Patent Application
20190010412
The present invention relates to a method and system for removing tar and in particular relates to a mineral looping method and system for removing tar. The invention has been developed primarily for the removal of tar from a synthesis gas using a mineral based chemical looping process.
The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its advantages to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an express or implied admission that such art is widely known or forms part of common general knowledge in the field.
Biomass, which is primarily composed of cellulose, hemicellulose and lignin, is a promising fuel resource. Biomass is available worldwide and its use is close to carbon neutral due to the biocycle of CO2, in which CO2 released after biomass combustion is re-absorbed via photosynthesis reactions. Biomass, as a potential feedstock for alternative gaseous and liquid fuels, has an important role in replacing fossil fuels on a global scale, with a critical factor to determining its applications its utilisation/conversion efficiency. One of the main applications for biomass utilisation is power generation, and it is expected that the primary global energy demand for biomass-derived electricity will grow strongly from 14% to 26% in 2030.
Gasification is considered one of the most promising bioenergy technologies for several reasons. One reason is that gasification can achieve higher thermal efficiencies, when integrated with combined cycle power plants, than conventional boiler systems. A second reason is that gasification has extremely lower NOx and SOx emissions due to the absence of nitrogen and excess oxygen. Despite such advantages, the technology is not being used at full commercial scale because of several problems, fuel gas (producer gas) cleaning being one of the major factors among them. Fuel gas cleaning is important as the fuel gas contains some impurities such as tar, particles and toxic gases including NH3 and HCl. Among the impurities, tars are the most notorious, which include chemically polyaromatic hydrocarbons (PAHs). Under the gasification temperature, tar exists as gas, while it condenses under ambient conditions (or below its dew point temperature) and deposits in the downstream equipment, blocking narrow pipelines. This tar deposition causes unwanted shutdown and major heat recovery losses. Tar particles also cause blockage and abrasion problems when the producer gas is used in downstream applications, such as engines and turbines. Therefore, for downstream applications of producer gas, the concentration of impurities must be below the maximum acceptable range for each individual application. Consequently, the development of an efficient tar removal process is highly desirable for successful biomass gasifier operation. Attempts to eliminate tar include the development of different types of gasifiers, cold gas filtration, hot gas filtration and catalytic gas cleaning.
Accordingly, a first aspect of the present invention provides a method for removing tar from a synthesis gas, comprising:
feeding the synthesis gas into a first reactor;
feeding mineral particles into the first reactor;
catalysing tar in the synthesis gas with the mineral particles to produce a mixture comprising hydrogen and a mineral carbonate;
feeding the mineral carbonate into a second reactor;
feeding oxygen into the second reactor to react with the mineral carbonate and produce a flue gas comprising carbon dioxide and mineral particles;
separating the carbon dioxide from the mineral particles; and
recycling the mineral particles to the first reactor.
Preferably, the method comprises reforming carbon from the mixture. More preferably, the carbon is reformed in the presence of steam. In one embodiment, the method comprises directing the mixture to a first chamber and feeding steam into the first chamber.
Preferably, the method comprises passing the mineral particles through a gas to reactivate the mineral particles. More preferably, the gas comprises steam. In one embodiment, the method comprises directing the mixture to a second chamber and feeding steam into the second chamber. In some embodiments, the reactivating step is performed before recycling the mineral particles to the first reactor.
Preferably, the method further comprises feeding a portion of the synthesis gas to a combustion unit for generating power to operate the second reactor. More preferably, the method comprises feeding the remaining synthesis gas into the first reactor.
Preferably, the method comprises connecting the first reactor to the second reactor to form a mineral-looping process.
Preferably, the mineral particles are depleted in the first reactor and regenerated in the second reactor. More preferably, the mineral particles are reduced in the first reactor and oxidised in the second reactor. Alternatively or additionally, the mineral particles are carbonated in the first reactor to form a mineral carbonate and the mineral carbonate is decomposed into the mineral particles in the second reactor. In one embodiment, the first reactor is a carbonator and the second reactor is a calciner.
Preferably, the method comprises gasifying a biomass to produce the synthesis gas.
