WO2013005239A1

WO2013005239A1 - Gasification process

Info

Publication number: WO2013005239A1
Application number: PCT/IT2012/000185
Authority: WO
Inventors: Vittorio DELL'ACQUA
Original assignee: Rewood S.R.L.
Priority date: 2011-07-05
Filing date: 2012-06-18
Publication date: 2013-01-10
Also published as: ITFI20110133A1; EP2748285A1

Abstract

A gasification process by which a biomass is gasified to produce syngas (SYN) through a reactor (2) of a gasification plant, and inside the said reactor (2) are placed pyrolysis gas (GP) or not purified syngas (SYN2), and waste carbon (CH, CA) with the possible addition of porous alumina (AL) in which the pyrolysis gas (GP) and the waste carbon (CH) come from a pyrolysis reactor (1) of the said gasification plant inside which is introduced a virgin biomass that is subjected to slow pyrolysis; the waste carbon (CH) produced by the slow pyrolysis consists of self produced activated carbon that is produced internally by the same plant which includes the said reactors (1) and (2).

Description

TITLE

Gasification process.

DESCRIPTION

The present invention relates to a gasification process. It is known that gasification consists in the incomplete oxidation of a biomass realized by means of gasifying agent and other reagents, in an environment at high temperature (800 to 1000 ⁰ C) for the production of a combustible gas, said gasogene gas, synthesis gas or syngas, composed of H₂, CO, C_xH_y, N₂, C0₂, in varying proportions depending on the type of biomass and the type of gasifier used. Oxidizing elements normally used are air, oxygen or steam.

The gas contains many impurities (char) including dust, tars and heavy metals. Therefore, in many cases, before to use this gas for combustion, it must be cooled and purified.

The unpurified syngas can be used in Rankine cycle boilers, either alone or together with traditional fuels (co-firing) .

In other cases it is cooled and purified for feeding gas powered turbines or endothermic engines. The syngas can also be used in endothermic engines (Otto cycle) coupled with generators for production of electricity. The gasification plants are classified by typology and divided into two categories:

- fixed bed gasifiers: downdraft (or cocurrent), updraft (or countercurrent ) , cross draft;

- fluid bed gasifiers.

In particular, the fixed bed gasifiers are constituted by a cylindrical container which has a grid installed in the bottom side, useful to support the overlying biomass and to allow the passage of air or gas and to allow the extraction of the ash produced by the gasification process . In updraft gasifiers, with top draw, the oxidizing agent that is inserted from the bottom side reacts with the fuel added from the top of the cylinder, at the center of the container.

In downdraft gasifiers with lower draw, the flow of biomass, combustive and gas is directed from the top to the bottom. The produced gas exits from the bottom side together with the ash.

The cross draft gasifiers work similarly to the previous mentioned gasifiers, except for the fact that the fuel is injected from the top, the oxidant is inserted transversely and the output of the gas is lateral.

It's also known that in the conventional plants the purification of the syngas provides the removal of the TAR with relatively high costs. Moreover, since the TAR has a calorific value of 8000 kcal/kg, its elimination represents a significant amount of energy subtracted' from the overall energy balance of the gasification process. It is also known that the conventional gasification plants provide, as a byproduct, the so-called "charcoal" that, having a calorific value of 5000 kcal/kg and being usually eliminated with the ashes, represents a further energy loss connected with the gasification process.

Examples of plants and/or gasification processes are disclosed in WO2005/113732 , WO2011/061299, US2010/223846, WO2008/107727,^' O2009/132449.

Mainly due to the above cited energy losses, the efficiency of a conventional gasification process is relatively low.

Therefore, it is strongly felt the need to have plants, and to implement gasification processes, energetically more efficient.

The main purpose of the present invention is to eliminate, or at least significantly reduce, the aforementioned drawbacks. These results have been achieved according to the present invention by adopting the idea of realizing an operative process having the features described in claim 1. Further features of the present invention are the subject of the dependent claims.

Thanks to the present invention, it is possible to produce TAR free syngas and produce ash-free charcoal, with the consequential advantages in energy terms.

