The present invention relates to a process for treating waste products generated during recycling of waste paper. In particular, the present invention relates to the thermochemical conversion of waste products generated during recycling of waste paper to generate fuel gas having a high hydrogen content.

Background

Paper reject and deinking sludge are the two major waste materials produced during the paper recycling process. Waste paper for recycling often contains fabric fibre and contaminants such as metals, wood, adhesives, ink and plastics. At the first stage of the recycling process, any metallic material is removed by a magnetic separator and the rest is pulped and washed to remove the ink. The resulting scum formed at this stage is called deinking sludge, and mainly consists of fillers, coating pigments, fibres, printing ink and adhesives. A sequence of screening operations also removes oversize material from the paper pulp; this material is referred to as paper reject and typically contains plastics, adhesive residues, large fibres and other coarse material.

In the United Kingdom, around 500,000 tonnes of paper reject and 150,000 tonnes of deinking sludge are produced as a result of paper recycling every year.

The current common method for disposal of the deinking sludge is combustion and incineration, while the paper reject goes directly to landfill. The deinking sludge is considered to be suitable for incineration and combustion due to its high heating value and the low levels of hazardous flue gas emissions.

A number of paper mills currently use the combustion of deinking sludge as an energy source for combined heat and power (CHP) systems. However, the process produces large amount of ash which is currently disposed of by landfilling/spreading. A small proportion of the ash is also sold to the cement industry for use as a cement additive.

With the steady rise in paper recycling and, consequently, in the amount of associated waste, there is a growing concern regarding the environmental impact of continued landfilling/spreading of paper reject and ash. The transport of the paper reject and ash to the landfill site is also a concern in terms of expense and exhaust emissions.

Accordingly, there is a desire to seek a more environmentally friendly solution for the disposal of deinking sludge and paper reject. The present invention aims to provide a new process for treating waste products generated during waste paper recycling which has a reduced environmental impact. The present invention also aims to convert the waste into a more valuable product, namely hydrogen-rich fuel gas.

Summary of the present invention

Accordingly, in a first aspect the present invention provides a method of generating fuel gas, the method comprising:

combusting deinking sludge in a combustion zone to generate a solid combustion product;

transferring at least part of the solid combustion product from the combustion zone to a gasification zone; and

gasifying carbonaceous material in the gasification zone to generate the fuel gas.

The solid combustion product generated during combustion of deinking sludge contains calcium oxide or lime (CaO) and/or aluminium oxide or alum ina (AI2O3). It is known that these oxides are useful gasification catalysts as they enhance catalytic cracking of heavy hydrocarbons and tar. Calcium oxide has been found to be particularly effective for removing carbon dioxide from hot gases. Accordingly, transferring the solid combustion product to the gasification zone enhances gasification of carbonaceous material because the solid combustion product containing lime and/or alumina catalyses the gasification of the carbonaceous material in the gasification zone to generate the fuel gas. Furthermore, the heat contained within the solid combustion product can be used to drive the endothermic gasification of the carbonaceous material within the gasification zone.

Gasification is a process that converts carbonaceous material such as biomass into fuel gas (also known as syngas) which comprises carbon monoxide (CO), hydrogen (H2), methane (CH ₄ ) and carbon dioxide (C0 ₂ ). Fuel gas produced from biomass is an important source of renewable energy since it can be burned directly in gas engines (e.g. for electricity generation), used to produce methanol and hydrogen or converted via the Fischer-Tropsch process into synthetic fuel. In the special case of pyrolytic gasification (in an oxygen starved environment) such as in steam gasification, the fuel gas produced is rich in H ₂ content, which is desirable as a source of cleaner energy.

