NEW HOT GAS CIRCUIT IN "LEPOL" GRATE PREHEATER FOR CEMENT ROTARY KILNS

Ziel

The aim of the project is to increase the efficiency of grate preheaters of cement rotary kilns and their production capacity (or to reduce their dimensions in the case of new plants) with a concurrent energy saving and a consistent reduction of plant emission per unit of clinker produced (referred to NOx in particular) all this without increasing mechanic and thermic stress of the equipment.
All the raw meal preheaters are using the hot (about 1000 deg. C) and dusty gas from the rotary kiln to preheat and partially calcine the raw meal.
The innovation which has been realized, refers actually to the circuit of the hot gas: a portion of this gas that is drawn from the kiln back end is cleaned from dust and alkalies ands put again into the circuit. In this way the permeability of the layer of raw meal nodules on the "Lepol" grate is improved and, as a consequence, the thermic efficiency of the whole plant is increased.
COMMISSIONING
The "Thermie" contract has started in August 1992 and the installation was completed in September 1995 (after the summer factory shutdown). Commissioning with cold plant was started at the end of September 1995 when tests for the motors, servo-drivers, electrical plant and controls were carried out.
The first tests with hot plant have initiated in the middle of October 1995. In this phase particular care was devoted to the check of the compensators for thermic expansion and to the verification of the perfect functioning of alarm and safety systems throughout the plant.
The first production tests have been carried out at the end of October. The first operation results are presented further on.
PERFORMANCE
After the first months of production tests, steady operation (i.e. without fluctuation of running, quality of the product and other) has been achieved during a period of some 60 days. The average performance data for the period in question compared with the data of previous periods are summarized in the following table :
ORIGINAL "THERMIE" %
PLANT MODIFIED VARIATION
PLANT
OUTPUT t clinker/day 730 830 + 13,70
HEAT CONSUMPTION kJ/kg clinker 3486 3265 - 6,34
HEAT CONSUMPTION TOE/t clinker 0,083 0,078 - 6,34
ENERGY CONSUMPTION kWh/kg clinker 32,20 31,89 - 0,96
- THERMAL SAVINGS E1 1.446 TOE/year
- ELECTRICAL SAVINGS E2 7,3 TOE/year.
DESCRIPTION OF THE FUNCTIONNING OF A TRADITIONAL GRATE PREHEATER
Grate preheater consists basically of a travelling grate similar to a big and very slow running apron feeder made of metallic plates provided with slots and placed in the body of the preheater itself. The raw meal nodules containing some 10-12% water are fed onto the grate forming a 20 cm thick layer. Dry raw meal cannot be fed directly onto the grate otherwise it would pass through the slots of the plates: hence nodulization is necessary. The uniform layer of nodules which obviously covers the upper part of the grate moves slowly with the grate from the feeding end to the unloading end, i.e. to the kiln back end.
Preheater is divided into 2 compartments: the layer of the nodules goes through both the preheater compartments one after the other whereas the hot gas drawn from the kiln flows in the opposite way: at first it passes through the nodules in the first compartment where the gas temperature drops from 1000 deg. C to 300-350 deg. C. After prededusting, it passes through the nodules in the second compartment where its temperature drops from 350 deg. C to about 100 deg. C. Then the gas is drawn into the electro precipitator for final dedusting before being let in the atmosphere.
LIMITS AND PROBLEMS OF THE GRATE PREHEATER
The problem that hinder apparently an easy realization of this improvement is the presence of dust and alkali. * A noticeable amount of dust leaves the kiln with the combustion gas
* The alkali and chlorine, mainly sodium and potassium compounds in form of gas above 1000 deg. C, condensing as fine powder at about 900 deg. C
THE SOLUTION WE HAVE STUDIED
The solution which has been studied to reduce at maximum the dust and alkali cycle inside the kiln, mainly of 2 circuits:
a) hot gas cleaning circuit
b) auxiliary thermic circuit
a) HOT GAS CLEANING CIRCUIT
We have essentially realized a new hot gas circuit with the aim of eliminating a large rate of dust and alkali from the preheater, thus favouring an easier passage of the gas through the nodules. As a result the efficiency and the performance of the burning plant are improved.
A considerable portion of dust and alkali rich gas is withdrawn from the kiln back end, cooled down up to 400-500 deg. C by mixing it with cool gas with 250 deg. C which is taken the lower part of the grate, dedusted by means of cyclones and blown into the 2nd chamber to dry the wet nodules. Finally the gas is dedusted in the electro precipitator.
b) AUXILIARY THERMIC CIRCUIT
Actually there is the possibility of introducing the necessary thermic energy at 2 different points of the plant, namely at the main burner located at the kiln discharge end and at an auxiliary burner located in a combustion chamber installed outside of the kiln itself and designed to obtain a perfect combustion of the fuel. Hot air from the clinker cooler is used as secondary air. The gas generated in this way is let into the first chamber of the "Lepol" preheater.

Programm/Programme

ENG-THERMIE 1 - Programme (EEC) for the promotion of energy technology in Europe (THERMIE), 1990-1994

Thema/Themen

2.3 - INDUSTRY

Aufforderung zur Vorschlagseinreichung

Data not available

Finanzierungsplan

DEM - Demonstration contracts

Koordinator

Cementeria di Cassago SpA

EU-Beitrag

Keine Daten

Adresse

Via Stazione 1
23893 Cassago Brianza Lecco
Italien

Gesamtkosten