ITRM20000157A1 - PROCEDURE FOR BIOMASS GASIFICATION. - Google Patents

Description

DESCRIPTION

accompanying a patent application for an invention entitled: "Process for the gasification of biomass"

The present invention relates to a process for the gasification of biomass which uses perovskite catalysts, producing gas rich in hydrogen and drastically reducing the content of condensable organic compounds (pitch or "tar").

The combustion of biomass (a term that includes wood, waste and / or agricultural products, municipal solid waste, cellulosic waste) has been used to generate heat and light since ancient times. Biomass was the predominant energy resource until fossil fuels took hold in industry during the industrial revolution.

Today the need to use renewable energy sources is increasingly felt, and biomass can be concretely used in small and medium capacity plants located close to the areas where they are produced, in order to minimize transport and storage costs, in order to to obtain liquid or gaseous fuels and chemicals (1). In the agricultural sector, both agro-industrial residues and crops specifically grown on agricultural land intended for the purpose can be sources of biomass at acceptable costs, and with characteristics compatible with the technical requirements of processing plants, as regards the level of humidity, sulfur content, particle size, etc (2).

Among the various thermochemical conversion processes of biomass, gasification is an economically competitive process for obtaining a clean, rich gas; in hydrogen, suitable for the production of energy or chemical compounds (3,4).

There are four conventional types of biomass gasifiers: fixed bed gasifier with bottom gas flow, fixed bed gasifier with top gas flow, moving bed gasifier and fluidized bed gasifier.

In particular, gasification in fluidized bed reactors in the presence of water vapor is able to maximize gaseous products compared to liquid ones (high molecular weight organic compounds, pitch or "tari <1>) and solid (coal or" char "), because it allows a high heating rate of the biomass particles used and an optimal residence time. Furthermore, this process is flexible enough to allow the treatment of solid materials, with variable chemical - physical characteristics.

The possibility of carrying out the process industrially depends essentially on gas purification technologies, aimed at separating the dust and transforming the organic compounds present in permanent gases, in order to make the gas usable in gas turbines (without condensation problems of the pitch in the plant), in fuel cells or for the production of chemical compounds such as methanol. Furthermore, the catalytic transformation of the "tar" increases the production of gas and therefore the efficiency of the thermochemical conversion process of biomass (5). The pitch can also be eliminated by washing the gas with water, but this solution is not attractive in terms of the thermal efficiency of the process and also creates environmental problems for the disposal of water containing organic compounds.

For the reasons just mentioned, biomass gasification processes increasingly make use of catalysts capable of transforming pitch into permanent gas, at acceptable temperatures (6). If it is desired to produce hydrogen or synthesis gas, then methane must also be catalytically reformed (7).

Numerous studies have been published regarding the catalytic treatment of gases obtained from the gasification of biomass (8-13). The main problems can be identified in the choice of the best configuration of the process and in the type of catalyst to be used.

In traditional gasification processes, air is used to burn a part of the biomass, in order to supply the heat necessary for the endothermic gasification reactions (15); this involves a dilution of the gas produced with nitrogen and the consequent reduction of its calorific value. To solve this problem, process configurations have recently been proposed which tend to separate the combustion and gasification phases by means of two parallel fluidized zones (16-20), one fed with air and the other with water vapor; the heat transfer from the combustion section to the gasification section is achieved by circulating the solid particles of the bed between the two sections, minimizing the contamination between the gaseous product of the gasification and the effluent from the combustion zone. In the latter, the carbon residue obtained from the biomass gasification process, possibly together with other fuels, is burned with air at a slightly higher temperature (about 50 ° C) than the temperature of the gasification zone where the biomass is introduced . By avoiding contamination between the gaseous phases present in the two zones, it is possible to obtain a fuel with a very low nitrogen content.

The artificial or natural catalysts (such as dolomite, olivine), which catalyze the cracking and reforming of pitches, intervene in the gasification of biomass through three possible process configurations (21):

- the catalytic substance is fed into the gasifier continuously, together with the biomass, jointly or separately;

- the catalyst forms an integral part of the fluidized bed of the gasifier (single-stage process, 22);

- a secondary catalytic reactor is used, with a fixed or fluidized bed, located downstream of the gasifier (two-stage process, 23).

These schemes are not alternative to each other, but they can coexist in the same process; sometimes, for example, dolomite is used in the gasifier to reduce the content of pitches, which are then subsequently eliminated by passing the gas through a catalytic reactor. When the catalysts are used directly inside the gasifier the. particles must be resistant to mechanical stress, friction and, in the case of circulating beds, must remain active even after several oxidation and reduction cycles. It is therefore difficult to define an optimal configuration for the catalytic gasification process, because it depends on the type of material used and on the specifications of the gaseous product that must meet the requirements of the users.

