GB2109212A - Agricultural method and composition - Google Patents

Abstract

Agricultural land is treated with a combustion product of a solid fossil fuel, particularly pulverised fuel ash, whereby heavy soil is rendered more permeable and light sandy soil less liable to drying out. A mixture of such a combustion product with a fertiliser material and/or soil is also claimed.

Description

SPECIFICATION Agricultural compositions and methods This invention relates to agricultural compositions and methods.

Agricultural compositions comprising a relatively minor amount of agricultural nutrient material and a relatively major amount of a filler, are known. The filler is a relatively cheap material. Commercially available compositions of this type often comprise sand as the filler. Fillers such as sand have little or no nutrient value.

Pulverised fuel ash (PFA) is a waste product produced in voluminous and troublesome quantities by electric power stations and other steam-raising plants. PFA is a combustion product of solid fossil fuel.

Various uses have been proposed for PFA, and British Patent Specifications Nos. 823,688; 1,241,941 and 2,061,241A describe a common present used, i.e. as a filler in the preparation of bricks and aggregates for the building industry.

One of the constituents of PFA is a particulate material which can be separated by flotation, e.g. as when PFA slurry direct from a power station is removed to a lagoon (this is a conventional waste disposal step).

These particles, known as cenospheres, have been themselves proposed as an industrial filler and as a raw material forthe manufacture of lightweight insulating products (PFATechnical Bulletin No.22, published by the Central Electricity Generating Board). Further, a method for growing grass is proposed in PFA Technical Bulletin No. 38; a layer of cenospheres combined with other floating material is spread over the surface of water, and a carrier sheet and a mixture of fertiliser and grass seed are placed on the floating layer.

The classification of PFA to give cenospheres is also a feature of British Patent Specification No.

2,061,241A. Here, cenospheres are preferably used as the sole PFA product, although a less preferred PFA product for use in producing the bricks described comprises both cenospheres and the "fine particles". The "large particles" are invariably removed. The product is mixed with a fuel, e.g. sewage sludge, and a binder before bricks are formed.

British Patent Specification No. 1,241,941 discloses the preparation of moulded compacts of materials in a "green" state, with subsequent firing to produce bricks of material analoguous to pumice. The materials dislcosed include mixtures of PFA and sewage sludge. The compacts are formed by pressure. The subsequent firing is an essential step in the formation of bricks for the building industry.

Despite the various proposals, PFA is still produced in large amounts which are of little or no value and which involve difficult disposal problems. It has now been discovered that PFA has physical and chemical properties which make it suitable for use in agricultural applications, without it being necessary to separate cenospheres from other PFA constituents. While PFA contains a wide range of trace elements on which plant life depends, in natural proportions, it has not previously been appreciated that this is of any practical utility.

According to a first aspect of the present invention, an agricultural method comprises applying a combustion product of a solid fossil fuel, in the green state, to an agricultural locus. This invention is also applicable to horticulture and references to agriculture herein apply to horticulture.

The preferred combustion product for use in the invention is PFA. For convenience, the invention will be illustrated herein with reference to PFA, and subsequent references to PFA apply to all combustion products as defined. PFA can be used in the invention, directly, in the form in which it is produced from the power stations. It need not be classified, and is preferably unclassified.

Loci to which PFA may be applied include areas where plants are growing, and areas where plants are intended to grow. For example, PFA may be ploughed or otherwise directly introduced into soil. One of the important properties of PFA is its ability to mix readily with other solids, even clayey soil. By way of example, PFA can be mixed with clayey soil, e.g. in a 1:1 ratio, to produce a friable composition having none of the soil's handling characteristics. Whereas a soil/sand mixture may have a pH of about 5, a comparable soil/PFA mixture can have a pH of about 6 which is much more appropriate for agricultural and horticultural use. By use of the invention, heavy clayey soils can be rendered more permeable, and light sandy soils less liable to desiccation.A mixture of soil, obtained directly from the ground or in the ground, and a combustion product as defined above, provides a further aspect of the invention.

It is thought that one of the primary modes of action of PFA in an agricultural locus is to render nutrients, and other materials beneficial to plant life, more accessible to plants. However PFA may not of itself provide all the fertilising constituents that plants may require. Therefore, certain fertiliser constituents and PFA, separately or together, may be applied to agricultural loci lacking such constituents. For ease of use, a single composition comprising PFA and a fertiliser material may be applied.

