Method and Apparatus for Generating Commodities from Biomass

Abstract

A method comprising: anaerobically digesting a first biomass feedstock 1 to produce biogas 3 and digestate 2 , feeding the digestate to a digestate reservoir 50, using a portion of the biogas to gener

Description

Technical Field The invention relates to a method and apparatus for generating commodities from biomass, 5 for example agricultural waste biomass such as cereal, manure, and silage. In particular, the apparatus and method relate to the generation of one or more of: biogas, fertiliser, electricity, bio-methane and CO2 in a system containing anaerobic digestion of a first biomass feedstock and the drying of a second biomass feedstock. 10 Background It is known in the art to use a system comprising anaerobic digestion for processing biomass, such as agricultural byproducts, to generate biogas. Anaerobic digestion takes the biomass as an input feedstock to a digester, which converts the biomass into a digestate of the digested feedstock and produces a biogas comprising a high proportion of 15 biogas. Biogas is widely used in a variety of applications. For example biogas is commonly used as fuel for combustion in order to generate electricity and heat. For example, biogas can be combusted in a Combined Heat and Power (CHP) generator to generate electricity for use 20 and commerce and recover heat.. Biogas is also often burnt to generate heat directly such as in a boiler to produce hot water. Anaerobic digestion produces large quantities of liquid digestate (digested feedstock), and it is conventional to deposit this digestate in large outdoor digestate reservoirs, also known 25 as digestate lagoons. As the digestate produced by anaerobic digestion typically has a high nutrient content, it serves as an effective organic fertiliser. Therefore, the digestate reservoir serves as both convenient disposal of the digestate byproduct and a storage location for fertiliser. 30 WO2023027590A1 discloses a method for treatment of organic waste comprising a stage of fermentation, followed by a stage of anaerobic digestion, to recover energy, nutrients, and carbon storage material. The process further comprises a stage where wet biomass is dried in a dryer to produce dry biomass. There is no heat exchanger to exchange heat between the dryer and the digestate produced by the first stage of digestion. 27 03 25 US20170275583A1 discloses a method and system for processing biomass comprising a single stage of anaerobic digestion to produce biogas, a drying stage to produce dry biomass to fuel electricity generation, and a heat recovery system. The additional biogas present in the digestate output of the anaerobic digestion is not released. The heat 5 recovery system does not dry a biomass feedstock separate to that which is used to produce biogas via anaerobic digestion. Summary of Invention The invention provides a method and apparatus for generating commodities from biomass 10 as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent subclaims. Method of Generating Commodities from Biomass 15 According to a first aspect of the invention there may be provided a method for generating one or more commodities from biomass, comprising the steps of: anaerobically digesting a first biomass feedstock to produce biogas and digestate, feeding the digestate to a digestate reservoir, using at least a portion of the biogas to generate hot air; drying a second biomass feedstock in a biomass dryer using the hot air, and recovering warm air 20 from the biomass dryer, directing the warm air through a heat exchanger to recover heat from the warm air, and transferring the recovered heat from the heat exchanger to the digestate reservoir, so that the recovered heat is used to increase the temperature of the digestate in the digestate reservoir; and capturing biogas generated from the heated digestate in the digestate reservoir. 25 The present invention may advantageously be a method of increasing the efficiency of biogas generation from biomass feedstock. The first biomass feedstock and / or the second biomass feedstock may be agricultural 30 waste biomass, or agricultural byproduct biomass. The method may be a method for generating one or more of: biogas, fertiliser, electricity, hot water, bio-methane and CO2 from biomass feedstock. Thus the commodities generated by the method may be one or more of: biogas, fertiliser, electricity, hot water, bio-methane 35 and CO2. Dried second biomass feedstock is also produced. 27 03 25 The method may be a method for generating biogas from biomass, or a method for generating biogas and fertiliser from biomass. The method may be a method for generating electricity and fertiliser from biomass. The method may be a method for generating bio- 5 methane, electricity, fertiliser and optionally CO2 from biomass. The method may be a method of generating biogas and electricity from a biomass feedstock. The method may in some embodiments be a method of generating biomethane, and optionally electricity, and optionally fertiliser, from a biomass feedstock. The method 10 may in some embodiments be a method of generating biomethane and CO2, and optionally electricity, and optionally fertiliser, from a biomass feedstock. The method may comprise the step of converting at least a portion of the biogas into electricity, for example in an electricity generator or combined heat and power (CHP) 15 system. The method may comprise the step of purifying at least a portion of the biogas to form biomethane. The method may comprise the step of capturing CO2 removed from the biogas during purification of biogas into bio-methane. 