WO1998038266A1

WO1998038266A1 - Coal additive

Info

Publication number: WO1998038266A1
Application number: PCT/AU1998/000119
Authority: WO
Inventors: Nian Fang Zhou; Liang Qin
Original assignee: COALCORP PTY Ltd
Current assignee: COALCORP PTY Ltd
Priority date: 1997-02-26
Filing date: 1998-02-26
Publication date: 1998-09-03
Anticipated expiration: 1999-08-26
Also published as: AUPO534397A0; CA2287239A1

Abstract

A base additive for coal comprising 9-11 % potassium or sodium permanganate or a mixture thereof, 27-33 % sodium or potassium carbonate or a mixture thereof and 54-66 % sodium chloride, potassium chloride or a mixture thereof. This base additive may be utilized as a base formulation or key additive with other components wherein the other components may be varied as may be required so as to accord with a desired specific type of coal which is to be fed to a boiler to provide an optimum combustion rate. The other components may be selected from: i) sulphur fixative; ii) catalyst to facilitate the action of the sulphur fixative; iii) combustion catalyst; iv) oxidant; v) loosening agent; vi) de-ashing and de-dusting agent; vii) anti-corrosive agent; and viii) calcium sulphate decomposition inhibitor.

Description

TITLE "COAL ADDITIVE" FIELD OF THE INVENTION THIS INVENTION relates to a coal additive which may be added to coal prior to combustion in a furnace of a boiler.

BACKGROUND OF THE INVENTION

Air pollution is a major problem in the world and, in particular, in those countries which utilize coal as a major fuel for industrial boilers or furnaces, such as China. The emission of air-borne pollutants forms an "acid rain" on a large scale which is threatening the ecological balance of agriculture, especially in South China. Coal is the main energy source in China. The emission of gases from the conversion of coal into energy is one of the reasons for this environmental damage. In China, the methods which are utilized in this country in relation to mining of coal and combustion of coal as well as storage and transportation of coal also cause air pollution problems, due to the emission of particulate matter.

Coal accounts for 74% of the sources of energy which are available in China and this demand will not change until the end of this century and the beginning of the 21st century. This is decided by the type of energy sources which are available and the level of economic development. In China, the majority of coal reserves are of marine origin and hence have a high sulfur content. After burning of the coal, the main pollutants which are emitted to atmosphere are sulfur dioxide, smoke, dust particles, nitrogen oxides and carbon monoxide. Thus, air pollution is mostly caused by incomplete combustion of the coal.

According to the available statistics, about 90% of SO₂ in the atmosphere and 96% of the available smoke and dust particles in the atmosphere result directly from coal combustion. Thus, air pollution is a critical environmental problem in China. It has a great effect on human life and on working conditions. It is therefore necessary to urgently institute appropriate procedures in relation to controlling the emission of sulfur dioxide, smoke and dust particles to atmosphere.

Coal is utilized in boilers for steam generation which then may be utilized for generation of electric power using, for example, a steam turbine. The electric power may then be applied to internal heating of buildings or for heating purposes in relation to various industrial processes which may include, for example, evaporation or distillation processes which occur, for example, in sugar juice processing and alcohol distillation. Boilers usually comprise a combustion chamber or furnace to which coal may be fed in any suitable fashion, such as by:- (i) manual feed utilizing a shovel;

(ii) belt feed wherein the coal is slowly passed through the furnace on a moving belt which may comprise a steel chain; or

(iii) jet feed wherein the coal in finely divided form is blown into the furnace through a door in a side wall thereof under the influence of compressed air. The furnace is also provided with a flue or chimney for discharge of gases to atmosphere. The furnace of the boiler usually surrounds or is located adjacent to a boiler chamber containing water from which steam may be passed to a header tank by a multiplicity of pipes in communication with the boiler chamber.

In most coal fired furnaces, 20-30% of the coal is not completely burnt or combusted and this may be due to inadequate air supply which may not reach internal regions of the coal being burnt. This may be demonstrated by the decomposition of the coal to elemental carbon which will result in black smoke being evolved which is an air borne pollutant. Incomplete combustion of the coal may also occur at relatively low temperatures, i.e. below 400°C.

The sulphur content in coal comprises both organic sulphur and inorganic sulphur and the presence of sulphur is also another factor in causing inadequate combustion of the coal because of the fact that the sulphur in the form of sulphate is difficult to decompose and is incombustible which may be demonstrated by the evolution of dust from the furnace. In general, however, the majority of organic sulphur and inorganic sulphur is combustible and this is demonstrated by the evolution of sulphur dioxide through the flue of the furnace which is another air borne pollutant. It is therefore desirable to carry out desulphurisation of coal prior to discharge into the furnace. However, this requires high capital investment in the form of complex technology. In developed countries, such as Japan, desulphurisation processes may amount to 30- 40% of total capital investment.

It is therefore imperative, at least in the case of China, that a new additive be developed which can improve combustion of coal which may then result in:-

(a) improvement in combustion efficiency which will then result in substantial savings in relation to reduction of coal being fed to the furnace;

(b) decrease in evolution of black smoke and dust;

(c) decrease in emission of sulphur dioxide from the furnace; and (d) to provide substantial savings in operating the furnace as well as to provide increased operational efficiency.