As regards the choice of catalysts, those based on nickel are the most suitable for both technical and economic reasons (24). However, many commercial catalysts that contain nickel deactivate more or less quickly, both due to the phenomenon of sintering and due to the deposition of carbon on their surface (25). Nickel deposited on alumina reacts with the support at high temperatures with a consequent decrease in its catalytic activity. Sintering favors the production of coke. The result of all this is that with the supported catalysts available on the market under the reaction conditions there is a rapid deactivation.

In order to increase the degree of dispersion of the active species in the catalyst particles, the insertion of the metal atoms into defined structures of the pyrochloric (A2B2O7), spinel (AB2O4) or perovskite (ABO3) type is known, in which A and B are compatible elements (2628). In this way the specific surface is lower than in the case of supported catalysts, but the strong interaction between the atoms reduces the phenomenon of sintering and carbon formation. The above catalytic systems have proved to be valid for the "reforming" of methane (29, 30), however they do not completely eliminate the phenomenon of sintering and carbon formation (31).

Catalytic systems are known in which a third element capable of stabilizing the system is inserted into the defined structures. US patent 5,447,705 describes a perovskitic catalyst containing this third element, having general formula Ι_η * Αι-yByC> 3, in which x is a number between 0 and 10, y is a number between 0 and 1, Ln is at least one rare earth, strontium or bismuth, A is a metal belonging to groups IVb, Vb, Vib, Vllb, or VIII of the periodic table of elements, B is a metal of groups IVb, Vb, Vib, Vllb, or VIII of the periodic table of elements, and A and B are two different metals. The catalysts of US patent 5,447,705 are used for the partial oxidation of methane or of gaseous mixtures containing methane. No application of these catalysts for the gasification of solid materials in general and biomass is described. The authors of the present invention have developed a process for the gasification of biomass which produces gases with a high hydrogen content and drastically reduces the content of condensable organic compounds (pitch). The process of the invention makes use of the perovskite catalysts described in US patent 5,447,705. The gases obtained with the process of the invention can be advantageously used for the supply of fuel cells or for the production of "Chemicals" or for any use that exploits the high content of hydrogen and the drastic reduction of the content of organic compounds. condensable (pitch). The authors have developed two types of process, single-stage and double-stage, as well as have found the absolute optimal quantities of the catalyst components. The object of the present invention is therefore a process for the gasification of biomass comprising a catalytic conversion step with a perovskitic catalyst having the formula: LnyAi-xBx03, in which y is a number between 0 and 10, x is a number between 0 e1, Ln is a rare earth, strontium or bismuth, A is a metal belonging to groups IVb, Vb, Vib, Vllb, or Vili of the periodic table of elements, B is a metal of groups IVb, Vb, Vib, Vllb, or VIII of the periodic table of elements, and A is different from B. Preferably it has formula LaNixFe1-xO3, with x defined as above. Even more preferably x is between 0.1 and 0.9, including the extremes. More and more preferably x is equal to 0.3. In a preferred aspect of the invention the catalyst is used after being reduced. ; A preferred object of the invention is a process for the gasification of biomasses which comprises the steps of:; a) thermo-chemical treatment of the biomasses in a gasifier at a temperature between 700 ° C and 900 ° C and in the presence of steam of water; ; b) catalytic conversion with the catalyst of the invention. ; Preferably, the process also includes the step of: c) condensation of the water vapor and pitch present in the gas thus obtained. In a preferred aspect, the thermo-chemical treatment step and the catalytic conversion step occur simultaneously in a single reactor. In a preferred aspect the reactor is a fluidized bed reactor. In a preferred aspect, the perovskite catalyst used in the process of the invention is supported on olivine. In a preferred aspect, the biomass thermo-chemical treatment step takes place in a first reactor and the catalytic conversion step takes place in a secondary reactor, located downstream of the first reactor. Preferably the first reactor is a fluidized bed and the secondary reactor is a fixed bed reactor; even more preferably the first reactor comprises olivine. ; A further object of the invention is a gas obtainable with the process according to the invention itself. ; Another object of the invention is the use of the gas according to the invention for the production of fine chemical products. ; Another object of the invention is the use of gas according to the invention for the power supply of electricity generators, in particular fuel cells. ; Another object of the invention is the use of the gas according to the invention as an energy vector. ; The present invention will now be described with reference to experimental tests to be considered exemplary and not limiting, together with the attached figures, in which:; figure 1 shows the elementary distribution of LaNio.3Feo.7O3 measured with EDX, using a localized ray of (1) 200 nm and (2-8) 14 nm; ; Figure 2 shows the Programmed Temperature Reduction Curves (TPR) for LaNio.3Feo.7O3 on calcined olivine and natural olivine; ; Figure 3 shows the results of the reactivity tests of the reforming of CH4 with LaNio.3Feo.7O3 as a function of temperature: (♦) conversion of CH4, (A) carbon deposited on the catalyst, (A) selectivity into CO, (* ) selectivity in CO2;

