Cross-Reference to Related Applications

This Patent Application claims priority from Italian Patent Application No. 102021000018125 filed on July 9, 2021, the entire disclosure of which is incorporated herein by reference.

Technical Field

The present invention relates to a process for the treatment of waste in which the hydrogasification step is integrated with the reforming step. In particular, the present invention makes it possible to recover energy from waste and produce, at discretion, hydrogen (¾ ) and/or methane (CPU).

The process of the present invention has the further advantage of being set up to produce PU and CPU as fuels with "green" energy content, as they are obtained with the use of a renewable energy source (e.g. solar energy) and without resulting in the emission of pollutants into the environment.

Background

The possibility of producing methane through the hydrogasification of carbon-based waste has long been known. In particular, the hydrogasification process of carbon-based waste is carried out with excess hydrogen and, therefore, in a reducing atmosphere, thus drastically allowing to limit the formation of toxic compounds, such as dioxins and furans. This treatment is part of the "waste-to-energy" processes, as it allows the transformation of a waste into a fuel (synthetic natural gas), which can be considered of a "green" nature as it is obtained from a biomass. The gas produced has characteristics that are very similar to those of natural gas and, depending on the operating conditions adopted, it can be introduced into the methane network directly or through simple further purification processes.

In the past, plants providing for the production of methane from biomass through a pyrolysis process were built. Such a solution suffers from the disadvantage of not being able to process waste comprising chlorine. In fact, the operating conditions, in particular the temperature and the presence of oxygen, which are characteristic of the pyrolysis process, are such as to cause the formation of dioxins if chlorine were present in the waste. Another disadvantage of this type of plant concerns the endothermicity of the pyrolysis process, which is necessarily reflected in the productivity of the plant itself.

To date, almost all of the hydrogen produced is obtained by steam reforming the methane (generally natural gas), through a thermo-catalytic process with mostly nickel-based catalysts. This is a process on which there is an abundance of literature. The chemical reaction involved is:

(1) CH ₄ + H ₂ 0 CO + 3H ₂ (DH°298K = +206 kJ/mol, SMR,

850-950 °C)

The fed methane is first purified of contaminants (possible deactivators for the catalysts used) and mixed with water steam before entering the catalytic reactor. As can be seen from the standard reaction enthalpy, the reaction (1) is very endothermic and is commonly carried out in large industrial furnaces where high process temperatures are reached by combustion of gaseous fuels. The mixture produced, consisting of CO and H ₂ (synthesis gas), is cooled and sent to Water-Gas Shift (WGS) reactors where the reaction (2) is carried out that allows CO to be converted into C0 ₂ and thus increase the production of hydrogen. (2) CO + H ₂ 0 C0 ₂ + H ₂ (DH°298K = - 41 kJ/mol, WGS,

200-450 °C)

Although steam reforming represents the most widely used industrial process for converting methane into hydrogen with the highest conversion yields, it suffers from the limitation due to high endothermicity. In this regard, the possibility of feeding the steam reforming process with "zero emissions" heat, for example through heat derived from solar systems or from electricity produced from other sources with low environmental impact, has been under consideration for several years. The problem relative to waste disposal is becoming more and more pressing year after year. In particular, despite the fact that technologies for the processing and the valorisation of waste have been developed for years, it is still extremely difficult to date to effectively use these technologies. Generally, the methods for the treatment of waste implemented to date suffer from the problem that they can lead to the emission of harmful substances into the environment and, for this reason, they meet with the hostility from the population living near the sites where the relevant plants should be built. In addition, many of the waste treatments available to date, requiring a constant supply of energy and reacting substances, do not result in a cost-effective energy gain. The inventors of the present invention have developed a process and a relative plant capable of integrating the hydrogasification of carbon-based waste with methane reforming, to produce at the complete discretion of the process operator ¾ and/orCIU depending on the convenience. The process object of the present invention, thanks to the integration of the reforming with the hydrogasification, can be self-sufficient in terms of reacting substances to be used.

Summary

Aim of the present invention is a process for the treatment of waste, the main characteristics of which are set forth in independent Claim 1, and the secondary and auxiliary characteristics of which are set forth in dependent Claims 2 - 7. A further aim of the present invention is a plant for the treatment of waste, the main features of which are set forth in independent Claim 8, and the secondary and auxiliary features of which are set forth in dependent Claims 9 - 15.

