WO2006003615A1

WO2006003615A1 - Particulate fuel

Info

Publication number: WO2006003615A1
Application number: PCT/IB2005/052139
Authority: WO
Inventors: Martin Jakobus Keyser; Johannes Christoffel Van Dyk; Johan Hendrik Slaghuis; Len Breytenbach
Original assignee: Sasol Technology (Proprietary) Limited
Priority date: 2004-06-29
Filing date: 2005-06-28
Publication date: 2006-01-12

Abstract

A particulate fuel includes an admixture of fine coal and fibrous cellulosic material. The fuel has a mechanical fragmentation value of less than about 60 % and a thermal fragmentation value of less than about 65 %. The fuel may optionally include hydrophobic organic material.

Description

PARTICULATE FUEL

THIS INVENTION relates to a particulate fuel. It also relates to a process for producing raw synthesis gas.

According to a first aspect of the invention, there is provided a particulate fuel which includes an admixture of fine coal and fibrous cellulosic material, the fuel having a mechanical fragmentation value of less than about 60 % and a thermal fragmentation value of less than about 65 %.

In fixed bed gasification, also known as moving bed dry ash gasification, one typically finds an almost stationary bed of particulate material through which a gas, comprising a gasification agent and gaseous gasification products flow.

Probably the best known estimation method for pressure drop through a bed of particulate material is the Ergun equation, which gives pressure drop as a function of bed voidage ε , viscosity μ, fluid density p, superficial velocity U_s and particle diameter d_p:

= 150 (i - g )² /^, + 1.75 (1 - ^S)PU ₁

L ε³dl ε³d .

...(1 )

When dealing with particle size distributions instead of uniformly sized particles, the particle size d_p has to be replaced by φd_p , where ψ is the particle sphericity and d_p the average particle size reflecting the mean surface area (also referred to as the Sauter diameter or Sauter mean diameter). The Sauter diameter of a particulate fuel such as a coal sample with a specific particle size distribution is calculated as follows: d_P = * ...(2)

V Z_ι JU

where i = screen number Xi = fraction (mass %) on screen i dpj = diameter (mm) of screen i

The Applicant believes that d_p is a useful parameter for predicting which particulate fuel particle size distributions are more likely to result in gasifier instability, with a smaller d_p being indicative of increased risk of instability.

It follows from the Ergun equation that for a fixed maximum allowable pressure drop through a bed of particulate material, the maximum allowable superficial velocity decreases with decreasing d_p . The applicant also believes that d_p is indicative of the maximum gasification load of a gasifier.

Thermal fragmentation of the particulate fuel is measured by placing a sample of the fuel with a specific predetermined size distribution into a pre-heated muffle oven at 100°C under atmospheric pressure. The sample is then heated at a rate of 10 ⁰C per minute to 700⁰C. After the sample is cooled under nitrogen and screened again, the change in size distribution is calculated. The percentage thermal fragmentation of coal is given as a percentage decrease in Sauter diameter ( d_p ). The smaller the percentage decrease, the better the thermal stability.

Thermal fragmentation is thus defined as:

_// before test - β after test

% Thermal fragmentation = ^ = ^ x 100 • • • (³) _ζj before test

The value of d is extremely sensitive to the smaller particle sizes, or the so- called "tail" of the particle size distribution. As illustrated in Table 1 for hypothetical coal samples, a 10% change in particle size to the coarser side resulted in a change of only 3% in the Sauter diameter, while a 10% change in particle size to the finer fraction resulted in a 7% change in the Sauter diameter.

TABLE 1 EFFECT OF CHANGE IN PARTICLE SIZE ON SAUTER DIAMETER

Weathering / oxidation and moisture content affect the thermal fragmentation of coal sources. An extensive study revealed that the effect of moisture contributes to +75% of the thermal fragmentation of coal. This is not only surface moisture, but a combination of surface moisture and inherent moisture captured within the pores and the coal structure. Although moisture contributes significantly towards fragmentation, a thermal fragmentation is also affected by a complex interaction of other factors.