Figure 4 shows the diagram of the gasification plant: 1-fluidized bed gasifier; 2-gas distributor; 3-air box; 4-oven; 5-biomass dispenser; 6-cyclone; 7-ceramic candle filter; 8-thermostat; 9-volumetric counter; 10-aspirator; 11 - water pressure gauge; 12-flame arrester; 13-detector of traces of water; 14-heat conductivity detector for H2 analysis; 15-infrared detector for the analysis of CO, CO2 and CH4; 16-computer; 17-temperature gauges; 18-metering pump; 19-fixed bed catalytic reactor; 20-boiler for the production of steam;

Figure 5a shows the content of “tar” in the gas with the configuration of the two-stage process pre-reduced fresh catalyst, B1 (O); catalyst used without pre-reduction, B2 (□), B3 (Δ), B4 (O), B5 (O);

figure 5b shows the "tar" content in the gas with the configuration of the two-stage process: pre-reduced fresh catalyst B1 (O); pre-reduced catalyst used: B6 (□), B7 (Δ), B8 (O);

Figure 6 shows the composition of the dry gas as a function of the gasification time for the E3 test;

Figure 7 shows the "tar" content in the gas with the configuration of the single step process: D1 (·), D2 (■), D3 (A), E1 (O), E2 (□), E3 (Δ) , E4 (O);

Figure 8 shows the "tar" content in the gas with the configuration of the single-stage process, as a function of the overall gasification time: E1 (0), E2 (□), E3 (Δ), E4 (O);

figure 9 shows the morphology of LaNio.3Feo.703 after the reactivity tests.

Figure 10a 10b illustrates the reactivity of different catalysts with x between 0.1 and 1.

Example 1 Preparation of La-Ni-Fe Catalysts

The catalyst having the formula LaNixFe (1-X) 03 with x varying between 0 and 1 was prepared by the sol-gel technique described elsewhere (33).

Briefly, the nitrates La (N03) 36H20, Ni (N03) 26H20 and Fe (N03) 39H20 were dissolved separately in propionic acid, in quantities such as to obtain the desired value of x. The solutions containing nickel and iron were mixed together and added to the solution containing lanthanum. The resulting solution was stirred for 30 minutes, evaporated to the consistency of a resin, which was then calcined in air at a temperature of 750 ° C (heating rate 3 <°> C / minute) for 4 hours.

Example 2 Preparation of Perovskite La-Ni-Fe supported on olivine

In order to be able to carry out biomass gasification tests with the catalyst placed in the fluidized bed of the gasifier (single-stage procedure), the trimetallic perovskite, whose aggregates are characterized by low hardness, must be impregnated on a suitable support. For this purpose olivine particles have been used, due to their high mechanical resistance to abrasion and the consolidated experience regarding their use in fluidized bed gasifiers. Olivine consists mainly of magnesium and iron silicates (MgO 48-50%, SiO2 39-42%, Fe203 8-10%), and has a good activity in the reforming of pitches (20). It is mined in Austria and northern Italy.

Both natural and calcined olivine were used to prepare different samples of supported peroskite. The nitrates of La, Ni and Fe were dissolved and mixed together as described above. After stirring the propionic solution for 30 minutes, olivine particles, having a diameter between 250 µm and 600 µm, were added to the solution. The excess solvent (propionic acid) was evaporated under vacuum (0.1 atm) at 60 ° C. The impregnated support was dried at 120 ° C and then calcined in air (4 hours at 800 ° C). X-rays confirmed the expected structure. Perovskite represents 4.5% of the total weight of the particles, as resulted from the elemental analysis.

Example 3 Characterization of the Catalysts

The catalysts were characterized by X-rays using a Siemens D-500 diffractometer having CuKa radiation, and by electron microscopy using a TOPCON EM 002-B apparatus connected with an X-ray spectrometer to determine the homogeneity of the sample.

The total quantities of La, Ni, Fe and the quantity of carbon deposited on the catalyst during the reactivity tests were evaluated by elemental analysis in the Vernaison center (CNRS), with the result of obtaining an error of less than 1% between the stoichiometric ratios and experimentally measured values.

All the results reported refer to the catalyst having a value x = 0.3 in the general formula of the perovskite structure: previous tests (28) have shown that for a Ni / Fe ratio corresponding to this value of x the best performances are obtained. in methane reforming reactions.