These and other advantages and characteristics of the present invention will be best understood by anyone skilled in the art from a reading of the following description in conjunction with the attached drawings given as a practical exemplification of the invention, but not to be considered in a limitative sense, wherein:

- Fig.l is a simplified block diagram concerning a plant for the implementation of a process in accordance with a first embodiment of the present invention;

- Fig.2 is a simplified block diagram concerning a plant for the implementation of a process in accordance with a second embodiment of the present invention;

As shown in the diagram in fig.l, a process in accordance with a first example of embodiment of the present invention mainly comprises two operative steps.

The first step is carried out in a first reactor (1) and consists in heating the biomass, which come from a loading hopper (3), up to a predetermined pyrolysis temperature comprised between 500°C and 550°C and maintaining such temperature for a predetermined time of at least 20 minutes, preferably for a time comprised between 20 minutes and one hour, in order to obtain a highly porous carbon or "charcoal" (CH) and pyrolysis gas

(GP) . The above-mentioned pyrolysis temperature is achieved preferably by means of predetermined increases each of which does not exceed 12 °C per minute. The temperature of 550 °C is also called "maximum temperature of pyrolysis". The heating step described above is a step of indirect heating.

Advantageously, the first indirect heating step is realized using the exhaust gases (G) of a motor (9) that is fed with the produced syngas (SYN) . Said exhaust gases come out from the first reactor, up to a chimney (not shown in the diagram) .

Moreover, advantageously, in the first reactor (1) the biomass is subjected to drying before the pyrolysis and the steam (V) thus produced is extracted from the reactor (1) and preferably is converted into a condenser (4) where the eventual steam produced in excess by the biomass is condensed. This allows the treatment of biomass with high humidity, up to 50%, without using a separated pre-drying. In addition, if necessary, to reach the above-mentioned pyrolysis temperature a small part of the syngas (SYN) is burned and the related combustion gases are recycled into a second reactor with a consequent heat recovery.

The charcoal (GH) produced in the first phase is essentially free from residual gases, is highly porous and assimilable to an activated carbon, and is in turn gasifiable. The high porosity of the charcoal (CH) is obtained thanks to the slow pyrolysis of the biomass made in the first reactor (1) as previously described at a predetermined temperature comprised between 500 °C and

550°C. In fact, as observed by the inventor, with a pyrolysis temperature lower than 500 °C there would be no full emission of the pyrolysis gas (GP) with a consequent low porosity of the charcoal (CH)thus obtained, while with a pyrolysis temperature higher than 550°C, the porous structure of the charcoal (CH) would be subjected to a collapse, with a consequent decay of the structural characteristics of the charcoal (CH) itself, which therefore would no longer have properties comparable or assimilable to those of an activated carbon.

In addition, the biomass pyrolysis occurs in the reactor (1) for indirect heating without the introduction of oxygen and/or air, hence using the exhaust gas (G) as previously described, in order to produce the greatest possible quantity of charcoal (CH) equivalent or assimilable to an activated carbon.

Finally, the pyrolysis realized in the first phase of the gasification process allows to produce the largest possible amount of a charcoal (CH) whose properties are comparable to those of an activated carbon. The charcoal (CH) thus produced is used, as described below, in the second step of the same gasification process to purify the pyrolysis gas (GP) which is also produced in the said first phase.

The second step is developed in a second reactor (2) and it consists of a updraft gasification in which, starting from the products of the first step (CH, GP) , it is produced syngas (SYN1) that contains a reduced quantity of TAR.

The charcoal (CH) is fed into the second reactor (2) from the top of the latter, so as to form a bed subdivided into three zones or layers, each of which having a predetermined height, the lower one consisting of an oxidation zone (20), the intermediate one consisting of a gasification and/or reduction zone and the upper one consisting of a zone of absorption of residual impurities of the pyrolysis gas (GP) .

The reactor (2) is constituted by three rings (21) and the said oxidation zone (20) is at the bottom of the lower ring (21) and the pyrolysis gas (GP) is entered in the same reactor (2) above the oxidation zone (20).

Therefore, the most quantity of heat is produced by the combustion of the charcoal (CH) at the bottom of the reactor (2), that is, in the oxidation zone (20), and not by the combustion of the pyrolysis gas (GP) , thereby increasing the overall efficiency of the process.

