EP2994438A2

EP2994438A2 - Fuel enrichment process

Info

Publication number: EP2994438A2
Application number: EP14732285.3A
Authority: EP
Inventors: Syed DAOOD; William NIMMO
Original assignee: International Innovative Technologies Ltd
Current assignee: International Innovative Technologies Ltd
Priority date: 2013-05-10
Filing date: 2014-05-12
Publication date: 2016-03-16
Also published as: RU2015153105A; US20160084500A1; JP2016527328A; GB201308472D0; CN105263880A; GB2516728A; GB2516728B; ZA201509033B; WO2014181133A3; WO2014181133A2; KR20160067811A; GB201408385D0

Abstract

A process for breaking hydrocarbon bonds during the combustion or heating of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen comprises the steps of: heating the particulate hydrocarbon with a fuel improver in a burner, the fuel improver comprising predominantly iron oxide and silicon dioxide. The proportion of fuel improver to fuel is up to 33% by weight of particulate hydrocarbon. The particulate hydrocarbon has a particle size distribution and the fuel improver has a particle size distribution, and particles of hydrocarbon at the 90th percentile of its particle size distribution are larger than the particles of the fuel improver at the 90th percentile of its particle size distribution, and wherein the particles of fuel improver at the 10th percentile of its particle size distribution are an order of magnitude smaller than the hydrocarbon particles at the 90th percentile its particle size distribution.

Description

Fuel Enrichment Process

Field of the Invention

The invention relates to a process for reducing emissions of nitrous oxides (NO_x) from the combustion of hydrocarbon fuels, and more specifically to an improved process for the combustion of fossil fuels which reduces NO_x emissions and results in an ash by-product with improved loss on ignition and lower carbon content.

Background of the Invention

In recent years, there has been a growing concern about air pollution from industrial processing units, and in particular from coal fired power stations.

The Large Combustion Plant Directive (LCPD) is a European Union Directive which requires EU Member States to limit emissions from certain combustion plants, including fossil fuelled power stations. The Directive specifies emission limits for sulphur dioxide and nitrogen oxides. In Europe the enforcement of the LCPD is going to close many conventional coal fired power stations. The noxious oxides of nitrogen, carbon and sulphur are causing fundamental damage to the climate, environment and human health.

In particular there has been substantial interest in finding systems to reduce or minimize the emissions, for which catalysts and additives have been developed.

Solutions to this problem have included coal fired power stations being fitted with low NOx burners. Whilst reducing NOx and SOx emissions, these burners also lead to a loss in combustion efficiency which can in turn lead to high levels of unburnt carbon in the ash, typically in the region of 20% carbon, rendering the ash an undesirable waste product.

The United States of America, India, China and Australia are the major producers of fly ash. In 2009, the USA alone produced 57.2 Million metric ton (Mton) of fly ash of which only 22.4 Mton was used in concrete /cement manufacturing (http://minerals.usgs.gov/ds/2005/140/coalcombustionproducts.pdf). Fly ash mainly comprises glassy spheres of oxides of silicon, aluminium together with unburnt carbon and some crystalline matter. The introduction of low NOx burners has led to a gradual rise in Loss on Ignitition (LOI) values of fly ash. The unburnt carbon is significant in air-entrained concrete mixtures because of its tendency of adsorbing air-entrained surfactant rendering less protection against freeze-thaw conditions. Similarly excessive carbon affects the optimum density and moisture content for filling applications. In order for fly ash to be used as a cement substitute it must have less than 7% LOI.

Catalysts and fuel additives have also been developed in order to reduce or minimise NOx and SOx emissions.

For example, US2006/0034743 (Radway) describes an additive aimed at reducing Sulphur emissions, NOx emissions and heavy metal deposits in coal-fired boilers. The additive contains predominantly magnesium oxide combined with oxides of calcium, silicon, iron and aluminium.

Ash is a by-product generated in the combustion of coal. Fly ash is generally captured from the chimneys of power stations and bottom ash is removed from the bottom of the furnace. In the UK, just over 1,000,000 tonnes of fly ash is produced annually.

Worldwide a large proportion of ash produced from coal fired power stations is disposed of in landfill or stored in slag heaps. Some countries impose a tax on the disposal of such waste in landfill. The recycling of ash has become an increasing concern in recent years due to increasing landfill costs as well as environmental costs.

A significant portion of this ash is pozzolanic in nature, which means that when combined with calcium hydroxide it exhibits cementitious properties. In principle fly ash can be used as a replacement for a proportion of Portland cement content of concrete mixtures. Production of Portland cement itself is energy-intensive and produces a large amount of carbon dioxide, approximately one tonne of carbon dioxide per tonne of Portland cement, so replacement of a proportion of this with an otherwise unused by-product could dramatically reduce carbon emissions. However, ash comprising a high percentage of unburned carbon is not useable as a Portland cement substitute since the ash then has a tendency to adsorb important cementitious chemical admixtures from the concrete during the mixing process. This renders admixtures unavailable to effect their intended purpose. Ash with a carbon content of 7% or less is desirable for use as a pozzolan.

Fly ash can be processed to reduce the carbon content to levels sufficient for use as a pozzolan. Examples of such processes include re-burning the fly ash to reduce the carbon content; electrostatic separation processes which produce low carbon fractions and the chemical treatment of fly ash to minimize the effect of the carbon content by reducing the adsorptive properties of the carbon. All of these processes require at least one additional processing step, adding to the overall cost of producing a useful by-product rather than a waste product.

It would be desirable to provide an improved process for the combustion of coal, reducing emissions of nitrous oxides, whilst also providing a better quality ash with a low carbon content that renders the ash a saleable and marketable by-product, rather than a waste product that would need to be disposed of in a manner to satisfy environmental regulations. In addition, it would be desirable to provide an improved process in which the amount of coal burned is reduced whilst not reducing the energy output and preferably increasing the energy output and reducing carbon emissions.