20 The method may comprise the step of removing at least a portion of the digestate from the digestate reservoir, and using the removed digestate as a fertiliser. Anaerobic digestion of the first biomass feedstock is a biological process which breaks 25 down the organic matter of the first biomass feedstock in the absence of oxygen. This produces biogas comprising a high proportion of methane, and a digestate which may be a liquid digestate or may comprise a liquid digestate. By directing the warm air through a heat exchanger to recover heat from the warm air, and 30 transferring the recovered heat from the heat exchanger to the digestate reservoir, the present method heats the digestate in the digestate reservoir. In conventional arrangements where an outdoor digestate reservoir stores the digestate, the temperature of the digestate is determined by the ambient temperature and weather 35 conditions. This results in the digestate being maintained at a fairly low temperature, such 27 03 25 that the bacteria responsible for biomass digestions cannot perform any further digestion after the digestate leaves the digester and enters the reservoir. Heating of digestate reservoirs is not done in the prior art, as heating large outdoor reservoirs would be expensive, and is also seen as unnecessary, as the primary function of the digestate 5 reservoir is to store digestate until it is used as fertiliser. The bacteria responsible for digesting biomass and generating biogas operate best at higher temperatures, ideally at least 28 degrees Celsius (°C), and cease to function at the low temperatures found in conventional digestate reservoirs. 10 By transferring recovered heat into the digestate reservoir, the aggregate temperature of the digestate in the digestate reservoir increases to a temperature higher than ambient temperature. This advantageously results in a second stage of digestion taking place in the reservoir, as in the warmed digestate reservoir the digestate can be further digested by 15 bacteria. The higher digestate temperatures created by this heating step are more optimal for the bacterial activity which drives biomass digestion. This second stage of digestion produces additional biogas in the digestate reservoir and this additional biogas is captured for use in a suitable application. This biogas may be purified to biomethane, for example. 20 Compared to prior art systems where the digestate is stored at low temperatures where no secondary digestion can occur in the digestate reservoir, the present method therefore produces more biogas from a given quantity of biomass feedstock, thus increasing the efficiency of the biogas production and having a positive environmental impact. 25 Unlike WO2023027590A1 or US2017275583A1, in the present system the digestate is delivered to a digestate reservoir, wherein the digestate reservoir is preferably an outdoor in-ground reservoir. These documents also do not teach the step of recovering waste heat and using that recovered heat to increase the temperature of digestate in a digestate reservoir. Neither of these documents therefore disclose the use of recovered heat to drive 30 a second stage of digestion in the digestate. As is well known in the art, the gases produced by anaerobic digestion of biomass typically contain a mixture of gases in varying amounts. In the present method, the anaerobic digestion of the first biomass feedstock produces a biogas mixture with a high methane 35 content, for example biogas containing at least 50 % methane and 40-50% CO2, preferably 27 03 25 biogas containing 50-60 wt% methane - this gas product is referred to herein as “biogas”. The second stage of digestion in the digestate reservoir also generates biogas containing, for example, 50-60% methane as well as 40-50% CO2. 5 Bio-methane typically refers to bio-originated gases with a methane content of at least 95%. Typically it is necessary to refine and purify biogas in order to increase its methane content to a level that qualifies as bio-methane. This can be done using bio-methane upgrading systems which are known in the art. 10 The present method advantageously provides a method of processing a first biomass feedstock to generate biogas, and of drying a second biomass feedstock, whilst recovering waste heat from the system and using that recovered waste heat to facilitate additional biogas production in the digestate reservoir. 15 The present method may preferably be a method of processing a first biomass feedstock to generate one or more of the following products: biogas, electricity, fertiliser, bio-methane and CO2. A wide variety of biomass feedstocks may be used in the present method. In a preferred 20 embodiment, for example, the biomass feedstock may be comprised of agricultural byproducts. Suitable agricultural byproducts include, but are not limited to, cereal, manure, or silage, or any combinations thereof. The step of using at least a portion of the biogas to generate hot air may be achieved in a 25 variety of ways. The hot air may be generated directly from the combustion of biogas or the hot air may be produced from the generation of hot water. In certain embodiments, the step of using at least a portion of the biogas to generate hot air comprises burning the at least a portion of the biogas in an electricity generator to generate 30 electricity, so that the method is a method of generating biogas and electricity. In such embodiments, the heat from the electricity generator may preferably be used to heat the hot air. Optionally, the method may comprise the additional step of delivering the captured biogas 35 from the digestate reservoir to the electricity generator to generate additional electricity. 27 03 25 The electricity generator may advantageously be a Combined Heat and Power (CHP) electricity generator. The inventor of the present system has realised that burning the at least a portion of the biogas in an electricity generator can advantageously provide both 5 process heat, and electricity which may be used to meet site-electricity requirements or may be sold to the grid to generate additional revenue. Unlike the present invention, in US2017275583A1, electricity is generated by the combustion of dried digestate, following the anaerobic digestion of a biomass feedstock. 