WO2006038002A1

WO2006038002A1 - Stabilisation of iron-impregnated biomass in storage

Info

Publication number: WO2006038002A1
Application number: PCT/GB2005/003828
Authority: WO
Inventors: Geoffrey Michael Whiteley
Original assignee: Geoffrey Michael Whiteley
Priority date: 2004-10-06
Filing date: 2005-10-05
Publication date: 2006-04-13
Also published as: GB2433410B; GB2433410A; GB0706690D0; AU2005291081A1

Abstract

An improved method for inhibiting variable yeast and mould growth and heating during storage of iron-impregnated biomass involves the supplementary application of appropriate combinations of fungistatic, fungicidal organic acid preservatives and other preventative measures in combination with iron impregnation of biomass. According to the improved methodology described herein, an iron impregnation process and additional inhibitors of microbial activity are used in combination with fine control of water content to reduce mould growth, heating and associated composting of processed biomass during storage. The improved methodology has been developed to increase the commercial value and saleability of products made from iron-impregnated biomass through greater stability and consistency of products in storage. The improved process makes it possible to store a processed biomass for up to 12 months without deterioration. The improved methodology can also advantageously remove the prior requirement for an intermediate storage period before packaging allowing the new process to be applied as a continuous throughput or batch production method. The iron impregnation of biomass treatment process can additionally be applied to the on-farm treatment of crop biomass prior to incorporation in farm soils or spreading as a mulch on farmland to increase the storage of carbon in agricultural ecosystems as a tool for increasing the sequestration of atmospheric carbon dioxide.

Description

STABILISATION OF IRON-MPREGNATED BIOMASS IN STORAGE

This invention relates to a method of stabilisation of biomass, particularly but not exclusively to facilitate storage. The invention also relates to biomass stabilised in accordance with this method.

WO9821292 describes a method of for treating crop and other plant biomass products with sesquioxide mineral-forming constituents to form protective coatings and to alter the bulk physical and chemical properties of the treated material. In particular, the addition of an aqueous solution of soluble iron increases microbial stability and retention of structure in composted plant material to yield a fibrous particulate physical form. This is important to the quality, including colour and appearance, of iron-impregnated biomass for use in composts and as a soil improver or mulch.

Example 1 of WO9821292 described the formation of a horticultural mulch from chopped and milled wheat straw by means of a spray treatment to apply iron salts and mineral nitrogen, followed by a resting period of up to 48 hours, after which the water content is raised to 52%. This was followed by an outdoor storage period, for example in black silage bags, at an elevated water content of 52% w/w for a period of between two weeks to three months duration. The outdoor storage period allowed time for a degree of microbial activity to take place in an open aerated stockpile to avoid the tendency for excessive heating and composting to take place. After this outdoor storage period the product was found to be safe to pack without a risk of composting or deterioration which might otherwise be encouraged by internal heat generation in an enclosed bag stacked on a wrapped pallet or in the centre a of larger stockpile.

Extensive trials by professional gardeners and growers have demonstrated the usefulness of mulches and growing media components produced from iron-impregnated biomass. In particular, the iron-impregnation treatment has been used as a mulch colourant and stabiliser to add value over and above basic crop straw mulching materials. Stabilisation is thought to be attributable to a number of mechanisms including: physical protection of the biomass through the formation of iron hydroxide coatings, binding of iron to active binding sites on biomass polymers such as lignin and cellulose - rendering these components more resistant to hydrolytic enzymes of microbial origin, mild acidification of the biomass through the oxidation of ferrous sulphate in water, iron-induced immobilisation of phosphate tending to produce phosphorus-limited rates of microbial biomass growth - reducing the rate of microbial nitrogen immobilisation. The persistence of surface mulching layers might also be influenced by a reduced palatability to earthworms associated with the altered properties of the biomass. Trial results underline horticultural potential of 'mineralised' straw (2000). Chronica Horticulturae 40 (3): 3-4. (Magazine of the International Society for Horticultural Science.)