Mechanical fragmentation of the particulate fuel is measured by means of a Micum tumble test. A sample of the fuel is placed in a steel drum and rotated at a speed of 60 revolutions per minute for 5 minutes. The sample is then sieved into specified standard particle size fractions. The results of the tumble test are calculated using the Ergun Index, which is also known as the Sauter Mean Diameter. As with thermal fragmentation, the mechanical fragmentation is thus defined as: sJ before test — _ζj after test

% Mechanical fi-agmentation = — = xlOO ■■■(4)

^ before test

From the above, it is clear that fragmentation of fuel particles in a gasification bed is undesirable and that mechanical and thermal stability of the fuel particles are desirable characteristics.

The fibrous cellulosic material present in the fuel particles may comprise or may have been obtained from biomass pulp produced by a paper mill.

The particulate fuel may include the fine coal and the cellulosic material in a mass ratio of between about 50 : 50 and about 80 : 20, usually between about 55 : 45 and about 75 : 25, e.g. about 60 : 40 or about 70 : 30.

The particulate fuel may include hydrophobic organic material. The hydrophobic organic material may be waste material, and may in particular be organic waste material generated by a petrochemical complex.

The hydrophobic organic material may be micro-organisms or microbes. When the hydrophobic organic material is in the form of micro-organisms or microbes, the micro-organisms or microbes present in the fuel particles may be obtained from or may comprise activated waste sludge. In one embodiment of the invention, the micro¬ organisms or microbes are obtained from or comprise activated waste sludge produced by aerobic water purification works.

Instead, or in addition, the hydrophobic organic material may be API sludge from an API gravity separator.

A further option is that the hydrophobic organic waste material may be dusty tar, i.e. the solids that remain once tars have been recovered from the organic component obtained from the water quenching of the gaseous product from a gasification stage. When the particulate fuel also includes hydrophobic organic material, e.g. micro-organisms or microbes, the fuel may include between about 58 % and about 95 % by mass fine coal, between about 1 % and about 25 % by mass fibrous cellulosic material and between about 1 % and about 26 % by mass hydrophobic organic material, e.g. the ingredients may be present in a mass ratio of 68 : 24 : 8. Naturally, all of the ingredients of the particulate fuel, including any moisture, will add up to 100 %.

Preferably, the particulate fuel does not include binders or other additives, e.g. thermoplastic or thermosetting materials or curable binders apart from the fibrous cellulosic material and fine coal and, optionally, hydrophobic organic material and is in the form of pellets produced by a mechanical process comprising mixing coal and cellulosic material and optionally hydrophobic organic material into an admixture and extruding the admixture to form pellets. The extrusion may be effected at a pressure in the range of 10 bar to 300 bar. Preferably, the mechanical process does not include the addition of significant heat to the admixture.

The particulate fuel may have a particle size ranging between about 4 mm and about 16 mm, preferably between about 6 mm and about 14 mm, e.g. about 8 mm or about 12 . When in the form of pellets, these dimensions may be pellet diameters.

The fine coal may have a maximum particle size of no more than 4 mm, preferably no more than 2 mm, more preferably no more than 1. 7 mm, most preferably less than 1 mm.

Preferably, the particulate fuel has a mechanical fragmentation value of less than about 55 %, more preferably less than about 50 %, e.g. about 45 %.

Preferably, the particulate fuel has a thermal fragmentation value of less than about 55 %, e.g. about 50 %.

When the fuel includes fine coal, fibrous cellulosic material and hydrophobic organic material, the mechanical and thermal fragmentation values may both be less than 30 %. The particulate fuel may have a moisture content of up to about 30% by mass, preferably less than about 25% by mass, more preferably less than about 20% by mass, e.g. about 18 % by mass. Typically, the particulate fuel has a moisture content of at least about 15% by mass.

According to another aspect of the invention, there is provided a process for producing raw synthesis gas, the process including, in a gasification zone, simultaneously gasifying a coal feedstock and a particulate feedstock comprising an admixture of fine coal and fibrous cellulosic material.

