DESCRIPTION

The object of the present invention is a gasifier for gasification of biomasses, provided with an improved combustion chamber in order to obtain a final product made up of poor gas, which after suitable treatments, can be fed to an internal combustion engine. In the literature of the field, "biomass" means everything having an organic matrix principally deriving from green plants, as for example algae, trees and cultivations as well as forest and agricultural residues excluding plastic materials deriving from petrochemical industry as well as traditional fossil fuels (oil, coal) ¹ . Biomass represents the most sophisticated form of storage of solar energy, which by means of photosynthesis, allows to turn atmospheric CO ₂ in organic substance useful for the growth of the same plant. In it the solar energy, transformed by means of the photosynthesis, is stored in form of chemical bonds between the atoms of carbon, hydrogen and oxygen of the principle molecules (essentially carbohydrates). In the conversion processes of the biomass as for example digestion, combustion

¹ Peter McKendry, Energy production from biomass (part 1): overview of biomass, Bioresource Technology 83 (2002) 37-46. or decomposition, such chemical energy stored in form of bonds is released to be used in a direct or indirect way ² . The benefits deriving from the use in energetic terms of the biomasses are many and the following ones are enlisted among the most important ones:

- reduction of the amount of solid waste to be disposed of;

- control of the CO ₂ emissions, since CO ₂ produced during combustion of biomass is reabsorbed by new biomass planted in the same amount; - it is a renewable energy source, thanks to its remarkable presence in nature and to the rapidity to regenerate biomass in relatively short times.

As for the conversion processes of biomass in energy, they comprise a wide range of conversion typologies, which can be distinguished in terms of fed biomass, final usages and required infrastructures. The factors influencing the choose of the most suitable conversion process are:

- the typology and amount of fed biomass;

- the form of desired energy, that is the final object; - environmental parameters;

- economic conditions;

- design specifications ³ .

² Peter McKendry, Energy production from biomass (part 1): overview of biomass, Bioresource Technology 83 (2002) 37-46.

³ Peter McKendry, Energy production from biomass (part 2): conversion technologies, Bioresource Technology 83 (2002) 47-54. The conversion processes of the biomasses in energy can be mainly classified in two great categories: thermochemical processes and biochemical processes. The thermochemical processes comprise combustion, pyrolysis and gasification; while the biochemical ones comprise aerobic or anaerobic digestion (with production of biogas) and fermentation (with production of ethanol).

Object of the present invention is to provide a gasifier for biomasses, provided with a new combustion chamber; therefore it will be lingered above all over this specific technology of energetic valuation in said thermochemical processes. The simple combustion of biomasses is a very widespread practice used to rapidly convert the chemical energy of the biomass in various "output" as for example heat, mechanical power or electricity. The final output depends on the used typology of device, as for example stoves, furnaces, boilers, steam turbine, turbo-generators etc.... The temperatures of the hot gases resulting from the combustion vary between 800 ⁰ C and 1000 ⁰ C, with the possibility to theoretically burn any kind of biomass. Anyway the combustion is possible only when the humidity content in the biomass is lower than 50%; greater humidity values make in fact the biomass more suitable to a treatment of chemical type. The dimensions of the combustion plant of biomasses vary between very small dimensions, as for example for the domestic usage, to great scale industrial plants with power between 100 MW and 3000 MW with yields variable between 20% and 40% ⁴ .

The pyrolisys is instead the process of thermochemical decomposition of organic materials contained in the biomasses, by means of heating at about 500 ⁰ C and strongly lacking oxygen. The products of pyrolisys are gaseous, liquid and solid, in proportions depending on the pyrolisys methods (rapid, slow, conventional) and on the reaction parameters. The pyrolisys is essentially used for the production of bio-oil by using the rapid pyrolisys (or flash pyrolysis) with a conversion yield of about 80%.

Finally, the gasification is a conversion process of the biomass in a gas mix by means of partial oxidation of the same biomass at high temperatures (800 ⁰ C - 1000 ⁰ C). The poor gas composition resulting from the gasification process depends on the gasifying agent used and on the type of reactor used. According to the application object of the present invention, reference will be made to a gasifying process with air, from which it is obtained a synthesis gas with low calorific power (about 4 - 6 MJ/Nm ³ ) which can be directly burned or constitute the fuel for alternative gas or turbine engines. In general, the synthesis gas produced can be alternatively used as raw material for the production of

⁴ Peter McKendry, Energy production from biomass (part 2): conversion technologies, Bioresource Technology 83 (2002) 47-54. chemical compounds as for example methanol, which in turn can be used for the production of biodiesel.

