Process and regulation device for ring furnaces

Details Process and regulation device for ring furnaces Process and regulation device for ring furnaces

1999-03-18

2002-01-15

Grant, William (Department: 2121)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C432S047000

Reexamination Certificate

active

06339729

ABSTRACT:

DOMAIN OF THE INVENTION
The invention relates to the domain of ring furnaces for baking blocks containing carbon and more particularly a process and a device for regulation of these furnaces.
STATE OF THE ART
Regulation methods for this type of furnace are already known, for example as described in French applications FR 2 600 152 and FR 2 614 093 submitted by the Applicant, and in international application WO 91/19147.
This type of furnace, also called an “open section” furnace comprises several preheating, baking and cooling sections in the longitudinal direction (as described in the referenced documents), the composition of each section in the transverse direction consisting of flue walls through which combustion gases circulate alternating with pits in which blocks containing carbon to be baked are stacked, the blocks being immersed in dust containing carbon.
This type of furnace comprises two bays whose total length may exceed a hundred meters. Each bay comprises a series of sections separated by head walls and open in their upper part, through which unbaked blocks are loaded and cooled baked blocks are unloaded. Each section includes a set of thin flue walls parallel to the longitudinal direction of the furnace, in other words its major axis, through which the hot gases or combustion exhaust gases which provide the heat for baking will circulate, alternating in the transverse direction of the furnace with pits in which the blocks to be baked are stacked.
Closable openings called “peep holes” are placed in the upper part of the flue walls. They are also provided with baffles to extend and more uniformly distribute the trajectory of combustion gases or exhaust gases.
The furnace is heated by burner ramps, the length of which is equal to the width of the sections, the injectors for these burners being inserted through peep holes in the flue walls of the sections concerned. On the upstream side of the burners (upstream considering the direction in which combustion is advancing), combustion air blowing openings are placed on an air blowing ramp equipped with fans, these blowing openings being connected to the said flue walls through the peep holes. On the downstream side of the burners, combustion exhaust gas openings are installed on an exhaust ramp supplying the exhaust gas collection centers equipped with dampers which close off the said exhaust openings to the required level. Heating is applied by combustion of the fuel injected in the baking sections, and by combustion of tar vapor released from the blocks during baking in the preheating sections, which due to the negative pressure in the preheating sections, leaves the pits by passing through the flue wall and burns with the oxygen remaining in the combustion exhaust gases circulating in the flue walls in these sections.
Typically, there are about ten sections “active” at the same time; four in the cooling area, three in the heating area and three in the preheating area.
As baking continues, the “blowing openings—burners—exhaust openings” assembly will be moved forward by one section, for example every 24 hours, the sequence of operations in each section consisting of loading an unbaked block containing carbon in front of the preheating zone, then natural preheating in the preheating zone due to combustion exhaust gases and combustion of tar vapors, then heating the blocks to 1100-1200° C. in the baking zone, and finally cooling the blocks by cold air in the cooling zone at the same time as preheating combustion air for the furnace, the cooling zone being followed by a zone in which the cooled blocks containing carbon are unloaded.
The most frequently used method of regulation for this type of furnace is to regulate the temperature and/or pressure in a number of sections in the furnace. Typically, out of the ten sections that are active at any one time, four will be provided with temperature measurements and two will be provided with pressure measurements. Firstly, the three burner ramps are regulated as a function of the temperature of the combustion exhaust gases, the fuel injection being adjusted to follow a temperature rise curve (typically the temperature of the combustion exhaust gases but possibly the temperature of the blocks containing carbon). Secondly, the fan speed on the air blowing ramp is typically regulated as a function of a static pressure measured on the upstream side of the burners, but it may also be kept constant. Finally, the exhaust gas dampers are regulated as a function of a negative pressure measured in a section located between the burners and the exhaust openings. But more frequently (particularly in more recent furnaces) the said negative pressure is itself controlled by a set temperature, which is typically the temperature of combustion exhaust gases such that the said dampers are controlled by a temperature measurement and its comparison with a set temperature.
The furnace may also be regulated by other complementary means:
French application FR 2 600 152 also describes a device for optimizing combustion in the baking area in order to measure the opacity of exhaust gases in the exhaust openings and to regulate this exhaust correspondingly;
French application FR 2 614 093 also describes a method of optimizing combustion in the furnace by continuously injecting the necessary and sufficient air quantity to obtain complete combustion of volatile materials released during baking of the blocks containing carbon and the fuel injected in the burners;
application WO 91/19147 also describes a check on the oxygen/fuel ratio in the furnace by measuring the oxygen content in the furnace.
PROBLEM THAT ARISES
Regulation methods used in the past are based mainly on temperature measurements and pressure measurements in a large number of sections, and in the various flue walls in the same section. As indicated in the mentioned state of prior art, these basic measurements may be complemented by other measurements.
Furthermore, temperature and pressure set values are known for each section, and must be respected so that the quality of the resulting blocks containing carbon is satisfactory and to ensure that the furnace operates correctly, particularly in the preheating area. Volatile materials contained in the tar escape while the blocks containing carbon to be baked are being preheated. It is important that these gases or vapors are drawn in towards the flue walls and burn immediately in the presence of the residual oxygen present in combustion exhaust gases. Otherwise, these tar vapors could form a deposit on the openings, the exhaust ramp and pipes leading to the collection system. These deposits can ignite on contact with incandescent particles of carbon dust. These fires damage flues and their hot exhaust gases burn the filters and fans in collection centers. Considering these risks, safety margins are adopted by increasing the flows of drawn in combustion exhaust gases, which in turn cause excess fuel consumption and reduce the energy performances of the furnace.
Furthermore, it is observed that current regulation of furnaces results in instabilities and generates sudden random variations in the flows of drawn in combustion exhaust gases and fuel flows, such that heat transfer conditions in the furnace are not stable, which has an adverse effect on the efficiency of the heat exchange or heat transfer between the combustion exhaust gases and the said blocks containing carbon.
Finally, this dispersion of the various flows leads to a dispersion in baking levels which makes it necessary to overbake some of the blocks containing carbon or anodes to guarantee the minimum quality in all anodes, which automatically reduces the energy performances of the furnace.
Finally, the current methods used for furnace operation and regulation are characterized firstly by a considerable increase in the number of measurement sensors, and secondly by adoption of large safety margins for each of the main three parameters used to operate the furnace; blowing air on the upstream side of cooling sections, fuel injection in bak

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