Patent Application
20200270539
The invention relates to the use of biomass, in particular waste coffee grounds (“WCG”), including those produced by Instant Coffee Factories (“ICFs”) as a biofuel, particularly as an advanced sold biofuel. The biofuel may be provided, for example, as a WCG-derived biomass pellet (“pellet”) and a WCG-derived biomass briquette (“briquette”). The invention provides the biofuel itself, as well as methods and apparatus for producing it.
WCG are a type of biomass. Biomass is organic material produced through the reactions between carbon dioxide in the atmosphere, water and chlorophyll, via photosynthesis, to produce carbohydrates. During photosynthesis, agricultural crops absorb solar energy that is stored in the form of chemical energy in the bonds of the biomass components. The stored energy in these bonds can be released via combustion of the biomass, and by digestion of the plant for the nutritional content contained within the carbohydrate structures of the biomass.
There is a variety of types of biomass, from a number of different sources, including wood, leaves and other plant material. The inventors have found that biomass from WCG is a more than effective equivalent biofuel to woody biomass. The WCG biomass has a backbone of carbon, hydrogen and oxygen molecules. These components are chemically bound with one another to form a lignocellulosic structure.
Biomass has been utilised for energy generation for centuries via the burning of wood to provide a source of heat. As an alternative to wood, other biomass can be converted into a variety of solid, liquid and gaseous biofuels, for different uses depending on the conversion process used.
The UK bioenergy strategy (DEFRA, 2012) states that sustainably sourced bioenergy could contribute around 8-11% of the UK's total primary energy demand by 2020. A similar level of contribution could be expected in other countries.
There have been significant initiatives in the UK to encourage the uptake of solid biofuel combustion, including Feed-in Tariffs (“FiTs”) and the Renewable Heat Incentive (“RHI”). As a result, there has been a large uptake in the number of domestic, commercial and industrial scale biomass fired boilers installed and operated across the UK. At present, most of these small scale biomass boilers use wood pellets or wood chip. The highest internationally recognised current standard for wood pellets is ENplus, which includes ISO 17225-2:2014 (formerly ISO 14961-2), that specifies the properties required to make an effective wood pellet. This standard includes properties such as size, moisture content, ash content, mechanical durability, fines, net calorific value, bulk density, ash melting behaviour and elemental composition.
It would be advantageous to provide an alternative solid biofuel to those currently available. It would be particularly advantageous to provide an alternative to the wood pellets that are used in biomass boiler systems. The inventors have identified that such pellets may be prepared from waste materials, such as WCG. Previously it had been understood that using WCG to produce pellets or briquettes for burning in biomass boilers or in stoves or the like would not be possible, due to the high oil content of the WCG. The prior art on the processes used for making pellets and briquettes indicates that it is very difficult to form pellets or briquettes from compositions containing more than about 8% oil or fat. Nevertheless, the inventors have developed pellets and briquettes that contain high levels of WCG and that are durable. The pellets and briquettes produced by the inventors contain 20%-25% oil and fat. The pellets and briquettes have also been shown to meet the relevant UK ISO standards for biofuels of their nature (non-woody solid biofuels (pellets): ISO 17225-6:2014; and non-woody solid biofuels (briquettes): ISO 17225-7:2014).
European patent application EP0962515A, describes a burnable biofuel log comprising at least 50% WCG. The logs further comprise a combustible binder, such as paraffin wax, a carrier, usually sawdust, and a coker, such as starch or molasses. The logs described in the patent application contain around 35% binder. Such logs would not be suitable for use in a biomass boiler, as the amount of ash that would be produced by burning the logs would rapidly fill the boiler. Problems with ashing may also be seen if the logs are used with stoves or in fireplaces, the ash having to be cleared regularly.
