This invention relates to the hydrothermal treatment of coals, and is particularly though not exclusively useful in the treatment of low rank coals such as brown coals and lignites. The invention involves a more efficient method of pressurization of the coal slurry fed to the hydrothermal reactor. This improvement reduces the energy consumption for the process and allows the use of more dilute slurry concentrations without incurring large pumping losses. This increases the ability to add coal cleaning technologies (which require dilute slurries), to reduce the mineral content of the coal either before or after hydrothermal treatment, without additional dewatering steps.

BACKGROUND OF THE INVENTION

There is rapidly growing pressure for stationary sources of C0 ₂ emissions such as power stations to make step reductions in greenhouse gas (GHG) emissions. In the case of coal-fired power stations, one method of achieving this is by increasing the thermal efficiency of coal utilization. For conventional technology involving pulverized fuel firing of boilers with low rank coals, this usually entails reducing the water content of the coal prior to combustion. One method of partially dewatering coal is by hydrothermal treatment. Hydrothermal treatment is a known method for dewatering low rank coal to improve its utilisation efficiency in boilers, and for producing a slurry fuel with fluid properties suitable for injecting into internal combustion engines, gasifiers and other combustion devices.

A schematic of the process currently practiced for hydrothermal treatment is shown in Figure 1. Low rank coal is milled to typically -1mm, and then mixed with water to produce a pumpable slurry. The slurry is pressurized and heated (typically to 250-350 C and at a pressure above the saturation pressure to prevent boiling) for a period of 15-30 minutes. After heating, the slurry is cooled, the pressure is reduced via let down valves or other frictional pressure loss devices, the gases evolved during the treatment are removed, and then the slurry is dewatered as much as practical using a range of separation devices such as centrifuges or filtration.

The main objective of the hydrothermal treatment process is to liberate bound water to enable it to be removed as a liquid, which avoids the latent heat losses that would occur if the water was removed by drying. Other effects include changes to the pore structure and surface properties of the coal which improve the rheology of the slurry after dewatering.

The energy for hydrothermal treatment is typically 1-3% of the energy content of the coal, and depends mostly on the efficiency of the heat exchangers, the solids content of the feed slurry to the hydrothermal reactor, the hydrothermal treatment temperature and pressure, and the energy for subsequent dewatering processes. The energy for pumping represents most of the actual processing energy, and generally amounts to 1-4% of the energy of the coal. This reduces the overall efficiency of the fuel cycle using the fuel, and increases the overall C0 ₂ emissions.

As this pumping energy loss is inversely proportional to the coal concentration of the slurry to the hydrothermal reactor, the coal concentration is normally made as high as possible (generally around 20-25 wt%) consistent with slurry heat transfer and flow characteristics in the heat exchangers and hydrothermal reactor.

When coal cleaning is required a much more dilute slurry is required (say 5-10 wt%). Current approaches to hydrothermal treatment would mean that an additional dewatering stage would be required for processes using a pre-clea ing step before the high pressure feed pump to the hydrothermal reactor.

A number of methods have been previously considered to reduce the energy required for pressurizing the hydrothermal reactor. These include 1) a hydrostatic reactor which uses a long vertical reactor/heat exchanger set into the ground, and 2) devices to utilize the pressure energy for hydraulic milling.

The disadvantage of the hydrostatic reactor- -arrangement- is the need for ^_ a ^~ vertrc¾l ^— hole ^" sufficiently deep (typically 1,000- 1,500m deep) to provide the necessary hydrostatic head, which may limit maximum operating pressure and temperature unless an auxiliary pump is used. This reactor has other disadvantages including access for inspection, repairs and maintenance. The disadvantage of the hydraulic milling method is this that this produces a finer particulate size before the dewatering step, which increases the difficulty of dewatering. It also requires the use of an hydraulic mill which may not be optimal for all coals.

It is an object of the invention to provide an improved or at least alternative method of reducing the overall energy losses to pressurizing the coal slurry for the hydrothermal treatment process. SUMMARY OF THE INVENTION The invention comprises an improved method for hydrothermal treatment of coals (especially low rank coals such as Victorian and other brown coals, and Indonesian, German, Eastern European and other lignites) for producing dewatered solid fuel and coal water fuel suitable or adaptable for diesel engines, gasifiers and other combustion devices.

The process directly utilizes the pressure and flow of the hydrothermally treated slurry to provide a proportion, advantageously a substantial amount, of the pumping energy for the high pressure feed slurry into the hydrothermal treatment stages.

