GB2528965A

GB2528965A - Anaerobic digestion

Info

Publication number: GB2528965A
Application number: GB1414046.1A
Authority: GB
Inventors: Edgar Blanco Madrigal
Original assignee: ANDIGESTION Ltd
Current assignee: ANDIGESTION Ltd
Priority date: 2014-08-07
Filing date: 2014-08-07
Publication date: 2016-02-10
Also published as: WO2016020673A2; EP3177411A2; WO2016020673A3; GB201414046D0

Abstract

A method of preparing feedstock for anaerobic digestion comprises heating green biomass 3 to a temperature of at least 6o Â°C and no more than 135 Â°C, preferably no more than 105 Â°C to produce a dried green biomass 8 having a dry solid content of at least 8o %. The green biomass may be waste from gardens, parks, farms, agricultural sites, woods or forests and includes material such as grass, plant cuttings, surplus fruit and vegetables, stalks, shrubs and tree branches. The dried green biomass may be separated into digestible leafy biomass which is transferred to an anaerobic digester and a woody biomass. Drying the green biomass renders the green biomass inert and inhibits natural decay during storage so maintains biogas potential. An apparatus for producing feedstock for anaerobic digestion comprises a dryer, which may be a conveyor belt drier, configured to heat the green biomass to a temperature of at least 6o Â°C and no more than 135 Â°C.

Description

Anaerobic digestion

Field of the Invention

The present invention relates to a method of and apparatus for preparing feedstock for anaerobic digestion. The present invention also relates to a method of anaerobic digestion.

Background

Anaerobic digestion is being increasingly used to convert food and animal waste into fertilizer and generate methane (often referred to as "biogas").

Anaerobic digestion can also be used to process plant material, such as maize and grass, and examples of anaerobic digestion processes of biomass can be found in Glenn Johnson et a!.: "The production of methane by the anaerobic decomposition of garbage i and waste materials", American Chemical Society Division Fuel Chemistiy, volume 16, page 70 (1972), Kestutis Navickas et al.: "Influence of different biomass treatment technologies on efficiency of biogas production", Engineering for rural development, page 586 (2012) and LV 14665 A which describes production of biogas using biomass having a relatively high dry matter content (>30%).

Although anaerobic digestion of green waste is desirable, the digestible fraction of green waste is not usually readily separable from the indigestible fraction thereby putting these potential sources of feedstock beyond the reach of most anaerobic digestion processes. Furthermore, the yield of biogas is poor.

Summary

According to a first aspect of the present invention there is provided a method of preparing feedstock for anaerobic digestion. The method comprises heating green biomass to a temperature of at least 6o °C and no more than 135 °C.

This produces dried green biomass having a dry solid content of at least 80 % or at east 8%. This can facilitate further processing of the green biomass, for example by making it easier to separate digestiNe leafy biomass from woody biomass, and can help to render the green biomass inert thereby inhibiting natura' decay during storage and so jo maintain biogas potential.

Leafy biomass may include grass, leaves, soft stalks, petals, fruit, vegetables, tubas and/or other soft plant tissue. Woody biomass may comprise branches, twigs and/or bark.

The method may comprise heating green biomass for sufficiently long so to result in the dry solid content of at east 8o % or at least 85%. The method may comprise heating green biomass for at least 1 minute, at least 2 minutes, at least 5 minutes, at least at least 10 minutes, at least 20 minutes or at least 50 minutes.

The green biomass may comprise green waste. The green waste may include grass, flowers, shrubs and/or trees. The green waste may include mass may inc'ude arable by-products or waste, such as sugar beet tops, potato tops, rapeseed tops and leaves and/or soft stalks of other similar crops. The green waste may include forestry by-products or waste. The green waste may include fruit and vegetables which are below

specification, e.g. incorrect size and/or shape.

The method may comprise heating the green biomass to a temperature of at least 70 DC, at Icast 75 °C, at Icast 80 °C or at Icast 8s°C. Thc grccn biomass is hcatcd to a temperature which is preferably at least 75 DC, more preferably at least 80 °C and still more preferaNy at least 8 °C. The method may comprise heating the green biomass to a temperature no more than 125 °C, no more than ii °C, no more than no °C, no more than io DC, or no more than 100 DC. The green biomass is heated to a temperature which is preferably no more than io °C and more preferably no more than 100 °C.

Heating the green biomass may comprise heating the green biomass in a conveyer belt dryer. Heating the green biomass may comprise heating the green biomass in a drum dryer, a cyclonic dryer, a fluidised bed dryer or other suitable dryer.