Reference may therefore be made to Chinese Specification

90106347.9 which refers to development of a coal additive which attempts to comply with objectives (a), (b), (c) and (d) above. This coal additive comprises a composition of:-

(1 ) 14-16% of sodium nitrate which is described as a combustion catalyst;

(2) 0.2-0.4% of potassium permanganate which functions as an oxidant;

(3) 0.5-1.5% of sodium carbonate which is utilized as a sulphur fixative;

(4) 0.3-0.7% of potassium oxide; and

(5) the balance comprising sodium chloride wherein both of potassium oxide and sodium chloride are described as loosening agents.

Sodium nitrate as a combustion catalyst is described in this reference as improving combustion efficiency of the coal which therefore will facilitate oxygen diffusing to internal regions of the coal and reducing the amount of surplus air externally of the coal. This also enables the oxygen to mix with other combustible gases thereby expanding the range of ignition points of the combustible gases. This will result in the complete combustion of suspended carbon particles and other incompletely combusted coal particles as well as complete combustion of the combustible gases present in the furnace with a resulting increase in furnace temperature to increase boiler efficiency.

Potassium permanganate as an oxidant increases combustible efficiency of the coal whereby volatile combustible components of the coal as well as free carbon particles and smoke may also be more readily burnt or combusted in the furnace. The permanganate, as described in this reference, is decomposed to form a corresponding oxide or salt having a lower Mn valency at higher temperatures thereby evolving oxygen which strengthens the combustion of half coked or fully coked coal layers to burn without flame and raise the furnace temperature. Sodium carbonate is described as a sulphur fixative whereby the boiler may be deodorized and remove the boiler of ash and dirt. As further described in Chinese Specification 92106423.3 referred to hereinafter, sodium carbonate can render the coal cinders porous, loose and powdery so as to facilitate removal of the cinders from the walls of the furnace.

Sodium chloride as the main component of the adjuvant is a loosening or bulking agent which functions by cracking of coal layers and also making such coal layers fluffy which will enhance coal combustion. The melting point of sodium chloride which is above 800°C will result in exploding of the sodium chloride crystals to agitate the coal and air layers in the furnace. Potassium oxide may also function as a loosening agent.

Reference may be made to Chinese Specification 92106423.3 which refers to a coal additive comprising 30-38% of sodium nitrate as a catalyst, 0.3-1.0% of potassium permanganate as an oxidant, 2-4% sodium carbonate as a sulphur fixing agent and the balance being sodium chloride which is a loosening agent. This additive is described in Chinese Specification 95100922.2 referred to hereinafter as having only a limited coal saving and combustion improving function and low sulphur fixing efficiency.

Reference may also be made to Chinese Specification 95100922.2 which refers to a coal fixing additive which includes:-

(a) as sulphur fixing agent, calcium carbonate or calcium oxide;

(b) as oxidant potassium permanganate or sodium nitrate; (c) as bulking or loosening agent, sodium chloride;

(d) as a stabilizer silica or strontium oxide;

(e) as a catalyst ferric t oxide; and

(f) sodium carbonate or carborundum which may be utilized as an auxiliary agent. Various formulations of the additive are described in different ranges of proportions which may be utilized for different end purposes. Such purposes include sulphur fixation alone where sulphur fixation is the sole concern of the customer or coal saving alone where coal saving is the sole concern of the customer. Another end purpose described is that of a combined coal saving-sulphur fixation application.

However, the practice of supplying different formulations dependent on the relevant end use required by the customer is disadvantageous in that coal deposits in different parts of the world will vary widely in chemical composition and physical characteristics and thus will have widely differing requirements in relation to obtaining optimum combustion efficiency. In this regard, it must be realized that coal is a generic name given to a dark burnable solid which is usually layered and which has resulted from accumulation and burial of partially decayed plant matter over earlier geological ages and later effects of temperature and pressure. This explains why coal characteristics vary widely. Differences in type are caused by variations in the amounts of different plant parts exemplified by common-banded, splint, cannel and boghead coals. The degree of coalification is referred to as rank and makes up a natural series with increasing carbon content and comprises brown coal and lignite, sub-bituminous coal, bituminous coal and anthracite. The impurities in coal cause differences in grade, such as in lignite and brown coal. The higher rank coals, according to the ASTM classification system, are specified by fixed carbon for volatile matter < 31 % on a dry, inorganic matter free basis and lower rank coals are classified by calorific value on a moist, inorganic matter free basis. In another classification set by European Economic

Community, coals with volatile matter up to 33% are divided into classes 1-5. Coals with volatile matter greater than 33% are each divided into classes 6-9. These classes are each divided into four groups determined by caking properties as measured through the free swelling index or the Roga index. These tests indicate properties observed when the coal is heated rapidly. The brown coals and lignites have been classified separately and have been defined as those coals with heating values less than 23.85 MJ/kg (10, 260 Btu/1 b, 5700 kcal/kg).