Instant coffee is one of the world's most popular drinks. There are several hundred ICFs globally, collectively producing several million tonnes of instant waste coffee grounds (“IWCG”) each year. Those IWCG are difficult to store and hard to transport for disposal. IWCG are wet when produced. Storing IWCG when wet encourages fungal and bacterial growth. To dry the IWCG requires significant energy input, which may not be desirable from a cost point of view. Further, storing dry IWCG poses some risks since the IWCG have a tendency to self-ignite. Directly incinerating IWCG is an expensive and inefficient activity due to a high moisture content and the requirement for flawless, continuous 24/7/365 processing. Additionally the machinery sometimes used for incineration of IWCG—fluidised-bed boilers—are very expensive to build, install and very inefficient, expensive and complex operate; indeed, they often require teams of highly experienced engineers and operators. It would therefore be advantageous to provide a cost-effective, environmentally beneficial method of treating IWCG produced by ICFs in order to enable them to be used as a solid biofuel, and therefore stored and/or transported easily before an efficient, stable and easy to operate combustion process. It would be beneficial for this solid biofuel to be combusted within conventional biomass boilers, to generate heat, steam and power in order to ensure the system is efficient, cost-effective and easy to operate.
ICFs range in scale, from factories producing over 200 kT/yr of IWCG, to those processing less than 5 kT/yr of IWCG. Due to large-scale, outdated, monolithic engineering, regional waste management schemes and inconsistent local waste disposal infrastructure, no single solution for treating the IWCG produced exists that can be applied across this potential 40-fold shift in scale. This results in IWCG being disposed of off-site in most scenarios, costing both the ICF and the environment as this waste usually gets sent to landfill or incinerators. IWCG do not perform well in anaerobic digestion due to low biomethane potential, low levels of protein, gritty nature and particle size.
ICFs require significant amounts of energy (heat, steam and power) to run. Currently, most of this energy is generated from conventional, fossil-derived fuels. On sites where IWCG are incinerated only a fraction of this energy is recovered. When dried, IWCG still retain a high calorific value (“CV”). It would be advantageous, both environmentally and commercially, if that CV could be captured and stored for energy generation at the ICFs discretion. Utilising IWCG as a biofuel would allow ICFs to provide significant amounts of useful energy to the ICF and in some cases the amount of biofuel produced may exceed the amount of energy consumed by the ICF.
Biomass based fuels are required, in many countries, to meet government set standards, in terms of emissions, particularly of CO2, NOx and particulate matter, such as those set out in the RHI in the UK. It would be advantageous to be able to produce a biofuel that meets as many of the standards as possible, or that can be tailored to meet the relevant standard in the country of use.
The inventors have identified that it is possible to prepare a solid biofuel from IWCG. As mentioned on previously it had been understood that using IWCG to produce a solid biofuel would not be possible, due to the high oil content of the IWCG (IWCG and WCG compositions typically contain between 20 and 25% oil and fat) and the lack of fibre in the material. The prior art on the processes used for making solid biofuels, particularly from biomass, indicates that it is very difficult to form solid biofuels from compositions containing more than about 8% oil or fat. Further, IWCG comprise particles of a fairly uniform, large size and high elasticity, making it even more difficult to produce a solid biofuel with high mechanical durability. The large particle size increases the likelihood of fissures. It is much easier to pelletise WCG with a mixture of particle sizes due to increased inter-particle bonding from mechanical interlocking. Further, the extent of processing applied during the production of instant coffee means oils usually held at cellular level may be released, and starches contained in the grounds may have been gelatinised, making bonding the particles together even more difficult. Nevertheless, the inventors have developed a solid biofuel with a suitable chemical composition, high levels of mechanical durability and a high CV.
A first aspect of the invention provides a combustible biofuel composition comprising WCG.
The term WCG refers to the spent grounds that remain after ground roasted coffee beans have been used to make coffee. Any coffee grounds may be used, including those from espresso, filter and other coffee production methods, such as instant coffee production.
The composition is preferably in the form of a pellet or briquette. The composition is typically for use in burning in a biomass boiler system. Such boiler systems are well known in the art, but are usually run on primarily wood based biofuel such as wood pellets, chips and logs.
