WO2006043112A1

WO2006043112A1 - Process and plant for treating biomass

Info

Publication number: WO2006043112A1
Application number: PCT/GB2005/050177
Authority: WO
Inventors: Michael Joseph Bowe
Original assignee: Compactgtl Plc
Priority date: 2004-10-18
Filing date: 2005-10-10
Publication date: 2006-04-27
Also published as: GB0423037D0

Abstract

Solid biomass such as wood chips are supplied to a fluidised bed gasifier (10), while also feeding a gas stream comprising hot steam at above 800°C into the gasifier to fluidise the bed of solid material. The hot gas mixture produced from the gasifier (10) may be cooled (22) so as to generate high-pressure steam to drive a turbine (46). The gas mixture is preferably cooled to below 100°C, compressed (28) to at least 1.7 MPa, and then subjected to Fischer-Tropsch synthesis (30). This generates a liquid hydrocarbon product (40) and tail gases (42). The hot gas stream for the gasifier (10) may be provided by subjecting the tail gases (42) to combustion in a compact catalytic reactor heat exchanger (42). Hence the biomass is used to generate electricity and to provide a liquid hydrocarbon which may be used as a fuel.

Description

Process and Plant for Treating Biomass

The present invention relates to a process and a plant for treating biomass, in particular solid biomass materials, so as to enable a liquid hydrocarbon to be produced, and also to generate electricity.

Many different sources of biomass are suitable for use in the present invention, such as forestry waste, corn husks, wood chips, and products such as willow grown and coppiced for fuel. All such biomass contains complex organic compounds (such as lignin and cellulose in the case of such plant materials) , and can be provided in the form of solid particles, chips or pieces. Hitherto it has not been easy to obtain useful and valuable products economically from such sources of biomass.

According to the present invention there is provided a process for treating solid biomass, the process comprising supplying the biomass in the form of solid pieces into a fluidised bed gasifier, while also generating a gas stream comprising hot steam, heating this gas stream to a temperature above 800⁰C, and feeding this gas stream at above 800⁰C into the gasifier to fluidise the bed of solid pieces and to produce a gas mixture including hydrogen and carbon monoxide, subjecting this gas mixture to Fischer-Tropsch synthesis, separating the output from the Fischer-Tropsch synthesis into a water stream, a liquid hydrocarbon stream and a gaseous output stream, and using the water stream to provide steam for the gas stream to be fed to the gasifier.

The gasification reaction between the complex organic compounds in the biomass and the steam generates a mixture primarily comprising carbon monoxide and hydrogen. If the gas stream contains only steam, the reaction is endothermic, but the thermal energy required for the reaction is provided by the very high temperature of the steam. Preferably the gas stream also contains a small proportion of air, not more than 30% by mass and more preferably not more than 10% by mass, as this enables an exothermic oxidation reaction to occur, and so provides more energy for gasification. However, the addition of air should be minimised, as it may lead to the production of carbon dioxide (which is undesirable) , and dilution of the gas stream with nitrogen.

Preferably the resulting gas mixture from the gasifier is cooled by passage through a heat exchanger, the heat being used to generate steam for the gas stream. Preferably the steam is generated at high pressure and a temperature of above 25O⁰C, for example at 300⁰C, and this high-pressure steam can be passed through a turbine to generate electricity and to reduce its pressure to approximately atmospheric pressure for supplying to the fluidised bed gasifier.

The gas mixture is cooled to below 100⁰C, and then the remaining gases are compressed to above 17 atmospheres pressure, and subjected to Fischer-Tropsch synthesis. Preferably the Fischer-Tropsch synthesis is performed in two successive stages, each stage being performed at an elevated temperature above 215⁰C and with a gas hourly space velocity greater than 10 000 hr^"1, so as to provide a conversion of carbon monoxide that is no greater than 60%. The space velocity, in this specification, is defined as the volume flow rate of the gases supplied to the reactor (measured at STP) , divided by the void volume of the reactor.