The previously described process is suitable for local horticultural use where there is access to outdoor storage facilities and the end product has been shown to be acceptable for use as mulch by professional gardeners. However, the requirement for a multi-stage process, particularly a process that includes an extended maturation period, adds time and costs associated with storing intermediaries for a period of weeks between production and packaging. Additionally, variable microbial activity, for example fungal blooms by species of Streptomycetes and other saprophytic colonisers, have been found to occur sporadically during storage, leading in some cases to spoilage and variable quality. Yeast and mould growth when the product is stored in bags prior to distribution can sometimes result in damaged and returned stock, wastage and inconsistent product quality.

According to the present invention, a method of making an iron impregnated biomass comprises the steps of: adding or removing water to a quantity of biomass to provide a predetermined water content; adding an iron composition comprising a solution of a ferrous or ferric salt to the biomass to provide a concentration of 0.4 to 0.8g of soluble iron per lOOg of biomass by weight; and adding an effective amount of an anti-fungal agent to the biomass; wherein the anti-fungal agent is selected from the group consisting of: formic acid (methanoic acid), acetic acid (ethanoic acid), carboxyethane (propionic acid), butyric acid (n-butanoic acid), valeric acid (n-pentanoic acid), caproic acid (n-hexanoic acid), enanthoic acid (n-heptanoic acid), caprylic acid (n-octanoic acid), alpha-ethylcaproic acid (2-ethylhexanoic acid), valproic acid (2-propylpentanoic acid, ), pelargonic acid (n-nonanoic acid), capric acid (n-decanoic acid), salts of the aforegoing acids and mixtures thereof.

Many waste-derived types of compost have a neutral to alkaline pH that can benefit to a degree from acidification during the preparation of growing media. In addition, during the early stages of decomposition or composting, rapid microbial population growth is often associated with a depletion of nitrate, a rising pH and subsequent volatilisation of mineralised ammonium. UK-A-2293374 shows the benefits of using free nitric acid and phosphoric acid to ameliorate the pH of growing media. The authors described adding free phosphoric and/or nitric acid to an alkaline composted material in sufficient amount to lower the overall pH of the composted material to a value in the range 4 to 7. This earlier patent also described a modified composted material obtainable by the said method, and a plant growth medium comprising the modified composted material in admixture with a low nutrient bulking agent. Phosphoric acid is not a suitable acidifying agent where iron is present because it will immobilise soluble iron species in preference to their binding to biomass and result in visible bleaching and reduced colour development. Nitric acid may be useful, but only as part of an optional or supplementary acidification of the biomass as described. Where mulches are made from wheat straw that has been grown to organic standards addition of nitric acid is appropriate. Nitrogen applications are not recommended where stability in storage is intended since the nitrogen addition might stimulate microbial activity.

Inhibition of microbial action by in situ production and/or addition of organic acids form the well known basis for preserving cut grass and other green crop residues in silage making. (Wilkinson, M 1990, Essential silage: Silage UK. Imprint Marlow : Chalcombe, 6^th Edition). Anaerobic bacteria ferment water soluble carbon with varying degrees of efficiency to end products consisting of acetic acid, lactic acid, ethanol and carbon dioxide. This acid production causes a decline in pH thus preserving the silage. Lactic acid bacteria may additionally produce inhibitory bacteriocins and other antimicrobial substances during silage production. (Hartnett DJ, Vaughan A, van Sinderen D (2002) Antimicrobial- producing lactic acid bacteria isolated from raw barley and sorghum. Journal of The Institute of Brewing 108 (2): 169-177). The stabilised biomass products referred to in this current patent are not ensiled materials (which depend for their preservation on relatively anaerobic conditions for the acidic by-products to persist). The present invention relates to aerated materials of generally lower moisture content where the access to gas exchange in perforated bags or an open stockpile and the generally lower moisture contents would not be conducive to such ensilage fermentation reactions. Furthermore, the typically low pH of ensiled biomass (pH 3 to 4) would be unsuitable for use as garden mulch (pH 6 to 6.5).