Typically, the composition comprises at least 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% 85%, 90% or 95% WCG. Where the composition is a pellet, it may typically comprise at least 60%, 65%, 70%, 75%, 80% 85%, 90% or 95% WCG. Where the composition is a briquette, it may typically comprise at least 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% 85% WCG.
When considering the features of the composition, in one embodiment, the components and proportions mentioned herein relate to those found in the composition itself. In another embodiment, they relate to those found in a feedstock used to make the composition.
The composition may further comprise a combustible filler. The filler is typically a calorific material. It may be solid and, if so, is usually fibrous. The filler typically contains less nitrogen than WCG, this helps to minimize the NOx emissions produced when the biofuel is burned. Optionally, the filler may be plant-based, typically an organic waste material such as barley waste, hops waste, cocoa husk, sugar beet, straw, wood, nut husks, reeds, bread waste, maize, wheat chaff, barley chaff. In one embodiment, the filler is sawdust. When the filler is sawdust, it may be sawdust from soft or hard wood. In some embodiments, soft wood sawdust is preferred as it contains more lignin than hardwood sawdust. When the filler is sawdust, it may be milled, for example hammer milled or pin-wheel milled. Other fillers may also be milled, such as by hammer milling or pin-wheel milling. The particle size of the filler may be selected to improve durability. For example, the filler may have a particle size, in particular, a maximum average length or diameter of less than 2 mm, less than 1.5 mm, less than 1 mm, less than 0.75 mm, less than 0.5 mm. The filler may be milled or otherwise treated to obtain this particle size.
The composition typically comprises at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or 55% filler. In some embodiments, it contains less than 30%, 25%, 20%, 15%, 10%, 7.5% or 5% filler.
When the composition is a pellet it typically comprises between 0% and 30% filler, between 5% and 25% filler, between 10% and 25% filler or between 15% and 25% filler. Alternatively, it may comprise at least 10% filler, more typically at least 15% filler, more typically at least 20% filler, even more typically at least 22% filler. It typically contains less than 30% filler. This is particularly the case when the filler is sawdust.
When the composition is a briquette it typically comprises between 0% and 50% filler, between 5% and 50% filler, between 10% and 50% filler or between 15% and 50% filler. Alternatively, it may comprise at least 10% filler, more typically at least 15% filler, more typically at least 20% filler, even more typically at least 25% filler. This is particularly the case when the filler is sawdust.
The inclusion of a combustible component allows the emissions produced when burning the biofuel to be controlled within desired limits, whilst maximizing the calorific output.
The composition may further comprise a binding agent. The term binding agent is used herein to mean a component that is added to the composition either to enhance the ability of the composition to be made into pellets or briquettes, or to increase the durability of the composition once made into pellets or briquettes. The binding agent is particularly useful when the composition is in pelleted form. The pellet must be sufficiently mechanically durable to allow it to remain substantially intact during storage, transportation and use in a boiler. Durability may also affect emission release, low durability resulting in dust and fine emissions being generated.
The binding agent may be selected to achieve the desired level of durability, whilst avoiding increasing unwanted emissions on combustion of the pellet. The binding agent may, for example, be or comprise a surfactant, an emulsifier, or a gelling agent. Examples of possible binding agents include polysaccharides, such as gum or starch, glycerol, natural paraffin, plant oil, lignosulphonate and molasses. The binding agent may be selected according to the amount and type of oil in the WCG. Binding agents useful in this composition may be more or less hydrophilic or lipophilic. The hydrophilic-lipophilic balance of binding agents, particularly surfactants, is measured on the HLB scale. Binding agents that are particular useful in compositions where the WCG have low oil levels are found at the lower end of the HLB scale. As the oil levels increase, binding agents further up the HLB scale may be more appropriate.
The composition typically comprises at least 0.5%, 0.75%, 1%, 1.5% or 2% binding agent. The composition typically comprises no more than about 10%, 7.5%, 7%, 6.5%, 6%, 5.5% or 5% binding agent. The composition may comprise more than one binder. When that is the case, it typically comprises no more than about 10%, 7.5%, 7%, 6.5%, 6%, 5.5% or 5% binding agent in total.