A second aspect of the present invention provides a system for removing tar from a synthesis gas, comprising:
a first reactor for receiving the synthesis gas;
a first conduit for feeding a mineral particles into the first reactor, wherein tar in the synthesis gas is catalysed in the first reactor to produce a mixture comprising hydrogen and a oxygen depleted mineral compound;
a second reactor for receiving the mixture; and
a second conduit for feeding oxygen into the second reactor to regenerate the oxygen depleted mineral compound and produce a flue gas comprising carbon dioxide and the mineral particles;
wherein the mineral particles from the second reactor is recycled to the first reactor.
Preferably, the system comprises a gasifier for gasifying a biomass to produce the synthesis gas.
Preferably, the system further comprises a first chamber for reforming carbon from the mixture. More preferably, the first chamber has an inlet for receiving steam to reform the carbon from the mixture. In one embodiment, the first chamber comprises a steam reformer unit.
Preferably, the system further comprises a second chamber for reactivating the mineral particles. More preferably, the second chamber has an inlet for receiving steam to reactivate the mineral particles. In one embodiment, the second chamber comprises a polisher unit.
Preferably, the system further comprises a third conduit for feeding a portion of the synthesis gas to a combustion unit for generating power to operate the second reactor. More preferably, the system further comprises a fourth conduit for feeding the remaining synthesis gas into the first reactor.
Preferably, the first reactor is connected to the second reactor to form a mineral-looping process.
Preferably, the second conduit feeds air into the second reactor.
Preferably, the first reactor has an outlet for removing the hydrogen the hydrogen from separated from the mineral carbonate in the mixture.
Preferably, the second reactor has an outlet for removing the hydrogen the hydrogen from separated from the mineral carbonate in the mixture.
Preferably, the mixture further comprises carbon monoxide, carbon dioxide and water vapour.
Preferably, the mineral particles comprise a metal or a metal oxide that is suitable for a carbonation and/or oxidation reaction. More preferably, the mineral particles comprise a mineral carbonate. In some embodiments, the mineral particles are selected from the group consisting of: PbO; CaO; MgO; Na; K; ZnO; MnO; CoO; Li2O; Sr; Fe; CuO; Mg olivine (Mg2SiO4); Mg serpentine (Mg3Si2O5(OH)4); wollastonite (CaSiO3); basalt; bauxite; magnetite (Fe3O4); brucite (Mg(OH)2); forsterite (Mg2SiO4); harzburgite (CaMgSi2O6); orthopyroxene (CaMgSi2O6); dunite (Mg2SiO3 with impurities); ilmenite (FeTiO3); dolomite (CaMg(CO3)2) and combinations or mixtures thereof.
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
Furthermore, as used herein and unless otherwise specified, the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings of which:
The present invention will now be described with reference to the following examples which should be considered in all respects as illustrative and non-restrictive. In the Figures, corresponding features within the same embodiment or common to different embodiments have been given the same reference numerals.
Biomass gasification is a process in which carbonaceous fuels are converted into synthesis gas (or the well known term, syngas) via a thermochemical route. The produced syngas should ideally have a high lower heating value (LHV) in order to benefit the downstream energy/power conversion processes. The syngas quality, however, is affected by the use of different gasification agents. For instance, biomass gasification using air as the gasification agent only produces syngas with a low LHV of about 4.4 MJ/m3, while using pure oxygen, a much higher LHV (about 9.6 MJ/m3) can be achieved. Nevertheless, using pure oxygen as the gasification agent requires additional costs associated with an air separation unit (ASU). On the other hand, biomass gasification using steam as the gasification agent has also been considered as a way to improve hydrogen content in syngas.
Steam gasification in a dual fluidised bed gasifier is the most suitable for biomass in comparison to other gasifier types, such as fixed/moving bed and entrained flow, due to its scale and compatibility with many different fuels. Biomass steam gasification is an endothermic process in which a small amount of oxidant (e.g., pure oxygen, air and etc.) is required to combust a fraction of the char produced to provide the energy for the gasification reaction. Without N2 dilution, the volatile matter and char can directly react with steam and generate higher HHV syngas. Dual fluidised bed steam gasification, therefore, is a promising technology to produce higher quality syngas which mainly consists of H2 and CO.
