Philipp Stolzenburg, LOESCHE GmbH
Dr. Holger Wulfert, LOESCHE GmbH
Introducing new products has long been the tradition of the Dusseldorf-based company Loesche. In keeping with their motto “Innovative Engineering” the first coal dust-operated LOMA® Hot Gas Generator with an output of 30 MWth was delivered to a client in Bangladesh.
The combustion chamber without refractory lining has a coal burner which is operated without a supporting flame – a first in hot gas generator technology.
The basis for this development was a research project with a European university.
To meet all the requirements in the combustion of solid fuels from different sources for large industrial plants, Loesche has a 1 MW test plant which is connected to a Loesche coal mill. All parameters, including the fineness of the coal can be analysed and optimised during the combustion of solid fuels.
A new energy consciousness and the scarcity of precious energy sources has led to the development of hot gas generators which use cheaper fuels, such as solid fuel and gaseous industrial by-products. The cost-saving potential lies between 50% and 70%.
Loesche technology — always one step ahead
For over one hundred years Loesche GmbH has been successfully building machines, such as mills, classifiers,
hot gas generators,rotary gates etc., and been involved in plant construction around the world. Loesche develops, plans and delivers plant components and complete grinding plants to the cement, iron & steel, power station, industrial mineral, ore and wood industries and for nonferrous metallurgy applications.
The first Loesche hot gas generators were developed, built and delivered in 1960, and were available both with and without refractory linings. Which hot gas generator was used depended on the desired outlet temperature for the downstream processes and on the dust content of the process gas to be heated.
Since then these hot gas generators have been subject to continuous further development, always represent the latest know-how and conform to the current technical standards. They are characterised by a clean, complete burning process and low emissions.
Loesche hot gas generators are ideally suitable for direct drying processes and are used for example in conjunction with:
» grinding plants
» drum-type driers
» fluidised-bed furnaces/driers
» flash driers
» spheroidisers
The hot gas generators can run on a wide range of different fuels:
» Gas (such as e.g. natural gas, biogas, coke gas, blast furnace gas, lean gas, synthesis gas and LPG)
» Light crude, diesel and heavy oils, and other liquid refinery waste
» Wood dust, pulverised lignite, pulverised hard coal, pulverised petroleum coke
Loesche combustion chambers come in two different versions: The LOMA® heater, a combustion chamber made from high temperature steel for maximum hot gas outlet temperatures up to 750 °C.
This steel combustion chamber is — thanks to its industrial, economical and ecological advantages — used in most cases. The LOMA® heater is adapted to the relevant combustion and technological requirements, i.e. there are design variants depending on the fuel used and depending on the respective burner type on account of different flame geometries.
This high flexibility on the part of the Loesche combustion chamber demonstrates the innovative approach of Loesche and has been utilised around the world in more than 600 plants with thermal output figures of between 0.1 MW and 60 MW.
The LOMA® heater
The perforated jacket heater developed by Loesche back in 1960 – a steel combustion chamber made of high-temperature steel and a burner muffle — was launched onto the market under the name LOMA® heater. For decades the LOMA® heater has been used around the world in a wide range of thermal processes in order to provide an optimum thermal configuration of the process. It is used to heat:
» Fresh air
» Return gases
» Cleaned exhaust gases
» Hot blast furnace gases
» Process gas with low dust content
These gases to be heated can be introduced at temperatures of up to 350 °C into the spiral housing.
Features of the LOMA® heater
» Combustion chamber manufactured from high-temperature steels, no refractory lining necessary (outlet temperatures up to 750 °C)
» Minimal heat losses on startup of the hot gas generator due to the absence of heating of a refractory lining, therefore startup at full load possible
» Virtually delay-free adaptation of output in the case of rapid load changes
» High combustion chamber cooling rate, preventing thermal overloading of downstream components
» There is no need for an EMERGENCY STACK in EMERGENCY SHUTDOWN SITUATIONS and during startup up and shutdown
» Short-term accessibility for inspections
» Longer service life than plants with refractory linings
» Short installation times, low weight, minimal space requirements, can be installed in existing plants, complete pre-assembly also for larger LOMA® heaters
» Horizontal and vertical erection possible
» Combustion chamber virtually adiabatic – more than 99 % of the heat can be utilised for the process
» Operates both in vacuum and overpressure modes
» No moving parts in the combustion chamber
» Low susceptibility to faults/repairs
» Optical monitoring of perforated jacket temperatures
» Customised plants for individual applications Operational reliability and economic efficiency considerations play amajor role in layout, design and manufacture.