In practice, the pyrolysis gas (GP) - together with the steam developed in the first reactor (1) - is entered into the second reactor (2) by means of a corresponding duct, in correspondence of the said intermediate zone for the gasification and/or reduction, subsequently being subject to a final purification during its passage through the said upper zone, where the charcoal (CH) absorbs the impurities in the pyrolysis gas (GP) .

By entering the pyrolysis gas (GP) in the second reactor (2) together with charcoal (CH) , which is maintained at a temperature comprised about between 720°C and 850°C, the charcoal (CH) acts as a catalyst transforming the TAR into coke and syngas , or causing the so-called "cracking" of the TAR. The temperature of the charcoal (CH) in the second reactor (2) is controlled by sensors (S2) and it is increased or decreased respectively introducing air or steam, through corresponding supply sleeves (Ma) and (Mv) distributed along the reactor (2) . The steam may be the one coming out from the drying of the biomass in the first reactor (1). Preferably, the air introduced into the reactor through the sleeves (Ma) is preheated air so as to recover the sensible heat of the produced syngas. The reaction time for the above mentioned transformation of the TAR, has been experimentally estimated in 0.3 seconds.

The reactor (2) is sized to ensure that the charcoal (CH) will stay in the second reactor for a time corresponding to said transformation time at the aforementioned temperature and it has been experimentally evaluated that the ratio between the diameter and the length of the part of the reactor (2) to obtain such result is preferably

1/3.5. In addition, a default level of charcoal (CH) inside the reactor (2) is maintained. In the diagram of fig.l it is visible a primary filter (6) on the reactor (2), placed at the output of the SYN1, that presents a filtering net, with meshes having a diameter of 0.25 mm, that is mechanically cleaned by scraping, thus provoking the fall-out of the intercepted dust in the same reactor

(2) . SYN1 is the syngas coming out from the second reactor (2) .

From experimental measurements it results that in the aforesaid first phase, using a same quantity of biomass, the charcoal (CH) that is produced can vary from 5% to

25%, in function of the type of biomass. In particular, the quantity of charcoal (CH) decreases proportionally to the lignin contained in the biomass. Therefore, the quantity of charcoal (CH) produced in the first phase could be insufficient to act as a catalyst for the transformation of the TAR contained into the corresponding quantity of the pyrolysis gas (GP) that has been produced. Therefore, to compensate this lack of charcoal (CH) , a porous alumina (AL) can be used as catalyst. At the working temperature of the reactor (2), the coke that is deposited from the TAR on the surface of the porous alumina (AL) is partially gasified. The porous alumina (AL) regeneration is completed at a temperature higher than 900 °C, that is in the oxidation zone (20) of the reactor (2) where the residual coke present on the porous alumina (AL)is oxidized. In order to be used again, the porous alumina (AL) is separated from the ash (CN) by means of a vibrating sieve (5) . The amount of porous alumina (AL) is calculated by considering that the weight of the charcoal (CH) , added to the weight of alumina (AL) be equal to 20 ÷ 25% of the biomass used in the process. Thanks to its high mechanical and thermal resistance, regenerated alumina (AL) can be reused many times .

The efficiency of the described process is greater than that of a conventional gasification (GT) , since all the charcoal (CH) is gasified and the majority of the pyrolysis gas (GP) is transformed into syngas and coke that is in turn gasified, thus producing limited quantities of waste carbon and in particular, of tars and charcoal, with consequent advantages in energy terms. The residual TAR into the syngas (SYNl) has a condensation temperature of 200°C lower than that of a TAR contained in a syngas (SYN2) obtained by conventional gasifiers (GT) . This allows a greater recovery of the sensible heat in the syngas (SYNl) leading to a further advantage in energy terms.

The syngas (SYNl) outgoing from the reactor (2) can be further treated or purified, by means of filtering and washing operations schematically represented in Fig.l and Fig.2 and implemented by means of cyclones (8), by which the amount of dust (P) can be decreased, and by means of scrubbers (7), thanks to which the TAR (T) can be removed from the gas.

The waste (P, T) produced by the purification of the syngas (SYNl) are convoyed to the oxidation zone (20) of the second reactor (2) thus increasing the efficiency of the process and reducing the quantity of waste (P, T) to dispose. Thanks to the described process, it is therefore possible to obtain the gasification of a biomass producing a syngas (SYNl) that contains a limited quantity of TAR and to obtain gasification efficiency much higher than that obtainable by conventional systems.

