WO2012010846A1

WO2012010846A1 - Gasification of carbonaceous feedstock

Info

Publication number: WO2012010846A1
Application number: PCT/GB2011/001099
Authority: WO
Inventors: Martin Groszek; Feng Jin Goh; Pasqualino Lannetelli
Original assignee: Mortimer Technology Holdings Limited
Priority date: 2010-07-23
Filing date: 2011-07-22
Publication date: 2012-01-26
Also published as: GB201012429D0

Abstract

The present invention provides a process for the gasification of a carbonaceous feedstock to produce a syngas, the process comprising: (a) providing a toroidal bed reactor comprising a reaction chamber and means to generate a substantially circumferentially directed flow of fluid therein; (b) preheating the reaction chamber by the introduction of an at least partially combusted gas; (c) introducing a pre-heated oxygen-depleted gas into the reaction chamber to provide a substantially circumferentially directed flow of fluid; (d) introducing a carbonaceous feedstock into the preheated reaction chamber, wherein the carbonaceous feedstock circulates rapidly about an axis of the reaction chamber in a toroidal band and is heated by the flow of fluid; and (e) gasifying the carbonaceous feedstock in the toroidal bed reactor to produce a syngas, wherein the oxygen-depleted gas is preheated without being combusted.

Description

Gasification of carbonaceous feedstock

The invention relates to a process for the gasification of a carbonaceous feedstock to produce a syngas. In particular, the invention relates to a process for the gasification of a carbonaceous feedstock in a toroidal bed reactor.

Gasification is a process that converts carbonaceous materials, such as biomass, petroleum and coal, into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen and/or steam (see EP 230 324 B1). The resulting gas mixture is called synthesis gas or syngas.

Fluidised beds are well known for use in gasification processes since the turbulent mixing of the carbonaceous material leads to fast and effective heat transfer and low processing times. Toroidal bed reactors, which are a class of fluidised bed reactor, are well known in the art for the thermal treatment of various materials. For example, the treatment of a carbonaceous material is described in GB2416583. The high turbulence in the toroidal bed reactor allows for express and more precise treatment of the feedstock through increased heat transfer. This helps minimise undesirable oxygen contact which might lead to scorching and allows for a higher temperatures and shorter residence times.

While it is known to gasify carbonaceous materials in conventional fluidised bed reactors, this has been found to be associated with a number of disadvantages. For example, such systems typically have a small turndown ratio, i.e. the ratio of maximum fuel input rate to minimum fuel input rate or the ratio of the maximum and minimum processing rates. As a consequence, conventional fluidised bed reactors do not cope well with deviations from the preferred rates of fuel or oxygen supply. Accordingly, the present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. In a first aspect, the present invention provides a process for the gasification of a carbonaceous feedstock to produce a syngas, the process comprising:

(a) providing a toroidal bed reactor comprising a reaction chamber and means to generate a substantially circumferentially directed flow of fluid therein;

(b) preheating the reaction chamber by the introduction of an at least partially combusted gas;

(c) introducing a pre-heated oxygen-depleted gas into the reaction chamber to provide a substantially circumferentially directed flow of fluid;

(d) introducing a carbonaceous feedstock into the preheated reaction chamber, wherein the carbonaceous feedstock circulates rapidly about an axis of the reaction chamber in a toroidal band and is heated by the flow of fluid; and

(e) gasifying the carbonaceous feedstock in the toroidal bed reactor to produce a syngas,

wherein the oxygen-depleted gas is preheated without being combusted. Conventional fluidised bed reactors are typically heated using a combustion heater, which results in a fast heating rate. However, the present inventors have found that when the process of the present invention involves recycling the syngas produced, the presence of a flame in the closed circuit containing flammable gases is a serious safety risk. However, it has been found that the use of a combustion heater allows for the most efficient heating of the system.