10 In certain embodiments, the step of using at least a portion of the biogas to generate hot air comprises burning the at least a portion of biogas in a biogas boiler to produce hot water, and generating hot air from the hot water. In such embodiments, the method may optionally comprise the additional step of delivering the captured biogas from the digestate reservoir 15 to the biogas boiler to generate additional hot air. In preferred embodiments, the step of using at least a portion of the biogas to generate hot air comprises burning at least a portion of the biogas in an electricity generator to generate electricity, in which heat from the electricity generator heats the hot air; and burning at least 20 a portion of biogas in a biogas boiler to produce hot water, wherein hot air is generated from the hot water. In such embodiments, the method may optionally comprise the additional step of delivering the captured biogas from the digestate reservoir to the electricity generator to generate 25 additional electricity. Alternatively, or additionally, the method may comprise the step of delivering the captured biogas from the digestate reservoir to the biogas boiler to generate additional hot air. The hot air generated from the biogas is used to dry a second biomass feedstock in a 30 biomass dryer. This use for the hot air advantageously improves the overall efficiency of the system, as instead of exhausting the hot air, the heat energy in the hot air is used to dry a second feedstock which requires drying. By using the hot air as a heat source to dry a second biomass feedstock in a biomass 35 dryer, the drying process evaporates moisture from the second biomass feedstock, and 27 03 25 exhausts warm air from the biomass dryer at a high relative humidity. Typically this humid warm air would be exhausted to the atmosphere, as the use of the hot air for the drying step has already improved the efficiency of the system and used up much of the waste heat from the hot air. In the present system, however, the thermal efficiency is yet further 5 improved by directing the warm air to a heat exchanger. The terms “hot air” and “warm air” are used in the present disclosure to designate the relative temperatures of the heated air before and after the second feedstock drying step. As this drying step involves the loss of thermal energy from the air into the second biomass 10 feedstock, the temperature of the air exhausted form the biomass dryer is inevitably lower than the temperature of the hot air before the drying step. In some preferred embodiments, the term hot air may refer to air at a temperature of at least 50 degrees Celsius (°C), or at least 55 °C, or at least 60 °C, for example between 50 15 °C and 70 °C, or between 55 °C and 65 °C. In some preferred embodiments, the term warm air may refer to air at a temperature of at least 25 °C, for example between 25 °C and 45 °C, or between 30 °C and 40 °C. 20 The warm air may have a temperature of at least 25 °C and a specific humidity of at least 20 g / kg. The warm air may be “high specific humidity air” which has a specific humidity greater than 20 g / kg (>20g water per kg air). The warm air may have a temperature of at least 25 °C, or at least 27.5 °C, and a specific humidity greater than 20 g / kg, or greater than 22 g / kg, or greater than 24 g / kg. 25 The present applicant has found that such high specific humidity warm air is particularly effective for heat recovery in the heat exchanger, as the condensate which forms in the heat exchanger aids in heat transfer. 30 Preferably, the method comprises the step of refining or purifying the captured biogas from the digestate reservoir, and / or the captured biogas from the anaerobic digestion, to produce bio-methane. In such embodiments a first portion of the biogas generated by anaerobically digesting a first biomass feedstock may be used to generate hot air, and a second portion of the biogas generated by anaerobically digesting a first biomass feedstock 35 may be converted to bio-methane. Optionally, the method may comprise the additional step 27 03 25 of converting at least a portion of the captured biogas from the digestate reservoir to biomethane. The inventor of the present system has realised that refining, or upgrading, the generated 5 biogas or captured biogas to produce bio-methane may advantageously provide a more energy-dense and versatile output product. Unlike unrefined biogas, bio-methane comprises over 95% methane which allows it to be used as a direct substitute for natural gas due to its similar methane content and consistent combustion characteristics. Additionally, bio-methane comprises significantly less Carbon Dioxide than unrefined 10 biogas which provides a positive environmental impact. The steps of recovering heat from the warm air, and transferring the recovered heat from the heat exchanger to the digestate reservoir, may be accomplished in a heat exchanger that is configured to exchange heat between the warm air and a heating loop, or in a heat 15 exchanger that is configured to exchange heat between the warm air and digestate liquid being circulated to the digestate reservoir. Preferably, the step of transferring the recovered heat from the heat exchanger to the digestate reservoir comprises transferring heat via a heating loop. For example, a closed 20 loop filled with heating liquid may extend between the heat exchanger and the digestate reservoir, so that pumping the heating liquid around the heating loop transports heat from the heat exchanger to the reservoir. More preferably the step of transferring the recovered heat from the heat exchanger to the digestate reservoir comprises transferring heat through a water-filled heating loop. 25 Optionally, the method comprises a step of collecting condensate generated from the warm air in the heat-exchanger. As warm air passes through the heat exchanger and loses heat, condensate will be condensed out of the warm air. This condensate may be collected from the heat exchanger, for example in a container, or the condensate may be recirculated for 30 use nearby, for example in the anaerobic digester. The condensate may advantageously condense impurities such as ammonia out of the warm air stream that is exhausted from the biomass dryer. The use of the heat exchanger to encourage condensation may thus advantageously reduce the proportion of odours and polluting impurities which are exhausted to the atmosphere. 