Numerous modelling of biomass steam gasification in a dual fluidised bed for different purposes has been performed. It has been found that, for a 10 MW biomass gasification power plant integrated with a gas turbine, the gasification temperature and the oxygen content of the fuel significantly affected the gasification chemical efficiency and the net power efficiency achieved was 18%. It has also been found that a combined heat and power steam cycle system results in a 10% power efficiency when biomass gasification is combined with a steam turbine. It has further been found that a biomass integrated gasification combined cycle (BIGCC) for heat and power production at ethanol plants can generate process heat and significant amounts of electricity, with a power efficiency of about 24%. Where a corn ethanol plant is used the BIGCC results in a net power efficiency was in the range of 18% to 22%. However, the sensible heat loss during tar trapping, which exists in a real BIGCC process, was not considered as it greatly affects the net power efficiency. Moreover, the sensible heat loss is required to understand the influence of fuel and operating parameters on the performance of a plant in terms of the design and operation of a gasifier.
While the preferred embodiments will be described using biomass as the fuel source for the gasification of synthesis gas, it will be appreciated that the synthesis gas can be produced from the gasification of other fuel sources, such as coal, crude oil or methane. Similarly, the gasification of the biomass is not limited to the application of steam, but can include air or pure oxygen. However, for the reasons stated above, it preferred that steam is used for gasification of the biomass due to its advantages in improving the hydrogen content of the synthesis gas.
The bio-syngas 10 produced then passes through a heat exchanger 12 to preheat the air 13 fed into a reactor. In this embodiment, the reactor is a regenerator 15. In other embodiments, the reactor may be a moving bed reactor, a fluidised bed reactor (bubbling or circulating bed), an oxidiser or a calciner. After the heat exchanger 12, the bio-syngas 10 is divided into two streams using conduits 17, 18. In one conduit 17, a small portion of the produced syngas (bio-syngas) is combusted with preheated hot air 19 to provide the required energy to operate the regenerator 15, while the other conduit 18 transfers the remaining (and greater) portion of the syngas 10 and feeds it into another reactor. In this embodiment, the reactor is a tar cracker unit 20. In other embodiments, the reactor may be a moving bed reactor, a fluidised bed reactor (bubbling or circulating bed), a carbonator or reducer. In the tar cracker unit 20, the LHV of syngas is improved via a series of primary chemical reactions; generally, carbon oxidation or reforming; combustion of synthesis gas; calcination of mineral particles; and oxidation of mineral particles. More specifically, they are reactions (R3), (R5), (R6) and (R7) from Table 1 above. More importantly, bio-tars are decomposed in the tar cracker unit 20 by catalysis using a mineral oxide, which in this embodiment is CaO, resulting in the formation of H2 rich syngas 22, thereby increasing the overall LHV of syngas.
The regenerator 15 and tar cracker unit 20 are connected to form a calcium looping process, where the calcium based particles are transferred between the calciner and carbonator to regenerate the CaO particles for the tar cracking process. More specifically, the consumed CaO is converted into CaCO3 in the tar cracker unit 20 as part of the tar removal process and the CaCO3 is then transferred by the loop 23 to the regenerator 15, where the hot air 1 and the small portion of syngas reacts with the CaCO3 to regenerate CaO that is then recycled back to the tar cracker unit 20.
Some corrosive gases such as H2S and HCl in syngas will be adsorbed by the CaO in the tar cracker unit 20, which can greatly decrease the workload of later gas cleaning operations. An additional advantage over conventional BIGCC technology is that CO2 in the flue gas 25 generated by the regenerator 15 can be greatly concentrated by the MLTR process 2. The removal of H2S, HCl and the gas cleaning operations are not shown for the sake of clarity and because there are only trace amounts of corrosive gases produced. The hot H2 rich syngas 22 after the tar cracker unit 20 is compressed and subsequently fed into a combined cycle CC, which in this embodiment comprises a gas turbine 28 to generate power. Exhaust gases 29 from the gas turbine 28 are released into the ambient environment. Alternatively, the combined cycle CC may also comprise a steam-driven turbine so that steam can be generated from the hot flue gas 25 eluted from the regenerator 15 can be used to generate power. In this alternative, the steam is fed directly into the steam turbine by mixing it with the hot exhaust gas 29 from the gas turbine 28.