The present inventors have arrived at a solution that allows for the economic use of a toroidal bed reactor while avoiding this risk. The inventors have found that by using the combustion heater before use it is possible to bring the toroidal bed reactor up to temperature surprisingly quickly. The system can then be kept at temperature using a heat exchanger. Once the process is running the heat of the gasification process in combination with a heat exchanger can be used to maintain the process temperature without requiring the use of the combustion heater. Accordingly, the present invention provides a highly efficient process for bringing the reactor quickly and safely to an operating temperature. This allows for energy efficient on-demand carbonaceous feedstock processing. Accordingly, the system can preferably be used to provide the fuel (the syngas; H2 and CO) for a gas turbine power generator. This is particularly useful, for example, as a system to supplement wind-powered energy generation since it allows near instant generation from standby to full output within minutes when the wind dies away.

In addition, the present inventors have surprisingly discovered that the turndown ratio that can be achieved with a toroidal bed reactor is greater than that of conventional fluidised bed reactors. In particular, the toroidal bed reactor can work at very low levels of carbonaceous feedstock. Without wishing to be bound by theory, it is believed that this is because of the intimate mixing that is achieved between the feedstock and the gas. Furthermore, since the toroidal bed reactor does not require a fluidised bed material, even small amounts can be processed without inefficient reaction speeds or requiring difficult removal of residues from the reactor.

Typical fluidised bed gasifiers make use of a bed material in order to promote heat transfer between solids and the heated gases in the gasifier. As a consequence, additional equipment is required in order to separate the bed material from the remaining ash. Further problems occur in conventional systems when biomass is used as the fuel because biomass has a low ash sintering temperature; this leads to blockages unless fast and precise temperature control is used to reduce the amount of ash sintering. Although the use of fluidised beds ensures reasonable temperature control, temperature differences still occur. Accordingly, regular maintenance of such systems is required. Since the toroidal bed reactor does not use a bed material, these problems have been obviated.

Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The term "carbonaceous feedstock" as used herein refers to a feedstock which comprises carbon. Examples of carbonaceous feedstocks are coal, petroleum, biomass and biofuel.

The term "biomass feedstock" as used herein refers to a biological feedstock derived from living or recently living organisms, such as plant matter, waste, landfill gases and alcohol fuels. Biomass is carbon based and is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth and heavy metals. Biomass does not include organic materials such as fossil fuels which have been transformed by geological processes such as coal or petroleum..

The term "toroidal bed reactor" as used herein refers to a reactor in which a material to be treated is embedded and centrifugally retained within a compact, but turbulent, toroidally circulating bed of particles which circulate about an axis of the processing chamber.

The term "oxygen-depleted gas" as used herein refers to a gas comprising a lower percentage of oxygen than in atmospheric air. Preferably the oxygen depleted gas has less than 20% oxygen, preferably between 1 and 15% and most preferably between 5 and 10% by volume. The oxygen-depleted gas may contain steam in addition to the depleted levels of oxygen. In one embodiment, the oxygen level in the depleted gas is from 17 to 13% and preferably about 15%. Provided the residence times are controlled, this allows for optimal treatment and minimal difficulty in preparing and providing the oxygen depleted gas. The gasification process according to the present invention is preferably conducted at a temperature of from 500°C to 1200°C, more preferably from 600°C to 950°C and most preferably from 650°C to 900°C. These are the temperatures to which the combusted gas and oxygen depleted non-combusted gas are preferably heated/preheated.

Preferably the carbonaceous feedstock is biomass feedstock. Suitable biomass feedstocks include wood, plant matter and waste (including sewage sludge and agricultural residues). Wood includes forest residues such as dead trees, branches and tree stumps, yard clippings, wood chips and process residues. Plant matter includes biomass grown from, for example, miscanthus, switchgrass, hemp, corn, poplar, willow, sorghum or sugarcane, and includes straw and husks. Preferably the biomass treated has a solid form and a useful calorific value. If the calorific value is too high or too low then the biomass may be initially

homogenized to provide a feedstock of substantially uniform calorific value.