The main reactions taking place during pyrolytic gasification are as follows:

Pyrolysis

(1 ) Carbon + heat (from the solid combustion product) → ash + char + gases (CO, C0 ₂ , H ₂ 0, CH ₄ , H ₂ ) + tar Gasification

(1 ) C + 1 /20 ₂ → CO

(2) C + 0 ₂ → C0 ₂

(3) C + H ₂ 0→ CO + H ₂

(4) C + C0 ₂ → 2CO (5) C + 2H2→CH ₄

(6) CO + H ₂ 0→C0 ₂ + H ₂

(7) C0 ₂ + CH ₄ → 2CO + 2H ₂

(8) CH ₄ + 2H ₂ 0→ C0 ₂ + 4H ₂

The C0 ₂ in the fuel gas will be eliminated by chemisorption at the surface of the lime in the solid combustion product:

CaO(s) (from solid combustion product) + CO ₂ (g) <→ CaCOs(s) (in a solid gasification product)

Preferably, the method comprises combusting the deinking sludge at a temperature equal to or greater than 800°C (e.g. 800-1000°C), more preferably at a temperature equal to or greater than 800°C (e.g. 800-1000°C), yet more preferably at a temperature equal to or greater than 850°C (e.g. 850-1000°C) and, most preferably, at a temperature equal to or greater than 900°C (e.g. 900-1000°C). At these elevated temperatures, calcium carbonate contained within the deinking sludge is calcined during combustion such that the solid combustion product comprises an increased amount of lime. Accordingly, the catalytic enhancement of the carbonaceous material gasification by the solid combustion product is increased as is the chemisorption of carbon dioxide.

Preferably, the method comprises pyrolytic gasification of the carbonaceous material at a temperature between 700-800°C, more preferably at a temperature between 720-800°C and, most preferably, at a temperature around 750°C. At these temperatures, the chemisorption of carbon dioxide onto the surface of the lime is optimised. The temperature used for gasification is lower than the temperature used for combustion.

Preferably, the carbonaceous material comprises paper reject generated during recycling of waste paper. In this way, the two main waste products arising from paper recycling can be disposed of in a single process without resorting to any significant landfill and without the associated environmental and transportation disadvantages. The paper reject is optionally mixed with other carbonaceous material, such as biomass (e.g. wood chips/pellets). In preferred embodiments, the step of combusting deinking sludge comprises combusting the deinking sludge in a first fluidized bed reactor. In these embodiments, the method further comprises flowing a combustion fluidizing gas into the combustion zone. Preferably, the combustion fluidizing gas is air or oxygen.

In preferred embodiments, the step of gasifying carbonaceous material comprises gasifying the carbonaceous material in a second fluidized bed reactor. In these embodiments, the method further comprises flowing a gasification fluidizing gas into the gasification zone.

The step of combusting deinking sludge in the combustion zone preferably further generates a gaseous combustion product. This gaseous combustion product typically comprises carbon dioxide and nitrogen. In some embodiments, at least part of the gaseous combustion product is transferred from the combustion zone to the gasification zone. For example, the carbon dioxide fraction of the gaseous combustion product may be separated from the gaseous combustion product and transferred from the combustion zone to the gasification zone. In some embodiments, the gasification fluidizing gas comprises at least part of the gaseous combustion product (e.g. the carbon dioxide fraction of the gaseous combustion product) and the method comprises flowing at least part of the gaseous combustion product (e.g. the carbon dioxide fraction of the gaseous combustion product) into the gasification zone. The gasification fluidizing gas may comprise steam or steam and air. The gasification fluidizing gas may comprise a combination of the gaseous combination product (e.g. the carbon dioxide fraction of the gaseous combustion product) and steam. It has been shown that char burn-out is optimised in steam gasification if carbon dioxide (from the gaseous combustion product) is introduced into the gasifier. It has also been shown that the hydrogen content of the fuel gas is increased through the following reactions:

C + CC-2→ 2CO

CO + H ₂ 0→ H ₂ + C0 ₂

Preferably, the method further comprises generating steam for use as the gasification fluidizing gas by allowing heat exchange between water (e.g. obtained from drying the carbonaceous material) and the solid and/or gaseous combustion product. Most preferably, the method comprises allowing heat exchange between water and the gaseous combustion product.