The fungistatic and fungicidal action of fatty acids, carboxylic acids and related compounds are well known (Wyss O, Ludwig BJ, Joiner RR (1945). The fungistatic and fungicidal action of fatty acids and related compounds. Archives of Biochemistry 7 (3): 415-425) and formulations from this group form some of the most common preservatives used in the food industry and in agriculture. These include the use of propionic acid and neutral propionate salts in products ranging from medicines, to bread and feed grain preservation on the farm.

A combined iron impregnation treatment and antimicrobial additives applied to biomass in combination with close control of water content during mixing and packing can produce mulches, growing media and their intermediate products that will not deteriorate during storage. A benefit of this combination of an iron impregnation treatment and supplementary preservative is that it enables products to be manufactured to a more consistent quality and distributed to end users or customers for later use without the risk of spoilage by heating and mould growth during storage and transport.

The process of this invention may make it possible to store the processed biomass in bulk or in enclosed plastic bags with an extended shelf life of up to twelve months without the quality deteriorating below that required for the intended use as mulch, soil conditioner or growing media constituent. Advantageously, it is possible to manufacture a garden mulch by a factory manufacturing process that is able to convert a raw material feedstock, for example fresh wheat straw, into a finished product that may be packed into bags and stacked on pallets ready for distribution on the same day. The mixing may be performed as either a single pass continuous or batch process, in both cases without the need for an extended period for storage of intermediates.

Examples of biomass materials which maybe used include: crop biomass straws, for example, wheat straw, barley straw, oat straw, rye straw, rape straw, pea straw, Lucerne, rice straw, flax, maize, miscanthus, sorghum and the like; raw and composted forest products, for example, wood, woodchips, wood flakes, sawdust, bark, composted wood and bark products and the like; forest leaf litter such as leaf and pine needle materials and the like; green wastes and other plant derived composts, mixed wastes, for example, garden and other green waste materials either before or after mechanical processing, composting or grading; processing wastes, for example, particulate and pulped wastes from food processing, sugar cane bagasse, residuals after extraction of oils, fibres and other non-food products for crops; blended composites, for example prepared mulches, growing media, mushroom composts (before and after composting); animal bedding waste containing crop biomass straws or raw and composted forest products may also be used.

The biomass material may be brought to a required particle size for example by chopping for the end product, dependant on the material and the intended end use.

Mulches are manufactured to a larger mean particle size or maximum chop length than constituents of plant growing media. This may require mechanical processing, such as chopping or milling in the case of crop straws. In other cases the biomass material may already be a process waste that does not require further milling or pulverisation. This initial biomass preparation stage may also require, mechanical separation, for example dust, seed removal, removal of other fines or of a coarse fraction for discard by screening.

A water content of 45 to 55 % w/w maybe used for growing media constituents designed to be stockpiled for blending into compost mixes.

For mulches destined for immediate bagging, a lower water content is necessary to give the required level of stability in storage. The lower water content during mixing of mulches for direct packaging may be in the range of 25% to 35% w/w. To produce a sufficiently intimate mix at this reduced water content, many basic types of mixing device commonly used in agricultural and food processing industries, such as a rotating drums with fixed blades or in- line spray equipment, may not be suitable for giving a sufficiently intimate mix of the added iron through the biomass material at lower water contents . The ability to produce a satisfactory and homogenously treated product mix at lower water contents requires the use of an appropriate mechanical mixing chamber (with augurs or an effective alternative mechanical homogenising mixing process) to rigorously force the additives into intimate association with the biomass at the reduced water contents.

The iron composition is preferably ferrous sulphate in an amount of 0.4 to 0.8g per lOOg of biomass by dry weight, equivalent to 4 to 8kg per tonne of biomass. Ferric sulphate or other iron containing compositions may be used. A larger amount may be applied where the iron is incompletely soluble or only partially in particulate or crystallised form.

In a preferred embodiment propionic acid or other carboxylic acid may be used to dissolve or partially dissolve a waste iron source such as scrap iron or iron hydroxide sludge. The resulting ferric propionate or other carboxylate can then form part or all of the iron added to the biomass. In this way the electrical conductivity of the finished product can be reduced by using less sulphate or other inorganic salt or salts.