Suitable fertiliser materials include soil conditioners, and agriculturally acceptable sources of nitrogen and/or phosphorus. Examples of such materials are known and include superphosphates, ammonium phosphate, lime, potash and soda. A particularly preferred composition for use in the invention comprises PFA and sewage sludge, since both materials are cheap and the latter comprises many agriculturally and horticulturally useful constituents.

Owing to the ability of PFA to mix readily with other solids, a composition of the type described is easy to prepare, in any desired proportions. For ease of transport, such a composition, or PFA alone, may be aggregated, e.g. by the application of pressure, to prepare moulded compacts (as is described in British Patent Specification No. 1,241,941). For use in the invention, disintegration of such compacts, if desired, is simple to achieve.

PFA is particularly suitable for containerised horticulture, e.g. in "growbag" methods, an important consideration being its low cost. According to a further aspect of the present invention, an agricultural product comprises a container containing a fertiliser material in admixture with a combustion product of a solid fossil fuel. The container may be rigid or flexible, e.g. a plastics sack. The container may have, or be modified to have, an aperture in at least one face, through which a plant may be introduced and planted in the composition. This use of PFA may be of especial importance in areas of poor soil and extreme climatic conditions.

Apart from compositions which contain sewage, farm wastes, green vegetable matter or other materials liable to decompose on storage, compositions of, and for use in, the invention can have a long shelf-life and present no storage or transport problems.

A particularly important discovery leading to the present invention relates to the structure of the smallest particles of PFA, i.e. the particles which have previously been suggested as unnecessary. The discovery is that a primary constituent of PFA is minerals having a zeolitic or open-molecular structure which enables them to render the pH of acidic and basic soil substantially neutral, thereby creating an ideal milieu for biogenesis in the soil. The zeolitic structure is lost if PFA is heated above about 500"C. It is thus important that PFA should be used in the "green" state, since it is not suitable for use in the invention after having been fired in the manner previously proposed for construction brick formation.

Discoveries, generally relating to the mineralogy and physical chemistry of PFA, enable the formulation of conventional fertilisers to be carried out on an accurately balanced molecular scale which reduces the possibility, in use, of under-dosing or over-dosing soils in the field.

The chemical composition for PFA produced by electrical power stations, consuming a wide variety of ranks of coal, may be, for example, as given in Table 1: TABLE 1 Element %w/w Element %w/w Silicon as SiO2 48.80 Strontium as SrO 0.17 Aluminium as A1203 25.43 Nickel as NiO 0.10 Iron as Foe203 10.8 Cobalt as CoO 0.10 Calcium as CaO 5.76 Beryllium as BeO 0.10 Zine as ZnO 0.50 Lead as PbO 0.10 Potassium as K2O 2.47 Copper as CuO 0.07 Germanium as GeO2 0.06 Zirconium as ZrO2 0.07 Sodium as Na2O 1.42 Vanadium as V2O5 0.07 Magnesium as MgO 1.16 Molybdenum as MoO3 0.05 Boron as B203 0.5 Gallium as Ga2O3 0.05 Arsenic as As203 1.00 Tin as SnO2 0.05 Titanium as TiO2 0.91 Chromium as Cr203 0.05 Phosphorus as P205 0.36 Manganese as Mn3O4 0.02 Barium as BaO 0.25 ~~~~~~~~ 100.96 In general, while variations occur in the percentages of the major elements, there is a significant degree of uniformity among those elements comprising less than 1% of the whole, i.e. among the so-called trace elements. This is due to their origin as essential constituents of the biomass from which the fossil fuel is geologically derived. Originally, the elements were systematically absorbed from soil by the roots of plants and trees, and transmitted into the plants and trees during growth. During decay, the trace elements were maintained in this biologically balanced condition, and remained as such throughout the geological processes which ultimately resulted in the formation of fossilised solid fuels.This balanced assembly of cations is transferred into the zeolitic minerals derived from the combustion of the solid fuel.

PFA contains, in a biologically balanced ratio, the essential micro-nutrients, i.e. Fe, Cu, Mn, Zn, Mo, Co, V, Ga, Sr, B, Al and Si. PFA is therefore a powder having properties which it would be impossible to reproduce artificially at reasonable cost. In particular, biogenesis has created a balanced amount of Mo and V which are thought to be interchangeable trace elements preferentially used by different plants, and available in PFA as a feature of the zeolitic base-exchange potential of its inherent structure. Thus the use of a composition of the invention can be described as a form of recycling of the cations which were assembled by the metabolism of vegetable material in distant geological times.