From the foregoing, therefore, it will be appreciated that supplying several different formulations dependent upon the relevant end use does not go far enough in providing a coal additive which can provide for optimum coal combustion efficiency so as to take into account the widely differing chemical and physical properties of the coal being mined or the source of the coal.

Another disadvantage in regard to providing several different formulations dependent upon the relevant end use is that this practice is extremely expensive as it requires plant and equipment which has to be modified in regard to having different production lines which produce each of the relevant formulations required or, alternatively, a single production line will have to be modified when a different formulation is required. Another disadvantage of the prior art coal additives is that such additives only have limited value in regard to lower quality coal having a low volatility due to a high sulphur content.

It is therefore an object of the present invention to provide a base additive for coal which may alleviate to some extent the aforementioned disadvantages.

SUMMARY OF THE INVENTION The base additive of the invention includes 9-11 % (more preferably 9.5-10.5%) potassium or sodium permanganate or a mixture thereof, 27-33% (more preferably 28.5-31.5%) sodium or potassium carbonate or a mixture of thereof and 54-66% (more preferably 57-63%) sodium chloride, potassium chloride or a mixture thereof. This base additive may be utilized as a base formulation or key additive with other components wherein the other components may be varied as may be required so as to accord with a desired specific type of coal which is to be fed to a boiler to provide an optimum combustion rate.

Suitably the coal additive of the invention comprises 8-12% of the base additive and more preferably 10% with the remainder comprising the other components.

The potassium permanganate or sodium permanganate functions as an oxidant as described above in relation to Chinese

Specification 90106347.9. The sodium chloride or potassium chloride functions as a loosening or bulking agent as described in relation to the prior art. The loosening or bulking agent may also function as a "de-ashing or de- dusting" agent which facilitates the removal of ash, dust or dirt from the walls of the furnace.

The sodium carbonate or potassium carbonate may also function as a sulphur fixative as described in Chinese Specification 90106347.9 and also to facilitate removal of the coal cinders from the walls of the furnace as described in Chinese Specification 92106423.3. The other components may comprise:-

(i) calcium oxide or a calcium oxide source such as calcium carbonate for use as a sulphur fixative preferably in finely divided form having a particle size of 250 micon or less; and (ii) ferric oxide or ferric trioxide as a catalyst which may facilitate the function of the calcium oxide or calcium oxide precursor as sulphur fixative; and (iii) sodium nitrate or potassium nitrate or mixture thereof as a combustion catalyst. The other components may also include silica, borax

(Na₂B₄0₇) and zinc metal powder. Silica is a good high temperature corrosion inhibitor and its inclusion may facilitate the action of NaCI as a de-ashing, de-dusting and loosening agent as hereinafter described. Borax may also facilitate the action of NaCI. Zinc powder is also a good corrosion inhibitor. Ferric oxide and silica may also inhibit the decomposition of calcium sulphate at high temperatures to inhibit the possibility of sulphur dioxide being evolved from the furnace.

Suitably the coal additive of the invention comprises 8-12% of the base additive and 25-40% sodium nitrate or potassium nitrate or mixture thereof, 2-3% sodium or potassium carbonate or mixture thereof,

5-20% calcium carbonate or calcium oxide, 3-7% ferric oxide, 30-60% sodium or potassium chloride or mixture thereof, 0.5-1.5% zinc oxide and 0.5-1.0% potassium or sodium permangante or mixture thereof.

As described in Chinese Specification 95100922.2, sulphur content in coal comprises inorganic and organic sulphur. The organic sulphur may be provided with the formula R - S where R is an organic group. Inorganic sulphur may be represented by the compound iron disulfide or FeS₂. When coal is subject to combustion, the following reactions may take place:-

S + O₂ _ S0₂ 4 FeS₂ + 11 0₂ → 2 Fe₂0₃ + 8 S0₂

7 FeS₂ + 6 0₂ → Fe₇S₈ + 6 S0₂ 3 Fe₇S₈ + 38 0₂ → 7 Fe₃0₄ + 24 S0₂ R - S + n 0₂ - n S0₂ + n H₂0. It is also noted in Chinese Specification 95100922.2 that if calcium oxide is included in a coal addition prior to combustion of the coal then calcium oxide will combine with S0₂ to form calcium sulphite under the influence of Fe₂O₃ as a catalyst. Calcium sulphite will then be oxidized to form calcium sulphate. Fe₂0₃ can also catalyse the oxidation of S0₂ to form sulphur trioxide which will subsequently react with calcium oxide to form calcium sulphate. In another less probable reaction, sulphur dioxide and oxygen and calcium oxide will form calcium sulphate. Calcium sulphate will only be decomposed above 1450°C and as the maximum temperature in a coal furnace will usually not exceed 1100°C then this means it will not be decomposed. In relation to the formation of calcium sulphate from calcium sulphite, less calcium sulphate is formed when the furnace temperature is above 760°C. The rate of sulphur fixation is increased by the use of Fe₂0₃ which catalyses the formation of sulphur trioxide from sulphur dioxide.