The method may further comprise separating the separated dried green biomass into dried leafy biomass (such as grass and leaves) and dried woody biomass (such as branches, twigs and bark). The method may comprise storing the dried leafy green biomass.

The method may comprise burning the dried woody biomass in a burner or boiler. The jo method may comprise using the burner or boiler to heat the green biomass. This can help increase energy efficiency.

The method may further comprise mixing the dried leafy green biomass with a different digestible material to form a feedstock suitable for anaerobic digestion.

According to a second aspect of the present invention there is provided a method of preparing feedstock for anaerobic digestion. The method comprises mixing leafy biomass which has been heated to a temperature of at least 60 °C and no more than 135 °C with a different digestible material to form a feedstock suitable for anaerobic digestion.

According to a third aspect of the present invention there is provided a method of preparing feedstock for anaerobic digestion. The method comprises dried leafy biomass having a dry solid content of at least 60 % and no more than 135 °C with a different digestible material to form a feedstock suitable for anaerobic digestion.

The digestible material may comprises food waste, sewage and/or digestate. The feedstock may have a dry solid content of no more than 15 % (i.e. the feedstock may be suitabic for a wet anaerobic digestion process) or no more than 40 %.

According to a fourth aspect of the present invention there is provided a method of anaerobic digestion. The method comprises preparing feedstock and feeding the feedstock into an anaerobic digester.

Preparing the feedstock may occur at the same site or plant as the anaerobic digester, or may occur at a different site or plant as the anaerobic digester.

According to a fifth aspect of the present invention there is provided a method of anaerobic digestion. The method comprises feeding a feedstock comprising a mixture of]ea1r biomass which has been heated to a temperature of at least 60 °C and no more than 135 °C and a different digestible material into an anaerobic digester.

The method may further comprise collecting biogas from the digester. The method may comprise burning at least some of the biogas in a gas burner or gas boiler. The method may comprise using the gas burner or gas boiler to heat the green feedstock.

o The method may comprise supplying at least some of the biogas to a gas grid.

The method may further comprise removing digestate from the digester and storing the digestate. The digestate may be stored for at least 6 months. The method may comprise using the digestate as a fertilizer.

According to a sixth aspect of the present invention there is provided apparatus for preparing feedstock for anaerobic digestion comprising a dryer configured to heat a green biomass to a temperature of at least oo °C and no more than i °C. This produces dried green biomass having a dry solid content of at least So %.

The dryer may be configured to heat the green biomass to a temperature of at least 70 °C, at least 75°C, at least 80 °C or at least 85°C. The dryer maybe configured to heat the green biomass to a temperature no more than 125 °C, no more than 115 °C, no more than no °C, no more than 105 °C, or no more than 100 °C. The dryer may be a conveyor belt dryer. The dryer may be a drum dryer, a cyclonic dryer or fluidised bed dryer or other or other suitable dryer.

The apparatus may further comprise a separator configured to separate the dried green biomass into dricd lcal' biomass and dricd woody biomass. The separator may bc a vibrating screen separator. The separator may be a cyclonic separator.

The dryer and separator maybe unitary.

The apparatus may further comprise storage for storing the dried leafy green biomass.

The apparatus may further comprise a burner or boiler for burning the dried woody biomass. The apparatus may be configured such that the burner or boiler is operable to heat the green biomass.

The apparatus may further comprise a mixer for mixing the dried leafy green biomass with a different digestible material to form a feedstock suitable for anaerobic digestion.

The mixer maybe a proportional pump.

According to a seventh aspect of the present invention there is provided apparatus for jo preparing feedstock for anaerobic digestion. The apparatus may comprise a mixer configured to mix dried leafy feedstock leafy biomass which has been heated to a temperature of at east 6o °C and no more than 135 °C and a different digestible material to form a feedstock suitable for anaerobic digestion.

The digestib'e material may comprise food waste.

The digestib'e material may comprises food waste, sewage and/or digestate. The feedstock may have a dry solid content of no more than 15 % or no more than 40 %.

According to an eighth aspect of the present invention there is provided an anaerobic digestion system comprising the apparatus for preparing feedstock for an anaerobic digester arranged to receive the feedstock.

The apparatus for preparing feedstock and the anaerobic digester may be disposed at the same site or plant or at a different site, for example, which may perform separation but not anaerobic digestion.