The particulate feedstock may be a particulate fuel as hereinbefore described.

The coal feedstock and the particulate feedstock may be gasified in a mass ratio greater than about 90 : 10, usually greater than about 95 : 5, e.g. about 99 : 1.

The gasification zone may be a fixed bed or moving bed dry ash gasification zone.

The particulate feedstock preferably has an ash content of less than about

40% by mass, more preferably less than about 35% by mass, most preferably less than about 30%, e.g. about 25% by mass.

The invention will now be described by way of the following Examples and the drawings.

In the drawings,

Figure 1 shows a graph of mechanical fragmentation versus the composition of the particulate fuel in accordance with the invention where the fuel consists of fine coal and biomass pulp produced by a paper mill;

Figure 2 shows a graph of thermal fragmentation versus the composition of the particulate fuel in accordance with the invention where the fuel consists of fine coal and biomass pulp produced by a paper mill;

Figure 3 shows a graph of thermal fragmentation versus gasifier feedstock composition where the feedstock consists of various mass ratios of lump coal and the particulate fuel of the invention;

Figure 4 shows a graph of mechanical fragmentation versus the composition of the particulate fuel in accordance with the invention where the fuel consists of fine coal, waste activated sludge produced during the purification of water and biomass pulp produced by a paper mill; and

Figure 5 shows a graph of thermal fragmentation versus the composition of the particulate fuel in accordance with the invention where the fuel consists of fine coal, waste activated sludge produced during the purification of water and biomass pulp produced by a paper mill.

EXAMPLE 1

Particulate fuel, in the form of pellets, was produced from coal with a particle size of less than 1.7 mm and fibrous cellulosic biomass material received from a paper mill. The pellets were produced by admixing the coal and the biomass and extruding the admixture at a pressure of between 10 bar and 30 bar through a die with a plurality of 12 mm diameter apertures to produce pellets with a 12 mm diameter. The admixture was not heated, i.e. was at ambient temperature, and no additives, binders or other ingredients, apart from the coal and biomass material, were used.

The pellets comprised coal and biomass pulp in a mass ratio of 70 : 30 and had a moisture content of about 15% by mass and an ash content of 25% by mass.

The mechanical and thermal fragmentation of the pellets, as a function of the mass ratio of coal : fibrous cellulosic material was determined by producing pellets with a coal : fibrous cellulosic material mass ratio ranging between 50 : 50 and 80 : 20. After testing for the mechanical and thermal fragmentation of the pellets, the graphs shown in Figure 1 and Figure 2 were produced.

Particulate fuel, in the form of pellets, was also produced from coal with a particle size of less than 1.7 mm, fibrous cellulosic biomass material received from a paper mill and micro-organisms (waste sludge) received from an aerobic water purification works. The pellets were produced by admixing the coal and the biomass and the waste sludge and extruding the admixture at a pressure of between 10 bar and 30 bar through a die with a plurality of 12 mm diameter apertures to produce pellets with a 12 mm diameter. The admixture was not heated, i.e. was at ambient temperature, and no additives, binders or other ingredients, apart from the coal and biomass material and micro-organisms, were used.

The pellets comprised coal, biomass pulp and bio-sludge in a mass ratio of 68 : 8 : 24 and had a moisture content of about 15% by mass and an ash content of 25% by mass.

The mechanical and thermal fragmentation of the pellets were determined for pellets with a coal content of between 58 % and 74 % by mass, a fibrous cellulosic content of between 7 % and 25 % by mass and a micro-organism content of between

13 % and 24 % by mass. After testing for the mechanical and thermal fragmentation of the pellets, the graphs shown in Figure 4 and Figure 5 were produced.

The mechanical fragmentation of the pellets gives an indication of the fragmentation that can take place during handling and conveying of the pellets. Thus, it gives an indication of the fine particulate matter generation that can take place before the pellets are used, e.g. gasified.

