Patent Application
20200222874
The invention relates to a plant for converting biogas into chemical products with a high added value and related operating modes.
Biogas is a gas that originates from fermentation/anaerobic digestion and is predominantly characterized by methane (CH4) and carbon dioxide (CO2). The ratio between CH4 and CO2 varies according to specific power supply (first generation biomass, recovery or second generation biomass, zootechnical waste such as sewage or pollen, or FORSU (Organic Fraction of Urban Solid Waste)), to seasonality, micro-organisms, operating conditions of digesters and plants, as well as according to ancillary units connected to the purification of biogas from any undesired compounds. The percentage of methane can generally vary from 55% by volume up to 80%, even if plants capable of supplying compositions out of this nominal range (20%-85%) are known.
Biogas in Italy is a well-established product with 1,823 operating plants, mainly concentrated in the Po Valley (about 450 plants in Lombardy alone), but it is even more so in Europe, with about 18,500 plants, more than half of which in Germany The most widespread plants of this type are those capable of generating electricity with a power in the order of 1 MW.
In fact, nowadays biogas is exclusively used to produce electricity and to supply it to the national power network after having used the electricity required for the management of the farm where it is installed. In order to generate electricity, the combustion engine burns biogas with air and at the same time generates heat that brings the combustion chamber to over 800° C.
The biogas plant is advantageous above all for the farms where it is installed, as well as for multi-utilities, due to the incentives received from the electricity supply companies.
In fact, an electric 1 MW plant that supplies, e.g., a biogas with 60% CH4 and 40% CO2 by volume, leads to a profit in electricity sold at a flat market price in Italy equal to 0.28€/kWh or 0.23€/kWh, depending on the year of installation. In the first case, the annual revenue is equal to 2.24 M€/year, in the second case (more recent biogas plants) it is equal to 1.84 M€/year.
Furthermore, biogas has significantly contributed to the achievement of European standards for renewable energy generation.
In environmental terms, it is a zero-impact process: the atmospheric CO2 that has been fixed in the biomass is completely re-introduced into the atmosphere thanks to the combustion in the power generation engine. It is estimated that an electric 1 MW biogas plant reintroduces 7852 t/year of CO2 into the atmosphere.
One of the most promising and discussed future potentials of biogas is its bio-methane purification (also called upgrading, according to the Bio-methane Decree of Mar. 2, 2018). Purification means the removal of the CO2 present in the biogas in order to obtain a methane stream of renewable origin to be introduced into the gas network currently in use. This would solve the complex problem linked to the non-transportability of biogas (too expensive to be applied) thanks to the exploitation of the national gas network infrastructure. Purification technologies are the most varied. Among the most promising, there are absorption with water, amine softening systems, scrubber-type systems or membrane systems and intermittently operating zeolite filters. Each of the aforementioned systems has advantages and disadvantages, which are not relevant to the present patent.
Bio-methane purification is a less desirable solution than biogas. An electric 1 MW plant that supplies, by volume, a 60% CH4 and 40% CO2 biogas is equivalent, in mass, to 35% CH4 and 65% CO2. This inevitably leads to an exploitation of only ⅓ of the total biogas and, therefore, to a profit of 0.21 M€/year without incentives, which corresponds to an estimated methane value of 0.11€/kg. With the incentive defined in the Decree of Mar. 2, 2018, the revenue amounts to €1.35 million/y. Although interesting as a solution and as a prospect for this sector, it is less attractive for the farmer/breeder who runs the biogas plant, as well as being more invasive in terms of incentives.
Finally, from an environmental point of view, the situation in terms of CO2 reintroduction into the atmosphere does not change: for a 1 MW plant, 3141 t/y of CO2 are released directly into the atmosphere from the purification plant, while the remaining 4711 t/year of CO2 are reintroduced into the atmosphere as a result of domestic use.
An alternative solution is the synthesis of liquid chemicals with a high added value, easily transportable and ready to use, starting from biogas.
Already known plants allow the formation of chemical products with a high added value starting from biogas, e.g. those described in WO2012/151545, WO2016/101076, GB2545474, which contemplate reforming to obtain syngas as intermediate product. In particular, the first and third of said prior art documents provide that syngas is obtained by carrying out steam reforming and partial oxidation (POX), whereas the second of said prior art documents contemplates an anhydrous reforming (dry reforming).