The system maybe configured to collect biogas from the digester. The system may comprisc a gas burncr or a gas boilcr for burning at Icast some of thc biogas. The gas burner or gas boiler may be configured to heat the green feedstock. At least some of the biogas (for example, substantiafly all of the generated biogas) may be supplied to a gas grid.

The system may further comprise storage for storing the digestate.

According to a ninth aspect of the present invention there is provided dried leafy feedstock leafy biomass which has been heated to a temperature of at least 60 °C and no more than 135°C or which has a dry solid content of at least 80%.

Brief Description of the Drawings

Certain embodiments of the present invention will now be described, by way of examp'e, with reference to the accompanying drawings, in which: Figure lisa schematic block diagram iflustrating a system for processing and digesting green biomass in accordance with the present invention; Figure 2 illustrates relative amounts of food waste feedstock and digestate resulting from anaerobic digestion of the food waste feedstock in a comparative example; Figure 3 illustrates relative amounts of green waste and food waste, a mixture of dried green waste and food waste and digestate produced from the mixture of dried green jo waste and food waste in accordance with the present invention; Figure 4 shows plots of generated biogas vohime against time for mixtures of food waste and different types of dried green waste in accordance with the present invention and comparative examples; and Figure 5 shows plots of generated biogas volume against time for different types of dried green waste in accordance with the present invention and comparative examples.

Detailed Description of Certain Embodiments

Referring to Figure 1, a system 1 for processing and digesting green biomass 2 from one or more sources 3 of green biomass is shown.

Green biomass 2 in the form of green waste can originate, for example, from gardens, parks and other municipal or public places, farms and other agricultural sites, woods and forests, hedgerows, commercial premises and industrial plants. It may include, for instance, grass and other plant cuttings, surplus or leftover fruit and vegetables, under specification fruit and vegetables, stalks, shrubs, tree branches and the like.

The system 1 is generally divided into two parts 4, 5. A first part 2 of the system 1 processes the green biomass 3 by separating soft plant material (herein referred to as "lca' grccn matcrial") from thc woody matcrial thcrcby making it casicr to digcst anaerobically and a second part 5 of the system carries out anaerobic digestion. The two parts 4,5 of the system 1 can be located at the same site or facility. However, they can be divided between different sites.

The green biomass 2 is received at the processing site 2 where it is held in storage 6 for a short period (typically a duration of a few hours to a few days) until it is processed.

Storage 6 may comprise simple holding bays or areas.

A chopper, shredder or other mechanical processing unit (not shown) can be used to break up the green biomass 2 and reduce the size of its constituents.

The green biomass 2 is fed into a dryer 7, for example, in the form of a conveyer belt dryer, which heats the green biomass 2 to a temperature of at least 80 °C or at least 85 °C, but no more than 135 °C or, preferably, no more than about 105 °C.

Without wishing to bound by theory, heating the green biomass 2 to such a o temperature can cause plant cell walls (not shown) to rupture and allow intra-ceilular water to be driven out of the green biomass 2, together with extra-cellular water.

However, the temperatures is not so high so as to breakdown digestible components of the biomass 2.

Heating the green biomass 2 to such temperatures results in dried green biomass 8 having a dry solid content of at east 80 %. By comparison, hay (i.e. naturally-dried grass) tends to have a dry solid content of no more than about o % even if it is dried in hot, dry climates.

Some types of green material (such as potato tops and sugar beet tops) and/or parts of a given green material may need to be heated to a higher temperature than other types green materials (such as rapeseed tops) and/or parts of a given green material to achieve the same minimum dry solid content.

The drying time can be of the order of seconds, minutes or even hours. The drying time varies according to the type, power and throughput of the drier 7.

A minimum drying time can be found by routine experiment by measuring the dry solid contcnt of thc matcrial bcfore and aftcr drying at differcnt tcmpcraturcs for diffcrcnt times. Dry solid content can be measured as described on page 2-55 in "Standard Methods for the Examination of Water and Wastewater" (20th Edition, American Public Health Association, 1998).

The dried green biomass 8 is substantially inert. Thus, the dried biomass 8 can be stored without it undergoing spontaneous decay. This can help to maintain the biogas potential of the green biomass 8.