From Figure 1 , it was deduced that the mechanical stability of the pellets compares favourably with that of coal sources currently used for gasification purposes by the Applicant, over most of the composition range of the pellets.

From Figure 5, it can be deduced that the mechanical stability of the pellets exceeds the mechanical stability of coal sources that have been successfully commercially gasified by the Applicant over the entire composition range investigated.

It is known that lump coal from certain sources tend to undergo fragmentation

(primary and secondary fragmentation) when exposed to temperatures of the order of 700 ⁰C, as are experienced during gasification of the coal. Primary fragmentation occurs during devolatilization, while secondary fragmentation occurs during combustion of char by burnout of carbon bridges connecting parts of the coal particle. In the case of fixed bed gasification, the fine material thus formed in the gasifier may lead to hydrodynamic problems, as well as carryover of fine coal particles into a raw gas stream produced by the gasifier. For use in a gasifier, the thermal fragmentation of the pellets is thus important.

From Figure 2, it can be deduced that the thermal fragmentation of the pellets, although not at a preferred level for compositions with lower fibrous cellulosic material content, is comparable with that of coal available to the Applicant for gasification. This however is not a major concern, as the fuel pellets, when used with lump coal in a gasifier in a relatively large mass ratio of coal : pellets of 90 : 10 or higher has a combined thermal fragmentation which is very similar to that of wet coal from the coal sources available to the Applicant for gasification purposes, as shown in Figure 3 of the drawings. Figure 3, which shows results for two tests, also illustrates that the pellets can repeatedly be produced with physical characteristics such as thermal fragmentation varying only to a small and acceptable degree.

From Figure 5, it can be deduced that the thermal stability of the pellets exceeds the thermal stability of coal sources that have been successfully commercially gasified by the Applicant over the entire composition range investigated.

EXAMPLE 2

Tests were conducted on biomass obtained from a commercial paper mill to determine whether or not the biomass can be gasified and the carbon contained therein converted to valuable products. The biomass was in the form of pulp with a fibrous structure and was tested on full scale in a test gasifier in order to obtain insight into the effect, if any, of the low ash melting temperature, high volatile and water content and other characteristics of the biomass which could cause unstable gasifier operation or downstream problems. Three tests were conducted with a coal : biomass blend where the biomass was derived from bark, in a mass ratio of coal : bark of 90 : 10 and one test was conducted with a coal : biomass blend where the biomass consisted of a mixture of bark and pulp and where a mass ratio of coal : biomass of 90 : 10 was used. These tests proved that wood products can be co-gasified with coal in a fixed bed gasifier, such as a Sasol-Lurgi fixed bed gasifier and that the addition of up to 10 % bark and pulp did not affect the properties of the total gasification feed significantly. Pure gas yields were as expected and the addition of the biomass did not have a significant effect on pure gas yield when compared with reference tests. Utility usage, such as steam, oxygen and high pressure boiler feed water and gas liquor production were similar to that of reference tests conducted with normal coal feedstocks. Similar tar yields were measured with the feedstock blend compared to tar yields from the reference tests. The liquid hydrocarbon fractions produced from the feedstock blend however differed from that produced from coal, although the difference was negligible.

EXAMPLE 3

About 100 tons of a particulate fuel in accordance with the invention was produced from coal and discarded paper pulp. The coal to pulp ratio used for the particulate fuel was 70 : 30.