The BET surface area is 5.8 m <2> (g cat) <-1>. the X-ray diagram was compared with the corresponding LaFe03 and LaNi03 diagrams. The diagram shows that only a perovskite phase is obtained. There is also a regular and progressive drift of the structure of the peaks between those of LaFe03 and those of LaNiO3 '(20 = 32.4, while for LaFeO3 the value is 32.2 and for LaNiO3 32.8). The cell parameter is equal to 3.90 A, while that of LaFeO3 and LaNi03 are worth 3.92 A and 3.84 A, respectively. The values refer to a pseudo-cubic structure which confirms the formation of a solid solution (37).

X-ray spectroscopy together with transmission electron microscopy was performed on different areas of the samples using both a 200 nm incident beam and a more localized 14 nm beam, in order to obtain information on the homogeneity of the sample (38). The results are reported in Figure 1. Both analyzes confirm the presence of a solid solution of La-Ni-Fe.

Example 4 Reduction of the catalyst to programmed temperature

The results of the programmed temperature reduction (TRP) of the LaNio.3Fe0.7O3 catalyst indicate the presence of two peaks, the first at a temperature of 370 ° C probably due to the loss of oxygen from the structure, which corresponds to the transition from Ni '' to Ni "(29). The second peak, at a temperature of 870 ° C, is due to the formation of nickel and metallic iron which form a Ni-Fe alloy (~ 60% Ni) deposited on LaFeO3 and La2O3 as demonstrated by Mòssbauer's experiments (39).

As for the perovskite supported on olivine, Figure 2 shows a comparison between the TPR curves of LaNio.3Feo.7O3 impregnated on uncalcined olivine and of the same olivine, respectively. Three peaks can be distinguished for the La-Ni-Fe / olivine system, at 340 ° C, 550 ° C and 880 ° C. The one at intermediate temperature (550 ° C) clearly corresponds to the olivine reduction peak (630 ° C), and the difference in temperature between the two can be traced back to the effect of nickel on the olivine structure. The just pronounced peak at 340 ° C can be related to the reduction of nickel oxide to form metal nickel particles, or to a loss of oxygen by the structure. The peak at 880 ° C is also similar to that encountered previously in the reduction of the solid solution LaNio.3Fe0.7O3 and leads to the formation of metallic nickel; it is also possible to generate iron atoms from the olivine structure, which lead to the formation of the Ni-Fe alloy. Example 5 Standard functionality tests of the catalysts: methane "steam reforming" reaction in a fixed bed catalytic reactor The activity of the catalyst in the methane reforming reaction was evaluated using a quartz reactor with a diameter of 6.6 mm . The operating conditions of the tests were: temperature at which the gases pass through the catalyst equal to 800 ° C, volumetric flow rates of the gases equal to: Ar 15 Lh <-1> (g cat) <-1>, CH4 1.5 Lh <-1> (g cat) <-1>, H2O 4,5 Lh <-1> (g cat) <-1>, (CFU / H203), H2 3 Lh <'1> (g cat) <- 1>. The water is sent by means of a micropump to a container kept at 120 ° C where it vaporizes, and is subsequently mixed with the other gases. The gas leaving the reactor was analyzed by means of two gas chromatographs: the first to determine CH4, CO and CO2, and the second for CO and H2.

Before carrying out the reactivity tests, the catalyst was pre-reduced with a stream of nitrogen and hydrogen at a flow rate of 2 L h <-1> (g cat) <- 1>, with heating of the sample from room temperature to temperature of 600 ° C at a heating rate of 10 ° C / minute. Subsequently, in the presence of the reaction stream, the catalyst was further heated to a temperature of 800 ° C at a rate of 3 ° C / minute. The results of the tests refer to a temperature of 800 ° C and to stable reaction conditions that exceed 150 hours. The conversion of CH4, the selectivity of CO and the production of CO were calculated as follows:

The reactivity of the catalyst was carried out in the presence of hydrogen and in excess of water vapor (H20 / CH4 = 3; H2 / CH4 = 2) in order to simulate a gas having the characteristics of the gas produced by the biomass gasifier, such as described later. After the reduction of the catalyst with H2 at 600 ° C, the CH4 conversion is about 90% and the CO selectivity of 80% (20% of C02). Under these conditions, the formation of carbon on the surface of the catalyst is negligible (<0.3% by weight after 48 hours of reaction) and the H2 / CO ratio is equal to 3.5. Figure 3 illustrates the conversion of methane, the selectivity of CO and CO2, and the formation of coal as a function of temperature. The H2 / CO ratio progressively decreases from the value 8 at the temperature of 600 ° C to the value 3.5 at 800 ° C.