CaO is preferred as a sulphur fixative as it is cheap and readily available. However, CaC0₃ may be utilized if required. Fe₂0₃ as catalyst is also preferred as it is cheap and is easy to obtain. The efficiency of absorption of SO₂ by CaO will increase proportionately with increase in temperature and this will also increase the speed of the reaction in the beginning of the reaction. However, as the duration of the reaction increases, the rate of absorption will decrease because S0₂ decreases its rate of diffusion into internal layers of the coal and this effect is also caused by the formation of CaS0₄. Therefore, combustion efficiency will be increased if the CaO particles have a smaller diameter because this will mean the CaO particles have a larger surface area per unit volume which will promote absorption efficiency of S0₂ by CaO and thus bring the reaction between CaO and S0₂ to a more rapid conclusion.

Use of Fe₂0₃ mainly accelerates the reaction between CaO and S0₂ to form CaSO and due to the high temperatures within the furnace oxidation of CaS0₃ to form CaSO₄ will increase. Therefore, use of Fe₂0₃ will increase the sulphur fixation rate. Sodium carbonate or potassium carbonate has a similar function.

It is also preferred in relation to the fixation of sulphur that the reaction temperature be maintained below 500°C. This means that before coal is passed into the furnace, the temperature of the coal should be maintained at a temperature of below 500°C and thus sulphur fixation may occur in a period of less than 30 minutes. The ignition temperature of R - S is less than 402°C. Therefore, if the temperature is greater than this temperature then the rate of formation of S0₂ will increase and this will mean that the contact time before SO₂, CaO and the catalyst will be too short and thus sulphur fixation will not occur at a satisfactory rate.

The role of the oxidant, such as permanganate, is to produce oxygen under the high temperatures attained within the furnace which complements the limited air which is available as referred to previously. This also promotes efficient exhaustion of floating carbon particles from the furnace and thereby reduces the amount of black smoke which is evolved from the furnace. The oxidant facilitates the complete combustion of the coal within the furnace.

In the burning layers of coal within the furnace, the coal is usually present in the form of coke which accumulates between the layers of coal. Therefore access to the coke from the oxygen in the air is substantially reduced and this promotes combustion inefficiency. However, within the high temperatures of the furnace, the use of a loosening agent can form a micro-explosion to thereby agitate or "crack" the coal layers and thus make the coke available for combustion. The "loosening" of the coal layers will therefore improve their porosity to facilitate more efficient combustion. Another effect of the cracking of the coal layers is the facilitate contacting of the end product of the coke which is usually a grey ash after combustion to contact the available oxygen. Also formation of "point cavities" within the coal or coke by the "loosening agent" will promote combustion efficiency. Therefore, the combined effect of the oxidant and the loosening agent will be to markedly reduce the consumption of coal within the furnace. It also reduces the amount of carbon in the coal cinders after combustion and the amount of black smoke emission is also substantially reduced.

Suitable "loosening" agents include NaCI which may also tolerate a minor amount of potassium oxide. KCI may also be utilized for this purpose if required.

It will also be appreciated that as a coal furnace is subjected to continued usage, the thickness of the furnace wall may reduce owing to corrosion and also the amount of contaminants, such as coal dust, ash and smoke which impacts on the furnace wall will increase with the passage of time. These effects will cause in turn a decrease in thermal efficiency of the boiler together with an increase in energy consumption, as well as coal consumption. It will also be appreciated that the available area for air flow will decrease and the amount of smoke that is emitted will increase. It has been ascertained, for example, that coal consumption will increase 5% each day as the dust thickness on the furnace wall will increase 1 mm. An analysis of the dust on the furnace wall has demonstrated that the main constituents are CaS0₄.AI₂(S0₄)₃, MgS0₄ and K₂S0₄. .These sulphates mainly originate from the reaction of SO ₂ with Ca, Al, Mg and K in the coal and CaO when used as the sulphur fixative. Water will also react with the smoke to cause staining of the furnace wall. It has been found that NaCI will react with these sulphates to produce sodium sulphate and the relevant chloride which includes MgCI₂, KCI, AICI₃ and CaCI ₂ Each of these chlorides have melting points of 714°C, 776°C, 780°C and 772°C which are all lower than 801 °C which is the melting point of NaCI. MgCI₂, for example, reacts with water to produce MgO and HCI and thus easily dissolves in water. The formation of these chlorides makes the dust on the furnace wall loose, powdery and porous and thus facilitates the removal of the dust from the furnace wall and associated boiler apparatus. Borax is decomposed at 741 °C to produce sodium metaborate which increases the alkalinity of the dust and also decreases the melting point of the dust thereby facilitating its removal.

Silica is a good high temperature corrosion inhibitor and reduces the possible corrosive activity of sodium sulphate. Its use, therefore, extends and protects the furnace wall of the boiler and associated apparatus.

The use of powdered metallic zinc also inhibits corrosion of the furnace wall and associated apparatus. Zinc has a melting point of 419°C and zinc is oxidised to zinc oxide during operation of the furnace so as to provide a protective layer of zinc oxide or zinc which has not been oxidised. The zinc or zinc oxide neutralises any sulphuric acid that is generated to prevent corrosion by sulphuric acid. It has been found that after inclusion of powdered zinc in the additive that the output of the boiler will increase with a resulting efficiency a thermal efficiency of the boiler and a resulting saving in energy consumption.