Thus, the method 1 enables the syngas 10 to be “cleaned” by the MLTR process 2 by reducing or removing the tar present in the syngas prior to its subsequent downstream use, such as the combined cycle CC. In comparison with conventional BIGCC processes, the method 1 has the following advantages:
Another embodiment of the invention is illustrated in
The characteristics of the biomass 5 used in the embodiment is summarised in Table 2 below.
As shown in
In this embodiment, 15 wt. % of the carbon content (char) in biomass leaves the gasification zone 33 via separator 37. In the combustion zone 35, the embodiment handles the mass and energy balance for complete combustion assuming an air to fuel ratio of 1.12:1. The flue gas 25 produced in the combustion zone 35 is used to preheat the water into steam for gasification using a heat exchanger 38 and is subsequently fed into the combined cycle system 3 in the form of a steam turbine. Also, energy released during combustion of char will be used to preheat the sand. A conduit 39 directs the sand and char into the combustion zone 35 while conduit 40 returns hot sand back to the gasification zone 33.
In other embodiments, the FICB reactor 30 is replaced by two separate reactors embodying the reaction zones 33, 35. That is, in one reactor the biomass 5 is subject to gasification while combustion occurs in the other reactor. Gasification is generally endothermic reaction and requires additional energy input. In standard bubbling bed or entrained flow reactors this energy input is provided by partial combustion by providing air or oxygen into the reactor. However, such air dilution may reduce the energy density of the synthesis gas and using pure oxygen may be extremely expensive. Therefore, for these reasons it is preferred to use a dual circulating fluidised bed where gasification and combustion reactions are separated.
The initial operating conditions for the MLTR process for the embodiments of
aThe gas agent to fuel ratio was considered according to the design of a 10 MW thermal power station in Austria.
The MLTR process was modelled using the following assumptions:
In the embodiment, the effects of various parameters including the compression ratio of the gas turbine, air/fuel ratio entering the gas turbine, mass ratios of CaO to biomass (Ca/B), steam to biomass (S/B), and temperatures of the carbonator and calciner (T) on the thermodynamic performance of the CL-BIGCC process were assessed. The ratios Ca/B and S/B were defined as follows:
where
In addition, the compression ratio (Rp) is defined as:
where
The gross power efficiency (η) and net power efficiency (φ) of the whole process was calculated by Equations (4) and (5), as set out below. In some instances it is more important to calculate the unit power production per kg of biomass, and this quantity can be calculated by Equation (6), as set out below.
where
A series of preliminary biomass gasification (i.e. partial oxidation in 1% O2) experiments with and without CaO were completed to demonstrate the tar cracking ability of the carbonator in the MLTR process. A thermo-gravimetric analyser coupled with a Fourier Transform Infrared Spectrometer (TGA-FTIR) was used to allow for online mass loss and gas evolution characterisation.
Due to its abundant availability in Australia, radiata pine (75-150 μm particle size) was the biomass sample used in all experiments, with its proximate analysis presented in Table 4. Omya limestone was the source of CaO of which the XRF analysis is presented in Table 5.
TGA conditions for all experiments consisted of 5 mg biomass sample, 100 mL/min flow rate of 1% O2 in nitrogen, heating rate of 10° C./min and final gasification temperature of 800° C. FTIR scans were taken at 10° C. intervals and operating conditions consisted of a gas cell length of 10 cm and temperature of 240° C., transfer line temperature of 240° C., 32 scans per spectra for a scan range of 500-4000 cm−1 and resolution of 4 cm−1. Experimental scenarios examined were biomass gasification in 1% O2, and a 1:1 mass ratio of CaO to biomass gasification in 1% O2.