Where the binding agent is glycerol, the composition typically comprises between about 1 and about 7.5% glycerol, more typically between about 2.5 and about 7.5% glycerol, more typically between about 4 and about 6% glycerol, even more typically about 5% glycerol.
Where the binding agent is lignosulphonate, the composition typically comprises no more than about 3% lignosulphonate, so as to avoid excessive sulphur emissions.
Where the binding agent is paraffin, the composition typically comprises no more than about 10%, particularly no more than about 5% paraffin.
When the biofuel is in the form of a solid or compressed structure, such as a pellet, briquette or puck, it may be preferable to use a binding agent other than paraffin. The temperatures at which it is ideal to prepare pellets or briquettes to maximise durability are too cold for paraffin wax to be used. Accordingly, in one embodiment, the binding agent is not paraffin wax.
When the composition is in the form of a pellet, the pellet is typically generally cylindrical in shape. It is typically between about 4 and 8 mm in diameter, more typically between about 5 and 7 mm in diameter, even more typically between about 5.5 and 6.5 mm in diameter. The length of the pellet is typically between about 30 and 50 mm, more typically between about 35 and 45 mm.
When the composition is in the compressed form, particularly in the form of a briquette, it may additionally comprise a coating. The coating may be made of any appropriate combustible material, such as a wax or resin. In particular it may be paraffin wax. The coating may be applied by any suitable method, such as dip coating or spray coating.
The composition may contain one or more additives.
A second aspect of the invention provides a feedstock composition suitable for use in making the composition of the first aspect of the invention.
A third aspect of the invention provides a method for producing a solid biofuel composition comprising WCG, the method comprising the steps of:
a) providing a feedstock composition according to the second aspect of the invention; and
b) compressing that feedstock into pellets or briquettes.
The step of providing a feedstock composition may comprise the step of providing WCG.
The step of providing the feedstock may also comprise the step of decontaminating the WCG to remove unwanted contaminants, such as plastic and paper.
The step of providing a feedstock composition may comprise the step of drying the WCG. Any appropriate method of drying may be used, such as convective drying, conductive drying, radiant drying, dielectric drying, mechanical drying and natural drying. Particular examples of drying methods include drying in a rotary dryer, drying in an industrial centrifuge, microwave vibratory drying, mechanical pressing, drying in a fluidised-bed dryer, drying in an agitated underflow dryer and drying in a screw press. The feedstock composition is typically dried to a moisture content of between 6 and 20%.
The step of drying the feedstock may itself comprise a pre-drying step and a drying step. The pre-drying step may comprise removing water using a centrifuge, a drying floor, a dewatering press or using in vessel composting. The pre-drying step may comprise the step of storing the WCG and allowing them to self-heat, via bacterial or fungal growth. A settling tank may further be used to remove suspended solids from the water. In the pre-drying step, up to 30%, 40% or 50% of the moisture may be removed. Inclusion of a pre-drying step is advantageous, as it allows a significant amount of moisture to be removed in a low energy intensity step. The drying step may be significantly more energy intensive than the pre-drying step. Combining the two reduces the overall amount of energy used to dry the WCG.
The method may also comprise the step of extracting coffee oil from the WCG. The step of extracting the coffee oil may comprise the step of centrifuging the WCG. The step of extracting the coffee oil may take place after the WCG has been dried, or between the pre-drying and drying steps. The method may further comprise the step of refining the extracted coffee oil.
The step of providing the feedstock may also comprise the step of sieving the WCG to remove smaller contaminants missed by the decontaminating step.
The step of providing a feedstock composition may comprise the steps of mixing WCG with one or more other components, such as a binding agent or filler.
The method may also comprise the step of agitating the feedstock to avoid any blockages.
When the feedstock is to be made into pellets, the step of compressing the feedstock may comprise the step of pressing the feedstock through a pellet press. The pellet press may be any appropriate press, for example it may comprise a die, with boreholes through which the feedstock is pressed. Passing the feedstock through the press results in long strands of compressed biomass composition, which may then be cut into pellets.