The amount of corrosion inhibitor included in the coal additive of the invention is 0.5-1.5%.

Calcium sulphite can decompose at temperatures over 1200°C to form CaO and S0₂ and calcium sulphate can also decompose at temperatures greater than 1450°C to form CaO, S0₂ and 0₂. These events inhibit boiler efficiency and also promote the causing of air borne pollution because of the evolution of S0₂. Such decomposition of calcium sulphite and calcium sulphate can be prevented by the addition of Fe₂0₃ and Si0₂ which forms a stable macromolecular complex -Ca-Fe-S-S₂-0-. Therefore, the addition of Fe₂0₃ and Si0₂ can greatly increase the sulphur fixing efficiency of the additive of the invention.

By using the coal additive of the invention which may include one or more of the following:-

(i) sulphur fixative;

(ii) catalyst to facilitate the action of the sulphur fixative; (iii) combustion catalyst;

(iv) oxidant; (v) loosening agent; (vi) de-ashing and de-dusting agent; (vii) anti-corrosive agent; and (viii) calcium sulphate decomposition inhibitor, it has been found that the following results may be obtained.

• The temperature of the flame in the furnace chamber is increased by over 10%;

• The combustion efficiency of the boiler is improved by over 10%;

• The power output of the boiler is enhanced by over 5- 11 % dependent upon the formulation which is utilized;

• The carbon content in the furnace cinder is reduced by over 10%;

• The emitted smoke blackness is decreased by 0.5-2 Lingerman magnitude;

Emitted S0₂ is reduced by 25-60% dependent upon the type of formulation which is utilized; and • The coal saving rate is 5-25% dependent upon the type of formulation which is utilized.

EXPERIMENTAL EXAMPLE 1

In regard to manufacture of the base additive, 10% of potassium permanganate, 30% sodium carbonate and 60% sodium chloride were pulverized to have a resulting particle size of between 100- 200 mesh and then feed into a stirring machine to provide a homogeneous mixture. The base additive was then supplied to users on an experimental basis. Such users also carried out an initial analysis of the coal being mined or which was available in their region and upon such analysis being provided, the identity of other components and their relative proportions were then supplied to the user who added the other components to the base additive. In each case, the other components were pulverized to have a resulting particle size of between 100-200 mesh and were added to the base additive by stirring. When being fed into the stirring machine, the feeding speed was slow. After 10 minutes stirring, the resultant products should be light red or dark brown and also the colour and texture of the product should be uniform.

Five different recipes of coal additive were chosen and these types are as described in Table 1 herein as Recipes 1-5. Five different applications were also designed for Recipes 1-

5 and these are described in Table 1 as Types I to V. Type I is designed primarily as a sulfur fixative and also for use in chain boilers, Type II is designed primarily for coal saving and also for use in chain boilers, Type III is designed as a comprehensive type for both sulfur fixation and coal saving and also for use in chain boilers, Type IV is designed primarily for a coal powder jet boiler and Type V is a general additive for use in chain boilers.

Table 2 shows each of the different Recipes 1-5 showing the amount, of base additive or key adjuvant that is employed together with remaining ingredients. Tests were then carried out in regard to Recipes 1-5 and such tests are reported in Tables 3-7. Each of these tests were carried out using the specific boilers described in the column headed "BOILER TYPE" and the percentages in relation to percentage decrease in S0₂, percentage increase in output and the percentage decrease in coal saving are also provided. These results are in accordance with the ranges given above.

It was found in relation to the tests that were employed that at most, a 10% tolerance could be employed. Outside this tolerance, the advantages discussed above were not obtained. It was also found that when a 5% tolerance was utilized that the relevant advantages as discussed above were obtained with less than 1% variation.

In Tables 3-7, the results set out in regard to Recipe 4 were carried out in a laboratory environment and not in an industrial environment. The results are still highly encouraging. The test coal was crushed down to 90 microns and the base additive was added at the rate of 0.2% by weight.

One principle advantage which is achieved by the use of a base additive as described above is that not only can the end additives be utilized with a wide variation in boiler type but also the use of a base additive means that low quality coals having a low volatility due to a high sulphur content can now be processed economically in boilers. In countries such as China, Russia, England and other European countries, due to the extensive mining that has taken place, only low quality coal deposits remain to be mined. Other advantage of the base additive of the invention may, in some cases, provide an increase in the combustion temperature of up to 200°C which means that less coal can be consumed with the same power output. EXAMPLE 2

Measurements of combustion gas emissions, corrected where appropriate to reference conditions of 0°C, 101.3 kPa, dry gas, 6% oxygen are given in Table 8. The coal testd was lignite from Germany having 2% ash, 47% volatile matter and 0.25% sulfur content.

The furnace utilized for combustion of the coal incorporated a combustor section, air and water cooled heat exchangers for cooling the flue gases and a bag filter for flue gas clean-up.