Moreover, unlike the known processes of gasification of the type described in the cited prior art documents, in the present invention, a charcoal (CH) with physical and chemical properties equivalent to an activated carbon is produced in a first phase and used in a second phase of the same gasification process. In other words, a coal with properties similar to that of an activated carbon is "self produced" and used in the same gasification process of the present invention.

In accordance with a further example of embodiment of the present invention shown in Fig.2, considering its low cost, the reactor (2) can also be advantageously used in place of a traditional system to decrease the TAR contained into the syngas (SYN2) produced by traditional gasifiers (GT) . The reactor (2) so used, allows also to burn the charcoal (CA) produced by the gasification, so recovering the energy contained in the latter. This is achieved by introducing in the same reactor (2) the unpurified syngas (SYN2) coming out from a conventional gasifier (GT) and feeding the reactor (2) with the charcoal (CA) produced in the same gasifier (GT) , added with porous alumina (AL) , that is used as a catalyst.

For example, the porous alumina (AL) cited with reference to the examples described above may be in the form of spheres with a diameter of 3 mm.

In the diagram of fig.2 the rings (21) of the reactor (2) are not represented for simplification reasons.

The biomass used to feed the gasifier, may come from forests, from agricultural, or from waste treatment.

It is to be understood that the drawing shows only an example provided solely as a practical demonstration of the invention, and that this invention may be varied in its forms and dispositions without departure from the scope of the guiding concept of the invention. The presence of any reference numbers in the enclosed claims has the purpose of facilitating the reading of the claims with reference to the description and to the drawing, and does not limit the scope of protection represented by the claims .

Claims

1) Gasification process by which a biomass is gasified to produce syngas (SYN) through a reactor (2) of a gasification plant, and inside the said reactor (2) are placed pyrolysis gas (GP) or not purified syngas (SYN2) , and waste carbon (CH, CA) with the possible addition of porous alumina (AL) in which the pyrolysis gas (GP) and the waste carbon (CH) come from a pyrolysis reactor (1) of the said gasification plant inside which it is introduced a virgin biomass that is subjected to slow pyrolysis characterized in that the waste carbon (CH) produced by the slow pyrolysis consists of self produced activated carbon that is produced by the same plant which includes the said reactors (1) and (2).

2) Gasification process according to claim 1, characterized in that the pyrolysis is realized at a pyrolysis temperature comprised between 500°C and 550°C and the pyrolysis time is at least 20 minutes.

3) Gasification process according to claims 1 and 2, characterized in that the said pyrolysis temperature is reached by means of increases not higher than 12°C/min.

4) Gasification process according to claims 2 and 3, characterized in that the pyrolysis is carried out by indirect heating using as heating means the exhaust gases (G) of a motor (9) that is fed by means of the produced syngas (SYN) . '

5) Gasification process according to claim 1, characterized in that the reactor (2) from which the syngas (SYN) comes out, includes three rings (21), of which the lower ring delimits a zone of oxidation (20) and characterized by the fact that the said pyrolysis gas (GP) is entered into the reactor (2) in a zone that is separated and is upstream with respect to the oxidation zone (20) .

6) Gasification process according to claim 5, characterized in that internally to said rings, the charcoal (CH) is arranged on three superimposed layers of which the lower one consists of an oxidation zone (20) , the intermediate one consists of a gasification and/or reduction zone and the upper one consists of a zone of absorption of residual impurities of the pyrolysis gas (GP) .

7) Gasification process according to claims 5 and 6, characterized in that the said pyrolysis gas (GP) is introduced into the reactor (2) in correspondence of the said gasification and/or reduction zone.

8) Gasification process according to claim 5, characterized in that in that in the oxidation zone (20) are placed TAR and /or dust (P) coming from the not purified syngas (SYN1) washing cycles.

9) Gasification process according to claims 1 and 5 characterized by the fact that porous alumina (AL) passes through the said zone of oxidation (20) before being recovered by separation from the ashes (CN) so as to be regenerated, said ashes (CN) being produced in the said oxidation zone (20) .

10) Gasification process according to anyone of the previous claims characterized in that the pyrolysis is obtained by indirect heating.