WO2007000724A1

WO2007000724A1 - Particulate material

Info

Publication number: WO2007000724A1
Application number: PCT/IB2006/052111
Authority: WO
Inventors: Johannes Christoffel Van Dyk; Len Breytenbach
Original assignee: Sasol-Lurgi Technology Company (Proprietary) Limited
Priority date: 2005-06-27
Filing date: 2006-06-27
Publication date: 2007-01-04

Abstract

A wet-screenable particulate fuel includes an admixture of fine coal and hydrophobic organic material as a binder. The fuel has a mechanical fragmentation value of less than about 60 % and a thermal fragmentation value of less than about 65 %. The invention extends to a process for producing the particulate fuel, and to a process for producing raw synthesis gas.

Description

PARTICULATE FUEL

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

Gasification plants utilising fixed bed gasification technology, also known as moving bed dry ash gasification technology, are typically associated with an upstream solids handling facility. Such solids handling facility includes classifying the solid feed material into various size fractions and reconstituting a uniform gasifier feedstock by blending together stockpiles of the various size fractions. The classification is preferably done using a wet screening process.

According to a first aspect of the invention, there is provided a wet- screenable particulate fuel which includes an admixture of fine coal and hydrophobic organic material as a binder, the fuel having a mechanical fragmentation value of less than about 60 % and a thermal fragmentation value of less than about 65 %.

The particulate fuel may have a water fragmentation value of less than 40 %, preferably less than 20 %.

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 U₃ and particle diameter d_P:

When dealing with particle size distributions instead of uniformly sized particles, the particle size d_p has to be replaced by φd_p , where φ 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:

where i = screen number

Xi = fraction (mass %) on screen i d_pj = 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 100⁰C under atmospheric pressure. The sample is then heated at a rate of 10⁰C 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:

A before test - A after test

% Thermal fragmentation - — = xlOO -(3)

A before test

The value of d_p 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, 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:

A before test - A after test

% Mechanical fragmentation = — = x 100 ■ ■ ■ (4 )

A 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.

Water fragmentation of the particulate fuel gives an indication of the wet- screeanability of the particulate fuel. The pellets are subjected to sieving with water spray for 5 minutes while shaking, in order to simulate a wet screening process. Similar to thermal fragmentation and mechanical fragmentation, the water fragmentation is determined using the Sauter diameter, as follows:

A before test - A after test % Water fragmentation = — = x 100

A before test

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

The hydrophobic organic material may include 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 microorganisms 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 include API sludge from an API gravity separator.

A further option is that the hydrophobic organic material may include dusty tar, i.e. the solids that remain once tars have been recovered from an organic component obtained after water quenching of a gasification stage gaseous product.

The particulate fuel may include the fine coal and the hydrophobic organic material in a mass ratio of from about 50 : 50 to about 99.5 : 0.5, often, depending on the nature of the hydrophobic organic material, from about 50 : 50 to about 90 : 10, usually from about 55 : 45 to about 75 : 25, e.g. about 60 : 40 or about 70 : 30.

When the hydrophobic organic material includes micro-organisms or microbes, the particulate fuel may include at least 10 % by mass of the hydrophobic organic material, preferably at least 15 % by mass, e.g. about 20 % by mass. Typically, the fine coal is the major component of the fuel, making up at least 50 % by mass of the fuel.

Preferably, the particulate fuel does not include other binders, e.g. thermoplastic or thermosetting materials or curable binders apart from the hydrophobic organic material (although the particulate fuel may include other organic combustible material which does not function principally as a binder). Typically, the particulate fuel is in the form of pellets produced by a mechanical process comprising mixing coal and 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 from about 4 mm to about 16 mm, preferably from about 6 mm to 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.

The fine coal may be a bitumous coal or a sub-bitumous coal.

Preferably, the particulate fuel has a mechanical fragmentation value of less than about 45 %, more preferably less than about 40 %, e.g. about 35 % or about 25 %.

Preferably, the particulate fuel has a thermal fragmentation value of less than about 45 %, more preferably less than about 40 % e.g. about 35 % or about 15 %.

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 hydrophobic organic 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 : 10, 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.

According to yet another aspect of the invention, there is provided a process for producing a wet-screenable particulate fuel, which includes forming an admixture by admixing fine coal and a hydrophobic organic material; and extruding the admixture at elevated pressure to produce pellets.

The process may include effecting the extrusion without transferring heat to the admixture prior to or during extrusion.

The admixture preferably does not include other binders apart from the hydrophobic organic material.

The fine coal may be a butimous or sub-bitumous coal. The hydrophobic organic material may include micro-organisms or microbes from activated waste sludge, or the hydrophobic organic material may include API sludge, or the hydrophobic organic material may include dusty tar. The hydrophobic organic material and the coal may thus be as hereinbefore described, and may be admixed in the ratios as hereinbefore described.

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 hydrophobic organic material;

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 hydrophobic organic material; and

Figure 3 shows a graph of water fragmentation versus the composition of the particulate fuel in accordance with the invention where the fuel consists of fine coal and hydrophobic organic material. EXAMPLE 1

Particulate fuel, in the form of pellets, was produced from a sub-bitumous coal with a particle size of less than 1.7 mm 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 micro-organisms, were used.