The combustor section has overall dimensions 1 m x 1 m x 5 m high and consists of a fluidized bed module, a transition section and an extended freeboard section. The mild steel casing is refractory line throughout. The furnace incorporated a fluidized bed module having an access door and a series of ports for instrumentation monitoring temperature and pressure. An offtake port in the access door acts as an overflow weir to allow bed material to be taken off when the nominal fluidized height is equal to 0.35 m. The internal cross section area of the bed is 0.19 m². A 68 mm diameter air classifier in the base of the fluidized bed can be used to withdraw either whole bed or a selected coarse size fraction. The transition section above the bed contains two ports for feeding solids onto the surface of the bed, a gas fired ignition burner, a viewing port and a bursting disc/vent duct. Fluidizing/combustion air supplied from a forced draught fan enters the bed through a standpipe distributor. Recycled flue gases taken from downstream of the bag filter can also be introduced to the bed through the same distributor. The air flow rate is controlled by a manual damper. This air can be preheated (usually as a means of start-up) using an in-duct natural gas burner. Natural gas for start-up or continuous firing purposes can also be supplied to the bed through an independent pipework grid.

The refractory lined freeboard continues above the transition section,for a further 3.6 m, providing adequate residence time for a wide range of combustible materials and operating conditions. Instrumentation and secondary/purge/air inlet ports are sited throughout these two sections. Secondary air, if required, is supplied by the same fan as the primary (fluidizing) air; the flow rate is controlled by a manual damper.

Recycled flue gases can be introduced above the bed through the same ports as secondary air. Flue gases and suspended solids exit the combustor via a refractory lined crossover duct at the top of the test rig and pass into a ceramic heat exchanger. This heat exchanger consists of 48 air cooled silicon carbide tubes. Gases pass through the heat exchanger in three passes, reducing in temperature by up to 600°C, depending on operating conditions,.

The water cooled heat exchanger consists of 15 off 1¹/2 inch nominal bore smoke tubes 900 mm long, enclosed in a cylindrical water jacket. This reduces the flue gas temperature by approximately 100°C.

Most of the suspended solids are separated from the flue gases leaving the heat exchangers in a cyclone. The solids are discharged to atmosphere via the induced draft fan.

The bag filter contains 16 off 1.5 m length porous rigid ceramic elements, with on-line and off-line cleaning capabilities.

The above-bed feed system consists of a variable speed 100 mm diameter screw plus hopper. This discharges into a 125 mm diameter screw, which is set to run at a constant high speed and act purely as a part-filled transfer screw. This transfer screw discharges material onto the surface of the fluidized bed.

The additive feed system used in most of the tests was a self-contained weigh hopper discharging via a variable speed 25 mm diameter screw into a compressed air-driven venturi. The metered powder feed was pneumatically conveyed into the body of the 125 mm diameter transfer screw at a point 1 m prior to its discharge. This allowed adequate time for the additive in the form of a powder to mix intimately with the coal in the screw before their simultaneous discharge onto the fluidized bed surface.

Furnace exit flue gas oxygen concentrations were measured and indicated on a portable electrochemical cell type "Servomex" analyzer. The sample point was located at the top of the above-bed combustion chamber prior ro the air cooled heat exchanger. Emissions of S0₂ were measured using a "Testoterm" portable analyzer sampling from the plant exit downstream of the heat exchangers, cyclone and bag filter. In this analyzer waste, gases were withdrawn from the outlet duct via a stainless steel probe and passed through a heated line to a gas preparation unit. Within the unit, the flue gas is cooled rapidly within a Peltier element to remove condensate whilst preventing absorption, and hence loss, of S0₂. The gas is then passed to the analyzer which uses specific electrochemical cells for the detection and measurement of the required species.

Data obtained from the "Testoterm" instrument was processed as follows: a scan of all analyzer readings was taken every 30 seconds for a test period of at least 30 minutes. The mean values for all scans in each period was calculated and reported. Tests 1 , 2 and 3 were carreid out without additive and tests 4, 5 and 6 were carried out with additive. Upon a review of the results of this experiment shown in

Table 8, it was noted that there was a very significant reduction (i.e. 89%) in the emission of sulphur dioxide when additive comprising Recipe 1 from Example 1 was fed with the lignite. The mean sulphur dioxide concentration of the flue gases for lignite alone was 142 mg.m^"3, and for lignite plus additive 15 mg.m ³. This represents an 89% decrease in sulphur dioxide emission from one test to another. The amount of sulphur retained as a solid as opposed to being emitted as S0₂ in Test 3 (without additive) was 74%, indicating that just under half (46%) of the calcium in the ash actually combined with sulphur to form inorganic solid sulphur compounds. The additive represented a source of alkali (mainly sodium

Na) which is also known to react with sulphur or sulphur dioxide to form solid compounds under the combustion conditions employed. The Na:S ratio for the additive and lignite (assuming 50% Na by weight in the original additive) was 2.4:1. Less than 10% of this sodium would need to have reacted with the sulphur in the fuel in order to increase the overall retention from 74% (Test 3) to 98% (Test 4). EXAMPLE 3

A first test run utilizing the additive of the invention comprising Recipe 1 from Example 1 was carried out in a 7.5 t/h boiler at a factory near Xi'an in Shaanxi Province, People's Republic of China, manufacturing 120,000 industrial sewing machines annually. The boiler provides low pressure saturated steam (1.4 barA) for heating process. The boiler was run at its maximum capacity.