Preferably at least a portion of the syngas produced is recycled into the oxygen- depleted gas. Preferably from 10 to 00% of the syngas is recycled, more preferably from 70 to 95% and most preferably from 75 to 85%. The syngas may be recycled several times. The toroidal bed reactor is designed to operate using a near constant airflow which provides oxygen to carry out the gasification process and forms part of the fluid to mobilise the biomass feedstock. When the bed is run in a batchwise manner it would be expected that the use of a constant airflow would lead to changes in the oxygen to fuel ratio as the feedstock is gasified. If the flow of air were reduced to maintain the oxygen to fuel ratio the bed would eventually lose fluidisation. Both of these changes would have a negative effect on the gasification process. However, the inventors have now realised that by recycling the syngas into the reaction chamber the oxygen to fuel ratio is kept constant. This ensures thorough processing of the feedstock with requiring complicated monitoring and control of the gas into the reaction chamber. Preferably the oxygen depletion of the oxygen-depleted gas is controlled by adjusting the percentage of syngas recycled into the pre-heated oxygen-depleted gas introduced into the reaction chamber. Preferably the oxygen levels present in the gasifier are less than 50%, more preferably less than 40% and most preferably less than 30% (by volume) of the amount required for complete combustion of the carbonaceous material. In one embodiment the oxygen content is at least partially provided in the form of steam which can decompose to provide a source of oxygen and hydrogen under the gasification conditions.

Preferably the oxygen depleted gas is atmospheric air blended with syngas due to the simplicity of the system that this allows.

Preferably the gasification of the biomass feedstock is controlled by regulating the rate at which the oxygen-depleted gas is introduced into the reaction chamber. The presence of greater oxygen levels allows for faster rates of gasification and hence greater exothermic energy production.

Preferably the oxygen-depleted gas is pre-heated via a heat exchanger by the syngas produced. This allows for energy saving by recovering otherwise lost heat from the syngas produced. Advantageously, the heat exchanger reduces the temperature of the syngas so that it can be used as a fuel. Furthermore, the heat exchanger can be used to cool the gases passed to the reactor to reduce the gasification and shut-down the reactor, or to control the gasification reaction if it is proceeding too rapidly.

It should be appreciated that the steps may be performed in any appropriate order and may be performed sequentially or in parallel, including partially overlapping. Accordingly, the system may be run continuously or batchwise. In particular, steps (a) and (b) are preferably performed before the remaining steps. Steps (c)-(e) are preferably performed simultaneously.

According to a second aspect, the present invention provides an apparatus for the gasification of a carbonaceous feedstock according to the process described herein. The apparatus comprises:

(i) a toroidal bed reactor comprising a reaction chamber and means to generate a substantially circumferentially directed flow of fluid therein;

(ii) a combustion heater configured to provide a source of at least partially combusted gas;

(iii) a heat exchanger configured to provide a source of pre-heated oxygen depleted gas; and

(iv) means to direct gas from the combustion heater and the heat exchanger to the reaction chamber,

wherein, in use, the apparatus is switchable between:

(1) a first state in which the combustion heater provides combusted gas to heat the reaction chamber; and

(2) a second state in which the heat exchanger provides a source of preheated oxygen depleted gas.

Preferably the combustion heater is configured so that it can be isolated from the reaction chamber and/or switched off. This prevents the risk of having a live flame in the system contacting the syngas produced. Preferably the combustion heater is configured so that when isolated from the reaction chamber the combustion heater can provide heat to the heat exchanger. This allows for efficient double use of the combustion heater and saves in equipment cost and system complexity. Preferably the heat exchanger has surfaces which come into contact with the syngas and these are temperature-controlled to minimise condensation of tar within the heat exchanger. The inventors have found that gases produced from the gasification of biomass are often heavily laden with tars. These tars often lead to fouling of equipment where cold spots occur, which eventually leads to plant shutdown. The inventors have overcome this problem in the heat exchanger, and also throughout the system, by ensuring that the surfaces that contact the gas are kept at an elevated temperature.

Preferably the heat exchanger is arranged to be, in use, in thermal

communication with the oxygen-depleted gas to be introduced into the reaction chamber and at least one of the combustion heater, the syngas produced and a supply of air. The air supply can be used for easy cooling if the gases used become too hot.