Table 1 shows the values obtained previously and those obtained in the absence of hydrogen in the reaction gas, with a water vapor / methane ratio varying between 1 and 3.

Table 1

It is evident that the results obtained with the H2O / CH4 ratio = 3 and with the presence of hydrogen in the feed are very similar to those obtained with the H2O / CH4 ratio = 1. This indicates that the addition of hydrogen partially limits the oxidizing character due to the excess of water vapor, without causing carbon formation (Fig. 3).

These tests therefore show that the characteristics of the gas leaving the gasifier are such as to favor the reforming of the methane on the trimetallic perovskite and furthermore the formation of carbon on the active surface of the catalyst is completely negligible. The characterization after the reactivity tests, discussed below, confirms these results.

Example 6 Structure of the gasifier and composition of the biomasses The process configurations examined are, respectively, a two-stage system in which the catalytic reactor is located downstream of the gasifier, and a single-stage gasification and catalytic conversion system in which the catalyst it is placed directly in the fluidized bed of the gasifier.

The laboratory plant used is illustrated in Fig. 4 and essentially consists of the following units: a boiling fluid bed gasifier 1, a continuous biomass feeding system 15, a steam generator 20, a gas purification section comprising a cyclone 6 and a ceramic candle filter 7, a cooling and separation system for condensed substances 21, a volumetric meter for the product gas 9, temperature detectors 17 and pressure 11, 22, and a continuous analysis section of the gases 23. In the configuration of the two-stage process, the secondary catalytic reactor is inserted after the ceramic candle filter 7. Both configurations are schematically represented in Figure 4.

Gasifier 1 consists of a cylinder in special steel with an internal diameter of 60 mm and a height of 800 mm. The particle bed has a height of 150-200 mm and rests on a porous ceramic disc 2 which also acts as a distributor of the fluidization gas. The latter can be water vapor, air or nitrogen according to the requirements of the process; its flow rate is measured and controlled by means of rotameters and needle valves in the case of permanent gases, while the flow of steam is controlled by a metering pump 18 which feeds the water to the steam generator. The pressure inside the reactor is maintained at a value just above the ambient pressure by using a volumetric aspirator 10 placed after the condensation section. The gasification temperature is measured by type K thermocouples connected to an automatic controller 17 which manages the electrical power of the oven 4 consisting of three independent heating zones: in this way the temperature varies by a maximum of 10 ° C with respect to the average value during gasification tests.

The biomass is continuously introduced into the fluidized bed through the use of an externally cooled probe with air. This injection method, despite the practical problems of operation that it entails especially at the low flow rates required by a laboratory equipment, was chosen because, as we have seen in previous works (34), the introduction of biomass beyond above the surface of the fluidized bed has a considerable influence on the gasification products obtained (increase in the pitch fraction) and therefore on the efficiency of any catalytic transformation. For this reason, the choice of biomass to be fed to a small gasifier must necessarily be limited to materials with high density, easily crushed into small and spherical particles, able to flow by gravity through the probe in the fluidized bed.

In this study, almond shell particles were used, whose composition and size are shown in Table 2.

Table 2

The feeding of the biomass particles was also facilitated by a continuous flow of nitrogen (1 liter / minute) introduced into the probe to further improve the mobility of the particles themselves.

In all the tests carried out, the gasifier contained 600 g of olivine particles, or of perovskite catalyst deposited on olivine as described in Example 2; the density of the particles, the average diameter and the particle size distribution are shown in Table 3, together with the value of the minimum fluidization rate with water vapor at a temperature of 800 ° C.

Table 3

The biomass flow rate was set at 0.3 kg h-1 and the water vapor flow / biomass flow rate ratio at 1. The effective contact time, defined as the volume of the secured bed occupied by the catalyst particles or of olivine divided by the volumetric flow rate of the gases under the reaction conditions, was estimated to be about 0.2 seconds.

In the two-stage process, the secondary catalytic reactor (internal diameter 60 mm) contains a fixed bed of perovskite catalyst (280 g, regular particles with an average diameter of 1 mm). The height of the bed is approximately 160 mm. The actual contact time is close to that reported for the fluidized bed reactor. The catalyst is reduced before starting the gasification, by sending a gaseous stream composed of 4% hydrogen in nitrogen, heating from room temperature to 800 ° C with a speed of 5 ° C min <'1>. The flow of hydrogen and nitrogen is stopped before starting gasification. The same procedure was used to pre-reduce the perovskite catalyst deposited on olivine placed inside the fluidized bed. In the tests carried out with the secondary catalytic reactor, the gasifier contained olivine particles.