Ferric sulphate is less soluble than ferrous sulphate and adds proportionately more sulphate to the finished product. Ferric sulphate can enhance colour development with some biomass materials. The amount used can be reduced proportionately when additional sources of iron are used. Ferric chloride may be used but is not preferred due to its corrosive nature.

Water soluble sesquioxide from a mineral species of iron and aluminium can be used as a partial or complete replacement for the ferrous or ferric salt. Suitable materials include waste from bauxite residue following aluminium extraction or soil materials with a high extractable iron content.

Iron hydroxide sludge maybe used in an amount of 50 to 300kg of wet dewatered sludge per tonne of biomass. A suitable sludge may be obtained from pumped ground water or from treatment of acid mine waste water. These materials can be applied in raw or treated form. Raw applications include slurries and dried powders of the product which may be physically mixed with the biomass. When the fraction of added total iron which is soluble and hence reactive with the biomass is low, the overall amount of iron is increased proportionately to give the required amount of soluble iron. Waste materials can be used to offset their disposal costs and also to add colour to mulches and to modify the water retention and wetting properties of growth media. Tannin may be added to assist in coagulation and binding of the iron hydroxide sludge to the biomass fragment surfaces.

Ferric propionate and related organic salt solutions including ferric citrate and ferric acetate may be used. The ferric salt may be obtained by reaction of the metallic iron waste or iron hydroxide. This has the advantage of neutralising potentially corrosive organic acids and avoids the need for corrosive acid compatable manufacturing plant. In addition sulphate or other anions are not required. The use of ferric propionate or an alternative ferric carboxylate has the advantage of combining preservation and stabilisation.

Ferric nitrate may be used where low electrical conductivity and neutral balance are important. Full or partial substitution of sulphate with nitrate salts provides a useful plant nutrient and replacement for an unwanted anion that would otherwise add to salinity. However, nitrate may not be used in circumstances where microbial activity may be a problem because it may stimulate mould growth and spoilage in storage of materials that would remain stable in the absence of added available nitrogen. Propionate or other carboxylate salts may be used, for example, sodium, calcium or ammonium salts. Use of calcium or sodium salts may have an advantage of supplying nutrients to the composition. Ammonium salts can be used to supply nitrogen.

Proprietary carboxylic acid formulations may be used, for example Sentinel-80 (Pathway Intermediates Ltd).

Iron may be applied in the form of ferrous sulphate (or other salt or derivative as described above) with a combined application rate of iron preferably of between 1 kg of Fe per tonne of dry weight equivalent biomass and 25 kg of Fe per tonne of dry weight equivalent biomass. Where a waste iron material is used and additional insoluble or un-reactive iron is added. A proportional amount of iron is used to achieve the same level of soluble iron. Iron can be applied in solution or only or partially in particulate or crystalline form. Propionic acid (or a alternative organic acid preservative) is used to dissolve (or partially dissolve) a waste iron source such as scrap iron or iron hydroxide sludge. The resultant ferric propionate, or other organic acid salts can then form all or part of the added iron for biomass treatment. This variation can be used to reduce the electrical conductivity of the finished product by reducing the amount of sulphate or other inorganic iron salt applied.

Application of octanoic acid (also known as caprylic acid), decanoic acid, propionic acid or a suitable combination of alternative antifungal agents from in Table 3 at an application rate appropriate for the type of biomass and water content. Optimal application rates vary with the specific formulation, biomass product and the need to minimise detrimental affects on plant growth when the product is used but are likely to lie between 0.1 kg per tonne of biomass and 10 kg per tonne of biomass.

Optional regulation of the pH of the finished product by adjustments to the absolute amounts of added iron and organic acids (Table 3) and the proportions of ferric to ferrous iron (subsequent oxidation releasing protons) and the proportions of organic acid to neutral organic salts. Optional removal of excess water by a heating and drying operation is permissible. In most cases this will not be required as it would be obviously wasteful to add more water than necessary for mixing only to have to go to the expense of adding a drying plant to the process. However, water removal maybe required when using biomass feedstock of high starting water contents, such as food process waste or sugar cane bagasse.