The particulate fuel was in the form of pellets and was produced in accordance with Example 1 above. The average properties of the pellets were similar to that of typical lump coal feed available to the Applicant for gasification purposes. The pellets had an average ash content of 27.9 %. When a coal/pellets blend was prepared in a ratio of 90 : 10, the blend had an average ash content of 25.9 %. During gasification of the blend in a test gasifier, ash which was coarser and which included a smaller middle fraction, compared to ash from coal only, was produced. The percentage fine coal (-6.3 mm) in the blend was 10.3 % in comparison with an average of 12.4 % for previous lump coal base case tests. The amount of fine material in the pellets varied from 11 % to 30 %, depending inter alia on the source of fine coal used. The amount of solids carried over in a tar stream from the test gasifier was within the variation observed for other tests with a coal feed only. Microscopic photos of solids in the tar indicated that no fibre structures were observed and the structures of the particles were that of coal which had already seen a temperature of higher than 500 ⁰C (char-like). This was observed in base case tests using coal only, as well as the tests using the coal and pellets blend. This indicated that thermal shock in upper regions of the gasifier did not play a role in the fragmentation of the pellets, as it would be expected that unreacted coal structures would be seen in the solids if this was the case. The microscopic photos also indicated that the particle size distribution of the solids in the tar from the gasification of the blend is comparable with the particle size distribution where normal coal is gasified without pellets. It was thus concluded that the characteristics of the coal pellets blend were not adversely affected by the properties of the pellets added to the coal and were very similar to that of normal coal blends tested previously by the Applicant.

During gasification, it was observed that the ratio of oxygen to pure gas was slightly higher or similar to the normal scatter of the data for gasification processes using normal coal blends. Carbon losses in the ash for the coal pellets blend were comparable with previous base case tests using coal blends only. Pure gas measured for the coal pellets blend was on the same trend line as that of previous coal blend base case tests. Similar pure gas production for the coal pellets feed is thus obtained in comparison with base case tests for slightly higher or similar oxygen loads. The gas outlet temperature for the gasifier was in the same range as the outlet temperature obtained during previous base case tests using normal coal blends and within the normal scatter of the data.

EXAMPLE 4

About 100 tons of a particulate fuel in accordance with the invention was produced from coal, discarded paper pulp and bio-sludge. The coal to pulp to bio- sludge ratio used for the particulate fuel was 68 : 8 : 24. When the fuel was used in a manner similar to example 3, similar results were achieved.

Further examples of the particulate fuel of the invention were prepared, comprising coal, discarded paper pulp and bio-sludge. Mechanical and thermal fragmentation tests were performed on the examples. A water fragmentation test was also performed for the examples. The purpose of the water fragmentation test was to determine the strength of the fuel pellets when exposed to water. The water fragmentation test was performed by sieving pellets with water spray for 5 minutes while shaking, in order to stimulate a wet screening process. The percentage water fragmentation was determined using the Sauter diameter, in the same fashion as for mechanical and thermal fragmentation. The following table illustrates the compositions and their mechanical, thermal and water fragmentation test results. Composition Mechanical Thermal Water Coal : Pulp ; Bio-sludge on fragmentation fragmentation fragmentation a dry mass basis (%) (%) (%)

94.6 2.2 3.3 22 19.91 16

90.2 6.2 3.6 19 17.71 12

92.8 4.8 2.5 24 25.55 18

94.9 2.2 2.9 22 15.51 10

92.4 4.4 3.3 22 18.24 17

90.2 6.9 2.9 18 8.55 14

88.0 8.8 3.3 21 23.88 16

92.0 4.0 4.0 29 17.62 16

92.0 6.6 3.3 25 8.81 19

92.0 4.0 4.0 24 17.62 11

From these tests, it can be concluded that a fuel pellet comprising an admixture of fine coal and fibrous cellulosic material and, optionally, hydrophobic organic material can be successfully gasified in a fixed bed gasifier, to produce raw synthesis gas and gas liquor which can be supplied to a conventional downstream plant, such as a Fischer-Tropsch hydrocarbon conversion plant.

Not wishing to be bound by theory, the Applicant believes that cellulosic fibres act to strengthen pellets by the mechanism of mechanical interlocking, while the hydrophobic organic material acts to strengthen pellets by the mechanism of immobile bridges.

The Applicant has successfully commercially gasified coals with mechanical fragmentation values of less than about 60 % and a thermal fragmentation values of less than about 65 %.