The water vapor and the pitch present in the gas, after the catalytic treatment, are condensed through the use of two heat exchangers in series, one cooled by water and the other with a refrigerant maintained at -10 ° C, respectively. The quantity of organic substances present in the condensate was determined by analyzing the total carbon, carried out on condensate samples taken every 10-30 minutes; the content of pitches in the gas produced (expressed in grams per cubic meter of dry gas under normal conditions) was calculated assuming naphthalene as a representative compound of the same. The conversion of water was also determined.

The quantity of gas produced is measured by means of a volumetric meter and reported to normal conditions. The concentrations of CO, CO2 and CH4 in the dry gas are measured instantaneously using an infrared system 15, while a sensitive element with thermal conductivity 14 is used for the determination of hydrogen. The values of the gas production and its composition have been corrected to take into account the quantity of nitrogen introduced into the probe feeding the biomass to the gasifier (the quantity of nitrogen represents about 10% of the final gas). The reliability of the analysis was periodically verified by using a cylinder containing a gas mixture having a known composition, similar to that of the gas produced during gasification.

Example 7 Catalytic gasification of biomass with the two-stage process

8 consecutive tests were carried out with the same catalyst sample and whose operating conditions, the resulting gas production and the content of pitch in the product are shown in Table 4 and Figure 5.

Table 4

The experimental tests were carried out both with and without the perovskite catalyst in the secondary fixed bed reactor, in order to quantitatively evaluate the effectiveness of the catalyst itself. The first column A0 of Table 4 illustrates the values of production and composition of the gas, content of pitch and conversion of water vapor, obtained in the absence of the catalyst.

Test B1 refers to the reactivity of fresh perovskite. After each test, the catalyst was exposed to a stream of air for 60 minutes at a temperature of 900 ° C in order to burn any coke particles deposited on the surface of the particles. In the first test (B1) and in the last three (B6-B8) the catalyst was also pre-reduced: this operation was not carried out before starting the gasification in tests B2-B5.

All the tests were carried out keeping the temperature in the fluidized bed gasifier constant and equal to 770 ° C. At this temperature, the "char" particles formed during the devolatilization phase are not completely consumed by reacting with the water vapor (and carbon dioxide): they accumulate inside the gasifier because the surface velocity of the gas does not is high (u / umf = 2.4).

From the analysis of the gasifier bed content after the AO test it was calculated that about 36 g of coal are formed per kg of treated biomass. The accumulation of "char" in the gasifier limits the duration of the tests, fixed at 90 minutes.

Figure 5 shows the trend, as a function of the gasification time, of the tar content in the gas after the catalytic treatment.

Similar results were obtained with different catalyst samples, and this confirms the reliability of the data reported in Table 4 and Figure 5.

When the perovskitic catalyst is present in the secondary reactor, the quantity and quality of the gas improve considerably: with the fresh catalyst (test B1), the typical value of the average gas production is about 2 normal m <3> of dry gas per kg of biomass, with a residual pitch content of less than 0.2 g m <'3> n. The net calorific value is approximately 11 MJ m <-3> n. The molar fraction of methane is considerably reduced in favor of hydrogen, while the H2 / CO ratio remains approximately equal to 2.3.

The substantial increase in the conversion of water vapor, compared to test A0, clearly indicates the activity of the catalyst: this is because it can be assumed that the reaction takes place between the water vapor and organic products (coal or hydrocarbons) deposited 0 adsorbed on the surface of the catalyst (5).

As regards the succession of catalytic tests, a progressive deactivation of the catalyst is noted in the case of absence of pre-reduction before gasification, with a consequent increase in the quantity of methane in the gas produced. These results are not surprising, because each test is followed by an oxidation treatment to eliminate any carbon deposited on the surface of the catalyst, and also during the test itself the conditions are rather oxidizing due to the presence of water vapor in the gaseous phase. concentration of about 30%. This suggests that the deactivation is linked to a progressive reoxidation of the nickel active sites. This thesis is supported by the comparison with the results obtained when the catalyst undergoes the pre-reduction treatment. However, it is important to note that, even in cases of non-pre-reduction of the catalyst, there is a notable activity in the transformation of the pitch, with the result of a considerable improvement in the quality of the gas, compared to the case of using olivine alone in the gasified. king.

Example 8 Catalytic gasification of biomass with the single-stage process

In these tests the perovskite catalyst (supported on olivine particles as described in Example 2) is used directly in the fluidized bed gasifier: in this case, therefore, the production and composition of the gases and the residual content of pitch refer to the fuel obtained in exit from the gasifier. Two different catalyst samples were used, obtained respectively by depositing LaNi0.3Feo.7O3 on previously calcined (sample D) and non-calcined (sample E) olivine particles. The operating conditions and the results related to each test are reported in Table 5 and Figures 6, 7 and 8; these values are representative of all the tests carried out with this experimental configuration.