Similar to our previous results,
With the above analysis in mind, the optimum S/B ratio should also consideration of the minimum required steam flow for fluidising the bed in the gasifier 30. When using steam as the gas agent, a good S/B ratio for both fluidisation and biomass gasification is 0.17. An S/B ratio of below 0.17, despite greater power production, may lead to poor fluidisation in addition to an elevated gas turbine inlet temperature which could damage the gas turbine blades (the gas turbine inlet temperature at an S/B mass ratio of 0.17 reaches 1322° C. as shown in
The previous parametric analyses have identified the most suitable operating conditions of the BIGCC/MLTR process, including the compression ratio, air/fuel mass ratio, Ca/B mass ratio, S/B mass ratio, carbonator and calciner temperatures. With these operating conditions, the performance of the CL-BIGCC plant was obtained and the results are summarized in Table 6 and Table 7. Table 6 compares the syngas flows before and after the carbonator. As Table 6 shows, the mass flow rates of the syngas before and after the carbonator are 7633 and 2757 kg/hr, respectively (i.e. a reduction of 64%), while the LHV of the syngas was found to increase by 2.7 times from 34.43 MJ/kg to 92.21 MJ/kg. This indicates that the integrated calcium looping process functions well in a BIGCC process and significantly improved the syngas quality. The H2 concentration was found to increase from 64 vol % to 94 vol % on a dry basis. The higher concentration of H2 in the syngas is believed to contribute to a more efficient power generation process as evidenced in the parametric analyses. Moreover, it enables the CL-BIGCC process to employ a compact gas turbine design which has a much smaller size and thus a much lower cost compared to the conventional process.
Table 7 below lists the calculated overall plant performance of the BIGCC/MLTR process and shows that the net power generation efficiency can reach 25%. With such efficiency, a BIGCC plant with a net power production of 47.5 MW would require a biomass consumption rate of 45,455 kg/hr, a steam flow of 7,727 kg/hr, and a CaO inventory of 22,727 kg/hr. The oxygen content in the flue gas of the gas turbine is 10%. Table 8 also compares the efficiency of the invention with other similar technology platforms using biomass gasification. It can be seen in Table 8 that the power generation efficiency of the BIGCC plant at 25% is among the highest of the parallel biomass steam gasification power generation processes.
a Considering combined heat and power application.
b All figures in this table are based on the LHV of the fuel.
The tar cracking capabilities of CaO were also assessed using preliminary gasification (i.e. 1% O2) experiments were conducted via a coupled TGA-FTIR apparatus. The FTIR volatile evolution profile for a CaO:B ratio of 1 is presented in
To gain a qualitative understanding of the tar cracking ability of CaO, the area under the curve of the carbonyl, phenol and aromatic peaks were taken when each peak reached its maximum at 350° C. The area under the CO2 peak at 350° C. was also taken for comparison between treatments. The area under the curve for each of the aforementioned peaks is presented in
From this discussion, it can be observed that the MLTR process can avoid separation of ash from CaO particles and improve the LHV of syngas through chemical reactions in the presence of CaO and clean the syngas by simultaneous removing H2S and HCl and inherently reduce the workload of the downstream gas cleaning unit. Moreover, it can produce syngas with a higher energy density. The MLTR process overcomes the problems of improving ash separation in a BIGCC process by separating the gasification and calcium looping operations allowing the CaO to be recycled and sensible heat losses to be minimised at certain temperatures under which tar can be thermodynamically cracked. The most favourable values of compression ratio, air/fuel mass ratio, Ca/B, S/B, temperatures of carbonator and calciner are 5.1, 15, 0.53, 0.17, 650° C. and 800° C., respectively. With the above inputs, the net power generation efficiency of BIGCC/MLTR process was found to reach 25%, which is higher than those of other parallel processes. In addition, TGA-FTIR experiments also confirmed that bio-tars formed during biomass gasification can be effectively cracked in the presence of CaO at higher temperatures.
The inventors also contemplate that the MLTR process lends itself to other gasification processes and is not limited to a biomass gasification process that includes a combined cycle. For example, the inventors believe that the MLTR process can be used with a biomass gasification process that has only a small-scale gas engine (an internal combustion engine) instead of a gas turbine combined cycle. In another example, the MLTR process may be applied to coal gasification plants.