The second and third of these prior art documents provide that the plants described therein are used for the production of gasoline according to the Fischer Tropsch method, whereas the plants disclosed in the aforementioned prior document are intended for the production of methanol or dimethyl ether.
However, this latter document does not provide the replacement of some parts of the plant, nor the replacement of the synthesis section of the chemical product with a high added value to obtain a different one.
Moreover, even if the second prior document envisages that system parts can be replaced, such substitution is only designed to remove the exhausted catalyst, or remove obstructions etc., so it is designed to replace a section identical to the broken one, but it is certainly not intended for the preparation of chemical products other than those obtained before the replacement.
The applicant has now found a plant that allows obtaining an easily transportable liquid chemical with a high added value and with a definitive CO2 sequestration (negative impact). This happens through the non-invasive installation of an extremely flexible module, able to transform the biogas, usually in its entirety (CH4 and CO2), into the aforementioned desired final product.
An object of the present invention is therefore said plant for converting biogas into a chemical with a high added value selected among methanol, dimethyl ether, formaldehyde, acetic acid by a process comprising the following steps:
a) reforming by converting methane and carbon dioxide contained in the biogas into syngas,
b) synthesis of methanol by using syngas from step a),
b′) synthesis of one of the aforesaid chemical products: formaldehyde, dimethyl ether or acetic acid by using as a reagent methanol from step b),
c) optional purification and separation of said chemical product from step b) or b′), wherein
i) said plant is located downstream of the biogas production plant and consists of a compact module comprising:
ii) at least the whole section B) or a part thereof can be substituted with a further and different section B) or part of section B), to obtain one of the aforesaid chemical products different from the one obtained before said replacement of the whole section B) or part of it,
iii) said at least one section B) or part of section B) is connected to at least one adjacent section selected among: A), the optional C), and at least one of said sections D) by means of hydraulic connection devices.
The applicant has indeed found that, thanks to the process object of the invention, the following advantages are achieved:
For the purposes of the present invention, the prefix bio before names of chemical products involved in the process of the present invention indicates that they have been obtained from biogas, but that their structure and chemical formula do not differ from the structure and chemical formula of the same compounds obtained by means of conventional industrial chemical processes.
The products obtainable with the plant object of the present invention are all hydrocarbon compounds with few carbon atoms and are selected among methanol, formaldehyde, acetic acid and dimethyl ether.
The module or plant object of the present invention preferably has dimensions 4 m×2 m×2 m and more preferably 4 m×1.5 m×1.5 m, namely dimensions such that the whole module or each section thereof can be easily transported by light means. This module is inserted downstream of the biogas plant as shown in
The purification section C) of the final product can be omitted or excluded from the plant object of the invention after upstream optimization of the feeding composition (e.g. by partial upstream removal of fed CO2), or alternatively if no high purity is requested in the final product, e.g. in the case of non-stringent market requests or in the event that the end user has his/her own further purification process.
Therefore, each single section of said module has its own autonomy as regards temperature and pressure, even if integrated solutions are possible.
The modularity is guaranteed by hydraulic-mechanical devices.
Hydraulic-mechanical devices mean those devices that allow a rapid removal/replacement of one or more sections of said module to close the lines in which the fluids connecting a section and the neighbouring one(s) flow, and of the mechanical means allowing the attachment/detachment of this section to or from the neighbouring section(s). A connection of this type may e.g. be of the valve/flange/valve type, like the one shown in
Each of said modules shown in
Since for the synthesis of dimethyl ether, formaldehyde and acetic acid, as discussed in detail in the following description, methanol is used as a reagent, the substitution of the whole section B) comprises both a part where the methanol synthesis is carried out starting from syngas and a second part where the desired product is synthesized.
For this reason it may be useful to replace only the portion of section B) for the production of the specific chemical product with a high added value through step b′). Therefore, in this case, the plant according to the present invention can provide that only the part of section B) used for the synthesis of the specific product is replaced.
In both cases, whether the whole section B) or only part of it is to be replaced, the plant object of the present invention is very versatile, since it allows producing a chemical product with a high added value able to satisfy not only the market requirements but also the local demand.
The reactions involved in the reforming are essentially three:
dry reforming or reaction (1)
CH4+CO2→2H2+2CO (1)
steam reforming or reaction (2)
CH4+H2O→3H2+CO (2)
and the balancing reaction (3) better known as water gas shift reaction (WGSR)
CO+H2OCO2+H2 (3)
The reforming operation implies a sharp increase in the number of moles and this is notoriously influenced by the pressure. Furthermore, a certain pressure allows reducing the volumes involved and ensures that the dimensions of section (A) are within the dimensions of the module in which the entire process of the invention is carried out.