Preferably the Fischer-Tropsch synthesis is performed at a pressure in the range 17-21 atmospheres (this being the absolute pressure) . With this comparatively low pressure, and the high temperature, the chain growth probability factor (Alpha) is only about 0.6 to 0.7. Consequently the hydrocarbons are primarily short chain; the selectivity to the production of C5+ hydrocarbons is less than 65% (the term C5+ referring to the hydrocarbons containing five or more carbon atoms) . Operation in this mode therefore generates a significant proportion of hydrocarbons that are gaseous under ambient conditions; but on the other hand it ensures that the proportion of waxy hydrocarbons (say above C17) is less than 1% of the product. Consequently the liquid hydrocarbons produced by condensing the products of the Fischer-Tropsch synthesis can be used directly as a vehicle fuel without requiring further chemical processing. The operating temperature is higher than would conventionally be appropriate, and the reaction kinetics increase rapidly with temperature, so it is necessary to operate at a high space velocity to limit carbon monoxide conversion and restrict the production of water and the associated risk of damaging the catalyst.

The conversion of carbon monoxide to hydrocarbons may be in the range 40% to 60%. This ensures that the proportion of water vapour does not reach the levels at which hydrothermal ageing of the catalyst is likely. Somewhat greater carbon monoxide conversion overall can be obtained by condensing and separating the liquids formed by the Fischer-Tropsch synthesis, and then subjecting the remaining gases to a further Fischer- Tropsch synthesis at the same operating conditions. Condensing and removing the water in this way hence has the benefit of avoiding hydrothermal ageing of the catalyst. Hence operation in two successive stages provides efficient conversion of carbon monoxide, while ensuring the catalyst for the Fischer-Tropsch synthesis does not need frequent replacement.

The Fischer-Tropsch synthesis generates both short chain hydrocarbons, which may be referred to as "lights", and longer-chain higher molecular weight C5+ hydrocarbons which are a useful product and may for example be blended with conventional diesel fuel. Preferably the gaseous output stream (which contains these lights) is used as the fuel to heat the gas stream for the gasifier.

Preferably this is done by catalytic combustion, as this avoids the presence of a flame front during combustion, thereby simplifying the heater, improving heat transfer and reducing the peak temperature.

It will typically be also necessary to clean the gas mixture emerging from the gasifier, before it can be subjected to Fischer-Tropsch synthesis. For example any suspended ash particles may be removed using a cyclone separator and/or a filter, and tar and sulphur-containing compounds may be removed by passing over a tar-cracking catalyst and a desulphurising adsorber material, respectively. Excess steam is preferably also removed at this stage, for example by cooling followed by a cyclone to remove water droplets. Partial water condensation can also be used to assist gas cleaning, as the droplets help remove ash particles.

The present invention also provides a plant for performing this process.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawing in which:

Figure 1 shows a flow diagram of a plant and process of the invention, showing the items of equipment forming the plant, and the connections between them.

Referring now to figure 1, a plant for treating wood waste chips or other solid particles of biomass includes a fluidised bed gasifier 10 into which the wood waste chips (or other solid biomass pieces) are fed through a loading chute 12. A gas stream consisting primarily of steam, and also with about 8% (by mass) air, heated to 900⁰C, is also fed into the gasifier 10 through a duct 14, and provides the fluidising medium. The gasifier 10 preferably also contains a bed of sand particles to provide thermal inertia and efficient mixing of the biomass particles, the particles of sand being sufficiently large not to be carried out of the gasifier 10. At the elevated temperature of the gas stream supplied through the duct 14 the organic compounds that make up the biomass react with the steam endothermically, generating carbon monoxide and hydrogen; the compounds also react with oxygen from the air exothermically, generating carbon monoxide, carbon dioxide and additional steam. Consequently a product gas stream emerges that consists primarily of carbon monoxide and hydrogen, with some carbon dioxide and excess steam, and with small particles of ash. There may also be longer-chain hydrocarbons that have been vaporised but have not fully reacted, which may be referred to as tar. Depending on the nature of the biomass there may also be some sulphur- containing compounds.