It has been discovered that the constituents of PFA are artificial minerals which are substantially the product of condensation of the inorganic constituents of the solid fuel, rendered volatile by the complex thermochemical reactions of combustion. Characteristically, the high-melting point cations released by combustion are the first to condense, forming alkali alumino-silicate artificial zeolites. The relatively low-melting point cations then condense into cenospheres. In addition, PFA contains particles of solid fuel which escape complete combustion, and resemble charcoal.

In more detail, the constituents of PFA may be divided into three clearly defined groups: (a) equant or approximately equi-dimensional white-to-grey coloured crystals of zeolitic molecular structure (typical characteristics: sp. gr. 1.9-2.4; refractive index 1,48-1,52); (b) cenospheres, i.e. alkali alumino-silicate hollow white glass beads; and (c) granular particles of activated carbon and hydrocarbons.

An understanding of the mode of formation, and the constitution, of PFA helps to explain one of its most important characteristics, i.e. the ability of PFA powder to mix freely with other materials of different and variable particle size and shape.

The zeolitic nature of the principal component of PFA resembles natural zeolites such as chabazite, but has a different water content as a result of the dehydrating conditions of combustion. The artificial PFA zeolites also differ from chabazite in their molecular geometry, being structured into distorted 8-membered silicate rings having elliptical apertures, e.g. 4.4 Ax 3.1 A. This structure allows these particles to behave as molecular sieves for organisms in soils, but their principal function lies in their ability to stimulate cation exchange, and to neutralise the air and water content of soils. It has been found that the base exchange potential of PFA can be as strong as that of artificial zeolites, by virtue of the narrow range and dimensions of the particles; these may have, for example, a specific surface index of 2200 to 2800 cm2/g.This factor also accounts for the ease with which PFA distributes itself homogeneously on mixing with other substances such as powdered phosphates, nitrates and lime. This characteristic of PFA is vividly demonstrated when it is mixed with vegetable matter in a green state, to form granules with obvious potential as a means of recycling silage and like farm wastes into the soil. It is also possible to slurry town sewage with PFA, to produce a granularfertiliser, containing active carbohydrates and enzymes, and an aqueous effluent which can be purified and sterilised by conventional methods. This application has the potential of dramatically reducing the cost of processing town sewage by recycling it directly into soils, without losing its active enzymes and associate biomass.

The cenospheres consist of hollow alkali alumino-silicate glass beads of low density (0.4-0.6 g/cm3) with a shell thickness of about 10% of their radius. The radius may vary from 20 to 125 microns, depending on the source, but the product of each furnace produces cenospheres of substantially uniform size within this range. The chemical composition of cenospheres is usually uniform, and an example is given in Table 2: TABLE 2 Component % wlw Silica (as SiO2) 55 to 61 Alumina (as A1203) 26 to 30 Iron oxides (as Fe203) 4 to 10 Calcium (as CaO) 0.2 to 0.6 Magnesium (as MgO) 1 to 2 Alkalis (as Na2O, K20) 0.5 to 4.0 Carbon (loss on ignition) 0.01 to 2.0 The principal role of the cenospheres is to assist in rendering heavy clayey soils more permeable to aeration and drainage.Owing to their resistance to chemical attack, they can assist in maintaining these desirable conditions for long periods of time. As described in British Patent Specification No.2,061,241 A, cenospheres can be separated by flotation. Separated cenospheres can be used for agricultural purposes but this use is likely to be of less advantage than the use of unclassified PFA.

The principal role of the incompletely combusted hydrocarbon-retention characteristics when introduced into soils. Thus, PFA can improve the coherence of sandy soils, and maintain them in a moist condition even under arid climatic conditions. The carbon and carbonaceous constituents of PFA behave in a manner similar to the constituents of peat, which means that PFA is suitable for the formulation of compost soils.

CLAIMS (Filed on 16.11.82) 1. A method which comprises applying a combustion product of a solid fossil fuel, in the "green" state, to an agricultural or horticultural locus.

2. A method as claimed in claim 1 in which the combustion product is pulverised fuel ash.

3. A method as claimed in claim 2 in which the pulverised fuel ash is unclassified.

4. A mixture of a combustion product of a solid fossil fuel, in the "green" state, together with a fertiliser material and/or soil.