A moderate pressure increase does not change, if not slightly, activity and selectivity of the main commercial catalysts. A pressure of 5-35 bar is preferable. More preferably, the pressure must be in a range of 10-20 bars; in this way, the certifications for operating the module do not change with respect to what is already required for the operation of the power generation engine currently installed on the plants. To facilitate the conversion of biogas into bio-syngas, the unit preferably operates in a temperature range of between 650-900° C., more preferably between 700-800° C., in the presence of platinum and rhodium catalysts and in accordance with the operating conditions of the engines currently installed on biogas plants. Furthermore, the use of steam hinders the formation of coke deposits on the catalysts and on their respective supports.
The reforming operation is endothermic. It must receive an enthalpic contribution to sustain itself and is usually coupled to a system of exothermic reactions (combustion or oxidation), or to energy sources of another nature (e.g. concentrated solar energy).
In the specific case, it is optionally provided a special apparatus that integrates the reforming section with the engine currently installed in the biogas plant, since their operating conditions are substantially identical, as shown for example in
The reactions involved in step b) carried out in section B) to obtain methanol are the following:
2H2+CO→CH3OH (4)
3H2+CO2→CH3OH+H2O (5)
CO+H2O→CO2+H2 (6)
If the synthesis of interest is the one of methanol, it must be also considered the high exothermicity of the reaction. In this sense, it can also be optionally envisaged an energy integration between the reforming section (A) and the synthesis section (B). Moreover, it is essential considering the low methanol conversion in the single passage in the reactor (about 7% by volume). This involves large gas recycling and expensive recompression, typical of large plants. To improve this aspect and reduce it for the biogas sector, catalysts based on iron and copper oxides have been developed, capable of guaranteeing yields of 25-35% by volume for each passage in the reactor. Furthermore, to further reduce the gaseous recycles inside the module, the methanol synthesis reactor is in stages, with an optional partialization of the power supply exiting from the reforming section A) to heat in situ the stream to be fed at the various reaction steps B) (an implementation method that can be also extended to other products, such as DME). The reactor is schematized in
The overall reaction for the synthesis of acetic acid is as follows:
CH3OH+CO→CH3COOH (7)
The acetic acid synthesis process requires the carbonylation reaction of methanol. The standard process includes two reactors, catalytic tubular for methanol and slurry for acetic acid. The typical catalysts used for the synthesis of methanol are based on Cu—ZnO—Al2O3 or Cu—Zn—ZrO2 in the presence of Ga2O3. The operating temperature and pressure conditions are of the order of 220-250° C. and 30-100 bar. In the case of biogas conversion, it is not optional to hypothesise such high pressures for reasons of safety, certification and regulations. Specific catalysts have therefore been developed with the same base as above, but in different concentrations, which are capable of favouring the synthesis of methanol at relatively low pressures, 10-40 bar, and preferably at 15-35 bar with high yields on the single passage in the reactor.
For the synthesis of acetic acid, commercial catalysts achieve conversions of methanol of 98% with high selectivity. All the results of the feasibility study are reported in the datasheet of
The WGS Section (D) is useful for correcting the H2/CO ratio of syngas to favour some chemical synthesis, such as methanol or Fischer-Tropsch. Methanol requires an H2/CO ratio of 2 (or slightly higher to avoid some parasitic reactions), since the reactions involved are the aforementioned reactions (1)-(3)
Only the first of the reactions is relevant to estimate the H2/CO ratio, since the other two reactions are linearly dependent on it. Acetic acid requires an H2/CO ratio of 1, since the overall reaction is the aforementioned reaction (7).
Similarly, formaldehyde, which is obtained with the reaction (8)
CH3OH→CH2O+H2
requires a ratio of 1, considering that the process actually is a dehydrogenation of methanol.
The use of a WGS unit is often combined with a CO2 removal system.
The WGS unit, however, becomes useless if the synthesis requires a ratio of H2/CO equal to 1, because the methane is almost always the main compound of the biogas (over 50% by volume) and this allows having a H2/CO ratio higher than 1. Any hydrogen surplus, if actually produced, can be fed to the generation engine.
SOFCs are one of the most useful systems for generating power in the event of a hydrogen surplus. They complete and energetically enrich the contribution of the module in cases of biogas with good level of methane and/or synthesis of chemical compounds that require low H2/CO ratios.