Summary of the Invention

One aspect of the invention provides a process for breaking hydrocarbon bonds during the combustion or heating of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of: heating the particulate hydrocarbon with a fuel improver in a burner, the fuel improver comprising predominantly iron oxide and silicon dioxide, wherein: the particulate hydrocarbon has a particle size distribution and the fuel improver has a particle size distribution, and wherein particles of hydrocarbon at the 90th percentile of its particle size distribution are larger than the particles of the fuel improver at the 90th percentile of its particle size distribution, and wherein the particles of fuel improver at the 10th percentile of its particle size distribution are an order of magnitude smaller than the hydrocarbon particles at the 90th percentile its particle size distribution; and wherein the proportion of fuel improver to fuel is up to 33% by weight of particulate hydrocarbon.

Preferably, the particle size of the fuel improver at the 90th percentile is 55 micron or less, the particle size of the particulate hydrocarbon at the 90th percentile is 60 micron or greater, and the particle size of the fuel improver at the 10th percentile is less than 6 micron.

Preferably, the particle size of the particulate hydrocarbon in the 100th percentile is between 60 micron and 700 micron.

Preferably the particle size of the fuel improver at the 90th percentile is less than or equal to 35 micron. More preferably, the particle size distribution of the fuel improver at the 90th percentile is less than or equal to 32 micron. Still more preferably, the particle size of the fuel improver at the 90th percentile is less than or equal to 25 micron.

Preferably, the volume weighted mean particle size of the fuel improver particles is 26 micron or less. More preferably, the volume weighted mean particle size of the fuel improver is 17 micron or less.

Preferably, the volume weighted mean particle size of the particulate hydrocarbon is in the range 29 micron to 90 micron.

Preferably, the surface weighted mean particle size of the fuel improver is 9 micron or less. More preferably, the surface weighted mean particle size of the fuel improver is 6 micron or less.

Preferably, the surface weighted mean particle size of the particulate hydrocarbon is in the range 9 micron to 35 micron.

Preferably, the particle size of the particulate hydrocarbon at the 90th percentile is not greater than 220 micron. More preferably, the particle size of the particulate hydrocarbon at the 90th percentile is not greater than 180 micron. Preferably, the proportion of fuel improver to fuel is in the range 1% to 33% by weight of particulate hydrocarbon. More preferably, the proportion of fuel improver to fuel is in the range 5%-12% by weight of particulate hydrocarbon.

Preferably, the fuel improver replaces between 1% and 5% of the particulate hydrocarbon and the proportion of fuel improver is equal to or greater than the amount of particulate hydrocarbon replaced. More preferably, the fuel improver replaces between 1% and 3% of the particulate hydrocarbon.

Preferably, the particulate hydrocarbon is coal.

Preferably, the fuel improver includes aluminium oxide and/or calcium oxide. More preferably, the fuel improver includes by weight up to 7% aluminium oxide and/or up to 10% calcium oxide. Still more preferably, the fuel improver includes by weight between 3% and 5.5% aluminium oxide and/or between 2% and 7.5% calcium oxide.

Preferably, the fuel improver comprises between 70% and 91% iron oxide and silicon dioxide, combined, by weight.

Preferably, the fuel improver comprises between 41 and 52% iron oxide, by weight.

Preferably, the fuel improver comprises between 32 and 40% silicon dioxide, by weight.

A second aspect of the invention provides a process for producing a pozzolanic fly ash during the combustion of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of: performing the process for cracking hydrocarbon bonds during the combustion or heating of a particulate hydrocarbon as hereinbefore defined; and

recovering the fly ash from the burner, wherein the particle size not more than 30% of the recovered fly ash is greater than 45 micron.

Preferably, the particle size of not more than 20% of the recovered fly ash is greater than 45 micron. Preferably, at least 70% by weight of the fuel improver is comprised of silicon dioxide, iron oxide and aluminium oxide. More preferably, at least 75% by weight of the fuel improver is comprised of silicon dioxide, iron oxide, aluminium oxide and calcium oxide.

Preferably, the pozzolanic fly ash has a loss on ignition of 7% or less.

A third aspect of the invention provides a process for producing a cementitious composition comprising performing the process for producing a pozzolanic fly ash as hereinbefore defined, and mixing the resulting pozzolanic fly ash with calcium hydroxide.

A fourth aspect of the invention provides a composite cement comprising a mixture of portland cement and a cementitious composition produced by the process for producing a cementitious composition as hereinbefore defined and/or a pozzolanic fly ash produced by the process for producing a pozzolanic fly ash as hereinbefore defined.

Preferably, the proportion of portland cement to cementitious composition produced by the process for producing a cementitious composition as hereinbefore defined and/or pozzolanic fly ash produced by the process for producing a pozzolanic fly ash as hereinbefore defined is between 30:70 and 70:30.

A fifth aspect of the invention provides a process for reducing slagging in a boiler and/or fouling of a heat recovery apparatus arising from combustion of a particulate hydrocarbon in a burner, the particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the step of:

performing the process for cracking hydrocarbon bonds during the combustion or heating of a particulate hydrocarbon as hereinbefore defined in a burner associated with a boiler and/or heat recovery apparatus and controlling the amount of fuel improver relative to the particulate hydrocarbon to provide a basic to acidic ratio and/ or a simplified basic to acidic ratio (R(B/A)) such that the slagging and/or fouling indices of the combination of particulate hydrocarbon and fuel improver are below the extremely high range.

Preferably, the slagging index is not greater than 2.

Preferably, the fouling index is not greater than 2.

More preferably, the slagging and/or fouling indices are 0.6 or less. Preferably, the particulate hydrocarbon is coal.

Preferably, the fuel improver includes Na20 and/or 2O and the particulate hydrocarbon includes Na20 and/or 2O and the relative proportion of fuel improver to particulate hydrocarbon is selected according to the proportions of Na20 and/or 2O in the fuel improver and the particulate hydrocarbon respectively.

A sixth aspect of the invention provides a process for reducing NOx emissions during the combustion or heating of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of:

performing the process for cracking hydrocarbon bonds during the combustion of a particulate hydrocarbon as hereinbefore defined to increase the proportion of volatile nitrogen to char nitrogen and volatile carbon to char carbon.