Aim of the present invention is therefore to provide a biomass- fed fixed bed gasifier with a new combustion chamber, which is neither classified in the category of updraft reactor nor in the downdraft reactor one.

The reactors for the biomass gasification belong in fact to two great categories: fixed bed reactors and fluidized bed reactors. In the fixed bed reactors, the different direction of the flow of gasifying agent (air) with respect to the bed implies mainly two different typologies of reactor: updraft, downdraft. In the updraft reactor the biomass is loaded at the top of the gasifier, while the gasifying agent (air) is introduced from the bottom of the unit through a grid. While falling down, the biomass is first dried by the hot gases produced which move upwards producing coal, which continues to fall down, and other vapors of pyrolisys which join the hot gases produced. In the following, in the area of pyrolisys, each volatile compound separates from the biomass and a remarkable amount of tar forms, which in part joins the synthesis gas produced in output and in part joins the solid residues. These latter, while forming, fall down to be collected; while in the upper portion of the gasifier, where the biomass is dried, the gases produced are cooled down to temperatures about 200 ⁰ C - 300 ⁰ C ⁵ .

⁵ Peter McKendry, Energy production from biomass (part 3): gasification technologies, Bioresource Technology 83 (2002) 55-63. The downdraft reactor is instead characterized by a parallel flow between biomass and gasifying agent: before coming out from the gasifier, the reaction products are mixed in a high temperature turbulent region, known as "diabolo". In such region it is realized the partial cracking of the tar, which limits its excessive production. Because of the high temperature in output of the produced gases (about 900 - 1000 ⁰ C), the total yield in energetic terms of such reactor is quite low; anyway the synthesis gas produced results with low content of dusts and tars. Aim of the present invention is to guarantee the highest yield possible of the gasification process by using an improved combustion chamber as well as of a system for mixing biomass. The combustion chamber, object of the present invention, provides in fact a top-down biomass flow with side air feeding, which is thus shaped as a cross flow, and the usage of organs for mixing the same biomass. The following detailed description makes reference to the following accompanying drawings 1/7 to 7/7, in which: Fig. 1 is a 3D view of the upper portion of the gasifier with the combustion chamber thereinto;

Fig. 2 is a side view of the upper portion of the gasifier with some inner details thereof, highlighted;

Fig. 3 is a top view of the upper portion of the gasifier; Fig. 4 is a longitudinal section of the gasifier, as indicated in fig.

3;

Fig. 5 is a 3D view of the lower portion of the gasifier, immediately arranged under the upper portion of the gasifier shown in fig. 1 ;

Fig. 6 is a side view of the lower portion of the gasifier;

Fig. 7 is a longitudinal section of the lower portion of the gasifier as indicated in fig. 6;

Fig. 8 is a cross section of the lower portion of the gasifier as indicated in fig. 6;

Fig. 9 is a cross section of the upper and lower portions of the gasifier with the mixing organs highlighted;

Fig. 10 shows a detail view of the moving organs and a longitudinal section thereof. As a whole the gasifier is made up of an "upper" portion 1 and a

"lower" portion 2 mounted the one on the other, respectively.

From top to the bottom, in the upper portion 1, to the height of the feeding holes 11 of the gasifying agent which is usually air, there are realized in the order the biomass drying and the combustion process lacking oxygen, which provides heating to the reactions of pyrolisys of the biomass occurring immediately thereunder. From this point on, there begin the actual gasification reactions, that is the reaction producing the gas useful for the following aims of the plant. The rotating grid 15, which suitably dimensioned for the biomass dimension prevents the same biomass from leaving the yet unburned reaction area, separates the lower portion 2 from the upper one 1. The lower portion 2 is the space where the produced gas is conveyed towards the output pipe; the shape of inverted cone of its bottom allows the the ashes to fall and to be collected for their removing by means of a suitable mechanical system (fig. 5).

The upper portion 1 of the gasifier is made up of an outer shell 3 of almost cylindrical shape, an inner section 4 accommodating the reactions of combustion, pyrolisys and gasification (or reaction chamber 9) and a series of air gaps 5, 5' and 5" between the outer surface 3 and the inner surface 4 of the gasifier. The air gaps 5 and 5' are crossed respectively by the gasifying agent, introduced as it will described in the following, and by the synthesis gas in output from the gasifier; while the air gap 5" is filled with a shim of any insulating material resistant to high temperatures (for example glass wool or mineral wool) or at least by a shim of "insulating air" acting as insulating in order to allow the formation of an area to guarantee the optimal conditions of temperatures for the reactions inside the reaction chamber 9. The inner surface of said reaction chamber 9 is inside coated with refractory cement.