Brief Description of the Drawings Below is an illustrative and non-limiting example of an embodiment of the present invention with the aid of the accompanying figure, which schematically shows a plant for the treatment of waste according to the present invention.

Description of Embodiments In the Figure 1 denotes as a whole a plant according to the present invention.

The plant 1 substantially comprises a hydrogasification reactor 2 (C + 2¾ CPU), a reforming reactor 3 (CPU + H2O CO + 3H2) and a carbon monoxide conversion reactor 4 (CO + H2O C0 ₂ + H ₂₎ .

The other units of the plant 1 will be introduced below during a description of the process according to the invention.

The description reported below was made following a flow sheet analysis using the Pro/II system simulation program. With this program, the compositions of the products of the hydrogasification reactions of a hypothetical carbon- based waste (in this case associable with a "Waste Fuel" or RDS) with hydrogen were calculated assuming that the conditions of thermodynamic equilibrium are reached relatively for all the compounds present in the database.

It was assumed by way of example that the hydrogasification reactor 2 operated at 300 °C and 10 bar. It was also assumed that the units downstream of the hydrogasifier operate at 10 bar, ignoring pressure losses.

The typical composition of the RDS can vary greatly from case to case: in the following analysis the composition of a RDS reported in % by weight in Table I was used.

Table I - Elemental composition of ash-free dried RDS. Such a composition could, for example, derive from a heterogeneous mass in which an organic fraction with polymeric compounds such as polyethylene, polypropylene, polyethylene terephthalate (PET), PVC, nitrogenous (mainly derived from textiles, such as nylon) and sulfurized (e.g. from rubbers) polymers, as well as aromatic substances, prevails.

On average, the Municipality of Rome produces about 740 thousand tons of RDS, equivalent to about 22% of the waste delivered. It is therefore an average of about 2000 tonnes/day .

Specifically, in the present analysis a plant of about 10 tons/day of RDS was considered, corresponding, therefore, to the average production of a community of 15 thousand inhabitants. From the chemical point of view, the complexity and heterogeneity of the fed RDS was modelled considering a mixture of reference "model" compounds of various nature, presenting the same functional groups that can be expected in such materials: mainly aliphaticchains (-CH2-CH2-CH3 bonds), aromatic rings and ester groups (-COOCH3), plus tracesof chlorinated organic (-CH2-CHCI-), nitrogenous (- CH2-NH2) and sulfurized compounds (-CH2-S-S-CH2-). It was assumed that approximately 10 tonnes/day of dry

RDF enter the plant according to the composition in Table I, and that the hydrogasification reactor 2 is fed with stoichiometric hydrogen (¾) to allow a quantitative conversion of the RDF components in the Gibbs reactor. Under the above conditions, a stream of 29 kmol/h (650 Nm ³ /h) of H2 was assumed. The hydrogasification reactor 2 was modelled as a Gibbs reactor, whose products, in addition to those present in the starting RDF and to ¾, may be CtU, H2O, CO, C0 ₂ , HC1, NH ₃ , H ₂ S.

Table II shows the material balance (kmol/h) and the specifications of the streams of the reagents (RDF and ¾) and of the products. In particular, the RDS stream will be referred to as the RDS leading line, the ¾ stream will be referred to as the "a-fh" leading line (¾ supply) while the stream of the products exiting the hydrogasification reactor 2 is referred to as the leading line 5.

It can be observed that at 300°C and 10 bar, a gaseous mixture is obtained containing essentially CPU (83%), H2O (8%), CO2 (8%), small amounts of ¾ and NH3 (< 1%) and traces of other components (< 0.15%).

Table

From the enthalpy values reported in Table II, it is clear that the hydrogasification step of the process object of the present invention has an exothermic character, with the relative advantages both in terms of lower operating temperature and greater productivity that this entails, both in terms of the possibility of making energy recoveries and therefore increasing the efficiency of the entire process.