The process of the present invention is carried out in a toroidal bed reactor. A toroidal bed (TORBED (RTM)) reactor and process is described in, for example, EP 0068853, US 4479920, and EP 1791632, the disclosures of which are incorporated here by reference. In the process, a material to be treated is preferably embedded and centrifugally retained within a compact, but turbulent, toroidally circulating bed of particles, which circulate about an axis of the processing chamber. Specifically, the material forms particles within the bed which may be circulated above a plurality of fluid inlets arranged around the base of the processing chamber. The fluid inlets are preferably arranged in

overlapping relationship and the particles are caused to circulate around the bed by the action of a processing fluid, for example a gas injected into the processing chamber from beneath and through the fluid inlets. The fluid inlets may, for example, be a plurality of outwardly radiating, inclined vanes arranged around the base of the processing chamber.

By way of example, Figure 3 shows a schematic diagram of a toroidal bed reactor V. The gaseous fluid (A) mixed with the feedstock enters through angled vents 9 in the base of the reaction chamber 3. The path of the turbulent flow in the reaction chamber 3 is shown by the spiralling arrows marked (E). The dotted arrows show the circulation pathway (in 2 dimensions only) taken by the feedstock that is to be processed. The toroidal bed reactor provides a rapidly mixing bed which can be used to circulate particulates toroidally through a zone in a process chamber where an interaction occurs with a gas stream.

Preferably a toroidal bed reactor for use in the present invention has a reaction chamber with a substantially circumferentially directed flow of fluid generated therein to cause the biomass feedstock to circulate rapidly about an axis of the reaction chamber in a toroidal band, and to heat the biomass feedstock, wherein the fluid comprises gas or gases introduced into the reaction chamber. Preferably the flow of fluid within the reaction chamber has a horizontal and a vertical velocity component. Preferably the chamber comprises a plurality of outwardly radiating inclined fluid inlets at or adjacent a base thereof, and wherein fluid is directed through the fluid inlets at the base of the chamber to generate the circumferentially directed flow of fluid within the chamber. Preferably the fluid directed through said fluid inlets is given both horizontal and vertical velocity components.

The carbonaceous feedstock may be introduced into the reactor(s) by injecting it through an inlet under the influence of a compressed gas such as compressed air and/or an inert gas such as nitrogen, CFC and other noble/mono-atomic gases. In a preferred embodiment of the present invention, the inlet is located above the fluid inlets at the base of the chamber and the carbonaceous feedstock is introduced into the chamber by a gravity feed mechanism, for example using an air lock device such as a rotary valve. The gravity feed mechanism may be provided in a vertical wall of the chamber. It will be appreciated that the flow of fluid may be generated either before or after the carbonaceous feedstock is introduced into the chamber. Alternatively, the flow of fluid may be generated at the same time as the carbonaceous feedstock is introduced into the chamber.

The flow of the fluid through the chamber may be generated in a manner as described in EP-B-0 382 769 and EP-B-0 068 853, i.e. by supplying a flow of fluid into and through the processing chamber and directing the flow by means of the plurality of outwardly radiating and preferably overlapping fluid inlets arranged in the form of a disc and located at or adjacent to the base of the processing chamber. The fluid inlets are inclined relative to the base of the chamber so as to impart rotational motion to the heating fluid entering the chamber, hence causing the heating fluid to circulate about a substantially vertical axis of the chamber as it rises. The fluid inlets may comprise, for example, a plurality of outwardly radiating vanes at or adjacent the base of the chamber. The vanes are typically inclined relative the base and preferably disposed in overlapping arrangement.

The present invention is described by way of example in relation to the following figures.

Figure 1 shows a schematic of the apparatus that may be used in the method of the present invention.

Figure 2 shows an example of a valve and combustion heater/heat exchanger configuration suitable for use in the present invention. The configuration comprises a process air inlet, a monoblock burner with separate air and gas connections, a process gas outlet which is "pre-heat exchanger", a process gas inlet, a process air exhaust, a reinforcing band, four sets of support legs, a process gas exhaust "after heat exchanger", a process air exhaust for bypass and a flange connection to access the compensator. Such a configuration allows - li the combustion burner to be used directly and indirectly in accordance with the present invention.