On the outer surface it is provided a plurality of circular openings 6, whose partial occlusion allows the intake of gasifying agent (usually but not exclusively air) to be adjusted during the production of synthesis gas from biomass. Yet, on the outer side surface on the tapered section of the reaction chamber, it is arranged the radial output conduit 7 of gas leading to the plant for treatment and feeding of the produced synthesis gas. As the longitudinal section of the "upper" portion 1 shows in fig. 4, inside it, from the top, there are an inlet area 8 shaped as a frustum of cone where the biomass is introduced and distributed uniformly by means of suitable mixing organs mounted on suitable support 12; a reaction chamber top-down characterized by sections which firstly decrease in a linear way 8 till the tapered section, then increase in a non-linear way 9 (barrel shaped), and finally increase linearly; and finally an output section 10, shaped as a frustum of cone as well, partially overlapping on the "lower" portion 2 at the grid and at the area where the movement of the synthesis gas inverts. The side feeding of the air flow inside the combustion chamber 9 occurs by means of suitable conduits 11 going radially (with or without inclination) from the combustion chamber 9 to the outermost preheating air gap 5 and the inlet gates 6. The air adjustment occurs by means of these inlet gates 6, whose section is suitably reduced in function of the load.

As it is shown in figs. 9 and 10, in a complete as well as improving embodiment of the invention, the mixing organs mounted on support 12 (suspended and guided by a system pin- bushing 22 with clearance at the bottom to allow both the shaft to be elongated without stresses deriving from thermal stresses and a correct and simple use of its rotating movement) of fig. 4, can be made up of a worm 18 with hollow vertical axis 19, electrically put in rotation and able to provide a slow mixing of biomass produced with a slow motion similar to the convective motion of the air generating in the recesses (vertical ascending at the centre and vertical descending at sides) in order to avoid the formation of voids (in jargon "bridges"), where undesired and dangerous synthesis gases accumulate, without significantly altering the correct stratification of the reaction areas. These accumulations cause discontinuity in the gas production and alter a fixed running of the gasifier with remarkable yield losses. The worm has a maximum diameter compatible with the absence of interference with the tapered section of the reaction chamber and with the easy mixing of biomass which has not to be opposed to the rotating movement. The hollow shaft 19, better if it is realized alternating structural materials 20 and with low thermal conductivity 21, allows the introduction of more temperature sensors at different heights useful for monitoring the actual and momentary operation conditions and for possibly realizing the setting logics. The propeller of the worm 18 can be provided with radial extensions to reach distances considered optimal to the aims of the slow mixing of biomass.

With such a configuration, inside the reactor 1 and in particular inside the combustion chamber 9, it is generated a "cross" flow between biomass fed from top moving downwards and air (gasifying agent) introduced laterally. In this way, it is formed a very hot area (the high and stable temperature is guaranteed by the presence of the insulating material resistant to high temperatures in the air gaps 5", which allows a correct stratification of the reaction area, thus favoring their correct sequence), where air enters the reaction chamber inside which the processes of combustion/pyrolisys and gasification occur; the areas of pyrolysis and pre-drying of biomass are instead formed immediately on said area of combustion/pyrolisys. With such a configuration it is obtained a type of fixed bed gasifier neither belonging to the downdraft one nor to the updraft one. As a whole, the shape of the chamber and the mixing system ensure the correct running of the plant while guaranteeing suitable stable conditions of temperatures for the reactions of combustion, pyrolisys and gasification and, therefore, to obtain high yields of gasification.

The "upper" portion 1 is as much as possible closed by means of a cover during operation to avoid air in excess in the combustion area. Figure 5 shows, instead, a tridimensional representation of the "lower" portion 2 of the gasifier, arranged immediately under the "upper" one, just described. As it is shown by the view in fig. 6, said "upper portion" 2 is made up of a first cylindrically shaped section 13 and a final section 14 shaped as an inverted frustum of cone, where the ashes are collected as byproduct of the entire gasification process. Figs. 7 and 8 show that inside the cylindrical section 13 it is provided a circular platform 15 rotating by means of a gear 16, on which a plurality of holed grids (according to the dimension of the biomass to be treated) 17 is arranged for the selective passage of the waste material already consumed by the reactions of pyrolisys and gasification.

The fluidized bed gasifiers for biomasses, not described herein, are surely more efficient in terms of total yield, but they are more expensive both in terms of initial costs and maintenance cost with respect to the fixed bed gasifiers. These latter remain the most convenient option for the production of synthesis gas with low calorific power to be used in plants for the generation of energy on small scale.