The method may also comprise the step of cooling the pellets or briquettes. Cooling can enhance the mechanical durability of the pellet or briquette.
The pellets or briquettes may then be sorted, for example by sieving to remove pellets or briquettes that are not the desired size. Any dust or particles drawn off from the pellets or briquettes may be returned to the production process, for example by being added back into the feedstock for the process.
A fourth aspect of the invention provides apparatus for preparing a biofuel from WCG biomass, the apparatus comprising a means for drying WCG, a means for mixing WCG with one or more additives to produce a feedstock and a means for compressing the feedstock.
The means for drying the WCG may be any dryer, such as a convection dryer, a conduction dryer, a radiant dryer, a dielectric dryer, a mechanical dryer and a means for natural drying. Particular examples of dryers include a rotary dryer, an industrial centrifuge, a microwave vibratory dryer, mechanical press, a fluidised-bed dryer, an agitated underflow dryer and a screw press.
The means for compressing the feedstock may be a die having boreholes through which the feedstock is pressed. The feedstock may be pressed through the die by rollers, which force the feedstock into the boreholes.
The apparatus may further comprise a means for decontaminating the WCG to remove unwanted contaminants. The decontaminating means may be a trommel or other appropriate means for separating material by size.
A fifth aspect of the invention provides a biofuel composition comprising WCG. In one embodiment, the WCG comprise at least 50% IWCG.
As indicated in relation to the first aspect of the invention, the term WCG refers to the spent grounds that remain after ground roasted coffee beans have been used to make coffee. It can include those from ICFs and from espresso, filter and other coffee production methods.
The biofuel of the fifth aspect of the invention typically comprises at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or about 100% WCG.
The term IWCG refers to the spent grounds that remain after ground roasted coffee beans have been used to make instant coffee, otherwise known as soluble coffee, for example in a spray drying or freeze drying instant coffee factory. Typically, the WCG in the biofuel comprises at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or about 100% IWCG.
The biofuel of the invention typically comprises at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or about 100% IWCG.
The content of the composition is preferably based on the composition itself, rather than on any feedstock used to produce the composition. Nevertheless, the features of the composition may also be applied to the feedstock.
The biofuel of the fifth aspect of the invention is typically solid. In particular, it is typically in the form of a pellet, briquette, puck or other compressed solid structure.
The biofuel of the invention preferably has a net calorific value of between about 16 and 24 MJ/kg, more preferably between about 17 and 23 MJ/kg, even more preferably between about 18 and 22 MJ/kg.
The composition may further comprise a binding agent. The term binding agent is used herein to mean a component that is added to the composition to enhance the ability of the composition to be compressed into a sufficiently durable solid to allow it to be stored and/or transported.
The particles in IWCG tend to be particularly uniform in size. This makes it somewhat surprising that the grounds can be compressed into a durable solid; however, the inventors have identified that it is possible to do so. As, in the first aspect of the invention, a binding agent may be used to increase durability, though. Durability also greatly impacts emission release, with low levels of durability typically resulting in dust and fine emissions being generated, producing high levels of soot, particulate matter, ash and resulting in incomplete combustion and creating explosion risk. The binding agent may, for example, be or comprise a surfactant, an emulsifier, or a gelling agent. Examples of possible binding agents may be as defined in relation to the first aspect of the invention, or may include polysaccharides, such as gum or starch, glycerol, natural paraffin, plant oil such as coffee oil, lignosulphonate and molasses. The binding agent may be selected according to the amount and type of oil in the IWCG. Binding agents useful in this composition may be more or less hydrophilic or lipophilic. The hydrophilic-lipophilic balance of binding agents, particularly surfactants, is measured on the HLB scale. Binding agents that are particular useful in compositions where the IWCG have low oil levels are found at the lower end of the HLB scale. As the oil levels increase, binding agents further up the HLB scale may be more appropriate.
When the biofuel is in the form of a solid or compressed structure, such as a pellet, briquette or puck, it may be preferable to use a binding agent other than paraffin. The temperatures at which it is ideal to prepare pellets or briquettes to maximise durability are too cold for paraffin wax to be used. Accordingly, in one embodiment, the binding agent is not paraffin wax.