In figure 2, the notation refers to:

61 - Interface 6, process air inlet;

62 - Monoblock burner with separate air and gas connection;

63 - Interface 1 , process gas inlet;

64 - Interface 2, process gas outlet pre H/E;

65 - Interface 5, process air exhaust 2 for bypass;

66 - Interface 3, process air exhaust;

67 - Reinforcing band;

68 - Four sets of support legs;

69 - Flange connection to access compensator;

70 - Interface 4, process gas exhaust after H/E.

Figure 3 shows a toroidal bed reactor of the type disclosed in EP1791632.

Referring to Figure , the gasification apparatus 1 that may be used in the present invention produces a gasified material 2 from a carbonaceous feed 0 or feedstock. The apparatus 1 comprises reaction chamber 5 (in this case a

TORBED (RTM) toroidal reactor), cyclone 15, recirculation fan 40, combustion heater 30, air fan 45 and air pre-heater/cooler 35 comprising a heat exchanger (not shown). The combustion heater 30 is connected to a supply of natural gas 20 and atmospheric air 25. The combustion heater 30 is in fluid communication with the air pre-heater/cooler 35. Accordingly, in use, natural gas may be ignited within the combustion heater 30 so as to supply a source of at least partially combusted gas to the air pre-heater/cooler 35. The reaction chamber 5 and air pre-heater/cooler 35 are arranged in a circuit such that, in use, gas exhausted from the reaction chamber 5 can be recycled back into the reaction chamber 5 via the air pre-heater/cooler 35. Alternatively, or in addition, gas exhausted from the reaction chamber 5 can be released to the atmosphere as exhaust gas 50 via the air pre-heater/cooler 35 and exhaust pressure control valve D.

The cyclone 15 is situated within the circuit between the reaction chamber 5 and air pre-heater/cooler 35 such that, in use, solid gasified material 2 contained in gas exhausted from reaction chamber 5 can be trapped in cyclone 15 before reaching the air pre-heater/cooler 35. The air pre-heater/cooler 35 is also capable, in use, of directing preheated oxygen-depleted gas to the reaction chamber 5 and/or the atmosphere, the latter via air exhaust control valve B. Air fan 45 is capable of controlling, in use, the flow of atmospheric air 25 to the gasification chamber 5 and/or combustion heater 30. The pressure and/or direction of gas flow, in use, within the apparatus 1 is controllable using a number of valves, namely primary air isolation valve A, air exhaust control valve B, air bypass control valve C, exhaust pressure control valve D, recirculation valve E and secondary air isolation valve F.

Although the reaction chamber 5 described above is part of a TORBED (RT ) reactor, it is envisaged that any other suitable reaction chamber may be used. An example of the start-up and operational procedure of an apparatus according to the invention is described by reference to Figure 1. The individual steps in the procedure are described as follows:

1. Valves A-C, E and F are closed. The recirculation fan 40 is started and the system exhaust pressure control valve D is released to control the gasification circuit pressure. Primary air isolation valve A and secondary air isolation valve F are opened; recirculation valve E is closed and the air fan 45 is started.

Natural gas 20 is ignited within the reaction chamber 30 and the system starts to heat up. The air pre-heater/cooler 35 is now operating in its direct heating mode.

Control of air recycle flow valve E can be released to the Programmable Logic Controller (PLC) to heat up the air inlet air temperature. This is to avoid condensation of tars and acids in the heat exchanger. Preferably this step is deferred until after step 11. This prevents the presence of too much recycled air in the air stream which can have a reduce the efficiency and functionality of the burner.

The combustion heater exhaust temperature is now ramped up in a controlled manner to prevent heat shocks to the equipment.

When the inlet temperature of the recycle air to the air pre-heater/cooler 35 reaches a set point temperature, the air exhaust flow control valve B is opened and the primary air isolation valve A is closed.