The dried green biomass 8 consists of two components, namely dried leafy green biomass (for example, leaves of a shrub or tree, or blades of grass) and dried woody green biomass (for example, twigs, branches and bark). Leafy materia' is generally digestible, whereas woody material is not digestible. The dried green biomass 8 is fed into a separator 9, for example, a vibrating screen separator. The separator 9 separates the dried green biomass 8 generally into dried leafy material 10 and dried woody material 11.

jo The dried woody material 11 can be fed into a burner or boiler 13 to be combusted and generate heat 14. Tt can be used to heat the green biomass either directly (e.g. using exhaust gas) or indirectly (e.g. via a heat exchanger). Additional fuel 3 may be fed to the burner or boiler 12, if required.

The dried leafy green biomass 10 is collected and may be held in storage 15. As explained earlier, the dried leafy green biomass lois substantially inert and so, provided it is not re-hydrated, can be stored for extended periods (e.g. weeks or months) without it undergoing unwanted spontaneous decay. Moreover, dried leafy green biomass 10 takes up a smaller volume.

The dried leafy green biomass 10 is fed into a mixer i6 and is mixed with digestible material 17 from another source 18 to produce a feedstock 19. The digestible material 17 may be food waste, animal waste or other type of waste, or a mixture of different types of waste. The material 17 may be or include active digestate, i.e. partially-digested feedstock. The biomass 10 and other digestible material 17 are mixed to yield a feedstock 19 for a dry anaerobic digestion process, i.e. one having a dry solids content of between 15 % and about 40 %.

Anacrobic digcstion produccs biogas 21. Thc biogas 21 may bc fcd to a gas burner or boiler 22 to be combusted and generate heat 14. The heat i4 can be used to heat the green biomass either directly (e.g. using exhaust gas) or indirectly (e.g. via a heat exchanger). Additionally or alternatively, the biogas 21 maybe supplied to the gas grid 23 for domestic or industrial use.

Digestate 24 is taken from the digester 20. It can be dried using a dryer (not shown) and/or maybe compressed and shaped into pellets using a pelletizer (not shown). -10-

The digestate 24 (in wet or dry form) can be stored in suitable storage 25, e.g. tanks.

The digestate 24 can be used as a fertiUzer for growing crops, which may be being cultivated for use as a green biomass.

Processing the green biomass 2 and using dried leafy green biomass can have one or more advantages.

First, it can enable material which, up until now has not been digestible by common o types of anaerobic digestion processes, to be digested. Secondly, it can reduce the mass of digestate 24 produced, while maintaining the volume of the biogas produced.

Third'y, it can maintain or increase the amount of biogas recovered from the green biomass. Fourtffly, it can reduce the volume of feedstock requiring storage.

Figure 2 illustrates relative amounts of food waste feedstock and digestate produced from anaerobic digestion of the food waste feedstock. Figure 3 illustrates relative amounts of green waste and food waste, a mixture of dried green waste and food waste and digestate produced from the mixture of dried green waste and food waste.

Referring to Figure 2, for every unit of mass of food waste, about 0.83 units of digestate is produced. For example, 80 kilotonnes of food waste results in 66 kilotonnes of digestate.

Referring to Figure 3, starting with equa' amounts (so/so volatile solids) of fresh green waste (including grass) and food waste (i.e. 0.5 units of mass of fresh green %sraste and 0.5 units of mass of food waste) drying the fresh green waste results in 0.20 to 0.22 units of dried green waste. The green waste is dried overnight at 105°C. Anaerobic digestion of a mixture of dried green waste and the food waste results in about 0.36 units of mass of digcstatc. Thus, for 1 unit of mass of food wastc and 1 unit of grccn waste, about 0.72 units of digestate is produced. Consequently, less digestate need be handled, e.g. stored and transported.

Figure 4 shows resulls of a kinetic study of biogas production for food waste, mixtures of food waste and different types of fresh green waste, and mixtures of food waste with different types of dried green waste.

-11 -The green waste includes mixed green waste, grass and elder. The green material was dried overnight at 105 DC in accordance with the present invention and mixed equally (i.e. 50:50 volatile solids) with the food waste. For comparison, biogas potentials were also measured food waste only, a mixture of food waste and mixed fresh green waste, a mixture of food waste and fresh grass and a mixture of food waste and fresh elder. The fresh green material is mixed equally (i.e. o:so volatile solids) with the food waste.

Referring to Figure 4, the measurements show that drying the green material helps to increase biomass potential.

Figure shows results of a kinetic study of biogas production different types of fresh green waste and corresponding dried green waste. The green waste includes grass, corn, elder and nellie.

Referring to Figure 5, the measurements show that samples using green waste that has been dried at io °C result in higher biomass potential.

It will be appreciated that many modifications may be made to the embodiments hereinbefore described.