Grain processing

Abstract

The present invention provides a process for producing biogas and/or methane from solid spent cereal products derived from, for example, the mashing process of malt whisk(e)y and/or beer production. There is also provided a system for producing biogas and/or methane from solid spent cereal products derived from, for example, the mashing process of malt whisk(e)y and/or beer production.

Claims

1. A self-sustaining method for fermentation of solid spent cereal waste products to produce a biogas and/or methane, comprising the steps of: (a) mixing the solid spent cereal waste products with a starter culture liquid to form a first slurry containing up to 20% of the solids by weight; (b) adjusting the pH of the mixture to a pH of 6.6-7.6 as necessary; (c) transferring the slurry to a digester and subjecting the slurry to anaerobic methogenic digestion to produce a biogas and a whole digestate; (d) removing whole digestate from the digester and separating the whole digestate into solid fiber and a digestate liquor; (e) reserving a portion of the digestate liquor; (f) adding the reserved portion of the digestate liquor to additional solid spent cereal waste products to form a second slurry; (g) transferring the second slurry to the digester without further pH adjustment; and (h) optionally repeating steps (d)-(g), wherein the starter culture liquid comprises a sludge from an existing anaerobic digestion that contains a mixture of microorganisms including methanogens; wherein the digestate liquor has high bicarbonate alkalinity buffering capacity; and wherein the solid spent cereal waste products comprise greater than 50% solid spent cereal waste and yeast.

2. The method of claim 1, wherein the method is continuous or semi-continuous.

3. The method of claim 1, wherein the method is performed in batches.

4. The method of claim 1, wherein the solid spent cereal waste products comprise solid spent cereal waste and yeast in a ratio of 1:3 to 3:1.

5. The method of claim 1, wherein the solid spent cereal waste products comprise greater than 90% solid spent cereal waste and yeast.

6. The method of claim 1, wherein the solid spent cereal waste products comprise greater than 20% protein by dry weight.

7. The method of claim 6, wherein the solid spent cereal waste products comprise 38-44% protein by dry weight.

8. The method of claim 6, wherein the solid spent cereal waste products comprise up to 44% protein by dry weight.

9. The method of claim 1, wherein the slurry comprises a dry solids content of up to 15% of the solids by weight.

10. The method of claim 1, wherein the slurry comprises a dry solids content of up to 10% of the solids by weight.

11. The method of claim 1, wherein the starter culture liquid and the digestate liquor contains at least 3000 mg/L calcium carbonate.

12. The method of claim 1, wherein the starter culture liquid and the digestate liquor contains 4000 mg/L-5000 mg/L calcium carbonate.

13. The method of claim 1, wherein the solid spent cereal waste products comprise draff.

14. The method of claim 1, wherein the solid spent cereal waste products further comprise a material selected from the group consisting of silage, spent pot ale, pot ale syrup, and any combination thereof.

15. The method of claim 1, wherein the starter culture liquid is obtained from a waste sewage sludge treatment plant or from a conditioned microbial population obtained from the anaerobic digestion of soluble distillery waste products.

16. The method of claim 1, further comprising subjecting the slurry to milling, grinding or homogenizing prior to its transfer to the digester.

17. The method of claim 1, further comprising subjecting the slurry to acidogenic digestion prior to anaerobic methogenic digestion.

18. The method of claim 1, further comprising subjecting the non-reserved portion of digestate liquor to evaporation to produce a liquid fertilizer or a solid fertilizer.

19. The method of claim 1, further comprising aerating the anaerobic digester with a portion of the biogas.

20. The method of claim 1, wherein the reserved portion of the digestate liquor comprises 3-10% of the digestate liquor.

Description

DETAILED DESCRIPTION

(1) The present invention will now be further described by way of example and with reference to the attached figure which show:

(2) FIG. 1 shows a schematic diagram of the malt distilling process and where draff and other waste materials may be derived and used in the present invention;

(3) FIG. 2 shows a schematic diagram of a grain distilling process and where solid waste products may be derived and used in the present invention;

(4) FIG. 3 shows a schematic diagram of a pot still all grains in distilling process and where solid waste products may be derived and used in the present invention;

(5) FIG. 4 shows a schematic diagram of a solid(s) digestion process in accordance with the present invention; and

(6) FIG. 5 shows the results of biogas production over time carried out by a process according to the present invention.

