GB2069668A - Improvements in or relating to lime kilns - Google Patents

Abstract

Pulverised or semi-pulverised solid carbonaceous or carbon-containing material is used as a fuel in firing vertical shaft lime kilns. The fuel is introduced into the kiln by injection preferably by being injected with the primary combustion air which may possibly contain recirculated waste gases.

Description

SPECIFICATION Improvements in or relating to lime kilns This invention relates to lime kilns particularly vertical shaft lime kilns.

Early designs of vertical shaft lime kilns were fired with solid fuel and were defined as mixed feed kilns. Alternate layers of limestone and fuel were charged into the kiln top to maintain a full shaft as the lime product was discharged from the kiln base. Common fuels used were wood, coal and coke and the kilns operated under natural draught.

In the particular case of coal firing, there was a serious problem with atmospheric pollution and loss of thermal efficiency as coal volatiles distilled off at the kiln top. Various designs were developed in an attempt to overcome the problem. These included the Spencer kilns and their derivatives where coal was fired into the kiln part way down the shaft. In general the limestone passed through a slot formed in the refractory in the upper part of the kiln and carried by twin arches across the kiln. Beneath the arches, the kiln returned to full diameter and the natural outward movement of the stone below the arches created exposed sloping surfaces on to which the coal was charged either manually or by machine.The principle was that the coal/limestone mixture was carried downwards into the main burning zone and coal volatiles released could burn as they passed up through the hot stone in the slotted section of the upper part of the kiln. This led to excessive temperatures in this region of the kiln and results were only partially successful. Serious pollution problems remained. The quality of lime was variable.

Another approach to the problem was to convert the coal to producer gas in a separate gasifier and use the gas so produced to fire the kiln. The producer gas kilns developed were the forerunners of modern oil and gas fired vertical shaft kiln designs and were capa ble of giving good quality lime. The problems with the gas producer were that if the gas was made in a single stage producer and used in a hot raw state to achieve maximum thermal efficiency, then depositions of carbon and tar occurred in the flues and these required regular "burning out". Again this resulted in se vere pollution. Also the older designs of single stage manually clinkered producer were difficult to control. Gas quality was variable.The use of a more modern two stage producer could overcome this problem, but such producers are high in capital cost and very selective in terms of coal qualities and sizes that may be used. They are therefore not seen as a practical solution to the problem of coal firing for lime kilns. It is well known that the use of simple burners whether oil or gas fired created difficulties in terms of lime kiln design and the solution to providing effective oil and gas firing for simple vertical shaft lime kilns has been the subject of a number of patents including Nos. 1,315,040 and 1,391,259.

The use of a pulverised fuel burner has the same limitations as the oil or gas burner with excessive heat release at the firing port and limited penetration of heat into the charge in the kiln.

The object of the present invention is to provide a method of firing a vertical shaft lime kiln with coal and other solid fuels in such a manner as to permit the use of such fuels for the production of high quality lime in an efficient design of vertical shaft kiln whilst overcoming the problem of pollution associated with earlier designs and at the same time avoiding the limitations of the simple burner systems.

According to the present invention we provide a method of firing a vertical shaft lime kiln comprising the step of injecting into the kiln as a fuel pulverised or semi-pulverised solid carbonaceous or carbon-containing material.

This provides a high efficiency kiln firing system similar to the oil and gas firing systems presently used in vertical shaft lime kilns.

The injection of the solid fuel into the kiln is preferably pneumatic, and preferably by way of the primary combustion air for the kiln, with the possible addition of the recirculated waste gases from the kiln to control combustion in the kiln.

The solid fuel may be for example coal, charcoal, husks or nuts, wood chips, or any other suitable carbonaceous material, or admixtures of any of these.

In an example of the present invention as applied to a vertical shaft lime kiln, the kiln construction is as disclosed in the aforesaid Patent Specifications.

Briefly, the kiln comprising a shaft surrounded by a ring manifold to which recirculated exhaust products are supplied. A fuel ring manifold is disposed below the ring manifold and is in communicaton with a number of fuel injectors around the shaft and angled downwards towards the centre of the kiln. The fuel is injected into the kiln through carburetting chambers having, for example an internal diameter of 1 foot 9 inches. A primary combustion air source is associated with the fuel injectors as is the recirculated waste gases from the kiln. The reaction, i.e. lime formation, occurs in the burning zone region of the kiln shaft.

The distinction over the aforesaid kiln lies in the fuel.

In this instance the fuel is solid carbonaceous material and for convenience we shall refer to coal as the fuel.

Coal is pulverised or semi-pulverised in a hammer mill and is injected into the kiln by way of the kiln's fuel injectors in stream with the primary combustion air for the kiln, and the recirculated waste gases.

In effect the primary air/pulverised coal waste gas mixture can be considered as a gas for combustion purposes.

The pulverised coal is preferably conveyed from the hammer mill to a convenient storage hopper from which it is distributed to the kiln fuel injectors in a controlled and metered manner when required.

The preferred particle size is less than 500 microns with a significant proportion, say upwards of 50% being - 75 microns.

The fuel is distributed in an arc in the upper part of the combustion chamber of the kiln either by a static or oscillating distributing means.

The ash content from the fuel is dissipated in part in the kiln in the form of dust through the top of the kiln with the exhaust products and in part with the fine lime discharged from the kiln.

The carbonaceous material selected for the fuel, it will be appreciated, determined the final quality of the lime.

With the present invention it is possible to employ in the firing of vertical shaft kilns an atmospheric polluting fuel in a non-polluting manner.

Claims (8)

1. A method of firing a vertical shaft lime kiln comprising the step of injecting into the kiln as a fuel pulverised or semi-pulverised solid carbonaceous or carbon-containing material.

2. A method as claimed in claim 1 comprising the step of injecting the fuel pneumatically into the kiln.

3. A method as claimed in claim 2 comprising the step of injecting the fuel into the kiln with the primary combustion air of the kiln.

4. A method as claimed in claim 3 comprising the step of adding to the primary combustion air recirculated waste gases from the kiln.

5. A method as claimed in any one of claims 1 to 4 in which the fuel is coal, charcoal, husks or nuts or wood chips or admixtures of any of these.

6. A method as claimed in any one of claims 1 to 5 in which the material constituting the fuel has a particle size of less than 500 microns

7. A method as claimed in claim 6, in which iipwards of 50% of the fuel material has a particle size of - 75 microns.

8. A method of firing a vertical shaft lime kiln, substantially as hereinbefore described.