The invention therefore provides, in one aspect, a process for treating coal in which coal, in the form of particles in an aqueous pumpable slurry, is treated at an elevated temperature while maintaining the pressure of the slurry at a pressure above the saturation pressure, whereby to at least partially release, as liquid water, bound water from the coal particles, wherein, after said treatment at an elevated temperature, energy is recovered from the pressure of the slurry and that recovered energy is utilised to contribute to raising the pressure of the pumpable slurry prior to said treatment to said pressure above the saturation pressure.

In a further aspect, the invention provides an apparatus for treating coal, comprising: . a hydrothermal reactor arrangement for treating coal, in the form of particles in an aqueous pumpable slurry, at an elevated temperature while maintaining the pressure of the slurry at a pressure above the saturation pressure, whereby to at least partially release, as liquid water, bound water from the coal particles; a pressurising mechanism upstream of said reactor arrangement for raising the pressure of the pumpable slurry to said pressure above the saturation pressure; an energy recovery device downstream of said reactor arrangement for recovering energy from the pressure of the slurry, which energy recovery device is coupled to the pressurising mechanism whereby said recovered energy contributes to said raising the pressure of the pumpable slurry.

Typically, the slurry after cooling and said recovery of energy, is treated to separate out at least a portion of the total liquid water of the slurry. Preferably, pressure energy in the hydrothermally treated slurry is recovered via a positive displacement slurry pump device operating in reverse as an energy harvesting device i.e. a regeneration motor. This pump device may be connected to a slurry pump using any of a range of mechanical, electrical or hydraulic devices. By virtue of the invention, the energy for pumping is substantially reduced, and the energy penalty for using more dilute slurries can thereby be also substantially reduced. This reduces the cost and energy consumption for the process. The arrangement also enhances the flexibility of the process for the integration and operation of steps for removal of minerals, dewatering- and milling. The positive displacement pump device may typically include a reciprocable piston element, which may be a linearly reciprocable or rotary element, that in part defines a variable volume chamber. Preferably, the positive displacement pump device has actively controlled valves for controlling intake and exhaust of the slurry to and from a variable volume chamber in part defined by the reciprocable piston element. BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be implemented using a number of arrangements, including, but not restricted to, the examples illustrated in the drawings, in which:

Figure 1 is a schematic of a conventional hydrothermal dewatering process for partial dewatering of low rank coal;

according to an embodiment of the invention;

Figure 3 is a similar view of an embodiment having a direct-coupled pump-motor configuration with hydraulic augment and control;

Figure 4 is a similar view of an embodiment having a direct-coupled pump-motor configuration with an electrically powered crank for augment and control;

Figure 5 is a similar view of an embodiment having an indirect-coupled pump-motor configuration using electrical or hydraulic means. Figure 6 is a similar view of an embodiment having a direct-coupled pump-motor configuration with an electrical augment and control using a compound slurry pump; and

Figure 7 is a similar view of an embodiment having a direct-coupled pump-motor configuration with an electrical augment and control using a tandem slurry pump. DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the exemplary process of the invention depicted schematically in Figure 2, low rank coal 110, for example a brown coal or lignite, of 55-70% water content, is processed in a mill 1 12 to a size typically of -1mm. The thus prepared particulate coal is passed to a slurry tank 116 in which the coal is mixed with water 114 to produce a pumpable slurry 118 which is, for example, 75% water. The slurry is drawn from tank 116, optionally via a pre-cleaning step 115, by a high pressure pump 120 and passed through a heater 122 to a hydrothermal reactor 124, which may, for example, be an autoclave. Here, in accordance with conventional hydrothermal treatment, the coal slurry is retained in a heated state at a temperature in the range at 250-350°C and at a pressure above the saturation pressure whereby the water released from the coal does not boil, for 20-30 minutes. Pump 120, heater 122 and reactor 124 together constitute a hydrothermal reactor arrangement 125. The process of hydrothermal treatment causes a number of dehydration and decarboxylation reactions to occur in the coal, which at least partially releases or liberates (chemically) bound water and some carbon dioxide (via the decomposition of hydroxide and carboxyl groups).

The hydrothermally treated slurry 127 is cooled 126 (including thermal regeneration heat exchange with the incoming slurry) and then passed to let-down valves 128 in which the high pressure slurry -water-isxpntcojlably released, effecting controlled pressure reduction. The gases evolved during the treatment, primarily carbon dioxide, are removed in a gas separator 130 and sent to waste 131. The slurry fraction 132 is passed through a centrifuge 134 (and optionally thickeners) to achieve a product slurry 140 having approximately 25-55% water. A post-cleaning step 129 is optionally included after pressure let-down.