The CtU-rich stream exiting the hydrogasification reactor 2 is sent via the leading line 5 to a mixer 6 where it is mixed with water steam superheated to 250°C and 10 bar and produced in a steam generator 7. As will be described below, a gaseous stream containing any unrecovered ¾ residues, as well as unconverted CtU and CO in the reforming 3 and carbon monoxide conversion 4 reactors, respectively, is also sent to the mixer 6. Through a leading line 8 the reforming reactor 3 is fed with a gaseous stream characterized by a H ₂ O/CH ₄ molar ratio equal to about 3.

It has been assumed that the reforming reactor 3 operates at 850°C / 10 bar, and that the carbon monoxide conversion reactor 4 operates at 350°C / 10 bar.

If the stream exiting the hydrogasifier contains an excessive content of traces of potential poisons for the catalysts of the reforming 3 and carbon monoxide conversion 4 reactors, the presence of a purification unit (not shown in the figure) will be required to rectify the content of potential contaminants (e.g. nitrogenous, chlorinated, sulfurized compounds, etc.). It is preferred that the purification unit operates at high temperatures (> 200°C) in order to avoid condensation and re-evaporation of the residual water steam with the obvious advantages in terms of productivity and energy efficiency that this entails.

A gaseous stream at a temperature of 850°C flows out of the reforming reactor 3 which, through a leading line 9, feeds the carbon monoxide conversion reactor 4.

The leading lines 8 and 9 engage a heat exchanger 10 to allow a recovery of the heat of the gaseous stream exiting the reforming reactor 3. In particular, the presence of the heat exchanger 10 divides each of the leading lines 8 and 9 into a respective upstream branch (8a and 9a) and into a respective downstream branch (8b and 9b) of the heat exchanger 10.

Table III reports the specifications of the streams of the leading lines 8 and 9 and of the stream exiting the carbon monoxide conversion reactor 4 through a leading line 11.

Table III

Through the leading line 11 the stream exiting the carbon monoxide conversion reactor 4 is sent to a carbon dioxide separation unit (C0 ₂₎ 12, from which a CO2 stream is produced which through the leading line 13 is transported outside the plant 1. The gaseous mixture separated from CO2 in the carbon dioxide separation unit 12 is sent via a leading line 14 to a hydrogen separation unit 15. The hydrogen exiting the separation unit 15 is conveyed in a leading line 16. From the separation unit 15, in addition to hydrogen, a mixture is also obtained composed of any residues of unrecovered hydrogen, water, as well as unreacted methane and carbon monoxide respectively in the reforming 3 and carbon monoxide conversion 4 reactors. This mixture is sent to the mixer 6 through a leading line 17.

The separation unit 15 can operate both at low temperature and at high temperatures (> 200°C). The high temperatures have the advantage of avoiding the condensation and re-evaporation of the water steam sent in excess and, therefore, not converted in the reforming reactor 3 and in the CO conversion reactor 4, with the obvious advantages in terms of productivity and energy efficiency that this entails. As shown in the figure, the leading line 16 branches into a hydrogen leading line 18 towards the outside of the plant 1 and into the a-H ₂ leading line previously described and characterized in Table II. The leading line 18 represents the net production of hydrogen by the process according to the present invention.

Table IV reports the specifications of the streams of the leading lines 13, 14, 16, 17 and 18

Table IV

From the above, it appears that about 10 tonnes/day of RDF is converted into about 3 tonnes/day of H ₂ . The latter corresponds to 57.4 kmol/h = 1286 Nm ³ /h, which as an order of magnitude is equivalent to the expected load for a hydrogen distribution system for mobility. Considering the lower heating power of ¾ (LHV = 244 kJ/mol) this corresponds to (57.4 kJ/mol)/3600*(244 kJ/mol) =3890 kW produced in the form of ¾ .

With regard to the heat balance, the system has two energy-consuming units: steam generator 7, with a gross load equal to 924 kW of heat to be supplied at 250°C reforming reactor 3 with a demand equal to 1151 kW of heat to be supplied at 850°C

However, it should be noted that the process also has two exothermic units operating at temperatures ³300°C, whose heat can then be recovered to (partially) feed the steam generator 7: the hydrogasification reactor releases 484 thermal kW at 300°C the CO conversion reactor releases 317 thermal kW at 350°C

Thus, 801 thermal kW will be available to be recovered in the process to feed the steam generator 7, which will require 123 kW from an external source.