Preferably, combustion of the particulate hydrocarbon produces a flame having a fuel rich zone and wherein the proportion of volatile nitrogen to char nitrogen and/ or volatile carbon to char carbon is increased in the fuel rich zone.

Preferably, the process comprises the further step of reacting volatile nitrogen with metal oxides of the fuel improver and volatile carbon oxides.

Brief Description of the Drawings

In the drawings, which illustrate preferred embodiments of the invention:

Figure la is a graph showing the distribution particle sizes of a sample of water cooled fuel improver after pulverisation using a roller mill;

Figure lb is a graph showing the distribution particle sizes of a sample of air cooled fuel improver after pulverisation using a roller mill;

Figures 2 a, b and c are graphs showing the distribution of particles sizes of three different coal samples after pulverisation using a roller mill. Figure 3 shows a series of graphs illustrating the effect of the mass fraction of the fuel improver on NO emissions from various commercial coals under un-staged flame conditions of stoichiometric ratio = 1.20; A: Water Cooled (WC) fuel improver with Russian Coal (RC); A': Air Cooled fuel improver (AC) with RC; B: WC fuel improver with Columbian Coal (CC); B' AC fuel improver with CC; C: WC fuel improver with Kellingley Coal (KC); AC fuel improver with KC.

Figure 4a illustrates a proposed schematic mechanism for the interaction of the fuel improver particles with the coal particles;

Figure 4b illustrates the NO_x reduction chemistry pathway in the presence of fuel improver;

Figure 4c illustrates a schematic mechanism for the thermal degradation of Fuel-C (coal) to Volatile— C (lighter hydrocarbons) in the presence of the fuel improver;

Figure 5 illustrates a two stage fixed bed reactor;

Figure 6a is a graph showing CO release during the combustion of different mixtures of AC fuel improver and coal;

Figure 6b is a graph showing hydrocarbon concentration during the combustion of different mixtures of AC fuel improver and coal;

Figure 7 shows a series of graphs illustrating NO emissions from various commercial coals with varying mass fraction of both Air Cooled fuel improver (AC) and Water Cooled fuel improver (WC). A to C: 6.4%, 8.8% and 13% mass fraction of WC fuel improver with Russian Coal (RC); D to F: 5.45%, 6.2% and 7.8% mass fraction of WC, AC and WC fuel improver with Columbian Coal (CC) respectively; and G to I: 8.9%, 9.1% and 10.3% mass fraction of AC, WC and AC fuel improver with Kellingley Coal (KC), respectively;

Figure 8 shows a series of graphs illustrating the LOI of fly ash plotted against the mass fraction of fuel improver for three different commercial coals: A: Russian Coal; B: Columbian Coal; C: Kellingley coal; Figure 9 illustrates a comparison between the particle size distribution of fly ash resulting from burners burning only RC, C or typical UK coal, with burners burning RC and 4.2% fuel improver and C with 4.8% fuel improver;

Figure 10 illustrates temperature measurements within the burner for different mass fractions of fuel improver with different commercial coals;

Figure 11 is a table showing calculated and predicted slagging and fouling indices for various coals and blends of coals and fuel improver; and

Figure 12 is a graph illustrating dust concentration in the exhaust gas downstream of an electrostatic precipitator in the west, centre and east legs of a boiler exhaust.

Detailed Description of the Preferred Embodiments

The improved combustion process of the invention involves the injection of a fuel improver into the main burner in a carbon-based fuel burner, for example a coal fired power station. The fuel improver is derived from a mixture of metal oxides typically sourced from slags, which are by-products of metal smelting processes, typically in the production of copper and nickel. The fuel improver includes a mixture of oxides of transition metals and other elements. The fuel improver mainly includes a mixture of iron, aluminium, calcium and silicon oxides. Two different types of fuel improver can be produced: one is air cooled, and the other is water cooled. Table 1 shows the X-Ray Fluorescence (XRF) analysis of two fuel improver samples. Table 2 shows the X-Ray Diffraction (XRD) analysis of the two samples.

Table 1: XRF analysis of two samples of fuel improver.

The fuel improver composition of the invention typically contains chemical elements and their oxides belonging to periods 3 and 4 (groups II-V) of the Periodic Table. As shown in Table 1 the fuel improver comprises predominantly iron oxide and silicon dioxide, meaning that the combined amount of iron oxide and silicon dioxide present in the fuel improver is greater than the amount of any other compound present in the fuel improver. The fuel improver is preferably pulverised using a mill suitable for producing fine powders from hard materials such as a ball mill or a roller mill as described in UK patent numbers GB2451299, GB2460505 and GB2471934. Preferably the fuel improver is milled such that the particle size at 90^th percentile is 55 micron or less [d(0.9) < 55]. Table 3 shows the particle size distribution, physical and chemical properties of both types of fuel improver milled in a roller mills, with the particle size distribution measured using various methods.

Table 3: Particle size distribution, physical and chemical properties of the Fuel Improver.

(Mhos)

Figures la and lb are graphs showing the range in diameter of particle sizes of fuel improver after passing through a mill, measured using a Malvern Mastersizer Scirocco 2000.

Figures 2 a, b and c are graphs showing the range in diameter of particles sizes of three different coal samples after passing through a mill. The particle sizes in these graphs were measured using a Malvern Mastersizer Scirocco 2000. Table 4 shows a summary of the particle size distribution of the three different coal samples.

Table 4: Particle size distribution of coal samples, measured using Malvern Mastersizer Scirocco

2000.

The fuel improver can also replace a proportion of the carbon-based fuel in the burner in an amount ranging from 1% to 5% by weight depending upon the improvement in loss on ignition (LOI). Carbon based firing boilers can either produce the same steam load by burning less fuel or can increase steam load by burning the same fuel input, depending upon the amount of fuel improver and improvement in LOI.