27 03 25 Preferably, the second biomass feedstock and the first biomass feedstock are different types of feedstock. More preferably, the second biomass feedstock comprises cellulosic pulp, for example paper pulp. The biomass dryer used to dry the second biomass feedstock may be a pulp dryer. 5 In a preferred embodiment, biogas and fertiliser are produced from the digestate in the digestate reservoir. In the method of the present invention, the digestate in the digestate reservoir is increased in temperature to facilitate the bacteria within the reservoir which drive a second digestion stage, producing biogas and a digestate. Therefore, the digestate 10 reservoir stores digestate from both the anaerobic digestion in the digester as it undergoes secondary digestion within the digestate reservoir. As this nutrient rich digestate is held in the digestate reservoir, it continually undergoes maturation which makes the nutrients present in the digestate more readily available for absorption. This makes for an effective organic fertiliser which can be removed from the reservoir as a system output. 15 Biogas is advantageously generated by anaerobic digestion from the heated digestate in the digestate reservoir. In such an embodiment, the digestate reservoir is heated to temperatures that enable higher digesting activity for multiple mesophilic bacterial groups. These multiple mesophilic bacterial groups drive anaerobic digestion in multiple stages 20 comprising, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Preferably, the digestate in the digestate reservoir is warmed by the heat transferred from the heat exchanger to a temperature of at least 20 °C, or at least 22 °C, or at least 24 °C, or at least 26 °C. More preferably, the digestate in the digestate reservoir is warmed to a 25 temperature of at least 27 °C. Most preferably, the digestate in the digestate reservoir is warmed to a temperature of at least 28 °C by the heat transferred from the heat exchanger. Temperatures above 28 °C have been found to generate significantly increased levels of secondary digestion in the digestate reservoir, as digesting bacteria are most active above this threshold temperature. Any increase in temperature above ambient atmosphere, 30 however, may advantageously create an increase in the bacterial activity in the digestate reservoir, and thus an increase in the quantity of biogas captured from the reservoir. Apparatus for Generating Commodities from Biomass 27 03 25 According to a second aspect of the invention there may be provided an apparatus for generating commodities from biomass. The apparatus may comprise an anaerobic digester for digesting a biomass feedstock to produce biogas and digestate; a digestate reservoir configured to receive and store digestate from the anaerobic digester; an electricity 5 generator and / or a biogas boiler configured to burn the biogas and to produce hot air; a biomass dryer, configured to receive the hot air, and to exhaust warm air from an exhaust; and a heat exchanger, configured to exchange heat between warm air received from the biomass dryer exhaust and the digestate reservoir. 10 The apparatus may be an apparatus for generating one or more of: biogas, fertiliser, electricity, bio-methane and CO2 from biomass. Thus the commodities generated by the apparatus may be one or more of: biogas, fertiliser, electricity, bio-methane and CO2. The apparatus may be an apparatus for generating biogas from biomass, or an apparatus 15 for generating biogas and fertiliser from biomass. The apparatus may be an apparatus for generating electricity and fertiliser from biomass. The apparatus may be an apparatus for generating bio-methane, electricity, fertiliser and optionally CO2 from biomass. Anaerobic digesters are well known in the art, and any commercially available digester that 20 is suitable for digesting biomass feedstock to produce digestate and biogas may be used in the apparatus of the second aspect. The apparatus may advantageously comprise an electricity generator configured to burn the biogas and to produce electricity and hot air, and / or a biogas boiler configured to 25 produce hot water, and to generate hot air from the hot water. Biomass dryers are also known in the art, and any commercially available biomass dryers which is suitable for drying wet biomass with a stream of hot air, and exhausting a stream of warm air, may be used in the apparatus of the second aspect. The biomass dryer is 30 preferably configured to receive the hot air from the electricity generator and / or biomass boiler, and to dry a second biomass feedstock using the hot air. The heat exchanger is configured to exchange heat between warm air received from the biomass dryer exhaust and the digestate reservoir. The heat exchanger may, for example, 35 be a gas-liquid heat exchanger that is configured to receive a stream of warm air through a 27 03 25 first flow path, and to receive a stream of liquid through a second flow path, and to transfer heat from the warm air in the first flow