The pellets comprised coal and bio-sludge in a mass ratio of 60 : 40 and had a moisture content of about 20 % by mass and an ash content of about 30 % by mass.

The mechanical and thermal fragmentation of the pellets as a function of the mass ratio of coal : hydrophobic organic material were determined for pellets with a coal content of between 60 % and 90 % by mass and a micro-organism content of between 40 % and 10 % by mass. Similarly, the mechanical and thermal fragmentations were determined for a 70 : 30 coal : API sludge particulate fuel and a coal : dusty tar particulate fuel with a dusty tar concentration ranging between 0.5 % and 10 % by mass. Again, a sub-bitumous coal was used. After testing for the mechanical and thermal fragmentation of the pellets, the graphs shown in Figure 1 and Figure 2 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.

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 dusty tar, is comparable with that of coal available to the Applicant for gasification. Even the less than desired thermal stability of the coal/dusty tar particulate fuel 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.

EXAMPLE 2

A water fragmentation test was performed for some of the 8 particulate fuel compositions of Example 1. The purpose of this 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 simulate a wet screening process. The percentage water fragmentation was determined using the Sauter diameter, as follows:

A beforetest - A after test

% Water fragmentation = — = xlOO

A before test

The results are shown in Figure 3. EXAMPLE 3

A 24 hour submergence test was also conducted to test the strength of the fuel pellets of various compositions as described in Example 1. The test merely comprised submerging the fuel pellets in water for a period of 24 hours whereafter the pellets were visually inspected. The fuel pellets of the 8 compositions referred to in

Example 1 all remained intact after having been submerged in water for 24 hours.

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

It has now been surprisingly found that fine butimous and sub-bitumous coal can be combined with hydrophobic organic material such as micro-organisms, API sludge or dusty tar 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. In addition, the pellets are wet-screenable. Advantageously, the pellets can thus be classified and blended with the coal feedstock using a common material handling facility. No special effort is thus required to keep the particulate fuel dry.

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, organic waste handling and landfill availability.

By employing hydrophobic organic waste material as a substituent of a fine coal particulate fuel, the organic waste material can be reincorporated into the value chain to produce synthesis gas.

Claims

1. A wet-screenable particulate fuel which includes an admixture of fine coal and hydrophobic organic material as a binder, 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 , which has a water fragmentation value of less than 40 %.

3. The particulate fuel as claimed in claim 1 or claim 2, in which the hydrophobic organic material includes organic waste material generated by a petrochemical complex.

4. The particulate fuel as claimed in any one of the preceding claims, in which the hydrophobic organic material includes micro-organisms or microbes obtained from activated waste sludge.

5. The particulate fuel as claimed in claim 4, in which the activated waste sludge is produced by aerobic water purification works.

6. The particulate fuel as claimed in any one of the preceding claims, in which the hydrophobic organic material includes API sludge from an API gravity separator.

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

8. The particulate fuel as claimed in any one of the preceding claims, which includes the fine coal and the hydrophobic organic material in a mass ratio of between about 50 : 50 and about 99.5 : 0.5.

9. The particulate fuel as claimed in any one of the preceding claims, in which the hydrophobic organic material includes micro-organisms or microbes and which includes at least 10 % by mass of the hydrophobic organic material.

10. The particulate fuel as claimed in any one of the preceding claims, which includes at least 50 % by mass of the fine coal.

11. The particulate fuel as claimed in any one of the preceding claims, which does not include other binders apart from the hydrophobic organic material.

12. The particulate fuel as claimed in any one of the preceding claims, which is in the form of pellets produced by a mechanical process comprising mixing coal and hydrophobic organic material into an admixture and extruding the admixture to form pellets without the addition of significant heat to the admixture.

13. The particulate fuel as claimed in any one of the preceding claims, which has a particle size in the range of from about 4 mm to about 16 mm.

14. 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 4 mm.

15. The particulate fuel as claimed in any one of the preceding claims, in which the fine coal is a bitumous coal or a sub-bitumous coal.

16. The particulate fuel as claimed in any one of the preceding claims, which has a mechanical fragmentation value of less than about 45 %.

17. The particulate fuel as claimed in any one of the preceding claims, which has a thermal fragmentation value of less than about 45 %.

18. The particulate fuel as claimed in any one of the preceding claims, which has a water fragmentation value of less than about 20 %.

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

20. 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 hydrophobic organic material.

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

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

23. The process as claimed in any one of claims 20 to 22 inclusive, in which the gasification zone is a fixed bed dry ash gasification zone.

24. The process as claimed in claim 23 in which the particulate feedstock has an ash content of less than about 40% by mass.

25. A process for producing a wet-screenable particulate fuel, which includes forming an admixture by admixing fine coal and a hydrophobic organic material; and extruding the admixture at elevated pressure to produce pellets.

26. The process as claimed in claim 25, in which the extrusion is effected without transferring heat to the admixture prior to or during extrusion.

27. The process as claimed in claim 25 or claim 26, in which the admixture does not include other binders apart from the hydrophobic organic material.

28. The process as claimed in any one of claims 25 to 27 inclusive, in which the fine coal is a butimous or sub-bitumous coal and in which the hydrophobic organic material includes micro-organisms or microbes from activated waste sludge, or in which the hydrophobic organic material includes API sludge, or in which the hydrophobic organic material includes dusty tar.