The boiler is a conventional balanced draft water wall boiler, with steam generated in both radiant and convection sections and with a separate economizer. It has a chain grate with water used to wash out fines, which drop through the grate, and to seal the system. The cinder drops down a chute where it is cooled by recirculated water before being flushed into an ash pit. The flue gas is scrubbed to remove solids in a counter-current wash tower for particulate removal before being ducted to a common stack. The bulk density of the coal was measured by weighing the same volume of coal and water.

The first test run was performed using the national boiler testing standard, which is based on a Russian standard. The boiler instrumentation is very basic, being mainly manual with the only automatic control being on the makeup water, which is controlled by the water level in the top drum. The instruments which exist have been used where practical, with additional instruments provided as appropriate. The latter instruments have all been calibrated and are known to be accurate. It is understood that the boiler instruments used were also calibrated. The water flow meter was unreliable and therefore an ultrasonic flow meter was mounted on the water pipe upstream of the main control valve. Samples of both coal and cinder have been retained for analysis for tests with and without catalyst. Flue gas was sampled downstream of the convection section and analyzed using equipment similar to an Orsat analyzer. Samples were retained for measurement of S0₂. Particulates were measured downstream of the water scrubbing tower. Steam quality was measured by condensing the steam and measuring its chloride ion content, which was compared to the fresh untreated makeup water analysis to determine the water content. The first test run was made with untreated coal. The feed hopper was partially filled to a preselected level, below which the hopper contains 5 t. The boiler was run until the hopper was empty. The bed of coal in the boiler was observed through a window. At the beginning of the chain grate, there was little precombustion with small quiet flames. In the main combustion zone, the flames were very luminous indicating significant combustibles present in the flames. In the third zone, where residual combustion occurs, the flames were moderate. The response of the level controller was slow and readings from the water feed flow were very variable. Readings were taken frequently and averaged for the experimental period. Preliminary data provided is given in Table 9.

Following the baseline test on the boiler, the additive was mixed with the coal feed at the proportion of 0.2 wt% in the second test run. The additive was manually mixed in powder form with the coal, each constituent being weighed before mixing. The improvement in performance of the boiler with the additive containing feed was very noticeable. No changes were made to the boiler prior to the addition of the new feed. The speed of the chain grate, the height of the coal bed and the settings on the forced draft air registers were kept constant.

Observing the combustion revealed that the precombustion in a first zone of the boiler was more significant with higher flames occurring earlier in the bed. The main combustion zone changed markedly with the flames being much less luminous with apparently better combustion occurring.

The bed appeared to "pop" with bright sparks jumping up from the bed.

The steam pressure rose sharply to the point where the operators were concerned about over pressure. The speed of the grate and the height of the bed were reduced, but due to the slow response of the boiler, the steam pressure continued to rise, so the boiler was shut down by cutting the air and stopping the feed coal.

On subsequent re-start, the reduced flow of coal was retained and the steam production was similar to the first test. The main combustion zone appearance reverted to that of the previous test in that the flames became more luminous, but the popping of the bed was again observed. Test results for the second run are included in Table 9.

The qualitative results show that there is a significant effect on the boiler performance by the addition of the additive. The amount of heat produced per unit of coal is increased and the amount of steam generated increased. Similarly, less coal was required for a similar steam production demonstrating an increase of boiler efficiency. EXAMPLE 4

The thermal efficiency of a boiler to assess its operational efficiency was carried out using untreated coal and subsequently using 0.3% of additive comprising Recipe 1 from Example 1 which had been added to the coal. The testing method was subject to the standard of GB 10180-88 and the output fluctuation of the boiler was not greater than 10%. During testing, the steam pressure at the exit of the boiler was held in the range of 0.3 - 0.4 Mpa and the boiler passage range 0 - 2.5 Mpa.

The output of the boiler was 6,500 kg/hr, the superheated steam pressure was 13.0 kg/cm², the water temperature was 105°C, the temperature of the emitted smoke was 184°C, the thermal efficiency was 76.9% and the volume.of the furnace chamber was 20.2 m³. The inducing fan had a flow rate of 24, 500 Nm³/h, the pressure of the inducing fan was 222 mmHg and the power of the motor of the inducing fan was 30 kw. The flow rate of the blower was 8720 Nm³/h, the pressure of the blower was 153 mmHg and the power of the blower motor was 7.5 kw. The boiler construction was of a double horizontal drum type with associated water pipes.