path into the liquid in the second flow path. A variety of heat exchangers are usable in the present apparatus. For example the heat 5 exchanger may be a plate heat exchanger or a shell and tube heat exchanger. In preferred embodiments, the apparatus may comprise a heating loop extending between the heat-exchanger and the digestate reservoir. The heating loop may be filled with a heating liquid which carries heat through the loop. In such embodiments, preferably, the 10 heating loop is a water-filled heating loop. The heat exchanger may thus be configured to transfer heat from a stream of warm air through its first flow path, into a stream of heating liquid in the second flow path. In preferred embodiments, the apparatus comprises a pump configured to circulate a 15 heating liquid through the heating loop. In preferred embodiments, the apparatus comprises a reservoir temperature sensor, the reservoir temperature sensor being configured to sense a temperature of digestate in the digestate reservoir. In such embodiments, the apparatus may further comprise a controller 20 which is configured to receive signals from the reservoir temperature sensor, and to control operation of the pump. Preferably, the controller is programmed to operate the pump so that the digestate in the digestate reservoir is warmed to a temperature at or above a reservoir target temperature. The reservoir target temperature may be a temperature of at least 22 °C , or at least 24 °C, or at least 26 °C. More preferably, the controller is 25 programmed to operate the pump so that the digestate in the digestate reservoir is warmed to a reservoir target temperature of at least 27 °C, or at least 28 °C. The controller may advantageously control or maintain the desired temperature of the digestate in the digestate reservoir by altering the flow rate of heating liquid to and from the 30 heat exchanger. If the controller receives a signal reading from the temperature sensor that the digestate is below the desired temperature it may turn the pump on, or increase the pump speed to increase the flow rate of heating liquid. If the controller receives a signal reading from the temperature sensor that the digestate is above the desired temperature it may decrease the pump speed, or turn the pump off. 27 03 25 By using a programmed controller to regulate the pump that circulates the heating liquid through the heating loop in the digestate reservoir based upon the reading of the reservoir temperature sensor, the apparatus may advantageously consume the minimum amount of power required to maintain the temperature in the digestate reservoir at or above the 5 reservoir target temperature. This ensures that the system uses as little additional electricity as possible to drive the pump at a rate at which the increased bacterial activity drives the secondary digestion in the digestate reservoir. The additional biogas generated by the secondary digestion is thus generated using the minimum required amount of electricity and heat. This advantageously reduces the financial and energetic cost of 10 running the apparatus, and has a positive environmental impact by maximising the efficiency of biogas production. In certain embodiments, the heat-exchanger is configured to collect condensate generated from the warm air in the heat-exchanger. As discussed above, this may advantageously 15 capture and remove pollutants such as ammonia which would otherwise be exhausted as airborne pollutants into the atmosphere. In preferred embodiments, the apparatus comprises a biogas purification system, also known as a bio-methane upgrading system, for producing bio-methane from biogas. In 20 such embodiments, the biogas purification system may be configured to receive biogas generated in the digestate reservoir and / or a portion of the biogas generated in the anaerobic digester, and to purify the received biogas into bio-methane. In certain embodiments, the biomass dryer is a pulp dryer. 25 The apparatus of the second aspect is an apparatus for carrying out the method of the first aspect. Features described in relation to the first aspect are therefore applicable to the second aspect, and vice versa. 30 Brief Description of the Figures Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment of the process according to the invention; 27 03 25 Figure 2 shows a preferred embodiment of the heat exchanger usable in the present invention; Figure 3 is a schematic illustration of an apparatus according to a first aspect of the present 5 invention; Figure 4 is a schematic illustration of an apparatus according to a second aspect of the present invention; 10 Figure 5 is a perspective view of an apparatus according to an embodiment of the present invention; Figure 6 is a process flow chart according to a preferred embodiment of the present invention; 15 Figure 7 is a process flow chart according to an alternative preferred embodiment of the present invention; Figure 8 is a process flow chart according to another preferred embodiment of the present 20 invention. Detailed Description Figure 1 is a flow diagram showing an embodiment of the process according to the present invention. 25 A biomass feedstock (1), which can be for example cereal (1a), manure (1b), or silage (1c), or any combination thereof, is delivered to an anaerobic digester (10) which digests the biomass feedstock and produces a liquid digestate (2) and biogas (3). 30 Inside the anaerobic digester (10) there is an oxygen-free environment which is favourable for anaerobic bacteria to break down the biomass feedstock (1). The metabolic process of the bacteria generates biogas with a high methane content and leaves behind a liquid digestate (2). The biogas (3) has a methane content of at least around 52-54 wt%. 