Experiments were conducted using three commercially available coals as the carbon based fuel. The coals had low, medium and high ash contents. Columbian coal is a low ash coal, Russian sub bituminous coal is a medium ash coal and UK Kellingley coal is a high ash coal. The chemical composition of these coals is listed in Table 5. Table 5: Chemical composition of tested

Experiments were conducted in a lOOkW Combustion Test Facility comprising a down-fired pulverised coal fired furnace of 4 metres length with an internal diameter of 400mm. The burner of the furnace was operated at an approximately 10-11.65 kg/hr of coal (depending upon types of coal) input feed rate resulting in a net thermal input of about 75-85 kW. The major flue gas species CO2, O2, NO_x, and CO were measured at the exit of the furnace. Gas samples were constantly drawn through a water cooled stainless steel probe to the gas sampling system in order that the correct combustion conditions could be set in the furnace. The extracted flue gas from the probe was transferred via polytetrafluoroethylene (PTFE) tubing through a series of filters and water traps for cleaning and drying purpose. The flue gas was later cooled to 2 °C by passing it through a chiller. The filters were frequently replaced along with cleaning of water traps in order to prevent any blockage of the gas sampling system. The flue gas was passed through a manifold that directed the sample gas to different gas analysers. The oxygen in the flue was measured by passing a part of the sample over the self-indicating silica gel. The gas sampling probe used to draw in the flue gas from furnace was attached with compressed nitrogen supply for purging to prevent any blockage during operation. On-line gas analysis systems monitor O2, CO2, CO, and NO (NO_x) and temperatures down the furnace are monitored and logged to PC during each test period.

The additive was fed with different types of coal to the furnace in mass fractions from 1.3 to 13%. Fly ash solids were collected by the fly ash catch pot connected to a cyclone separator. The samples and emissions were collected and measured after attaining steady state condition for each test. Fly ash samples were analysed for loss on ignition (LOT) in a muffle furnace by drying at 105°C for one hour followed by heating at 850°C for 2 hours. In boilers generally the LOI value is equated to un-burnt carbon.

NQ_X Emissions

Fuel bound nitrogen contributes to about 80% - 95% towards the NO_x formation in pulverized coal combustion. Fuel bound nitrogen during coal combustion is generally split into volatile-N and char- N. This division preferentially depends upon nitrogen content and volatility of coal along with the combustion conditions such as temperature, residence time, and heating rates. In the case of sub bituminous coals, the volatile-N comprising of tarry compounds decay rapidly to hydrogen cyanide (HCN) or soot-nitrogen. Whereas in contrast the low rank coals would preferentially release the light nitrogen species such as N¾. Combustion of nitrogenous species (N¾ and HCN) present in the released volatiles and oxidation of the char-nitrogen results in the formation of oxides of nitrogen. However, the HCN or N¾ may also be reduced to N2 after reacting with the available NO. This depends upon the available stoichiometric ratio near the burner, mixing of the evolved species in the furnace and fuel-N concentration.

Figure 3 shows the effect of addition of both water cooled (WC) and air cooled (AC) fuel improver towards NO_x emissions. In all of the examples the NO_x emissions decrease as the mass fraction of fuel improver increases from 0 to 12%.

Figure 4a illustrates a proposed schematic mechanism for the interaction of the fuel improver with the coal. The process of NO_x reduction under un-staged combustion observed during fuel improver addition is associated partly with the interaction of the fuel improver particles with the coal matrix and volatiles as they are released from coal particles, resulting in cracking of the heavier hydrocarbons favouring the split of fuel-N into volatile-N. As shown in Figure 4a the coal particles swell during the heating that occurs in the combustion chamber, but the fuel improver particles do not swell. As the coal particles swell the fuel improver particles enter the coal matrix and enhance volatile hydrocarbon cracking.

For this mechanism to proceed the particle size distribution of the coal particles must include larger particle sizes than the particle size distribution of the fuel improver particles. In addition, the particle sizes of the fuel improver at the 10^th percentile of the particle size distribution must be an order of magnitude smaller (10 times smaller) than the particles sizes of the coal particles at the 90^th percentile of the coal particle size distribution. For example, looking at the particle size distributions for Russian coal and Water Cooled Fuel Improver (both measured using the Malvern Mastersizer Scirocco 2000), the largest particles of coal are 631 micron in size, whereas the largest particles of fuel improver are 138 micron in size. In addition, the particle size of the fuel improver at the 10^th percentile, d(0.1), is 4.7 micron and the particle size of the coal at the 90^th percentile, d(0.9), is 179 micron. This means that the upper 10% of the coal particles are 38 times the size of the lower 10% of the fuel improver particles. The smaller fuel improver particles are able to react with the larger coal particles as shown in Figure 4a.

This mechanism favours the NO_x reduction pathway towards N2 formation rather than NO by oxidation, since this form of fuel-N is easier to control in the fuel -rich zones of the flame, as shown in Figure 4b which illustrates the NO_x reduction chemistry pathway in the presence of fuel improver.

Figure 4c illustrates how the fuel improver enhances the thermal degradation of Fuel-C (coal) to Volatile— C (lighter hydrocarbons). The lighter hydrocarbons are less likely to form Char-N.

Figure 5 illustrates a two stage fixed bed reactor 1 used to carry out a laboratory test. Nitrogen was used as a product carrier gas. A coal sample 2 was pyrolysed in the first reactor 3. The derived gases were reformed in the second reactor 4, where the fuel improver 5 was placed. Products after the second- stage reaction were condensed by air and dry-ice in a condenser system 6. The non-condensed gases were collected by a gas sample bag 7 and further analysed by gas chromatograph (GC). Both stages of the reactor were maintained at 950°C, with two grams each of fuel improver 5 and coal 2 in each of the stages. The gas and oil products were collected after an hour and all the products (gas and oil) were collected for analysis. It was found that gas production increased in the presence of fuel improver (Error! Reference source not found.). Similarly oil yield decreased, indicating the conversion of heavier hydrocarbons into more of gaseous fractions. The colour of the oil produced from coal was dark brown, whilst with fuel improver it changed to cleaner oil. The summary of the experiments with and without the fuel improver is tabulated in table 6 below.