Optional heat treatment may be used to reduce seed viability.

Optional additions of mimosa bark tannin or alternative natural product and synthetic tannins to enhance colour development and contribute to preservation.

EXAMPLE 1

Ferrous sulphate solution was applied to chopped crop straw on the farm at an application rate of between 4 and 8 kg of Fe per tonne of straw and a total application of water of between 50% w/w/ and 60% w/w. The water and ferrous sulphate solution were either applied as a single dilute solution spray application or, alternatively, a more concentrated application of ferrous sulphate was followed by a drenching of the treated biomass with water. In one example chopped straw from the combine taken during or after the harvesting of a field crop was spray treated with a concentrated 30% solution of ferrous sulphate using a conventional crop sprayer to give the required total Fe addition of 6 kg of Fe per Tonne of straw. The sprayed crop straw was left on the soil surface or windrowed into elongated heaps. The spayed straw was then drenched with a single or multiple passes from a water tanker with a drip line to bring the crop straw up to the 50% w/w to 60%w/w water content. The treated biomass was then incorporated into the soil by cultivations or spread and left on the surface as a mulching layer. The deposition of the treated biomass as a surface layer on the soil surface or shallow windrows is important because there will be insufficient volume for heating to develop. Advantageously, further measures described above for enhanced stabilisation can be avoided by this means of storing the treated product. Treatment of the straw by this iron stabilisation process renders the material more resistant to decay in the field environment resulting in longer residence times for carbon fixed into the crop straw biomass. The net increase in stabilised carbon residence in the agricultural system will include any net increase in retention of the biomass in a mulching layer plus any net increase in residual retention of iron stabilised fragments of crop straw and iron stabilised decomposition products remaining in the soil over subsequent years. Advantageously, the straw treatment process can be repeated regularly over successive years of wheat cultivation leading to a further net accumulation of stored soil carbon above that 5 found if the same farming system was practised without iron treatment. Advantages to the farming system include greater retention of mulches to protect the soil surface, longer term increases in the soil organic matter content and crop yield increases resulting from greater surface protection and improved soil structure. Any verifiable increase in net carbon storage in the system may be eligible for subsidy or rental payment for the rental value of the increased l o storage of carbon in the agricultural system. Depending on the current discounted rental value per year of the verified net increase in sequestered carbon (or farm incentive payment available), sequestered carbon income may be used to offset a proportion of the application costs.

EXAMPLE 2 - Horticultural garden mulch prepared from wheat straw

15 Fresh organically grown wheat straw at 16 % w/w water content was taken directly from bales, chopped to a maximum chop length of 30 mm and passed through a hammer mill. Dust was removed using a cyclone chamber.

1000 kg of the chopped straw was fed into a mixing chamber containing rotating augers for mixing. 1.1 litres of octanoic acid (for example Sentinel 80 proprietary formulation extracted 0 as a natural product by fractional distillation of cocoanut oil), 40 kg of ferrous sulphate heptahydrate in damp crystalline form and 200 kg of water are added to the mix to raise the water content to approximately 37% w/w.

The materials are mixed thoroughly for 10 minutes to give a uniform mixture of water and treatment chemicals.

5 The finished product is turned out directly into 8kg or 12kg bags for storage and distribution on pallets. Examples 3 through to 6 below were prepared according to the same protocol as Example 1 above but with the end products destined for stockpiling prior to incorporation in compost mixes for plant growing media.