27 03 25 The liquid digestate (2) is extracted from the digester after anaerobic digestion has taken place, and transferred to a digestate reservoir (50) for storage. The digestate reservoir (50) is an outdoor, in-ground reservoir in which the liquid digestion is kept until it is eventually removed for use as an agricultural fertiliser (4). 5 Although any suitable digestate reservoir may be used in the present invention, conventional digestate reservoirs can be very large - for example approximately 75 meters by 100 meters. 10 The digestate reservoir (50) is covered with an impermeable gas-tight membrane, so that gases generated in the reservoir are captured and collected. In the illustrated embodiment, the biogas (3) generated in the anaerobic digester (10) is delivered to a biogas boiler (60) and a CHP generator (70). In alternative embodiments, at 15 least some of the biogas may be delivered to a biogas purification system and converted into bio-methane. Inside the biogas boiler (60), a portion of the biogas (3) is combusted for hot water generation to produce hot water. 20 Inside the CHP generator (70), a portion of the biogas (3) is combusted for electricity generation and heat generation. During electricity generation a significant amount of heat is released as a byproduct which is transferred to a water circuit to produce heat recovery hot water (71). The CHP generator (70) preferably produces enough electricity to supply 25 electricity for site usage (72a) as well as electricity which is sold as grid electricity (72b). As well as hot water, the biomass boiler (60) generates hot air (20) which is supplied to a biomass dryer (30).The heat recovery hot water (71) from the CHP generator is also used to produce hot air (20b) for the biomass dryer (30). 30 The biomass dryer (30) is preferably a pulp dryer, but may optionally be a biomass dryer for drying non-pulped biomass. In the biomass dryer (30), a wet second biomass feedstock (32) is dried to dry second 35 biomass feedstock (33). The hot air (20) flows through the biomass dryer and dries the wet 27 03 25 feedstock (32) by evaporating moisture from the feedstock. The hot air (20) loses heat to the drying feedstock, and is converted to high humidity warm air (34), which is exhausted from the biomass dryer. 5 In the method and apparatus of the present invention, the warm air exhausted from the biomass dryer (30) is delivered to a heat recovery system (40). The heat recovery system (40) includes a heat exchanger (41) in which the low grade heat remaining in the warm air (34) is extracted and used to heat the digestate reservoir (50). 10 The commodities produced by this system are therefore electricity, hot water and fertiliser, as well as any biogas and / or bio-methane that is removed from the system. As shown in Figures 2, 3 and 4, a preferred embodiment of the heat recovery system (40) uses a fan (43) to draw the warm air (34) from the exhaust of the biomass dryer (30) into a 15 first flow path of a gas-liquid plate heat exchanger (41). The warm air (34) flows through the heat exchanger (41) due to the negative pressure created by the fan (43) where it is released as exhaust air (44) or recirculated for re-use elsewhere in the system. In the embodiment shown in Figure 3, a heating liquid (52, 53) is pumped through a second 20 flow path of the heat exchanger (41) by a pump (51). During the flow of the warm air (34) through the heat exchanger (41), the warm air (34) cools down, condenses condensate out of the warm air, and transfers heat to initially-cool heating liquid (52) in the second flow path of the heat exchanger (41), so that the heating 25 liquid is heated to produce warmed heating liquid (53). The warmed heating liquid (53) is pumped through a heating loop between the heat exchanger (41) and the digestate reservoir (50), so the warmed heating liquid (53) carries heat to the digestate reservoir. The warmed heating liquid (53) then loses heat into liquid 30 digestate (2) in the digestate reservoir (50). The temperature of the digestate (2) in the digestate reservoir (50) is gradually increased by cycling the heating liquid around the heating loop between the digestate reservoir (50) and the heat exchanger (41). 27 03 25 Inside the digestate reservoir (50), there is the stored liquid digestate (2) and bacteria in an oxygen free environment as a result of the air-tight membrane which covers the surface of the reservoir. The heat transferred from the warm heating liquid (53) to the digestate (2) in the digestate reservoir (50) increases the aggregate temperature of the lagoon from 5 ambient outdoor temperature to a reservoir target temperature which in a preferred embodiment is 28 degrees Celsius. This target temperature is favourable for the growth, reproduction, and activity of the bacteria which break down the digestate in a second digestion, thus producing additional methane-containing biogas inside the digestate reservoir. The biogas produced from digestion in the digestate reservoir (50) is captured, 10 and may either be combined with the biogas (3) as shown in the flow diagram of Figure 1, or purified to produce bio-methane. Maintaining the digestate temperature at or above a reservoir target temperature of about 28 degrees Celsius in the digestate reservoir (50) is preferably achieved by controlling the 15 heating liquid pump (51) which pumps heating liquid (52, 53) to and from the heat exchanger (41). In a preferred embodiment, a controller (not shown) receives signals from a reservoir temperature sensor (not shown) within the digestate reservoir (50). The controller is 20 programmed to increase the flow rate of heating liquid when the temperature is below the reservoir target temperature and to decrease the flow rate of heating liquid (52, 53) when the temperature is above the reservoir target temperature. In a preferred embodiment, at least a portion of the methane-containing biogas from the second digestion is fed to a biogas purification system for conversion to bio-methane. Some or all of the biogas (3) 25 produced by anaerobic digestion may also be fed to the biogas purification system. Inside the biogas purification system (also known as a bio-methane upgrading system), the biogas (3) and / or the biogas produced in the digestate reservoir is passed through a series of membranes for purification which remove Carbon Dioxide (CO2). The biogas produced in 30 the digestate reservoir, for example, may initially comprise about 46% CO2. The output of the biogas purification system is bio-methane with a methane content of around 97%. By purifying the gases produced in the digester (10) and / or the digestate reservoir (50), the gas produced by the present system can thus be purified into bio-methane that is usable in a variety of applications. 