By using heat contained within the solid and/or gaseous combustion product to heat water to form steam for use as the gasification fluidizing gas, input of external energy to form the steam is avoided.

In some embodiments, the carbon dioxide fraction of the gaseous combustion product is transferred to storage. The second fluidized bed reactor can be operated in bubbling (low flow rate of gasification fluidizing gas) or circulation (high flow rate of gasification fluidizing gas) mode.

Preferably, the first fluidized bed reactor is operated in circulating mode (with a high flow rate of combustion fluidizing gas) so that the solid combustion product is entrained within the gaseous combustion product as it exits the combustion zone. In these embodiments, the method further comprises separating the solid combustion product from the gaseous combustion product using one or more separators e.g. one or more cyclone separators or electrostatic precipitators. From the separator(s)/precipitator(s), the solid combustion product is transferred to the gasification zone whilst at least part the gaseous combustion product (e.g. the carbon dioxide fraction) is flowed through the gasification zone as the gasification fluidizing gas or is transferred to storage. In some embodiments, at least part of the gaseous combustion product (e.g. the nitrogen fraction) is scrubbed and then vented.

Gasification of the carbonaceous material will result in a solid gasification product which will typically comprise calcium carbonate (CaCOs), alumina and char. The calcium carbonate component is generated as a result of chemisorption of carbon dioxide on the surface of the lime during gasification. Preferred embodiments of the method comprise transferring the solid gasification product to the combustion zone. Heating of the solid gasification product in the combustion zone will result in calcination of the calcium carbonate to form refined lime which will exit the combustion zone as the solid combustion product for re-use as a catalyst and heat carrier in subsequent gasification. It can be seen that this preferred embodiment results in regeneration of the lime catalyst after use in gasification for subsequent reuse.

The method preferably further comprises cleaning the fuel gas exiting the gasification zone to remove any solid gasification product entrained within the fuel gas. This may be achieved, for example, using a separator such as a cyclone separator or an electrostatic precipitator. Any solid gasification product removed from the fuel gas may be returned to the combustion zone. Since, in preferred embodiments, the solid gasification product is returned to the combustion zone, it is preferable to remove a portion of the solid combustion product to avoid a build-up of solid products. Accordingly, in preferred embodiments, the method further comprises removing a portion of the solid combustion product. This portion of solid combustion product will be rich in refined calcium oxide (lime) and alumina as discussed previously and can be sold as a higher-cost catalyst (rather than a low cost material for use in the building industry e.g. as a cement additive).

In a second aspect, the present invention provides a system for generating a fuel gas, the system comprising:

a combustion zone for combusting deinking sludge to generate a solid combustion product;

a gasification zone for gasifying carbonaceous material to generate a fuel gas; and transfer means for transferring at least part of the solid combustion product from the combustion zone to the gasification zone. Preferably, the system comprises a combustion zone for combusting the deinking sludge at a temperature equal to or greater than 800°C, more preferably at a temperature equal to or greater than 850°C and, most preferably, at a temperature around 900°C. In preferred embodiments, the system comprises a combustor comprising a combustion zone and heating means for heating the combustion zone to a temperature equal to or greater than 800°C, more preferably to a temperature equal to or greater than 850°C and, most preferably, to a temperature around 900°C.

Preferably, the system comprises a gasification zone for gasifying the carbonaceous material at a temperature between 700-800°C, more preferably at a temperature between 720-800°C and, most preferably, at a temperature around 750°C. The heat in the gasification zone is provided by the solid combustion product.

Preferably, the carbonaceous material comprises paper reject generated during recycling of waste paper. In this way, the two main waste products arising from paper recycling can be disposed of in a single process without resorting to significant landfill and the associated environmental and transportation disadvantages. The paper reject is may be mixed with other carbonaceous material such as biomass (e.g. wood chips/pellets). Preferably, the combustor is a first fluidized bed reactor. In these embodiments, the combustion zone comprises a combustion fluidizing gas inlet for flowing a combustion fluidizing gas (e.g. air or oxygen) into the combustion zone.