Besides the compressors and the exchangers, the module where the process according to the present invention is carried out provides as further section D) also one or more heaters in which water is sent to generate medium-pressure steam, where a medium pressure means a range between 8 and 25 bars.
The module for carrying out the process according to the present invention can comprise one or more absorption swing adsorption columns for recovering hydrogen.
Wireless control systems are hypothesised for an internet connection, remotely or via cloud, for predictive performance management in order to guarantee a complete autonomy of the individual sections of the module, as well as to facilitate as much as possible their interchangeability.
Illustrative but non-limiting examples are given in which methanol, dimethyl ether and the production of acetic acid with three distinct versions of the energetically self-sustainable module are respectively obtained by means of the process of the invention.
The module receives the biogas coming from the existing washing section, already installed on the currently operating plants for the removal of H2S and other impurities. Biogas is fed to the first section of the module where the reforming operation takes place after compression, then passes into the synthesis section, where the syngas reacts to form methanol according to the reactions (1)-(3) above [Bozzano, Manenti, Efficient methanol synthesis: Perspectives, technologies and optimization strategies, Progress in Energy and Combustion Science, 56, 71-105, 2016. doi: 10.1016/j.pecs.2016.06.001]
The separation section recycles the unreacted syngas upstream of the reforming and removes the water by flash separation and membrane system.
The diagram in
In a configuration optimized for energy recovery, the amount of methanol produced is even higher than the supplied biogas (
In the aforementioned figure optimized for energy recovery, the amount of produced methanol is even higher than the one of the supplied biogas. This is due to the fact that also part of the steam used in the reforming unit is converted into a product.
The synthesis section B) must be completely replaced to allow both the production of methanol as an intermediate product and its subsequent dehydration reaction according to the reaction (9).
CH3OH→CH3OCH3+H2O (8)
in the presence of catalysts already known for this type of reaction.
As an alternative to section B) of the methanol production plant as provided for in Example 1, only the portion of section B) dedicated to dehydration can be added as described in Example 2.
In the plant object of the invention and intended for the production of dimethyl ether, it is also useful to redesign the separation section, since the bio-DME appears less volatile.
With 563 kg/h of biogas fed to the module, 416 kg/h of specific bio-DME are obtained with a total yield of 75%. Considering that bio-DME is part of the new renewable and innovative fuels decree, the economic impact for this production is also significantly higher.
Three different layout modes for the synthesis module are shown. These modes also include additional units compared to what shown in
Also in this case the synthesis section B) must be completely replaced to allow both the production of methanol as an intermediate product and its subsequent reaction (9) to give acetic acid.
As an alternative to the entire replacement of section B) of the methanol production plant as provided for in Example 1, only the portion of section B) dedicated to the formation of acetic acid can be added.
Option I
This alternative is shown in
This solution exploits the combination of burners and SOFCs to have an adequate generation of electrical and thermal energy to meet the requirements of the module, as well as those of the farm where the biogas plant is installed.
Option II
This alternative is shown in
The goal of this configuration is to maximize the recovery of CO2 by means of Sabatier-type hydrogenation systems.
Option III
This alternative is shown in
The biogas from the washing is fed to the reforming section together with a Medium Pressure Steam (MPS) stream. After heat exchange for energy recovery, the bio-syngas is fed to the acetid acid synthesis section.
This option aims at optimizing the thermal efficiency of the process and at the same time minimizing the process units involved. For this specific case, data sheets are reported.
In all the above cases, the burners can be replaced by the currently installed engine. The following table offers a comparison of the various options compared to the current biogas plant.
It is reported for illustrative but non-limiting purposes the economic effects of the process according to the present invention, in particular for the synthesis of acetic acid according to Option III compared with conventional biogas plants used for the production of electricity.
Starting from 20 kg/h of wet biogas with a CH4/CO2 ratio of 1.5, 13.88 kg/h of dry biogas are obtained (equivalent to 11.43 Nm3/h). The equivalent power generated by this amount is equal to 27.7 kW, starting from the assumption that the main generation efficiency is 40%. With the same equivalent energy potential, the process object of the present invention guarantees an income (not incentivized) comparable to what is perceived with the incentivized biogas:
| Number | Date | Country | Kind |
|---|---|---|---|
| 102017000073797 | Jun 2017 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2018/054895 | 7/2/2018 | WO | 00 |