The results of the tests carried out are shown in Table 10 and it was noted that the errors of efficiencies between the forward and backward balances with or without additive are both less than 5% so the test results were valid and effective. After addition of additive, the boiler efficiency was increased by 4.23% and the saving in usage of coal was 6.9%. In another aspect of the invention, there is provided a process for improving the combustion efficiency of coal which includes the steps of:-

(i) providing to users of coal the base additive as hereinbefore described; (ii) adding to the base additive other components selected from the group consisting of:-

(a) a sulphur fixative;

(b) a catalyst to facilitate the action of the sulphur fixative; (c) a combustion catalyst;

(d) an oxidant;

(e) a corrosion inhibitor;

(f) a calcium sulphate decomposition inhibitor;

(g) an oxidant; (h) a loosening agent; and

(i) a de-ashing and de-dusting agent; and (iii) adding to the coal prior to combustion a coal additive obtained from step (ii).

TABLE 1

% Measured in Weight

TABLE 2

% Measured in Weight

Π

TABLE 3

TABLE 4

TABLE 5

TABLE 6

TABLE 7

TABLE 8

TABLE 9

TABLE 10

LEGENDS TABLE 3

* Denotes test carried out in laboratory environment. TABLE 4 * Denotes test carried out in laboratory environment.

TABLE 5

* Denotes test carried out in laboratory environment. TABLE 6

* Denotes test carried out in laboratory environment. TABLE 7

^* Denotes test carried out in laboratory environment. TABLE 8

All data corrected to 6% O₂ dry unless otherwise stated. *am denotes "as measured".

Claims

1. A base additive for coal having 9-11 % potassium or sodium permanganate or a mixture thereof, 27-33% sodium or potassium carbonate or a mixture thereof and 54-66% sodium chloride, potassium chloride or a mixture thereof.

2. A base additive as claimed in Claim 1 having 10% potassium or sodium permanganate or a mixture thereof, 30% sodium or potassium carbonate or a mixture thereof and 60% sodium chloride, potassium chloride or a mixture thereof.

3. A coal additive incorporating the base additive of Claim 1 or

Claim 2 together with other components inclusive of calcium oxide or a calcium oxide source, ferric oxide and optionally sodium or potassium nitrate.

4. A coal additive as claimed in Claim 3 additionally including silica, borax and zinc metal powder.

5. A coal additive as claimed in Claim 3 wherein the calcium oxide source is calcium carbonate.

6. A coal additive as claimed in Claim 3 including 8-12% of the coal additive of Claim 1 or Claim 2 with the balance of the additive comprising the other components.

7. A coal additive as claimed in Claim 6 including 10% of the coal additive of Claim 1 or Claim 2.

8. A coal additive as claimed in Claim 6 or Claim 7 further including 25-40% sodium nitrate or potassium nitrate or mixture thereof, 2- 3% sodium or potassium carbonate or mixture thereof, 5-20% calcium carbonate or calcium oxide, 3-7% ferric oxide, 30-60% sodium or potassium chloride or mixture thereof, 0.5-1.5% zinc oxide and 0.5-1.0% potassium or sodium permanganate or mixture thereof.

9. A process for improving the combustion efficiency of coal which includes the steps of:-

(i) providing to users of coal the base additive of Claim 1 or Claim 2; (ii) adding to the base additive other components selected from the group consisting of:- (a) a sulphur fixative; (b) a catalyst to facilitate the action of the sulphur fixative;

(c) a combustion catalyst;

(d) an oxidant;

(e) a corrosion inhibitor; (f) a calcium sulphate decomposition inhibitor;

(g) an oxidant; (h) a loosening agent; and (i) a de-ashing and de-dusting agent; and (iii) adding to the coal prior to combustion a coal additive obtained from step (ii).

10. A process as claimed in Claim 9 wherein the sulphur faxative is calcium oxide or a calcium oxide source.

11. A process as claimed in Claim 9 wherein the catalyst (b) is ferric oxide.

12. A process as claimed in Claim 9 wherein the combustion catalyst is sodium or potassium nitrate or mixture thereof.

13. A process as claimed in Claim 9 wherein the oxidant is potassium or sodium permanganate or mixture thereof.

14. A process as claimed in Claim 9 wherein the corrosion inhibitor is silica or powdered metallic zinc.

15. A process as claimed in Claim 9 wherein the calcium sulphate decomposition inhibitor is ferric oxide and silica.

16. A process as claimed in Claim 9 wherein the loosening agent is sodium or potassium chloride or mixture thereof.

17. A process as claimed in Claim 9 wherein the de-ashing and de-dusting agent is sodium or potassium chloride or mixture thereof.

18. A process as claimed in Claim 17 wherein the de-ashing and de-dusting agent also includes borax.

19. _. A process as claimed in Claim 13 wherein there is included 0.5-1.0% of the potassium or sodium permanganate or mixture thereof.

20. A process as claimed in Claim 14 wherein there is included

0.5-1.5% of the corrosion inhibitor.

21. A process as claimed in Claim 15 wherein there is included 3-7% of the calcium sulphate decomposition inhibitor.

22. A process as claimed in Claim 16 wherein there is included 30-60% of the loosening agent.

23. A process as claimed in Claim 9 wherein in step (iii) there is provided a coal additive comprising 8-12% of the base additive.