In preferred embodiments, the gasifier is a second fluidized bed reactor. In these embodiments, the gasification zone comprises a gasification fluidizing gas inlet for flowing a gasification fluidizing gas into the gasification zone.

Combustion of the deinking sludge in the combustion zone preferably further generates a gaseous combustion product. This gaseous combustion product typically comprises carbon dioxide and nitrogen. In some embodiments, the transfer means are for transferring at least part of the gaseous combustion product (e.g. the carbon dioxide fraction of the gaseous combustion product) from the combustion zone to the gasification zone. The system may comprise separation means for separating the carbon dioxide fraction from the gaseous combustion product. In some embodiments, the gasification fluidizing gas comprises at least part of the gaseous combustion product (e.g. the carbon dioxide fraction of the gaseous combustion product) and the gasification zone comprises a gasification fluidizing gas inlet for flowing at least part of the gaseous combustion product (e.g. the carbon dioxide fraction of the gaseous combustion product) into the gasification zone. The gasification fluidizing gas may comprise steam or steam and air. The gasification fluidizing gas may comprise a combination of at least part of the gaseous combustion product (e.g. the carbon dioxide fraction of the gaseous combustion product) and steam. It has been shown that char burn-out and hydrogen production is optimised in steam gasification if carbon dioxide (from the gaseous combustion product) is introduced into the gasifier.

In some embodiments, the system comprises storage for storage of the carbon dioxide fraction of the gaseous combustion product.

Preferably, the system further comprises a steam generator for generating steam for use as the gasification fluidizing gas by allowing heat exchange between water (e.g. obtained from drying the carbonaceous material) and the solid and/or gaseous combustion product. Most preferably, the system comprises a steam generator allowing heat exchange between water and the gaseous combustion product. The steam generator preferably comprises a heat exchanger. The heat exchanger may be provided within the combustor.

In preferred embodiments, the second fluidized bed reactor is a bubbling or circulating mode reactor.

Preferably, the first fluidized bed reactor has a circulating mode so that the solid combustion product is entrained within the gaseous combustion product as it exits the combustion zone. In these embodiments, the system further comprises separation means for separating the solid combustion product from the gaseous combustion product. The separation means may comprise one or more cyclone separators or electrostatic precipitators. From the separator(s)/precipitator(s), at least part of the solid combustion product is transferred by the transfer means to the gasification zone whilst at least part of the gaseous combustion product (e.g. the carbon dioxide fraction) is flowed through the combustion zone as the gasification fluidizing gas or is transferred to storage. In some embodiments, at least part of the gaseous combustion product (e.g. the nitrogen fraction) is scrubbed and then vented.

Gasification of the carbonaceous material will result in a solid gasification product which will typically comprise calcium carbonate (CaCOs), alumina and char. The calcium carbonate component is generated as a result of chemisorption of carbon dioxide on the surface of the lime during gasification. Preferred embodiments of the system comprise second transfer means for transferring the solid gasification product to the combustion zone. Heating of the solid gasification product in the combustion zone will result in calcination of the calcium carbonate to form refined lime which will exit the combustion zone in the solid combustion product for re-use as a catalyst in subsequent gasification.

The system preferably further comprises cleaning means for cleaning the fuel gas exiting the gasification zone to remove any solid gasification product entrained within the fuel gas. The cleaning means may be, for example, a separator such as a cyclone separator or an electrostatic precipitator. Any solid gasification product removed from the fuel gas may be returned to the combustion zone. Since, in preferred embodiments, the solid gasification product is returned to the combustion zone, it is preferable to remove a portion of the solid combustion product to avoid a build-up of solid products. Accordingly, in preferred embodiments, the system further comprises collection means for removing a portion of the solid combustion product. This portion of solid combustion product will be rich in refined calcium oxide (lime) and alumina as discussed previously and can be sold as a higher-cost catalyst (rather than a low cost material for use in the building industry e.g. as a cement additive).