The air pre-heater/cooler 35 continues to heat up the system in indirect heating mode. As more heat is transferred to the system, the combustion heater 30 turns down until it is supplying only sufficient heat for the system to maintain the carbonaceous feedstock gasification temperature.

The air exhaust flow control valve B is now controlled by the PLC to produce a pre-combustion heater air pressure that is higher than the pressure within the reaction chamber 5 to ensure that air will be delivered into the reaction chamber 5. The presence and use of this valve is optional. Carbonaceous feed 10 is now added to the gasification chamber 5 and at the same time, the air bypass flow control valve C is opened. The carbonaceous feedstock feeder (not shown) is pre-loaded to ensure the shortest lag time possible. The carbonaceous feed 10 starts to gasify in the reaction chamber 5 and the inlet temperature to the air pre-heater/cooler 35 increases beyond the set-point temperature of the combustion heater 30. The air bypass flow control valve C is modulated to keep the temperature of the reaction chamber 5 at the correct carbonaceous feedstock gasification temperature. When this has been achieved, the natural gas supply 20 of the combustion heater 30 is turned off and control of the air fan 45 is given to the PLC. The PLC controls the temperature of the reaction chamber 5 by controlling the amount of air delivered by air fan 45. Now, the bypass flow control valve C is closed and primary air isolation valve E is opened. The air exhaust flow control valve B is now released for the control of the reaction chamber recycle temperature. If the recycle temperature is too high, the air exhaust flow control valve D is opened, less heated air will reach the reaction chamber 5 causing the temperature of the reaction chamber 5 to drop. The drop in temperature causes the air fan 45 to ramp up, resulting in an increased cooling of the recycle flow. The air pre-heater/cooler 35 is now operating in cooling mode. 16. The apparatus 1 is now operating in gasification mode, the gases that are being re-circulated will normally be gasification gases. In one alternative embodiment, all of the syngas can be passed through the heat exchanger. In such an embodiment, the syngas exhaust (which can pass through valve D) would preferably be after the recycle fan (40).

In general, the gas leaving valves B and D need not be combined. These gases may together be passed into a high temperature afterburner.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.

Claims

Claims:

1. A process for the gasification of a carbonaceous feedstock to produce a syngas, the process comprising:

wherein the oxygen-depleted gas is preheated without being combusted.

2. The process according to claim 1 , wherein at least a portion of the syngas produced is recycled into the oxygen-depleted gas.

3. The process according to claim 2, wherein the oxygen depletion of the oxygen-depleted gas is controlled by adjusting the percentage of syngas recycled into the pre-heated oxygen-depleted gas introduced into the reaction chamber.

4. The process according to any preceding claim, wherein the gasification of the carbonaceous feedstock is controlled by regulating the rate at which the oxygen-depleted gas is introduced into the reaction chamber.

5. The process according to any preceding claim, wherein the oxygen- depleted gas is pre-heated via a heat exchanger by the syngas produced.

6. The process according to any preceding claim, wherein the carbonaceous feedstock is a biomass feedstock.

7. An apparatus for the gasification of a carbonaceous feedstock according to the process of any of claims 1 to 6, the apparatus comprising:

(1) a toroidal bed reactor comprising a reaction chamber and means to generate a substantially circumferentially directed flow of fluid therein;

wherein, in use, the apparatus is switchable between:

8. The apparatus of claim 7, wherein the combustion heater is configured so that it can be isolated from the reaction chamber and/or switched off.

9. The apparatus of claim 8, wherein the combustion heater is configured so that when isolated from the reaction chamber the combustion heater can provide heat to the heat exchanger.

10. The apparatus according to any of claims 7 to 9, wherein the heat exchanger has surfaces which come into contact with the syngas and these are temperature-controlled to minimise condensation of tar within the heat exchanger.

11. The apparatus according to any of claims 7 to 10, wherein the heat exchanger is arranged to be, in use, in thermal communication with the oxygen- depleted gas to be introduced into the reaction chamber and at least one of the combustion heater, the syngas produced and a supply of air.