The hot product gases, initially at about 800⁰C, emerge from an outlet duct 16 and are then passed through a cyclone 18 to separate and remove ash. The gases are then passed over a tar cracking catalyst in a catalyst column 20. The product gases are then passed through a heat exchanger 22 such as a shell and tube heat exchanger, used to generate steam at elevated pressure and a temperature of about 300⁰C. The product gases are then passed through a desulphurising bed 24. They are then passed through a further heat exchanger 26 to cool them to about 5O⁰C, and then a further cyclone 27 to condense the excess steam. This may alternatively be carried out in two stages, the first stage condensing part of the excess steam, the resulting water droplets scrubbing any remaining ash particles and then being removed by a first cyclone; the second stage condensing the bulk of the excess steam, the resulting water droplets being removed by a second cyclone. In this alternative, the water from the second cyclone may be recycled as a source of water for generating the steam required for the gasifier 10.

The product gas stream emerging from the cyclone 27 consists predominantly of hydrogen and carbon monoxide, and may be referred to as synthesis gas. This synthesis gas is then compressed in an electrically driven multi^¬ stage compressor 28 to a pressure such as 18 atmospheres, and then supplied to a Fischer-Tropsch synthesis reactor 30.

In Fischer-Tropsch synthesis the gases react to generate a longer chain hydrocarbon, that is to say:

n CO + 2n H₂ → (CH₂) _n + n H₂O

which is an exothermic reaction, occurring at an elevated temperature, typically between 190° and 35O⁰C, and an elevated pressure typically between 2 MPa and 4 MPa, for example 2.5 MPa, in the presence of a catalyst such as iron, cobalt or fused magnetite, with a potassium promoter. The exact nature of the organic compounds formed by the reaction depends on the temperature, the pressure, and the catalyst, as well as the ratio of carbon monoxide to hydrogen.

A reactor suitable for the Fischer-Tropsch synthesis comprises a stack of plates defining coolant channels alternating with reaction channels, and with gas- permeable catalyst structures (such as corrugated foil, felt or mesh) in the reaction channels. The plates may be flat, with channels defined by grooves; alternatively some of the plates may be corrugated so as to define channels. The plates are bonded together typically by diffusion bonding or brazing, and are provided with suitable headers for the reactant gases and the coolant. Preferably the reactor 30 consists of a stack of flat plates each 1 mm thick, separated by plates corrugated in the form of rectangular castellations to define reaction channels alternating with coolant channels. The corrugated plates in the coolant channels are 0.3 mm thick and have 1.5 mm high castellations each of width 5 mm, while the corrugated plates in the reaction channels are 0.5 mm thick and have 6 mm high castellations each of width 25 mm. The coolant channels are transverse to the direction of the reaction channels, so the coolant is crossflow relative to the reactants. Preferably corrugated Fecralloy alloy foils 50 μm thick coated with a ceramic coating impregnated with a catalyst material are then be inserted into the reaction channels before the headers are attached, and can be replaced if the catalyst becomes spent.

A preferred catalyst comprises a coating of lanthanum-stabilised gamma-alumina of specific surface area 140 - 450 m²/g with about 10-40% (by weight compared to the weight of alumina) of cobalt, and with a ruthenium/platinum promoter, the promoter being between 0.01% to 10% of the weight of the cobalt. There may also be a basicity promoter such as gadolinium oxide. The activity and selectivity of the catalyst depends upon the degree of dispersion of cobalt metal upon the support, the optimum level of cobalt dispersion being typically in the range 0.1 to 0.2, so that between 10% and 20% of the cobalt metal atoms present are at a surface. The larger the degree of dispersion, clearly the smaller must be the cobalt metal crystallite size, and this is typically in the range 5-15 nm. Cobalt particles of such a size provide a high level of catalytic activity, but may be oxidised in the presence of water vapour, and this leads to a dramatic reduction in their catalytic activity. The extent of this oxidation depends upon the proportions of hydrogen and water vapour adjacent to the catalyst particles, and also their temperature, higher temperatures and higher proportions of water vapour both increasing the extent of oxidation.