In accordance with an embodiment of the invention, the cooled slurry is passed, downstream of the cooler/thermal regenerator 126, through an energy recovery device in the form of a regenerative slurry motor 150 comprising a positive displacement slurry pump operating in reverse. High pressure pump 120 is similarly a positive displacement pump device. The motor 150 and pump 120 are coupled via one or more mechanical, electrical or hydraulic coupling means 160 so that energy recovered at motor 150 in part drives pump 120. The positive displacement pumps can be linear or rotary reciprocatory piston pumps. If a piston-type regenerative motor is used, multiple pumping and motor elements and/or accumulators could be used to dampen pressure oscillations in the system.

Pump 150 has actively controlled intake and exhaust valves 152, 153 because the self-actuating valves or one way valves ordinarily fitted to positive displacement pumps will not act as needed in reverse operation. These actively controlled valves can also be used to control the phase between the elements of pump and motor to optimise efficiency and let down operation.

Small energy losses in the illustrated system typically require that the method of linking the elements of the pump 120 and motor 150 is also used to provide a small proportion of augmenting energy. Pressure let down valves 128 are employed to control the rate of pressure reduction in the slurry after the regenerative motor 150 to reduce decrepitation of the coal and cavitation in the motor. Additionally, regenerative motor 150 may be provided with a by-pass let down system to enable depressurization control independent of the regenerative motor.

Compressor and expander type pumps and turbines are less suitable to serve (in reverse operation) as regenerative motor 150, as these devices incur a significant efficiency loss due to the thermodynamics of compression and expansion. Positive displacement pumps do not in general encounter this loss mechanism, because they allow the compression / decompression phase to be separated from the transport phase. This reduces losses as large changes in pressure due to the required compression are not encountered during fluid flow. This is because the inlet and delivery valves on such a pump are passive non-return valves.

-Eig-ures-3-andAillustrate specific forms of the embodiment of Figure 2 in which the coupling between the regenerative motor 150 and the pump 120 is respectively a direct hydraulic coupling 160a, with energy augment 162, and an electrical crank 160b with auxiliary power 164 for providing augmenting energy. Figure 5 depicts an indirect coupling using electrical or hydraulic means 160c. It is envisaged that this linkage would include power augment and control.

Figure 6 illustrates an embodiment in which the motor 150 and pump 120 are directly coupled 160d without augment, but in which a compound slurry pump 121 is disposed between pump 120 and thermal regenerator 126 to provide electrical augment and control upstream of the reactor 124. This arrangement is effective to increase the slurry pressure to offset head and mechanical losses in the hydrothermal and regenerative steps. A range of means could be provided for recharging the pump, for example a pre-charging pump or a small return stroke actuator.

Figure 7 shows an embodiment having a direct-coupled pump-motor configuration with an electrical augment and control using a tandem slurry pump 123 to increase the volume of pressurized slurry to offset head and mechanical losses in the hydrothermal and regenerative steps. In this system the swept volume of the regenerative motor is larger than the main high pressure pump. The advantage of this method is that controlled valving of the regenerative motor may be used to allow expansion of evolved gases in the treated slurry (eg C0 ₂ produced from decarboxylation reactions) to provide additional pumping power. A range of means would be provided for recharging the pump, for example a pre-charging pump or a small return stroke actuator.

The illustrated configuration utilizes the pressure of the slurry leaving the hydrothermal reactor directly to substantially offset the energy for pumping without otherwise affecting the process. This gives greater energy efficiency without restricting the plant configurations available for the hydrothermal process, dewatering and pre- or post-cleaning steps. This allows the use of more dilute slurries with a smaller energy penalty, thereby enabling the overall process of precleaning, hydrothermal treatment, dewatering, post cleaning and milling to be optimized to produce a higher quality fuel together with reduced energy consumption. By way of example, the inventor has calculated the benefit of using the configuration of the invention as follows. The energy for pumping slurry to a hydrothermal reactor depends mostlym ^~ the ^" reaetor ^~ pressure-ahd-the-coal-eontent-of-the-sluiTy— For^^^

conditions of 320 deg C and 20 MPa pressure, and with a coal loading of 20 wt%, the energy for pumping would be 3-4% of the energy in the feed coal (as an equivalence, and depending on the source of the electricity). Using the regenerative pumping system of the present invention this can be reduced to less than 1% loss thereby giving a significant improvement in overall energy efficiency, a reduction in the power cost for the plant, and a reduction in C0 ₂ emissions.

The savings may be even greater for processes requiring more dilute feed slurry and higher pressures.