(7) FIG. 1 shows a schematic diagram of the malt distilling process. As can be see malted barley (10) is initially milled (12) before subjecting the milled malted barley to a mashing process (14). Following mashing, draff (16) is removed and the resulting liquid is subjected to fermentation (16). After fermentation, the resulting liquid is distilled (18) and the low wines fraction (20) separated for further distillation. The liquid remaining in the still is pot ale (22) which includes trace spent cereal solids and yeast, which may be in form of soluble solids or suspended solids. The pot ale (22) may be used directly to form a slurry comprising draff, or may be subjected to an evaporation process (24), in order to make pot ale syrup (26) and foul condensate (28), which can be further processed. Pot ale syrup (26) can be used to form slurry comprising draff, within a slurry vessel (28). The resulting slurry can be subjected to an initial acidogenic process, or directly to an anaerobic process to make biogas and liquid digestate.

(8) FIG. 2 shows schematically the essentials of a grain distilling process. The spent wash includes a soluble fraction and suspended solids which comprises spent cereals and yeast.

(9) The suspended solids may be recovered by decanting, filter pressing, membrane pressing, belt pressing or the like and the solids material comprising spent grain and yeast may be use in the present invention.

(10) FIG. 3 shows schematically the essentials of a pot still all grains in distilling process. Following pot still distillation, pot ale is removed and this pot ale includes soluble dissolved solids as well as suspended solids which comprise trace spent cereal solids and yeast. Akin to the grain distilling process described above, the suspended solids may be recovered by decanting, filter pressing, membrane pressing, belt pressing or the like and the solids material comprising spent grain and yeast may be use in the present invention.

(11) FIG. 4 shows schematically an example of a process in accordance with the present invention. Draff (17) is initially mixed in a slurry vessel (28) with high bicarbonate alkalinity anaerobic digestate (effluent stream from an anaerobic reactor or digester, see for example the attached appendix), pot ale (22) and/or pot ale syrup (26) to produce up to a 12% dry solids slurry on a wt/wt basis. The slurried draff then undergoes a reduction in particle size by mechanical means (42) to increase surface area and solubilisation rates in the anaerobic digester.

(12) By high bi-carbonate alkalinity digestate we mean digestate with a bi-carbonate alkalinity between 4,000 and 5,000 mg per litre expressed as calcium carbonate.

(13) Following particle size reduction the slurried draff is transferred to a digester vessel (44) and starved anaerobic sludge (taken from conditioned sludge prepared in accordance with the process described in WO2013104911) is added to start the anaerobic conversion process to biogas and liquid digestate. The digester vessel (44) is closed and contains an internal deformable gas hood (46), which expands within the vessel upon biogas production and evolution from the slurry.

(14) The starved anaerobic sludge was taken from an existing anaerobic reactor working on distillery spent solubles. Sludge addition rate is typically 5% of the total working volume of the digester. The sludge dry solids are measured for each experiment and are found to be approximately 3% dry solids.

(15) The operation is carried out under mesophilic conditions with digester maintained at 37 degrees centigrade+/−2C for the duration of the experiment. Some of the gas which is evolved is recirculated (50) in order to facilitate mixing of the slurry within the digester vessel (44).

(16) The anaerobic digestion process experiments are typically run for 35 days and both biogas and methane yields per tonne volatile dry substance are calculated.

(17) Gas which is collected can be sent to desulphurisation towers (50) to remove hydrogen sulphide, before being used to run a gas engine in order to produce electricity, or further cleaned in order to remove carbon dioxide and to provide clean methane which can be supplied directly to the national grid, for example, or as an alternative to scrubbing towers some oxygen may be introduced to the headspace of the digester to react with H.sub.2S and produce elemental sulphur.