It has now been surprisingly found that fine coal can be combined with fibrous cellulosic material and, optionally, hydrophobic organic material such as micro¬ organisms as binder to yield pellets of comparable, or even improved strength. Advantageously, the pelletizing process does not require a heat treatment step, which implies substantial economic benefits.

The generation of fine coal by coal handling facilities has a negative cost implication for an enterprise using the coal. The fine coal requires an additional cost to store or dispose of in dams which has to meet specific environmental requirements.

Advantageously, the invention, as illustrated, alleviates the problem of fine coal dumping, biomass waste handling and landfill availability.

Claims

CLAIMS:

1. A particulate fuel which includes an admixture of fine coal and fibrous cellulosic material, the fuel having a mechanical fragmentation value of less than about 60 % and a thermal fragmentation value of less than about 65 %.

2. The particulate fuel as claimed in claim 1 , in which the fibrous cellulosic material comprises biomass pulp produced by a paper mill.

3. The particulate fuel as claimed in claim 1 or claim 2, which includes the fine coal and the cellulosic material in a mass ratio of between about 50 : 50 and about 80 : 20.

4. The particulate fuel as claimed in claim 3, which includes the fine coal and the cellulosic material in a mass ratio of between about 55 : 45 and about 75 : 25.

5. The particulate fuel as claimed in any one of the preceding claims, which includes hydrophobic organic material.

6. The particulate fuel as claimed in claim 5, in which the hydrophobic organic material includes micro-organisms or microbes obtained from activated waste sludge.

7. The particulate fuel as claimed in claim 6, in which the micro-organisms or microbes are obtained from or comprise activated waste sludge produced by aerobic water purification works.

8. The particulate fuel as claimed in any one of claims 5 to 7 inclusive, in which the hydrophobic organic material includes API sludge.

9. The particulate fuel as claimed in any one of claims 5 to 8 inclusive, in which the hydrophobic organic material includes dusty tar.

10. The particulate fuel as claimed in any one of claims 5 to 9 inclusive, which includes between about 58 % and about 95 % by mass fine coal, between about 1 % and about 25 % by mass fibrous cellulosic material and between about 1 % and about 26 % by mass hydrophobic organic material.

11. The particulate fuel as claimed in any one of the preceding claims, which does not include binders or other additives apart from the fibrous cellulosic material and fine coal and, optionally, hydrophobic organic material and is in the form of pellets produced by a mechanical process comprising mixing coal and cellulosic material and optionally hydrophobic organic material into an admixture and extruding the admixture to form pellets.

12. The particulate fuel as claimed in any one of the preceding claims, which has a particle size ranging between about 4 mm and about 16 mm.

13. The particulate fuel as claimed in any one of the preceding claims, in which the fine coal has a maximum particle size of no more than about 4 mm.

14. The particulate fuel as claimed in claim 13, in which the fine coal has a maximum particle size of no more than about 2 mm.

15. The particulate fuel as claimed in any one of the preceding claims, which has a mechanical fragmentation value of less than about 55 %.

16. The particulate fuel as claimed in any one of the preceding claims, which has a thermal fragmentation value of less than about 55 %.

17. The particulate fuel as claimed in any one of the preceding claims, which has a moisture content of up to about 30 % by mass.

18. The particulate fuel as claimed in claim 17, which has a moisture content of less than about 25 % by mass.

19. A process for producing raw synthesis gas, the process including, in a gasification zone, simultaneously gasifying a coal feedstock and a particulate feedstock comprising an admixture of fine coal and fibrous cellulosic material.

20. The process as claimed in claim 19, in which the particulate feedstock is a particulate fuel as claimed in any one of claims 1 to 18 inclusive.

21. The process as claimed in claim 20, in which the coal feedstock and the particulate feedstock are gasified in a mass ratio greater than about 90 : 10.

22. The process as claimed in any one of claims 19 to 21 inclusive, in which the gasification zone is a fixed bed or moving bed dry ash gasification zone.

23. The process as claimed in any one of claims 19 to 22 inclusive, in which the particulate feedstock has an ash content of less than about 40 % by mass.