Methods of operation
The flow of process gas entering via the spiral housing ➂ cools the protective jacket ➄, the burner muffle ➁ and the perforated jackets as a result of flow control. The process gas passes through the ring slots ➃ and holes in the perforated jacket into the interior of the combustion chamber, where it mixes with the hot smoke gases from combustion. The combustion chamber geometry and flow control protect the perforated jacket against the flame and the hot smoke gases. The combustion chamber has an overall pressure loss of 2 – 3 mbar.
When process gases with a light dust content and powdery fuels are used, a vertical arrangement of the LOMA® heater is always chosen.
In processes which require a highly uniform temperature profile, specially shaped swirl constructions (e.g. baffle plate) with very low pressure losses (< 1 mbar) are installed in the area of the outlet ➆ or at a short distance behind this area. This results in a highly uniform temperature distribution after only a distance of 1.5 to 2 times the duct diameter.
Hot gas generators for lean gases
The Loesche steel combustion chamber in combination with a multiple-lance burner (MLB) constitutes a special development for burning lean gases and is characterised by the following features:
The multiple-lance burner comprises a number of individual nozzles for lean gas and combustion air, thereby enabling lean gas to be fully combusted without a supporting flame. Both media strike one another within the nozzle system at high velocity. This provides for an intensive mixing of the media, and thereby ensures that the basic requirement for efficient combustion is achieved. The hot smoke gases emerging from the burner muffle mix in the area of the perforated jackets with the process air to be heated and which is fed to the LOMA® heater via the spiral housing. The high proportion of inert gases of sometimes over 75 % in lean gases results in a low flame temperature. For this reason, a start burner installed coaxially in the main burner is used to start the LOMA® heater. The function of this start burner (run on natural gas, coke gas, other burnable gases but also light crude oil) is on the one hand to heat up the interior wall of the burner muffle to a specific surface temperature in order to ensure that the lean gas is safely ignited (the ignition temperature of e.g. CO is 605 °C).
On the other hand, the start burner is dimensioned in terms of its output in such a way that the entire plant can be heated up above the dew point. The start burner is switched off after the main burner has started. The output ranges of the main and start burners are designed to overlap so as to provide continuous operation. The main burner has a control range of 1:10, the start burner has a control range of 1:5 to 1:8. Every necessary operating point within the control range of 1:40 to 1:70 can therefore be easily achieved.
The excellent control performance of the multiple lance burner is a significant benefit to the downstream processes. More than 50 of these hot gas generators developed and patented by Loesche are deployed in conjunction with Loesche vertical roller mills and mills of other manufacturers around the world in dry-grinding plants (e.g. PCI plants) and coal gasification plants for preparing coal for the purpose of smelting iron ore.
Main features of lean gas combustion:
» Combustion of lean gases from a calorific value of 2800 kJ/m3 (standard conditions) without a supporting burner
» Thermal output of 0.5 MW to 60 MW
» Wide control range
» Short flame and stable combustion
» Low CO and NOx content in the exhaust gas
Hot gas generators for solid fuels
Since 1986 the Loesche-developed steel combustion chamber (LOMA® heater) has been successfully used in combination with multifuel burners of different manufacturers to burn wood dust and pulverised lignite. This steel combustion chamber is the prototype of all these combustion chambers which are used for different thermal processes around the world.
Since 1988 Loesche has supplied a large number of hot gas generators with outputs of up to 60 MW to burn wood dust from the chipboard industry. LOMA® heaters run on pulverised lignite are also successfully used. In order to burn solid fuels, a vertical arrangement of the combustion chamber should wherever possible be adopted so as to avoid slag caking.
In view of the increasing scarcity and thus increasing cost of rare energy sources, such as light crude oil and natural gas, customers are increasingly expressing a desire to use hard coal, brown coal or petroleum coke to generate process heat. Loesche has taken up this challenge by instigating an appropriate research programme in conjunction with university institutes, since pure steel combustion chambers for burning hard coal and petroleum coke are barely available on the market. The purpose of this programme has been to gain initial experience of burning solid fuels with a 400 kW dust-fired LOMA® heater.
The Loesche Technology centre also houses an HGG test rig for solid fuels with a heater output of 1 MW. What is unique about this test rig is that it is directly coupled to a Loesche mill. This rig enables Loesche to analyse customer-specific coal of all kinds and origins under real conditions and to determine the combustion parameters.
On the basis of the positive research results and the ensuing development of large-scale plants, it has been possible to successfully implement in Asia the first LOMA® hot gas generator run on pulverised hard coal with an output of 30 MW (pulverised hard coal as the primary fuel, heavy oil as the secondary fuel).
Loesche hot-gas generators for solid fuels are characterised by the following features:
» High degree of mixing between dust and combustion air
» Total combustion
» Pulsation-free flame
» Monitored flame temperature
» No caking on the combustion chamber walls
» Gases and oils can be used as the second fuel
» Complete redundancy with secondary fuel
» No heat-up phase required
» Pulsation-free dosing