(18) The liquid residue remaining after anaerobic digestion is known as whole digestate and can be transferred to a whole digestate vessel (56) This whole digestate may be separated further by mechanical means or filtration (58) into a solid fibre stream (60) and liquor (62), a portion (64) of which (being high in bicarbonate alkalinity) can be recirculated back to form new slurry.

(19) The remainder of the separated liquor digestate, which is the major portion, can be concentrated in order to provide a solution with desired levels of Na, K, P, for application to the land as fertiliser. The solid fibre portion can be applied to land as a soil conditioner, for example.

(20) Where filtration is used a partial suspended solids separation takes place followed by ultrafiltration and reverse osmosis. The retentate streams are richer in N:P:K as a result of concentrating up these fractions by filtration.

(21) Alternatively evaporation is also an option. Here the pH of the digestate should be adjusted to around pH to 5 to hold ammonium in solution for the evaporator process, otherwise this will lost as ammonia to the foul condensate fraction.

(22) Similar small scale experiments were carried out using 10 litre glass digester vessels with gas tubes leading to graduated water column collection vessels, where both total quantity of biogas and methane can be accurately measured. The experimental work for yield determination was typically carried out in duplicate digesters with a third digester operating on the same substrate that is used to examine the internal chemistry of the anaerobic conversion process. These experiments are repeated to ensure yields and rate of gas production can be replicated.

EXPERIMENTAL

(23) The key steps in the experimental protocol are outlined as follows: 1. Grain Distilling Spent Solids Preparation for Laboratory Anaerobic Trials (a) A sample of distillery spent grain comprising spent cereal solids and yeast is recovered from the heavy phase of distillery spent wash separation via: by decanter centrifuge or filter press or membrane press. (b) The spent grains sample is checked for moisture content and ash content. (c) The percentage volatile suspended solids dry basis is then calculated. (d) The distillery spent grains sample is made up to a 12%-20% dry solids slurry using digestate from an existing soluble stream anaerobic reactor or liquor stream from an anaerobic digester that is rich in bi-carbonate alkalinity and provides a natural slurry pH in range of 7.2-7.4. Note, that the bi-carbonate alkalinity of the digestate or liquor must lie in the range of 4,000-5,000 mg/litre expressed as calcium carbonate. (e) Alternatively if no digestate or liquor is available the sample may be made up to a 12%-20% dry solids slurry with water and pH adjustment by lime or sodium bi-carbonate to pH 7.2-7.4. 2. Milling of Grain Distilling Spent Solids (a) The slurry is then milled using a stick blender to reduce the particle size of the solids present. (b) Particle size reduction using the stick blender is carried out for 5 minutes per sample. 3. Anaerobic Sludge (a) Anaerobic sludge is sampled from an existing anaerobic reactor and stored under ambient conditions for a period of one week. The purpose of storage phase is to starve the sludge. (b) This is the seed sludge that will be added to the spent grains solids slurry. (c) The seed sludge dry solids are measured and are typically 5% suspended solids on a dry basis. 4. Anaerobic Fermentation—Biogas Volume and Methane Concentration (a) The distillery spent cereal solids slurry and sludge in pre-determined quantities are added to 10 litre glass anaerobic digesters. (b) The 10 litre glass digesters are placed in a water bath operating at a controlled temperature of 37 degrees centigrade. (c) The digester gas collection headspace is in turn connected to graduated water columns so that the biogas volume can be measured each day. (d) There is an additional connection from the digester that allows the carbon dioxide and methane content of the biogas to be determined. (e) Apart from total biogas volume and gas composition this method also allows the rate of gas production and methane production to be determined. 5. Anaerobic Fermentation Time (a) The anaerobic fermentation is allowed to run for a period of 35 days. 6. Evaluation of Biogas and Methane Yield (a) The total biogas volume collected is expressed as “x m3 biogas per tonne volatile dry solids.” (b) The total methane volume measured is expressed as “Y m3 methane per tonne volatile dry solids.” 7. Whole Digestate (a) The N:P:K values of the whole digestate are measured after 35 days anaerobic fermentation.