Table 5

In order to better evaluate the behavior of the catalyst, the results of the test carried out using olivine particles (CO test) in the gasifier are reported in the first column of Table 5, under the same operating conditions as the tests carried out with trimetallic perovskite: gasification temperature 820 ° C, 50 ° C higher than that used in the tests reported in the previous Table 4. This temperature increase has a positive effect on the catalytic activity of the olivine and on the quantity of carbon residue produced by the devolatilization process: this quantity is considerably reduced compared to to test A0, as it results from the comparison between the data reported in Table 4 and Table 5, respectively; furthermore, the carbonaceous particles are much smaller and lighter and therefore are mainly entrained by the gas and separated in the cyclone and in the filter.

Overall, the results show that when the plant operates with the fresh and pre-reduced catalyst, a gas with a very low "tar" content is obtained: this is an important datum, considering the relative simplicity of the procedure. The comparison between the results of the E1 and CO tests shows the positive effect that the catalyst has on gas production, pitch content and water conversion; on the other hand, there are no significant differences in the concentration values of permanent gases, in particular methane. The major difference between the results obtained with this configuration and the one described above consists precisely in a reduction of the catalytic activity for the methane reforming reaction. This involves a small decrease in gas production and a lowering of the concentration of hydrogen in the gas, which in any case remains close to 50% by volume. The gas composition does not change appreciably during the subsequent gasification tests and is well represented by that obtained in test E3 illustrated in Figure 6.

The almost negligible activity of the catalyst in the conversion of methane is probably due to the high concentration of water vapor in the gasifier, compared to the conditions found in the secondary catalytic reactor. This is confirmed by the result of the preliminary methane reforming tests, namely that an excess of steam negatively influences the transformation of CH4 due to the too oxidizing atmosphere determined by it, as previously discussed.

On the other hand, the experimental results confirm the positive action of perovskite in pitch hydrogenation processes: the concentration of condensable hydrocarbons in the gaseous fuel produced is only slightly higher than that obtained with the two-stage process configuration, with values similar of the contact time in the fixed bed converter and in the fluidized bed converter, respectively. The type of gas-solid contact in the first reactor is naturally more favorable to a high performance of the catalyst. Furthermore, as has already been pointed out with reference to the configuration of the two-stage process, in the analyzes carried out on the particles contained in the bed after tests D3 and E4, only very few traces of carbon deposited on the surface of the catalyst were observed (the production of "char" reported in Table 5 with reference to test E4 concerns the carbon produced by devolatilization of the biomass and separated from the cyclone and filter).

Test D1 also shows that the absence of a pre-reduction of the catalyst significantly decreases its activity. This is evident from Figure 7, which shows the quantity of pitch contained in the gas as a function of the gasification time: in the case of test D1 the concentration of organic products reaches a peak (more than 1 g m- <3> n and then subsequently it decreases, probably due to a progressive reduction of the catalyst by the hydrogen produced by the gasification process This behavior is not observed in any of the tests carried out by pre-reducing the catalyst.

Experimental tests highlight that the quality of the gas produced, in terms of purity from condensable organic compounds (pitch), tends to worsen as the time of use of the catalyst itself increases. This is evidenced by the progression of the tests carried out with sample E: Figure 8 shows that after about 11 cumulative hours of gasification the activity of the catalyst becomes almost indistinguishable from that of the olivine under the same operating conditions (test C0). From an examination of the content of the particles present inside the gasifier after test E4, and of the fines separated by the cyclone and the ceramic candle, it was concluded that the perovskite layer deposited on the olivine particles had been completely eroded; this is certainly due to friction and abrasion phenomena typical of fluidized bed reactors. This is confirmed by the results of complementary analyzes carried out on the bed particles at the end of the reactivity tests (see below).

Finally, the results obtained in each test were verified by means of a matter balance around the experimental equipment.

Example 9 Characterization of the catalyst after the reactivity tests The morphology of the catalyst after the reactivity tests is illustrated in Figure 9. The general appearance is similar to that of the catalyst before the tests. There are few areas covered with coal. The specific surface area (BET) increased slightly from 5.8 m <2> (g. Cat) - <1> to 6.7 m <2> (g. Cat) <-1>.

The X-ray spectra show the presence of NiO, traces of La203, LaFe03 and part of the structure still in the form of LaNio.1Feo.9O3, as also confirmed by the FT-IR analysis (a characteristic absorption band at 561 cm <- 1>). Iron is never present in the metallic state. The particle size of free nickel is about 30 nm; they are slightly larger than those obtained after reactivity in the absence of H2 (25 nm for a H2O / CH4 ratio equal to 1 or 3), which indicates a tendency to sintering in the presence of hydrogen, as reported in the literature (40,41) .