The synthesis gas flows through the reactor 30 at an hourly space velocity in the range 10 000 to 20 000 hr^"1, and a temperature in the range 215-26O⁰C. In the coolant channels a single phase coolant fluid is employed to remove the exothermic heat to maintain the temperature in the Fischer-Tropsch reaction channels as close to isothermal as possible. The coolant fluid is recirculated, its temperature being maintained at the required value by heat exchange with ambient air through a forced draft air cooler 32.

The reaction products from the Fischer-Tropsch synthesis, predominantly water and paraffins C17 and below, are cooled to condense the liquids by passage through a heat exchanger 34 and a cyclone separator 36 followed by a separating chamber 38 in which the three phases water 39, hydrocarbons 40 and tail gases 42 separate, and the liquid hydrocarbon product 40 is stabilised at atmospheric pressure. The hydrocarbon products produced by Fischer-Tropsch synthesis depend upon the chain growth probability factor (Alpha) ; the larger this probability factor, the higher the proportion of the product at longer chain lengths. In the present example, the reaction temperature and pressure lead to a comparatively low value of Alpha of about 0.6-0.7, so that the highest mass fractions are those of hydrocarbons that are gases under ambient conditions - methane, ethane, propane, butane - and which therefore emerge in the tail gases. On the other hand, the mass fraction of long chain hydrocarbons above about C22 is negligible. Hence the product 40 does not contain significant amounts of waxy hydrocarbons.

It will be appreciated that the tail gases 42 contain not only the short-chain hydrocarbons, but also hydrogen, and (because of the comparatively low conversion of carbon monoxide) also contain a significant quantity of carbon monoxide. They can therefore be subjected to a second Fischer-Tropsch synthesis by passage through a second such reactor (not shown) , so that the overall carbon monoxide conversion is increased, and somewhat more of the desired product 40 is obtained.

The hydrocarbons that remain in the gas phase and unreacted hydrogen and carbon monoxide (the Fischer- Tropsch tail gases 42) are passed through a pressure reduction valve 44. They are then mixed with air and used as fuel for catalytic combustion (as described below) .

The water 39 is fed through the heat exchanger 22, to form high-pressure steam at about 300⁰C, as described above. This high-pressure steam is passed through a steam turbine 46 used to generate electricity; this reduces the pressure down to about atmospheric pressure, so the steam can be fed to the gasifier 10. This electricity may be used to drive the compressor 28. The low pressure steam, mixed with a small amount of air (as described above) , is passed through a compact catalytic combustion heat exchange unit 48 (represented diagrammatically) , the steam passing through the heat exchange passages and so being heated up to the high- temperature (in the range 800° - 95O⁰C) to be supplied to the inlet duct 14. This unit 48 may comprise a stack of plates defining alternate reaction channels and coolant channels, and with a catalyst structure in each reaction channel (similar to those described above as suitable for the Fischer-Tropsch reactor 30), but the catalyst structure incorporating a combustion catalyst, and the unit 48 being bonded together by diffusion bonding or brazing. The plates may be made of a material such as Haynes HR-120 or Inconel 800HT (trade marks), iron/nickel/chromium alloys for high-temperature use, or similar materials. (Such materials may also be used for the gasifier 10, as this is at a similarly high temperature) . The tail gases 42 mixed with air are passed through the catalytic combustion channels of the unit 48, where they undergo combustion to provide the necessary heat. Preferably the dimensions and relative directions of the flow paths of the coolant (i.e. the steam) and combustion gases are such as to minimise the temperature difference between them, to ensure the combustion catalyst is not heated to excessive temperatures.