Table 6: Gas/Oil yields and concentration of gases.

The results suggest gas production and hydrogen concentration increased in the presence of the fuel improver. It is concluded that the gas yields increased by around 5-7% in presence of both types of fuel improver. Similarly, as shown in Figures 6a and 6b, the increase in the concentration of CO and hydrocarbon from coal has also been confirmed using thermogravimetric analysis coupled with Fourier transform infrared (FTIR) spectroscopy by blending 5% to 33% weight proportion of AC fuel improver in coal.

The combustion of the coal char and the behaviour of the unblended fuel improvers were investigated using a Stanton Redcroft TG782 thermo-gravimetric analyser (TGA) connected to a Nicolet Magna 560 FTIR spectrometer via a heated interface and heated transfer line. In this study, the FTir spectrometer was operated in scan range of 400-4000cm-l and a spectrum was taken every 45s during the course of the TGA run, with 100 background scans taken prior to the run to correct for ambient moisture and carbon dioxide. The transfer line was maintained at 170°C, while the TGA interface cell was held at 300°C, with a constant purge of nitrogen around the cell to minimise the effect of changes in atmospheric moisture and carbon dioxide levels. The intensity of absorbance in the wavenumber range 2000-2500cm-l, which in this study corresponds to the relative concentration of carbon monoxide and carbon dioxide in the sample gas during char combustion, was plotted against time. In addition, the intensity of wavenumber ranges 2170-2180cm-l (carbon monoxide only) and 2800-3200cm-l (a variety of C-H bonds, such as hydrocarbons, released during coal pyrolysis) were plotted from the spectral series data.

The onset of CO release is at a temperature of about 300°C reaching a plateau at under 600°C. CO release peaks at about 900°C followed by a reduction then complete burnout when the O2 containing mixture is introduced. As shown in Figure 6a, there appears to be an increase in the CO produced when the fuel improver is present compared to that measured from coal alone. Elution of hydrocarbons during the coal pyrolysis/combustion tests was followed by FTIR and the results are shown in Figure 6b. Hydrocarbon release from the coal (under N2) begins at about 300°C and reaches a maximum at about 500°C and is complete by about 900°C. The concentration of hydrocarbons in the presence of the fuel improver were higher than that from the coal alone. There does not appear to be significant difference in CO and hydrocarbon concentration measured between the amount of fuel improver in the coal-fuel improver blends. This increase in the gas yield supports the hydrocarbon cracking and release producing more of volatile which in turn facilitates the NO reduction into N2 as described with reference to Figures 4a and 4b.

Moreover, the presence of iron oxide in the fuel improver would also interact with coal to result in additional NO reduction reactions supplementing the existing pathway towards N2 formation. Fe203 can be reduced to Fe in presence of CO, and later on NO can oxidize iron to reproduce Fe203. The summary of reactions is as follows;

3 CO + Fe20 → 3 COz + 2 Fe

3

2Fe + 3 NO→ -N2 + FezOz

The net algebraic addition of reactions yield

1

CO + NO→ CO2 + - N2

2

Three different types of coals investigated for the study with a view to observing any variation in the behaviour of additive on NO_x reduction. The medium ash Russian Coal (RC) and high ash Kellingley Coal (KC) resulted in slightly higher reduction in NO_x as compared to Columbian Coal (CC) because of relatively higher volatile matter and lower fixed carbon compared to CC. The air to fuel ratio in the combustion test facility (CTF) was set at 20% excess air levels (stoichiometric ratio of 1.20) for un-staged flame firing condition. The optimum range up to 13% by weight of that of coal input was observed for both types of fuel improver. NO_x reduction of 15% & 16% for 13% & 12% mass fractions of WC and AC fuel improver were observed for RC, respectively. Whereas, 11% & 10% NO_x reduction was achieved for 11% and 13% mass fraction of WC and AC fuel improver with CC, respectively. KC with WC and AC fuel improver co-firing resulted in 14% & 15% reduction in NO_x for 10% and 13% added mass fractions, respectively. In general the following mechanisms can be summarised towards reduction of NO_x emissions using both types of fuel improver.

• It is associated partly with the interaction of fuel improver particles and coal matrix, resulting in cracking of the heavier hydrocarbons favouring the split of fuel-N into volatile-N. The increase in the gas yield supported by the higher concentration of CO and hydrocarbon would favour the NO_x reduction pathway towards N₂ formation rather than NO by oxidation, since volatile part of N is easier to control in the fuel -rich zones of the flame.

• The fuel improver, having higher surface area because of its finer particle size distribution compared to coal, would facilitate the thermal degradation of heavier hydrocarbon into lighter hydrocarbons and these lighter hydrocarbon are less likely to form Char-N.

• The presence of iron oxide in the additive would also interact with coal to result in additional NO_x reduction reactions supplementing the existing pathway towards N₂ formation.

Figure 7 represents the effect of change of stoichiometric ratio near the combustion zone on different co-firing blends of Fuel improver with RC, CC and KC. The in-furnace air staged combustion creates fuel rich zones due to the delayed mixing of fuel particles with air resulting in the abatement of NO. The reduced stoichiometric ratios i.e. 0.8, 0.9 in primary combustion zone restrain coal combustion, and large amount of unburned char enters the burnout zone resulting in poor carbon burnout.

The addition of Fuel improver resulted in an additional impact on increase in NO reduction with decreasing air to fuel ratio. WC Fuel improver with RC resulted in a range of 4.6% to 25.8% reduction in NO for range of 0.9 to 1.20 stoichiometric ratio. Whereas, a range of 4.7% to 23.9% was observed for WC/AC Fuel improver with CC for 0.8 to 1.16 changing air to fuel ratios. AC/WC Fuel improver with KC for 0.9 to 1.30 stoichiometric ratio resulted in 7.3% to 31.1% reduction in NO with respect to coal staged flame base lines.