In conclusion, the proposed process requires 1274 kW of heat from renewable heat source to produce 3890 kW of "clean" fuel, in the form of heating power of ¾ . The process therefore results in a "net gain" in terms of "thermal power" equal to 2616 kW, corresponding, therefore, to more than 200% valorisation of the renewable energy (in the case of an electrolysis process the net energy gain is about 60-70%). Even considering the heating power of the fed RDS, the process has a high efficiency: a RDS can have a heating power of the order of 23-31 MJ/kg. Therefore, 10 tons/day correspond to a thermal power input of around 2600-3600 kW and an energy efficiency of renewable energy conversion + RDS into hydrogen ranging between 80% and 99% is obtained. Although this is an "ideal" efficiency (it does not take into account, for example, the electrical consumptions and non-ideality of some process units), the results obtained are encouraging for future developments. The same reforming process of CtU, under the same ideal conditions, would have an efficiency of 89% (understood as the energy of the H2 produced, on an LHV basis, divided by the incoming energy in the form of LHV of CH4 and of the net heat transferred from the external source to the SMR and SG units). In the case object of the present invention, however, the starting point is not a fossil fuel such as methane, but a waste-derived fuel (RDF).

The problem with the traditional RDF waste-to-energy process stems from the limitations of using chlorine-free materials because the latter are among the main dioxin forming agents in combustion; the use of the hydrogasification at low temperatures (below 500°C and preferably 300°C) reduces or eliminates the dioxin problem. In contrast, processes using pyrolysis for the production of methane from biomasses require much higher temperatures. This characteristic together with the presence in the pyrolysis of an oxidizing environment (which instead is avoided in the hydrogasification process that operates with excess hydrogen and therefore in a strongly reducing environment) results in the production of dioxins once wastes comprising chlorine-containing materials are treated. Another important advantage of using hydrogasification instead of pyrolysis lies in the fact that the hydrogasification reaction is exothermic, while the pyrolysis reaction is endothermic, which benefits the energy balance of the process as a whole. In the case examined, only 33% of ¾ produced is recycled to feed the hydrogasification reactor 2, while the remaining represents a net production of hydrogen.

However, it is possible to consider further process schemes and case studies, in which instead of a net production of ¾ there may be a net production of methane or a methane/hydrogen mixture:

- case in which only the methane necessary to produce the hydrogen needed for hydrogasification is sent to the reforming reactor 3; therefore, with a net production of methane from RDS and renewable energy being self-sufficient with regard to hydrogen;

- combination of the previous configurations, with co production of methane and hydrogen in a flexible and controlled manner. The case examined above refers to the application of a steam reforming scheme at high temperatures, where the thermal input at 850°C will probably be supplied electrically and/or thermally from preferably renewable sources. Replacing the proposed scheme with a process scheme in which reforming takes place at a low temperature (< 600°C) by means of a membrane reactor (e.g. CoMETHy technology) would bring the following additional benefits:

- possibility of heat storage by means of molten salts for a better exploitation of the renewable (electrical/thermal) source; production of a stream with a higher CO2 concentration, thus easier removal and confinement of CO2 produced by the carbon dioxide separation unit 12;

- reduction of the heat to be supplied to the reforming reactor 3 due to its combination with the carbon monoxide conversion reactor 4 in a single reactor.

As anticipated above, the possibility of removing CO2 in the unit 12 and ¾ in the unit 15 under conditions of high temperature, at least higher than the dew temperature of the gaseous mixture in the leading line 11, would allow to avoid condensation and re-evaporation of residual H2O in this stream, resulting in a reduction of the thermal load on the steam generator 7 and, therefore, a further increase in the efficiency of the process as a whole. The process of the present invention also has the great advantage that it can also treat waste in wet form. Such a possibility would allow the hydrogasification reactor 2 to be operated in an almost autothermal way (zeroing the heat to be removed) with an easier control of the temperature and reduction of plant costs. In other words, the H2O present in the starting RDF could absorb the reaction heat of the hydrogasification reactor 2, be vaporized and, therefore, reduce the thermal load at the steam generator 7 in addition to the plant costs for the thermal recovery from the hydrogasification reactor 2 which could be operated in an almost autothermal manner.