It will be appreciated that it is important to maintain the steam flowing into the gasifier 10 at the desired temperature in the range 800° up to 95O⁰C, and so the flow of water into the heat exchanger 22 and so the flow of steam through the unit 48 must be adjusted in accordance with the calorific content of the tail gases 42. This calorific content generates the high- temperature steam, and at least some of the energy is recovered in the heat exchanger 22 in generating the high-pressure steam fed to the turbine 46. Thus, at least indirectly, the calorific content of the tail gases 42 is used to generate electricity. Typically it can be expected that of the total carbon present in the syngas (and derived from the biomass) , about half (say between 40% and 60%) will be converted to hydrocarbon product 40, and the other half will be subjected to combustion as a component of the tail gases 42, and so used in generating electricity.

Claims

1. A process for treating solid biomass, the process comprising supplying the biomass in the form of solid pieces into a fluidised bed gasifier, while also generating a gas stream comprising hot steam, heating this gas stream to a temperature above 800⁰C, and feeding this gas stream at above 800⁰C into the gasifier to fluidise the bed of solid pieces and to produce a gas mixture including hydrogen and carbon monoxide, subjecting this gas mixture to Fischer-Tropsch synthesis, separating the output from the Fischer-Tropsch synthesis into a water stream, a liquid hydrocarbon stream and a gaseous output stream, and using the water stream to provide steam for the gas stream to be fed to the gasifier.

2. A process as claimed in claim 1 wherein the gas stream also contains not more than 30% (by mass) of air.

3. A process as claimed in claim 1 or claim 2 wherein the gas mixture resulting from the gasifier is cooled by passage through a heat exchanger, the heat being used to generate steam for the gas stream.

4. A process as claimed in claim 3 wherein the steam is generated at high pressure and a temperature of above 25O⁰C, and is then passed through a turbine to generate electricity and to reduce its pressure.

5. A process as claimed in any one of the preceding claims wherein the gas mixture resulting from the gasifier is cooled, and the remaining gases compressed to a pressure of at least 1.7 MPa, before being subjected to Fischer-Tropsch synthesis.

6. A process as claimed in any one of the preceding claims wherein the Fischer-Tropsch synthesis is performed in at least two successive stages, each stage being performed at an elevated temperature above 215⁰C and with a gas hourly space velocity greater than 10 000 hr^"1, so as to provide a conversion of carbon monoxide that is no greater than 60%.

7. A process as claimed in claim 5 or claim 6 wherein the Fischer-Tropsch synthesis is performed in such a way that the selectivity to the production of C5+ hydrocarbons is less than 65%.

8. A process as claimed in any one of the preceding claims wherein the gaseous output stream from the

Fischer-Tropsch synthesis is used as a fuel to heat the gas stream supplied to the gasifier.

9. A process as claimed in claim 8 wherein the gaseous output stream is combined with air and subjected to catalytic combustion in a compact catalytic reactor heat exchanger.

10. A plant for treating solid biomass, the plant comprising a fluidised bed gasifier, and means to supply the biomass in the form of solid pieces into the fluidised bed gasifier, means to generate a gas stream comprising steam and superheated to a temperature above 800⁰C, and means to feed this gas stream at above 800⁰C into the gasifier to fluidise the bed of solid pieces and to produce a gas mixture including hydrogen and carbon monoxide, a reactor for subjecting this gas mixture to Fischer-Tropsch synthesis, separation means for separating the output from the Fischer-Tropsch synthesis into a water stream, a liquid hydrocarbon stream and a gaseous output stream, and means to vaporise the water stream to provide steam for the gas stream to be fed to the gasifier.

11. A plant as claimed in claim 10 wherein the vaporisation means is a heat exchanger such that the water exchanges heat with the gas mixture emerging from the gasifier.

12. A plant as claimed in claim 10 or claim 11 wherein the means to superheat the gas stream is a heat exchanger such that the gas stream exchanges heat with a combustible gas mixture undergoing combustion, wherein the combustible gas mixture comprises the gaseous output stream.