It will be appreciated that while the above embodiments have described the invention in terms of using calcium based particles in a calcium looping process, the invention is not limited to this particular mineral. Rather, the mineral particles that can be used in the MLTR process include a metal or a metal oxide that is suitable for a carbonation and/or oxidation reaction, and may include a mineral carbonate. These general reactions are shown in
There will be a slight variation in the reactions in the reactors, depending on the mineral oxide or metal oxide that is used. Examples of carbonator reactions include the following:
CxHy→xC+y/2H2 (7)
CxHy+MOn→MOn-1+xCO+y/2H2 (8)
CxHy+MOn→MOn-1+xCO+y/2H2 (9)
MO+CO2→MCO3 (10)
CxHy+H2O→xCO+y/2H2 (11)
Examples of calciner reactions include the following:
MCO3→MO+CO2 (12)
2MOn-1+O2→2MOn (13)
The mineral particles used as catalytic materials include both synthetic and natural minerals. In particular, dolomite, ilmenite and olivine are found to be more suitable due to their lower cost and superior performance.
As shown in
In the further embodiment of the invention illustrated in
A further embodiment is illustrated in
The raw fuel gas (syngas) 10 primarily enters the tar cracker unit 55, which preferably operates at temperatures in the range of 450° C. to 800° C. and at pressures of 1 to 100 bar. The tar cracker 55 performs catalytic cracking of the tar in the presence of the mineral/metal oxide particles or mixtures thereof. If a controlled amount of steam 77 is injected into the tar cracker unit 55, reforming reactions will also occur in the tar cracker unit 55. During tar cracking, several side reactions such as mineral carbonation (i.e. where the mineral oxide is lime or dolomite) and reduction (i.e. where the metal oxide is ilmenite or olivine) may occur based on the chemical-equilibrium conditions pertinent to the operating temperature of the tar cracker unit 55. Also, soot/carbon formation occurs on the surface of the minerals while any sulphur and chlorine present in the raw synthesis gas 10 is captured. The reactions that may occur in the tar cracker unit 55 are as follows:
Catalytic Tar Cracking: aCnHx→bCmHy+dH2
Catalytic Steam Reforming: CnHx+nH2O→(n+x/2)H2+nCO
Catalytic Dry Reforming: CnHx+nCO2→(x/2)H2+2nCO
Soot formation or carryover: CnHx→nC+(x/2)H2
Carbonation: MO+CO2→MCO3
MeO+CO2→MeCO3
Reduction: MeO→Me+½O2
Sulfation: MO+H2S→MS+H2O
MeO+H2S→MeS+H2O
Chlorination: MO+2HCl→MCl2+H2O
MeO+2HCl→MeCl2+H2O
In the above reactions, CnHx represents tar, CmHy represents hydrocarbons with smaller carbon number than CnHx, M represents minerals and Me represents metal.
These side reactions (especially carbon formation on the surface of the mineral/metal mixture) may reduce the performance of the tar cracker unit 55 and therefore the mineral/metal mixture is continuously transported to the steam carbon (steam-C) reformer 60 where, in the presence of steam, carbon is converted to produce additional mole of H2. The reaction that occurs in the steam-C reformer 60 is as follows:
Steam reforming of carbon: C+H2O→CO+H2
The operating temperature of the steam-C reformer 60 is in the range of 450° C. to 800° C. and the operating pressure of the steam-C reformer 60 is in the range of 1-100 bar.
The gaseous stream 80 produced in the steam-C reformer 60 is mixed with the clean fuel gas stream 22 generated from the tar cracker unit 55 and diverted to the combined cycle power plant 82 to generate heat and power. It will be appreciated that the combined cycle power plant 82 can be readily replaced with a gas engine, boiler-steam turbine or gas turbines to generate power.