5. A mixture as claimed in claim 4 in which the combustion product is pulverised fuel ash.

6. A mixture as claimed in claim 5 in which the pulverised fuel ash is unclassified.

7. An agricultural or horticultural product comprising a container containing a mixture as claimed in any of claims 4 to 6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. In more detail, the constituents of PFA may be divided into three clearly defined groups: (a) equant or approximately equi-dimensional white-to-grey coloured crystals of zeolitic molecular structure (typical characteristics: sp. gr. 1.9-2.4; refractive index 1,48-1,52); (b) cenospheres, i.e. alkali alumino-silicate hollow white glass beads; and (c) granular particles of activated carbon and hydrocarbons. An understanding of the mode of formation, and the constitution, of PFA helps to explain one of its most important characteristics, i.e. the ability of PFA powder to mix freely with other materials of different and variable particle size and shape. The zeolitic nature of the principal component of PFA resembles natural zeolites such as chabazite, but has a different water content as a result of the dehydrating conditions of combustion. The artificial PFA zeolites also differ from chabazite in their molecular geometry, being structured into distorted 8-membered silicate rings having elliptical apertures, e.g. 4.4 Ax 3.1 A. This structure allows these particles to behave as molecular sieves for organisms in soils, but their principal function lies in their ability to stimulate cation exchange, and to neutralise the air and water content of soils. It has been found that the base exchange potential of PFA can be as strong as that of artificial zeolites, by virtue of the narrow range and dimensions of the particles; these may have, for example, a specific surface index of 2200 to 2800 cm2/g.This factor also accounts for the ease with which PFA distributes itself homogeneously on mixing with other substances such as powdered phosphates, nitrates and lime. This characteristic of PFA is vividly demonstrated when it is mixed with vegetable matter in a green state, to form granules with obvious potential as a means of recycling silage and like farm wastes into the soil. It is also possible to slurry town sewage with PFA, to produce a granularfertiliser, containing active carbohydrates and enzymes, and an aqueous effluent which can be purified and sterilised by conventional methods. This application has the potential of dramatically reducing the cost of processing town sewage by recycling it directly into soils, without losing its active enzymes and associate biomass. The cenospheres consist of hollow alkali alumino-silicate glass beads of low density (0.4-0.6 g/cm3) with a shell thickness of about 10% of their radius. The radius may vary from 20 to 125 microns, depending on the source, but the product of each furnace produces cenospheres of substantially uniform size within this range. The chemical composition of cenospheres is usually uniform, and an example is given in Table 2: TABLE 2 Component % wlw Silica (as SiO2) 55 to 61 Alumina (as A1203) 26 to 30 Iron oxides (as Fe203) 4 to 10 Calcium (as CaO) 0.2 to 0.6 Magnesium (as MgO) 1 to 2 Alkalis (as Na2O, K20) 0.5 to 4.0 Carbon (loss on ignition) 0.01 to 2.0 The principal role of the cenospheres is to assist in rendering heavy clayey soils more permeable to aeration and drainage.Owing to their resistance to chemical attack, they can assist in maintaining these desirable conditions for long periods of time. As described in British Patent Specification No.2,061,241 A, cenospheres can be separated by flotation. Separated cenospheres can be used for agricultural purposes but this use is likely to be of less advantage than the use of unclassified PFA. The principal role of the incompletely combusted hydrocarbon-retention characteristics when introduced into soils. Thus, PFA can improve the coherence of sandy soils, and maintain them in a moist condition even under arid climatic conditions. The carbon and carbonaceous constituents of PFA behave in a manner similar to the constituents of peat, which means that PFA is suitable for the formulation of compost soils. CLAIMS (Filed on 16.11.82)

1. A method which comprises applying a combustion product of a solid fossil fuel, in the "green" state, to an agricultural or horticultural locus.

2. A method as claimed in claim 1 in which the combustion product is pulverised fuel ash.

3. A method as claimed in claim 2 in which the pulverised fuel ash is unclassified.

4. A mixture of a combustion product of a solid fossil fuel, in the "green" state, together with a fertiliser material and/or soil.

5. A mixture as claimed in claim 4 in which the combustion product is pulverised fuel ash.

6. A mixture as claimed in claim 5 in which the pulverised fuel ash is unclassified.

7. An agricultural or horticultural product comprising a container containing a mixture as claimed in any of claims 4 to 6.