27 03 25 Figure 4 illustrates an alternative embodiment of the system which operates in the same way as the embodiment of Figure 3, described above, with the exception that liquid digestate is recirculated between the heat exchanger and the digestate reservoir instead of using a closed heating loop filled with heating liquid. In the embodiment of Figure 4, cool 5 liquid digestate (152) is pumped to the heat exchanger (41), where it is warmed to become warm liquid digestate (153) that is returned to the digestate reservoir (50). Figure 5 shows a schematic perspective view of the embodiment of Figure 3, in which two heating loops (500) circulate heating liquid between heat exchangers (contained in building 10 (550)) and the digestate reservoir (50). The heating loops are positioned within the liquid digestate in the digestate reservoir to facilitate heat transfer to the liquid digestate. Figure 6 is a flow diagram illustrating an exemplary embodiment of the present invention, which operates as described above in relation to Figure 1. In the example embodiment of Figure 6, three first biomass feedstocks - Cereal, Manure and Silage - are fed to the 15 anaerobic digester, which produces digestate and biogas. The digestate is fed to the digestate reservoir (alternatively known as a digestate lagoon), while the biogas is used for electricity generation in a combined heat and power (CHP) unit, and for hot water generation in a biogas boiler. The electricity generated from the biogas can be used onsite, and / or sold to the electrical grid as a product of the process. Heat is recovered from both 20 the CHP unit and the hot water generation in the form of hot air, and the hot air is fed to an inlet of a pulp drying unit. In the pulp drying unit, a second biomass feedstock in the form of wet biomass pulp is fed onto a moving belt where it is dried by the hot air. The pulp dryer outputs dry pulp biomass, and warm high specific humidity air. The warm high specific humidity air has a temperature of at least 25 °C and a specific humidity greater than 20 25 g / kg (>20g water per kg air). The waste heat in the warm, wet air exhausted from the pulp dryer is recovered in the heat exchanger and used to warm heating liquid that circulates through a heating loop. The heating loop heats liquid digestate in the digestate lagoon to maintain its temperature at a higher level than would otherwise be possible. This higher digestate reservoir temperature helps to break down tough lignin walls in the digestate, and 30 produces additional biogas from the “spent” digestate in the reservoir. The additional biogas generated in the digestate reservoir is a further product of the process, and this additional biogas may be either collected and used, or combined with the biogas produced by the anaerobic digester and used for electricity and hot water generation. The digestate is another product of the process, which is eventually used as organic fertiliser. This 35 additional second digestion advantageously reduces ammonia emission when the fertiliser 27 03 25 is spread in use, and also increases the overall efficiency of biogas generation from the feedstock. The commodities produced by this system are therefore biogas, electricity, hot water and 5 fertiliser, as well as dried biomass from the pulp dryer. The apparatus and process illustrated in Figure 7 may thus be an apparatus and method for generating some or all of: biogas, electricity, dried biomass and fertiliser from the biomass feedstocks. 10 Figure 7 shows an alternative embodiment of this system, with the addition of a second heat recovery step, which recovers heat from the warm air and redirects it into the air heating step prior to the pulp dryer. 15 Figure 8 illustrates an alternative embodiment of the system which in addition to the features of Figure 6, also includes a bio-methane upgrading apparatus. In this preferred embodiment, some of the biogas produced by the anaerobic digester and / or from the secondary digestion in the digestate reservoir is fed to the bio-methane upgrading system, where it is purified through multiple membranes to increase the proportion of methane in 20 the biogas until the biogas qualifies as “bio-methane”. This process strips out the CO2 portion of the biogas in order to concentrate the methane in the biogas. As shown in Figure 8, the CO2 removed from the biogas may also optionally be collected and liquefied. In this embodiment, bio-methane and CO2 are additional products of the process, in addition to the biogas, the electricity that is generated, the dry biomass pulp, and the digestate- 25 fertiliser. The commodities produced by this system are therefore biogas, bio-methane, CO2, electricity, hot water and fertiliser, as well as dried biomass from the pulp dryer. The apparatus and process illustrated in Figure 8 may thus be an apparatus and method 30 for generating some or all of: biogas, bio-methane, CO2, electricity, hot water, dried biomass, and fertiliser from the biomass feedstocks. 27 03 25