The biofuel may further comprise an oxidiser, such as sodium nitrate or potassium nitrate.
The composition typically comprises at least 0.5%, 0.75%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% binding agent. The composition typically comprises no more than about 10%, 7.5%, 7%, 6.5%, 6%, 5.5%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5% or 1% binding agent. The composition may comprise more than one binder. When that is the case, it typically comprises no more than about 10%, 7.5%, 7%, 6.5%, 6%, 5.5%, 5%, 4.5%, 4%, 3.5%, 3%, 2.5%, 2%, 1.5% or 1% binding agent in total.
As in the first aspect of the invention, the composition may further comprise a combustible filler. The filler is typically a material with a high Net CV and may be as defined in relation to the first aspect of the invention. It may be solid and, if so, is usually fibrous. Optionally, the filler may be plant based, typically an organic waste material such as barley waste, hops waste, cocoa husk, sugar beet, straw, wood, nut husks, reeds, bread waste, maize, wheat chaff, barley chaff. In one embodiment, the filler is coffee chaff. The term coffee chaff refers to silverskin that comes away from a coffee bean after roasting; silverskin is the fine inner coating found on green coffee beans that remains after milling and hulling. In another embodiment, the filler is sawdust. In another embodiment the filler is bean husk. The composition typically comprises less than 60%, 55%, 50%, 45%, 40%, 35% 30%, 25%, 20%, 15%, 10%, 7.5%, 5%, 4%, 3%, 2%, 1% filler. The particle size of the filler may be selected to improve durability. For example, the filler may have a particle size, in particular, a maximum average length or diameter of less than 2 mm, less than 1.5 mm, less than 1 mm, less than 0.75 mm, less than 0.5 mm. The filler may be milled or otherwise treated to obtain this particle size.
The inclusion of a filler allows other waste products from the ICF, particularly coffee chaff to be utilised in the biofuel. It also allows the CV of the biofuel to be altered, depending on which filler is added. Inclusion of a filler also allows the emissions produced when the biofuel is combusted to be controlled. For example, where it is beneficial to have reduced NO emission levels, to meet governmental set emissions requirements, a filler with a lower nitrogen content than IWCG, such as sawdust, may be included. Many governments are now setting standards relating to fuel emissions, based on the emission of NOR, carbon dioxide, particulates and the like. It is advantageous to provide a compact, cleaning burning fuel with low levels of particulate emission.
The composition may contain one or more additives, such as a processing agent. The processing agent may be a hydrophilic agent which may improve binding of the biofuel. Further, the processing agent may be an agent that aids compression of the biofuel, such as an agent that improves the passage of the biofuel through a die or pelleting head. It may also reduce cracking of the solid biofuel. It may additionally aid cooling. The processing agent may be a starch, such as farina. Where the processing agent also aids binding, the composition may also comprise an additional binding agent, or it may simply comprise the processing agent.
When the composition is in compressed form, for example, in the form of a pellet or briquette, it may additionally comprise a coating. The coating may be made of any appropriate combustible material, such as a wax or resin. In particular, it may be paraffin wax. The coating may be applied by any suitable method, such as dip coating or spray coating.
A sixth aspect of the invention provides a feedstock composition suitable for use in making the composition of the fifth aspect of the invention. The feedstock typically comprises the same components as the biofuel of the invention.
Also provided by a seventh aspect of the invention is a method for producing a solid biofuel composition comprising WCG, particularly comprising IWCG, the method comprising the steps of:
a) providing a feedstock composition according to the sixth aspect of the invention; and
b) compressing that feedstock into a solid biofuel.
The step of providing a feedstock composition may comprise the step of providing WCG, particularly WCG comprising or consisting of IWCG. The feedstock typically comprises at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or about 100% WCG.
Typically, the WCG in the biofuel comprises at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or about 100% IWCG.
Accordingly, the feedstock typically comprises at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or about 100% IWCG.