(24) Biogas volume and gas quality (averaging 60% methane, 40% carbon dioxide and trace H.sub.2S typically 300-700 ppmH.sub.2S) were measured on a daily basis throughout the duration of the experiment. The third digester was used to determine the internal chemistry via soluble COD, VFA, bi-carbonate alkalinity and pH. After 35 days anaerobic fermentation the whole digestate was collected and both the total solids and total suspended solids were measured. Additionally, soluble N:P:K levels were also measured.

(25) Gas measurements for total biogas, methane, carbon dioxide and H.sub.2S were taken every 24 hours. Volatile fatty acids were measured every 24 hours from the third digester. Very low concentrations of VFA were found throughout the 35 days suggesting that conversion to biogas is rapid as substrate becomes available i.e. as the draff substrate solubilises. The pH of the third digester was also measured every 24 hours and found to lie consistently between pH 7.2 and 7.4.

(26) Bi-carbonate alkalinity was also measured every 24 hours with a small increase noted over the period of 35 days. The digestate by the end of 35 days would typically show a bi-carbonate alkalinity of 4,500 to 5,000 mg per litre as calcium carbonate.

(27) Again the soluble Chemical Oxygen Demand was measured every 24 hours. Reading of less than 1,000 mg per litre COD were noted. Again this would suggest that substrate is converted to biogas as soon as it becomes available.

(28) The rate of gas production and the overall biogas and methane yield from draff benefit from a very small addition of micro-nutrients. Supplementation with very small (up to 5 ppm) quantities of cobalt, nickel and iron in the form of the chloride were seen to benefit the overall methane yield by some 10%.

(29) After a period of 35 days of anaerobic digestion the experiment was stopped and the fertiliser N:P:K values of the digestate are measured. Potentially Toxic Elements, potential pathogens and residual methane production were also measured in the digestate. In the UK there is a code of practise with specific limits for these parameters. This is known as PAS 110—Specification for whole digestate, separated liquor and separated fibre derived from the anaerobic digestion of source segregated biodegradable materials. The draff digestate was found to meet the limits in all cases.

(30) For draff a typical biogas yield range of 710 to 750 m.sup.3 biogas per tonne draff volatile dry substance is obtained after 35 days in the digester (see FIG. 3) The methane yield after 35 days range lies between 410 and 450 m.sup.3 methane per tonne volatile draff dry solids. Biogas composition is typically around 60% methane and 40% carbon dioxide.

(31) Solubilisation of draff over the period of 35 days equates to approximately 70-80% of the original dry matter.

(32) The rate of solubilisation and conversion to biogas when digestate high in bicarbonate alkalinity is used as the source of slurry preparation is rapid. Some 30% of the total biogas is produced within the first 48 hours. Thus, shorter slurry retention times could be optimised to take account of biogas production and solid solubilisation.

(33) The present inventors have carried out digestion of a number of products in accordance with the present invention and the results are shown in the table below. As can be seen, a variety of starting materials have been digested in accordance with the present invention.

(34) TABLE-US-00001 Anaerobic Digestion Substrates Tested - Laboratory and Plant Digesters Digester Digester Digester Organic Dry Average Retention Matter Methane No. Substrate Time Loading Yield 1 Malt Distillery Draff 35 days >5 kg's 410 m3/ ODM/m3/day tonne VSS 2 Malt Distillery Draff and 35 days >5 kg's 390 m3/ Pot Ale Syrup Combined ODM/m3/day tonne VSS 3 Grain Whisky Distillery 35 days 5 kg's 440 m3/ Solid Co-Product ODM/m3/day tonne VSS (Vitagold from Girvan Distillery) 4 Vodka Distillery Solid 35 days 5 kg's 400 m3/ Co-Product - Decanter ODM/m3/day tonne VSS Heavy Phase Solids 5 Canadian Whiskey Solid 35 days 5 kg's 420 m3/ Co-Product - Decanter ODM/m3/day tonne VSS Heavy Phase Solids