The characterization of the catalyst was also carried out after the gasification tests. With the two-stage process configuration, the total mass of carbon present on the catalyst after the last gasification test (B8) was less than 0.3% by weight, i.e. a practically insignificant amount: only a few carbon filaments were observed through the scanning microscope. The system therefore proved not to be subject to the formation of coal, despite the experimental conditions being extremely favorable to the deposition of coke (presence of heavy aromatic hydrocarbons). X-ray examination shows a prevalence of the perovskite phase, with a small presence of La203 and free nickel. The intensity of the peaks indicates the formation of a few nickel metal particles with a diameter slightly larger than those observed in the methane reforming tests.

With the configuration of the single-step process, the carbon deposited on the catalyst after all the tests is <30 ppm in the case of the calcined olivine and then impregnated with the perovskite La-Ni-Fe (sample D), and 190 ppm in the case uncalcined olivine impregnated with La-Ni-Fe perovskite (sample E). The elemental analysis conducted on the particles of sample E confirmed that the percentage of lanthanum present on the olivine had dropped to 1.56%, which corresponds to a decrease in the concentration of perovskite to 2.7% (instead of 4.5% measured in the fresh catalyst). Similar analytical determinations conducted on sample D revealed a higher concentration of perovskite present on the olivine (3.6%), in line with the shorter overall duration of the tests conducted with this sample and its better performance in the destruction of pitches, such as results from the comparison between the gasification tests D3 and E4. Example 10 Reactivity tests with other catalysts

Reactivity tests were also carried out by reacting separately CH4 with CO2 and H2O on different catalysts having the general formula LaNixFei-xO3 with x between 0 and 1 at different temperatures, according to the methods described in Example 5. The results expressed in% yield of CO, as a function of x at the different reaction temperatures, are shown in Figures 10a and 10b. The operating conditions were as follows:

Fig. 10a: 0.3 l / h of CH4 '0.3 l / h of 002, 2.4 l / h of Ar, 200 mg of catalyst;

Fig.10b: 0.3 l / h of CH4, 0.3 l / h of H20, 2.4 l / h of Ar, 200 mg of catalyst.

It is evident that with the LaNixFe1-xO3 system the best CO yields are obtained in the temperature range between 600 ° C and 800 ° C.

Claims (17)

CLAIMS 1. Process for the gasification of biomass comprising a catalytic conversion step with a perovskitic catalyst of the formula: LnyA1-xBx03, in which y is a number between 0 and 10, x is a number between 0 and 1, Ln is a rare earth, strontium or bismuth, A is a metal belonging to groups IVb, Vb, Vib, Vllb, or VIII of the periodic table of elements, B is a metal of groups IVb, Vb, Vib, Vllb, or VIII of the periodic table of elements, and A is different from B. 2. Process for the gasification of biomass according to claim 1 wherein the catalyst has formula LaNixFei-XO3. 3. Process for the gasification of biomass according to (a claim 2 in which x is comprised between 0.1 and 0.9, including the extremes. 4. Process for the gasification of biomass according to claim 3 wherein x is equal to 0.3. 5. Process for the gasification of biomass according to any one of the preceding claims in which the catalyst is used after being reduced. 6. Process for the gasification of biomass according to any one of the preceding claims comprising the steps of: a) thermo-chemical treatment of the biomasses in a gasifier at a temperature between 700 ° C and 900 ° C and in the presence of water vapor; b) catalytic conversion with the catalyst according to one of claims 1 to 4. 7. Process for the gasification of biomass according to claim 6 also comprising the step of: c) condensation of water vapor and pitch present in the gas thus obtained. 8. Process for the gasification of biomass according to claim 6 or 7 in which the thermo-chemical treatment step and the catalytic conversion step take place simultaneously in a single reactor. 9. A process for the gasification of biomass according to claim 8 wherein the reactor is a fluidized bed reactor. 10. Process for the gasification of biomass according to claim 9 wherein the perovskite catalyst of claim 1 is supported on olivine. 11. A process for the gasification of biomass according to claims 6 or 7 in which the thermo-chemical treatment step of the biomass takes place in a first reactor and the catalytic conversion step takes place in a secondary reactor, located downstream of the first reactor. 12. A process for the gasification of biomass according to claim 11 wherein the first reactor is a liquidized bed and the secondary reactor is a fixed bed reactor. 13. Process for the gasification of biomass according to claim 12 the first reactor comprises olivine. 14. Gas obtainable with the process according to any one of the preceding claims. 15. Use of the gas according to claim 14 for the production of fine chemicals. 16. Use of the gas according to claim 14 for the supply of electricity generators, in particular fuel cells. 17. Use of the gas according to claim 14 as an energy carrier. SS