the technological process, the quality of the manufactured products, and the use of alternative types of fuel. The main disadvantages of this technology are the following: limitations on granulation of pure hematite raw materials, lower energy efficiency, increased costs for maintenance and refractories, limitations on the production of pellets with a basicity of more than 0.3units. If the production of pellets with a basicity modulus of up to 1.2units has been mastered on conveyor machines (SG), then the production of fluxed pellets using the GK technology is associated with the formation of accretions in the tube furnace, which leads to damage to the refractory lining, local overheating of the tube furnace casing, and long shutdowns. The schemes of pellet production using the GK and SG technologies are shown in figures. A comparison of the differences in the technological process of the grate-tube furnace-ring cooler and straight grate-type firing units is reflected in table.