Brief Description of the Drawings Figure 1 shows a first preferred embodiment of the present invention; and

Figure 2 shows a second preferred embodiment of the present invention.

Detailed description of preferred embodiments

Figure 1 shows a first preferred embodiment of a system 1 for generating a fuel gas. The system comprises a combustor 2 which is a circulating mode fluidized bed reactor comprising a combustion zone 3 for combusting deinking sludge.

In the combustion zone 3, deinking sludge obtained from a waste paper recycling process is heated to a temperature of around 900°C to generate a solid combustion product which contains calcium oxide (lime) and aluminium oxide (alumina) and a gaseous combustion product which mainly contains carbon dioxide and nitrogen. The combustion zone 3 comprises a combustion fluidizing gas inlet 4 for flowing a combustion fluidizing gas (e.g. air) into the combustion zone 3.

The system further comprises a gasifier 5 which is a second fluidized bed reactor (bubbling mode) comprising a gasification zone 6 for gasifying paper reject.

In the gasification zone, paper reject obtained from the waste paper recycling process is heated to a temperature of around 750°C to generate a fuel gas which is rich in hydrogen and a solid gasification product which contains calcium carbonate, alumina and char. The heat for the gasification reaction is provided by the solid combustion product. The gasification zone 6 comprises a gasification fluidizing gas inlet 7 for flowing a gasification fluidizing gas into the gasification zone 6.

The system further comprises transfer means for transferring at least part of the solid combustion product from the combustion zone 3 to the gasification zone 6 and for transferring at least part of the gaseous combustion product (i.e. the carbon dioxide fraction of the gaseous combustion product) to the gasification fluidizing gas inlet 7.

The transfer means is made up of a first portion 8 which transfers solid combustion product entrained in the gaseous combustion product to a first cyclone separator 9. A second portion 19 of the transfer means transfers solid combustion product separated by the first cyclone separator 9 to the gasification zone 6 where the heat contained in the solid combustion product acts to drive the gasification reaction and the lime and alumina catalytically enhance the cracking of heavy hydrocarbons and tar. Carbon dioxide generated by gasification is removed from the fuel gas (thus enhancing the proportion of hydrogen and reducing carbon dioxide emission) by chemisorption onto the surface of the lime.

A third portion 10 of the transfer means carries the gaseous combustion product from the first cyclone separator 9 to a second cyclone separator 11 where any fine particles of solid combustion product and/or ash remaining in the gaseous combustion product are removed and collected for disposal 12. A fourth portion 13 of the transfer means includes a separator (not shown) which separates the carbon dioxide fraction of the gaseous combustion product. The fourth portion 13 then carries the carbon dioxide fraction of the cleaned gaseous combustion product to storage. The nitrogen fraction is vented to air.

Prior to entry to the separator, the gaseous combustion product is used in a heat exchanger within a steam generator (not shown) to heat water to generate steam which is flowed into the gasification zone 6 via the gasification fluidizing gas inlet 7. The heat exchanger may be provided within the combustor.

Gasification of the paper reject in the gasification zone 6 results in fuel gas which is collected 14 after passing through cleaning means comprising a cyclone separator 15 to trap any solid gasification product which is subsequently returned 16 to the combustion zone. The system further comprises second transfer means 17 for transferring the solid gasification product back to the combustion zone 3. Heating of the solid gasification product in the combustion zone 3 results in calcination of the calcium carbonate to form refined lime. This lime then exit the combustion zone 3 in the solid combustion product for re-use as a catalyst in subsequent gasification.