After ensuring that carbon has been gasified to produce an additional mole of hydrogen (the gasification having occurred in the steam-C reformer 60 due to the presence of steam), the mineral/metal mixture is sent to a regenerator 70, where in the presence of hot air 19 and a portion of the raw fuel gas 10 diverted by conduit 17, the mineral/metal carbonates are decomposed to mineral/metal oxides. Also, reduced metal oxides are expected to be oxidised to their higher oxidation state. The operating temperature for regenerator 70 is between 750° C. and 1000° C. and the operating pressure is between 1 and 100 bar. The following reactions occur in the regenerator 70:
Calcination: MCO3→MO+CO2
MeCO3→MeO+CO2
Reduction: Me+½O2→MeO
In the embodiment of
Decomposition of sulphur and chlorine may be optional as this would require the flue gas cleaning step to be performed at the back end of the regenerator 70 before performing the heat recovery operation and/or exhausting the gases. Based on the fuel type and amount of sulphur and chlorine present in the original fuel, the extent of sulphur and chlorine decomposition can be controlled. For decomposition reactions in the regenerator 70, oxygen from air or steam can be used, although in this embodiment preheated hot air 19 is used. The decomposition reaction in the regenerator 70 is as follows:
De-sulfation: MS+O2→M+SO2
MeS+O2→Me+SO2
MS+H2O→M+H2S
MeS+H2O→Me+H2S
De-chlorination: MCl2+H2O→MO+2HCl
MeCl2+H2O→MeO+2HCl
Fresh mineral/metal mixture 90 can be added to the regenerator 70 to replenish spent mineral/metal mixture that has become saturated with sulphur and/or chlorine. The spent mineral mixture 95 (generally in the form of metal/mineral chlorides or metal/mineral sulphides) is purged off after several cycles from the system. The purging or makeup can be done from any location of the MLTR loop 23.
Finally, before sending the regenerated mineral/metal mixture back to the tar cracker 55, it passes through the polisher unit 75 where in the presence of steam, the pores of mineral/metal mixtures are reactivated with hydration reactions. The mineral/metal mixtures are deactivated due to the strong carbon/carbonate layer formation on the surface of mineral/metal mixture particles. This layer if not treated stays permanently and thus deactivates the pores which usually allow gases to diffuse through and enable the reactions to occur. As a pore activation process, the aim in the polisher unit 75 is to cause physical and chemical reactions between the deposits (carbon/carbonate) and water (in the steam) to liberate the carbon via reforming and consequently forming hydrates. The operating temperature of the polisher unit 75 is in the range of 750° C. to 1000° C. and the operating pressure of the polisher unit 75 is in the range of 1-100 bar. The polisher unit 75 ensures the longer term recyclability of the mineral/metal mixtures since it addresses the issues of catalyst deactivation due to carbon build up and poisonous gas adsorption on the catalyst surface, difficulty in regeneration, partial oxidation of fuel gas and carryover of fines that may occur in the use of mineral particles in catalytic removal of tar in the synthesis gas.
Experimental work has been performed on the embodiment of
In some embodiments, the tar cracker unit 55 comprises the tar cracker unit 20 shown in
It will be appreciated that the above described embodiments of the invention, primary products from the tar cracker unit 20, 55 are hydrogen, carbon monoxide, carbon dioxide and water vapour and a mineral carbonate.
In some embodiments, the synthesis gas is produced from sources other than biomass, such as coal, crude oil or methane. In other embodiments, the biomass is selected from the group consisting of but is not limited to Paulownia, Beema Bamboo, Melia Dubia, Casuarina, Eucalyptus, Leucaena and Prosopis.
The advantages of the MLTR process are as follows:
It will further be appreciated that any of the features in the preferred embodiments of the invention can be combined together and are not necessarily applied in isolation from each other. For example, the steam-C reformer 60 and/or polisher unit 75 may be used in the embodiments of
By providing mineral particles to catalyse tar from a synthesis gas and regenerating those mineral particles, the invention improves tar removal efficiency, reduces material consumption of the mineral particles and complexity in tar removal processes, increases the energy density of the synthesis gas and avoids ash separation. All these advantages of the invention result in improved efficiency in the gasification process, especially biomass gasification. Furthermore, the invention can be readily implemented to existing gasification systems, especially biomass gasification systems. In all these respects, the invention represents a practical and commercially significant improvement over the prior art.
Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015905382 | Dec 2015 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2016/051286 | 12/23/2016 | WO | 00 |