Claims

1. A method of generating commodities from biomass, the method comprising the steps of:5 anaerobically digesting a first biomass feedstock to produce biogas and digestate;feeding the digestate to a digestate reservoir;using at least a portion of the biogas to generate hot air;drying a second biomass feedstock in a biomass dryer using the hot air, and recovering warm air having a temperature of at least 25 °C and a specific humidity of at10 least 20 g / kg from the biomass dryer;directing the warm air through a heat exchanger to recover heat from the warm air, and transferring the recovered heat from the heat exchanger to the digestate reservoir, so that the recovered heat is used to increase the temperature of the digestate in the digestate reservoir; and15 capturing biogas generated from the heated digestate in the digestate reservoir.

2. A method according to claim 1, in which the method is a method for generating one or more of: biogas, fertiliser, electricity, bio-methane and CO2 from biomass.20 3. A method according to claim 2, in which the step of using at least a portion of thebiogas to generate hot air comprises burning the at least a portion of the biogas in an electricity generator to generate electricity, so that the method is a method of generating biogas and electricity.25 4. A method according to claim 3, in which the method comprises the additional stepof delivering the captured biogas to the electricity generator to generate additional electricity.

5. A method according to any preceding claim, in which the step of using at least a30 portion of the biogas to generate hot air comprises burning the at least a portion of biogas in a biogas boiler to produce hot water, and generating hot air from the hot water.

6. A method according to any preceding claim, in which the step of transferring the recovered heat from the heat exchanger to the digestate reservoir comprises transferring35 heat via a heating loop.27 03 257. A method according to any preceding claim, comprising the step of collecting condensate generated from the warm air in the heat-exchanger.5 8. A method according to any preceding claim, comprising the step of purifying thecaptured biogas to produce bio-methane.

9. A method according to claim 8, comprising the step of capturing CO2 removed from the biogas during the purifying process.1010. A method according to any preceding claim, in which a first portion of the biogas generated by anaerobically digesting a first biomass feedstock is used to generate hot air, and in which a second portion of the biogas generated by anaerobically digesting a first biomass feedstock is converted to bio-methane.1511. A method according to any preceding claim, in which the second biomass feedstock and the first biomass feedstock are different types of feedstock.

12. A method according to any preceding claim, in which the second biomass feedstock 20 comprises cellulosic pulp.

13. A method according to any preceding claim, in which biogas and fertiliser are produced from the digestate in the digestate reservoir.25 14. A method according to any preceding claim, in which the biogas is generated byanaerobic digestion from the heated digestate in the digestate reservoir.

15. A method according to any preceding claim, in which the digestate in the digestate reservoir is warmed to a temperature of at least 28 degrees Celsius.3016. An apparatus for generating commodities from biomass, comprising:an anaerobic digester for digesting a biomass feedstock to produce biogas and digestate;a digestate reservoir configured to receive and store digestate from the anaerobic 35 digester;27 03 25an electricity generator and / or a biogas boiler configured to burn the biogas and to produce hot air;a biomass dryer, configured to receive the hot air, and to exhaust warm air from an exhaust; and5 a heat exchanger, configured to exchange heat between warm air received from the biomass dryer exhaust and the digestate reservoir.

17. An apparatus according to claim 16, in which the apparatus is an apparatus for generating one or more of: biogas, fertiliser, electricity, bio-methane and CO2 from biomass 1018. An apparatus according to claim 16 or 17, comprising an electricity generator configured to burn the biogas and to produce electricity and hot air.

19. An apparatus according to claim 16, 17, or 18 comprising a biogas boiler configured 15 to produce hot water, and to generate hot air from the hot water.

20. An apparatus according to any of claims 16 to 19, comprising a heating loop extending between the heat-exchanger and the digestate reservoir.20 21. An apparatus according to claim 20, comprising a pump configured to circulate aheating liquid through the heating loop.

22. An apparatus according to any of claims 16 to 21, comprising a reservoir temperature sensor, the reservoir temperature sensor being configured to sense a 25 temperature of digestate in the digestate reservoir.

23. An apparatus according to claims 21 and 22, comprising a controller which is configured to receive signals from the reservoir temperature sensor, and to control operation of the pump in response to signals from the reservoir temperature sensor.3024. An apparatus according to claim 23, in which the controller is programmed to operate the pump so that the digestate in the digestate reservoir is warmed to a temperature of at least 28 degrees Celsius.

25. An apparatus according to any of claims 16 to 20, in which the heat-exchanger is configured to collect condensate generated from the warm air in the heat-exchanger.

26. An apparatus according to any of claims 16 to 21, comprising a bio-methane5 upgrading system for producing bio-methane.

27. An apparatus according to claim 26, in which the bio-methane upgrading system is configured to receive biogas generated in the digestate reservoir.10 28. An apparatus according to claim 26, in which the bio-methane upgrading system isconfigured to receive a portion of the biogas generated in the anaerobic digester.27 03 25

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