Effect on LOI Use of the fuel improver results in a substantial improvement in the LOI values of the fly ash of all three types of coal tested, as shown in Figure 8. The presence of the fuel improver has increased the hydrocarbon intensity and gas yield conversion from coal, which in turn intensifies the combustion and results in improved LOI. In the case of Russian coal, an overall net reduction for the LOI in the range of 19% to 63% for 1.3% to 13% mass fraction of added fuel improver was achieved. Similarly, a LOI reduction in the range of 20% to 70% was found for addition of 2.5% to 11% mass fractions of fuel improver for Columbian coal and a LOI reduction in the range 64% to 70% was found for 5% to 13% mass fractions of added fuel improver for ellingley coal. The optimum mass fraction of fuel improver ranges from 5% to 12%, resulting in fly ash having less than 7% LOI that can be used in cement manufacturing.

The particle size distribution (PSD) of resultant coal fly ash can potentially fluctuate depending upon the operation of the power station. Typically power stations are operated under a steady load to compensate for variation of the resulting fly ash. However, the PSD of the coal fly ash is also important when considering its use in concrete manufacturing. General purpose cement utilizes finer ash because finer ash is more reactive. The strength and water content of the resulting concrete is also dictated by the variability in the fineness of the fly ash.

Fly ash fineness is usually measured as the % retained on a 45 micron sieve. British standard BS EN 450 which governs particle sizes of fly ash suitable for mixing with cement states that the fineness of the fly ash must be≤ 40% retained on a 45 micron sieve.

The fuel improver has been found to improve the fineness of the resultant fly ash as shown in Figure 9. Figure 9 shows a comparison between coal fly ash resulting from burners both with and without fuel improver present. It can be seen from the graph that with addition of fuel improver (added at either 4.2% or 4.8% mass fraction) the fineness of resulting fly ash increased by about 36% to 85% in addition to that of RC and C fly ash baselines, respectively. Using the fuel improver of the invention the fineness of the fly ash is typically≤ 20-30% retained on a 45 micron sieve. Table 7 below illustrates compression strength tests carried out on cement cubes manufactured from fly ash from a burner burning just RC and a burner burning RC and 4.2% fuel improver. The tests were performed by a major cement manufacturer in the UK. It can be seen from the table that the cube strength results are reasonably close to each other without any major variation in the strength characteristics of the cement mixture. The addition of the fuel improver therefore results in an equally comparable strength mortar when prepared by mixing 30% of coal fly ash from a fuel improver/coal blend with Portland cement.

If the fly ash is to be used in cement manufacture it is beneficial that the fuel improver includes calcium oxide since the calcium oxide improves the cementitious properties of the fly ash. The fly ash may be added directly to the calciner during cement manufacture, or just mixed in with Portland cement to produce a ready mix/Portland fly ash mixed cement.

Table 7: Compression strength tests on RC fly ash, with and without fuel improver.

Effect on Temperature

The addition of fuel improver to the burner has been found to result in increased temperature measurements within the burner. The generation of extra temperature is due to the burning of the additional carbon of the coal feed, favouring the split of carbon into light volatiles rather than remaining in the char. Figure 10 illustrates the temperature differences calculated at axial distances downwards from the burner, with Tl being close to the burner and T7 near the flue section. The values were calculate for different mass fractions of fuel improver for all the studied coals against the corresponding coal baseline temperature measurements. As indicated from the general trend found in Error! Reference source not found.10, the different mass fractions of fuel improver produced a broad range of 12-30 °C increase in temperatures at Tl. These changes in Tl values are dependent upon the added mass fraction of fuel improver. The increase in the temperature also supports the improved values of LOI.

Effect on Slagging and Fouling

Slagging and Fouling characterizes the deposits on the radiant section of the boiler and heat recovery section, respectively. These deposits are formed through a series of complex mechanisms, forming a variety of compounds which cause corrosion and reduction in heat transfer rates.

Slagging and fouling indices are used for the assessment of the propensity of fuel ashes to form these deposits. These indices have been specifically developed for the assessment of coal ashes only, but these indices are widely used in literature for co-fired fuels as well. The most commonly used traditional indices used to calculate the fuel ash deposition tendency are shown in Figure 11. The predicted composition is calculated as a mass average of the metal oxides present in the known feed rate of coal and fuel improver. The actual ash samples collected during these combustion tests were used to measure the ash components and were reported as measured values in the table shown as Figure 11.

The predicted values of metal oxides are in close proximity to the actual measured concentration of metal oxides. The existing difference between the values is expected due to the + 1-2% combined variation in the actual feed rates of coal and fuel improver. However, irrespective of the predicted and calculated indices, there is an insignificant increasing trend in both the predicted and measured values of fouling and slagging indices, when compared with the metal oxide concentration of individual coal fly ash samples. The reported chemical composition of the fly ash samples show a narrow range of variety of alkali oxides between coal fly ash and coal fly ash plus fuel improver samples. A dominance of SiC½, AI2O₃ and Fe2C>₃ was found in all the fly ash samples. This is partly due to the inherited concentration of these oxides in the actual coal and fuel improver. The percentage of oxides of iron was found to have increased in the fly ash mix, whereas the percentage of alumina concentration decreased, slightly.

Generally, a substantial increase in the percentage concentration of Na20 and 2O results in higher fouling propensity in commercial boiler. The %age of 2O is relatively higher in RC and C fly ash as compared to the fuel improver, hence addition of the fuel improver delivers an overall positive impact towards lowering fouling propensities. Moreover, the tabulated overall measured concentrations show an insignificant variation concluding trivial effect on the actual boiler furnace wall. Moreover, the measured values of R₍B/A₎ were less than 0.75 indicating that ash flow temperature will be higher resulting in a decrease of slagging tendency.

The addition of the fuel improver delivers an overall positive impact towards lowering fouling propensities for the fuels which have relatively higher % of Na20 and 2O. It can also result in increasing the ash flow temperature resulting in decrease of slagging tendency depending on the type of fuel (coal).