Since the solid gasification product is returned to the combustion zone, the system is prone to a build-up of solids. Therefore, the system further comprises collection means for removing a portion of the solid combustion product. This portion of solid combustion product will be rich in refined calcium oxide (lime) as discussed previously and can be sold as a higher-cost catalyst (rather than a low cost material for use in the building industry e.g. as cement additive).

Figure 2 shows a second preferred embodiment of a system 1 for generating a fuel gas. Many components are identical to the components in the first embodiment and therefore share the same reference numerals. The system comprises a combustor 2 which is a circulating mode fluidized bed reactor comprising a combustion zone 3 for combusting deinking sludge.

In the combustion zone 3, deinking sludge obtained from a waste paper recycling process is heated to a temperature of around 900°C to generate a solid combustion product which contains calcium oxide (lime) and aluminium oxide (alumina) and a gaseous combustion product which contains carbon dioxide and nitrogen.

The combustion zone 3 comprises a combustion fluidizing gas inlet 4 for flowing a combustion fluidizing gas (e.g. air) into the combustion zone 3.

The system further comprises a gasifier 5' which is a second circulating mode fluidized bed reactor comprising a gasification zone 6' for gasifying paper reject. In the gasification zone, paper reject obtained from the waste paper recycling process is heated to a temperature of around 750°C to generate to generate a fuel gas which is rich in hydrogen and a solid gasification product which contains calcium carbonate, alumina and char. The heat for the gasification reaction is provided by the solid combustion product. The gasification zone 6' comprises a gasification fluidizing gas inlet 7 for flowing a gasification fluidizing gas into the gasification zone 6'.

The system further comprises transfer means for transferring at least part of the solid combustion product from the combustion zone 3 to the gasification zone 6'. The transfer means is made up of a first portion 8 which transfers solid combustion product entrained in the gaseous combustion product to a first cyclone separator 9. A second portion 19 of the transfer means transfers solid combustion product separated by the first cyclone separator 9 to the gasification zone 6' where the heat contained in the solid combustion product acts to drive the gasification reaction, the lime and alumina catalytically enhance the cracking of heavy hydrocarbons and tar and carbon dioxide generated by gasification is removed from the fuel gas (thus enhancing the proportion of hydrogen and reducing carbon dioxide emission) by chemisorption onto the surface of the lime.

A third portion 10 of the transfer means carries the gaseous combustion product from the first cyclone separator 9 to a second cyclone separator 11 where any fine particles of solid combustion product and/or ash remaining in the gaseous combustion product are removed and collected for disposal 12.

A fourth portion 13 of the transfer means includes a separator (not shown) which separates the carbon dioxide fraction of the gaseous combustion product. The fourth portion 13 then carries the carbon dioxide fraction of the cleaned gaseous combustion product to storage. The nitrogen fraction is vented to air.

Prior to entry of the gasification zone, the gaseous combustion product is used in a heat exchanger within a steam generator (not shown) to heat water to generate steam which is flowed into the gasification zone 6' via the gasification fluidizing gas inlet 7. The heat exchanger may be provided within the combustor.

Gasification of the paper reject in the gasification zone 6' results in solid gasification product which is entrained in fuel gas. The gasification products first pass to a cyclone separator 20 where the solid gasification product and the fuel gas are separated. The fuel gas is then collected 14 after passing through cleaning means comprising a cyclone separator 15 to trap any remaining solid gasification product. The system further comprises second transfer means 17' for transferring the solid gasification product back to the combustion zone 3. Heating of the solid gasification product in the combustion zone 3 results in calcination of the calcium carbonate to form refined lime. This lime then exit the combustion zone 3 in the solid combustion product for re-use as a catalyst in subsequent gasification.

Since the solid gasification product is returned to the combustion zone, the system is prone to a build-up of solids. Therefore, the system further comprises collection means for removing a portion of the solid combustion product. This portion of solid combustion product will be rich in refined calcium oxide (lime) as discussed previously and can be sold as a higher-cost catalyst (rather than a low cost material for use in the building industry e.g. as cement additive).