EXAMPLE 3 - Wood based (Light) without ferrous sulphate

1 Tonne of wood waste (<2.8 mm grade or less) 800 kg Water

300 kg Iron hydroxide sludge (dewatered iron removed by oxidation from a mine water treatment plant or dredged from drainage works but not allowed to dry out) 30 kg Mimosa tannin 2.2 litres octanoic acid 2.13 tonnes of total yield

EXAMPLE 4 - Wood based (Dark) with ferrous sulphate

1 Tonne of wood waste (<2.8 mm grade or less)

00 kg Water 800 kg Iron sludge (dewatered by not allowed to dry out) 30 kg Mimosa tannin 20 kg Ferrous Sulphate heptahydrate 2.2 litres octanoic acid

2.15 tonnes of total yield

EXAMPLE 5 - Wheat straw based (Light) low ferrous sulphate and tannin

1 Tonne of wood wheat straw meal (As for example used in pelleting processes) 1 Tonne Water

300 kg Iron sludge (dewatered by not allowed to dry out) 10 kg Mimosa tannin 10 kg Ferrous sulphate 2.2 litres octanoic acid 2.32 tonnes of total yield

EXAMPLE 6 - Wheat straw based (Dark) high ferrous sulphate and tannin

1 Tonne of wood wheat straw meal

1 Tonne Water

300 kg Iron sludge (dewatered by not allowed to dry out)

30 kg Mimosa tannin

15 kg Ferrous sulphate

2.2 litres octanoic acid

2.32 tonnes of total yield

Typical bulk densities and yields for Examples 2 - 5

EXAMPLE 7

This Example illustrates field treatment of crop residues of wheat cultivation in a conservation tillage management where the crop residues are left on the soil surface for erosion protection, water retention over the summer period and to protect the emerging wheat crop after planting. Chopped wheat straw leaving the combine during harvesting of a field crop was spray treated with a concentrated 30% solution of ferrous sulphate using a conventional crop sprayer mounted behind the straw chopper assembly on the combine. The spray bar application rate was adjusted to give the required total Fe addition of 6 kg of Fe per Tonne of straw. The sprayed crop straw was windrowed into elongated heaps. The spayed straw was then drenched with a single pass from a water tanker with a drip line to bring the crop straw up to the 50% w/w to 60%w/w water content. After 1 hour the treated straw windrows was spread and left on the surface of the field as a mulching layer. The recorded yield of straw for the harvest is estimated to be 1.80 Tonnes per hectare requiring the addition of 54 kg of Ferrous sulphate heptahydrate made up into a 30% solution, followed by drenching of treated windrows with water from a drip bar mounted behind a tanker regulated to deliver 2 cubic meters of water per hectare.

Claims

CLAMS:

1. A method of making an iron impregnated biomass comprising the steps of: adding or removing water to a quantity of biomass to provide a predetermined water content; adding an iron composition comprising a solution of a ferrous or ferric salt to the biomass to provide a concentration of 0.4 to 0.8g of soluble iron per lOOg of biomass by weight; and adding an effective amount of an anti-fungal agent to the biomass; wherein the anti¬ fungal agent is selected from the group consisting of: formic acid (methanoic acid), acetic acid (ethanoic acid), carboxyethane (propionic acid), butyric acid (n-butanoic acid), valeric acid (n-pentanoic acid), caproic acid (n-hexanoic acid), enanthoic acid (n-heptanoic acid), caprylic acid (n-octanoic acid), alpha-ethylcaproic acid (2-ethylhexanoic acid), valproic acid (2-propylpentanoic acid, ), pelargonic acid (n- nonanoic acid), capric acid (n-decanoic acid), salts of the aforegoing acids and mixtures thereof.

2. A method as claimed in claim 1 , wherein the biomass is selected from: crop biomass straws, raw and composted forest products, forest leaf litter, green waste, processing composites or animal bedding waste.

3. A method as claimed in claim 1 or 2, wherein the water content is 45 to 55%.

4. A method as claimed in claim 1 or 2, wherein the water content is 25 to 35%.

5. A method as claimed in any preceding claim, wherein the iron composition is ferrous sulphate.

6. A method as claimed in any preceding claim, wherein the carboxylic acid is propionic acid.

7. A method as claimed in any preceding claim, wherein the iron composition is ferric carboxylate.

8. A method as claimed in claim 7, wherein the iron composition is ferric propionate.

9. A method as claimed in any preceding claim, wherein the iron composition is an iron hydroxide sludge.

10. A method as claimed in any preceding claim, wherein the amount of antifungal agent is 0.1 to 10 kg per tonne of biomass.