The formulae used to calculate the fouling and slagging indices in the table shown in Figure 11 are as follows:

(B/A) is the base/acid ratio

Fu, the Fouling index is calculated as follows:

Where Fu≤ 0.6 there is a low fouling inclination; where Fu 0.6-40 there is a high fouling inclination; and where Fu > 40 there is an extremely high fouling inclination. Rs, the slagging (Babcok) index is calculated as follows:

Where S^d is the mass percent of total Sulphur in the dry fuel.

Where Rs < 0.6 there is a low slagging inclination; where Rs— 0.6-2.0 there is a medium slagging inclination; where Rs 2.0-2.6 there is a high slagging inclination; and where Rs > 2.6 there is an extremely high slagging inclination.

R₍B/A₎ is a simplified basic to acidic ratio calculated as follows:

R_b = Fe₂0₃ + CaO + MgO + Na₂0 + K₂0

The amount of fuel improver can be controlled in relation to the type of coal being burnt to ensure that the slagging and fouling indices are at the desired levels, and below the extremely high levels. For coals with poor slagging and fouling indices larger amounts of fuel impover may be added in order to bring the basic to acid ratio and/or the simplified basic to acid ration (R₍B/A₎) back into the desired range.

Effect of Fuel Improver on Efficiency of an Electrostatic Precipitator

Exhaust stacks of particulate hydrocarbon fired boilers typically include electrostatic precipitators. It has been found that by using the fuel improver of the invention the efficiency of these electrostatic precipitators can be increased resulting in a greater yield of fly ash, which is beneficial for two reasons. First, less dust is emitted to the atmosphere. Second, the fly ash resulting from the process of the invention is useful as a pozzolan. Table 8 below and the graph of Figure 12 show the results of dust collection at east, central and west legs of a coal fired steam producing boiler (230MWth) operating at (205 tons /hour of steam) during a 4 hour trial. Jie outj_.ejje^

precipitator situated at the outlet of the bofler-½- s~e¾haasfr-s?¾te«fc The boiler was fired with 23 tons /hour of milled coal. 6.8% by weight of coal was added to the coal and this is shown by the vertical line intersecting the time axis approximately mid way between 10.36 and 11.26. The second vertical line intersecting the time axis approximately mid way between 14.45 and 15.35 marks the point where the introduction of the fuel improver ceased.

In another shorter test lasting 45 minutes using the same boiler but operating at (252 tons/hour of steam) and fired with 30 ton/hour of coal, when 9% of water cooled fuel improver was added the reduction in dust concentration in the exhaust downstream of the electrostatic precipitator measured in the west, centre and east legs was 13%, 4.5% and 20% respectively when compared to the coal base line.

Table 8

It can be seen that the dust concentration post the electrostatic precipitator is less where the fuel improver is added, even though the combusted mass is increased by 6.8% (the amount of coal entering the boiler's furnace is not reduced when the fuel improver is added).

This reduction in dust concentration post the electrostatic precipitators is explained by a number of factors attributed to the fuel improver. First_s the improvement in LOI discussed above means that the fly ash has a lower content of carbon. Electrostatic precipitators function better where carbon concentration in the fly ash is lower. Second, the fuel improver has electrically conductive in particular the oxides of iron within the fuel improver and the presence of faylite and magnetite phases of the fuel improver post combustion in the fly ash enable the electrostatic precipitators to remove more material as fly ash rather than dust entrained in the fluecxhaust gas.

References to relative amounts in compositions are percentages ftfe-by weight.

The process of the invention provides for improved combustion efficiencies and improved slagging and/ or fouling in addition to reducing emissions and improves the pozzolanic properties of fly ash.

Claims

1. A process for breaking hydrocarbon bonds during the combustion or heating of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of:

heating the particulate hydrocarbon with a fuel improver in a burner, the fuel improver comprising predominantly iron oxide and silicon dioxide, wherein: the particulate hydrocarbon has a particle size distribution and the fuel improver has a particle size distribution, and wherein particles of hydrocarbon at the 90th percentile of its particle size distribution are larger than the particles of the fuel improver at the 90th percentile of its particle size distribution, and wherein the particles of fuel improver at the 10th percentile of its particle size distribution are an order of magnitude smaller than the hydrocarbon particles at the 90th percentile its particle size distribution; and wherein the proportion of fuel improver to fuel is up to 33% by weight of particulate hydrocarbon.

2. A process according to Claim 1, wherein the particle size of the fuel improver at the 90th percentile is 55 micron or less, and wherein the particle size of the particulate hydrocarbon at the 90th percentile is 60 micron or greater, and wherein the particle size of the fuel improver at the 10th percentile is less than 6 micron.

3. A process according to Claim 1 or 2, wherein the particle size of the particulate hydrocarbon in the 100th percentile is between 60 micron and 700 micron.

4. A process according to any preceding claim, wherein the particle size of the fuel improver at the 90th percentile is less than or equal to 35 micron.

5. A process according to Claim 4, wherein the particle size distribution of the fuel improver at the 90th percentile is less than or equal to 32 micron.

6. A process according to Claim 5, wherein the particle size of the fuel improver at the 90th percentile is less than or equal to 25 micron.

7. A process according to any preceding claim, wherein the volume weighted mean particle size of the fuel improver particles is 26 micron or less.

8. A process according to Claim 7, wherein volume weighted mean particle size of the fuel improver is 17 micron or less.

9. A process according to any preceding claim, wherein the volume weighted mean particle size of the particulate hydrocarbon is 29 micron to 90 micron.

10. A process according to any preceding claim, wherein the surface weighted mean particle size of the fuel improver is 9 micron or less.

11. A process according to Claim 10, wherein the surface weighted mean particle size of the fuel improver is 6 micron or less.

12. A process according to any preceding claim, wherein the surface weighted mean particle size of the particulate hydrocarbon is 9 micron to 35 micron.

13. A process according to any preceding claim, wherein the particulate hydrocarbon at the 90th percentile is not greater than 220 micron.

14. A process according to Claim 13, wherein the particulate hydrocarbon at the 90th percentile is not greater than 180 micron.

15. A process according to any preceding claim, wherein the proportion of fuel improver to fuel is in the range 1% to 33% by weight of particulate hydrocarbon.

16. A process according to Claim 15, wherein the proportion of fuel improver to fuel is in the range 5%-12% by weight of particulate hydrocarbon.

17. A process according to any preceding claim, wherein the fuel improver replaces between 1% and 5% of the particulate hydrocarbon and the proportion of fuel improver is equal to or greater than the amount of particulate hydrocarbon replaced.

18. A process according to Claim 17, wherein the fuel improver replaces between 1% and 3% of the particulate hydrocarbon.

19. A process according to any preceding claim, wherein the particulate hydrocarbon is coal.

20. A process according to any preceding claim, wherein the fuel improver includes aluminium oxide and/or calcium oxide.

21. A process according to Claim 20, wherein the fuel improver includes by weight up to 7% aluminium oxide and/or up to 10% calcium oxide.

22. A process according to Claim 21, wherein the fuel improver includes by weight between 3% and 5.5% aluminium oxide and/or between 2% and 7.5% calcium oxide.

23. A process according to any preceding claim, wherein the fuel improver comprises between 70% and 91% iron oxide and silicon dioxide, combined, by weight.

24. A process according to Claim 23, wherein the fuel improver comprises between 41 and 52% iron oxide, by weight.

25. A process according to Claim 23 or 24, wherein the fuel improver comprises between 32 and 40% silicon dioxide, by weight.

26. A process for producing a pozzolanic fly ash during the combustion of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of:

performing the process of any of Claims 1 to 25; and

recovering the fly ash from the burner, wherein the particle size of not more than 30% of the recovered fly ash is greater than 45 micron.

27. A process for producing a pozzolanic fly ash according to Claim 26, wherein the particle size of not more than 20% of the recovered fly ash is greater than 45 micron.

28. A process for producing a pozzolanic fly ash according to Claim 26 or 27, wherein at least 70% by weight of the fuel improver is comprised of silicon dioxide, iron oxide and aluminium oxide.

29. A process for producing a pozzolanic fly ash according to Claim 28, wherein at least 75% by weight of the fuel improver is comprised of silicon dioxide, iron oxide, aluminium oxide and calcium oxide.

30. A process for producing a pozzolanic fly ash according to any of Claims 27 to 29, wherein the pozzolanic fly ash has a loss on ignition of 7% or less.

31. A process for producing a cementitious composition comprising performing the process of any of Claims 27 to 30, and mixing the resulting pozzolanic fly ash with calcium hydroxide.

32. A composite cement comprising a mixture of portland cement and a cementitious composition produced by the process of Claim 31 and or a pozzolanic fly ash produced by the process of any of Claims 27 to 30.

33. A composite cement according to Claim 32, wherein the proportion of portland cement to cementitious composition produced by the process of Claim 31 and/or pozzolanic fly ash produced by the process of any of Claims 27 to 30 is between 30:70 and 70:30.

34. A process for reducing slagging in a boiler and/or fouling of a heat recovery apparatus arising from combustion of a particulate hydrocarbon in a burner, the particulate hyrdocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the step of

performing the process of any of Claims 1 to 25 in a burner associated with a boiler and/or heat recovery apparatus and controlling the amount of fuel improver relative to the particulate hydrocarbon such that the basic to acidic ratio and/or the simplified basic to acidic ratio (R₍B/A₎) to provide a slagging and/or fouling indices of a combination of particulate hydrocarbon and fuel improver are below the extremely high range.

35. A process for reducing slagging and fouling according to Claim 34, wherein the slagging index is not greater than 2.

36. A process according to Claim 34 or 35, wherein the fouling index is not greater than 2.

37. A process for reducing slagging and fouling according to any of Claims 34 to 36, wherein the slagging and/or fouling indices are 0.6 or less.

38. A process according to any of Claims 34 to 37, wherein the particulate hydrocarbon is coal.

39. A process for reducing slagging and fouling according to any of Claims 34 to 37, wherein the fuel improver includes Na20 and/or 2O and the particulate hydrocarbon includes Na20 and/or 2O and wherein relative proportion of fuel improver to particulate hydrocarbon is selected according to the proportions of Na20 and/or 2O in the fuel improver and the particulate hydrocarbon respectively.

40. A process for reducing NOx emissions during the combustion or heating of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of:

performing the process of any of Claims 1 to 25 to increase the proportion of volatile nitrogen to char nitrogen and volatile carbon to char carbon.

41. A process for reducing NOx emissions according to Claim 40, wherein combustion of the particulate hydrocarbon produces a flame having a fuel rich zone and wherein the proportion of volatile nitrogen to char nitrogen and/ or volatile carbon to char carbon is increased in the fuel rich zone.

42. A process according to Claim 40 or 41, comprising the further step of reacting volatile nitrogen with metal oxides of the fuel improver and volatile carbon oxides.

43. A process for reducing the dust concentration in an exhaust gas exiting an exhaust system, the fluecxhaust gas resulting from the combustion of or heating of a particulate hydrocarbon containing fuel bound carbon and fuel bound nitrogen, the process comprising the steps of:

providing the exhaust system with an electrostatic precipitator; and

performing the process of any of Claims 1 to 25 to reduce the mass of dust in the fluecxhaust gas downstream of the electrostatic precipitator.

44. A process according to Claim 43, comprising the step of controlling the concentration of dust in the exhaust gas by adjusting the amount of fuel improver heated in the burner.

45. A process for cracking hydrocarbon bonds during the combustion or heating of a particulate hydrocarbon substantially as shown in, and as described with reference to, the drawings.

46. A process for producing a pozzolanic fly ash substantially as shown in, and as described with reference to, the drawings.

47. A process for producing a cementitious composition substantially as shown in, and as described with reference to, the drawings.

48. A process for producing a composite cement substantially as shown in, and as described with reference to, the drawings.

49. A process for reducing slagging and fouling substantially as shown in, and as described with reference to, the drawings.

50. A process for reducing NOx emissions substantially as shown in, and as described with reference to, the drawings.