The method may comprise the step of decontaminating the feedstock to remove any unwanted contaminants.
The method may comprise the step of drying the feedstock. The feedstock composition is typically dried to a moisture content of between 6 and 20%. Any appropriate method of drying may be used, such as convective drying, conductive drying, radiant drying, dielectric drying, mechanical drying and natural drying. Particular examples of drying methods include drying in a rotary dryer, drying in an industrial centrifuge, microwave vibratory drying, mechanical pressing, drying in a fluidised-bed dryer, drying in an agitated underflow dryer and drying in a screw press. It is advantageous to dry the IWCG rather than attempt to simply burn the wet IWCG, as the amount of energy required to efficiently dry the IWCG is significantly lower than the amount required to burn off the water from the wet IWCG.
The step of drying the feedstock may itself comprise a pre-drying step and a drying step. The pre-drying step may comprise removing water using a centrifuge, a drying floor, a dewatering press or using in vessel composting. The pre-drying step may comprise the step of storing the IWCG and allowing them to self-heat, via bacterial or fungal growth. A settling tank may further be used to remove suspended solids from the water. In the pre-drying step, up to 30%, 40% or 50% of the moisture may be removed. Inclusion of a pre-drying step is advantageous, as it allows a significant amount of moisture to be removed in a low energy intensity step. The drying step may be significantly more energy intensive than the pre-drying step. Combining the two reduces the overall amount of energy used to dry the IWCG.
The method may also comprise the step of extracting coffee oil from the IWCG. The step of extracting the coffee oil may comprise the step of centrifuging the IWCG. The step of extracting the coffee oil may take place after the IWCG has been dried, or between the pre-drying and drying steps. The method may further comprise the step of refining the extracted coffee oil.
The method may also comprise the step of combining the IWCG with one or more other components, such as a filler.
The method may also include the step of conditioning the feedstock. The conditioning step may comprise the step of treating the feedstock with steam. The steam may be wet or dry steam. The conditioning step may also comprise the step of adding a binding agent to the feedstock.
The step of compressing the feedstock may comprise the step of extruding, briquetting, pucking or pelleting the feedstock.
The method may also comprise the step of cooling the compressed feedstock.
The method may also comprise the step of sorting the compressed feedstock, for example by sieving to remove pellets, pucks or briquettes that are not the desired size. Any dust or particles drawn off may be returned to the production process, for example by being added back into the feedstock provided in step a) of the method.
The method may also comprise the step of fuelling the dryer using solid biofuel produced by the method of the invention or biodiesel from the coffee oil extracted by the method of the invention.
The method may also comprise the step of recovering heat from any of the other steps of the method of the invention, in particular from the drying or pre-drying steps.
An eighth aspect of the invention provides an apparatus for preparing a biofuel from IWCG, the apparatus comprising a means for drying IWCG and a means for compressing the feedstock.
The means for drying the IWCG may be any dryer, such as a convection dryer, a conduction dryer, a radiant dryer, a dielectric dryer, a mechanical dryer and a means for natural drying. Particular examples of dryers include a rotary dryer, an industrial centrifuge, a microwave vibratory dryer, mechanical press, a fluidised-bed dryer, an agitated underflow dryer and a screw press.
The apparatus may also comprise a conditioning means, for conditioning the feedstock.
The means for compressing the feedstock may be a die having boreholes through which the feedstock is pressed or otherwise extruded. The feedstock may be pressed through the die by rollers, which force the feedstock into the boreholes. Alternatively the means for compressing the feedstock may be a cold press, a block press or any other device capable of compressing the feedstock into a solid, particularly into a puck, pellet or briquette.
The apparatus may also comprise a sorting means for sorting the biofuel, such as a sieve.
The apparatus may further comprise a heat recovery means, for recovering heat from other parts of the apparatus.
The apparatus may be arranged to be used in an ICF.
The invention further provides an ICF comprising an apparatus according to the eighth aspect of the invention.
The invention will now be described in detail, by way of example only, with reference to the drawings.
A process for preparing a biofuel composition according to the invention.
The process comprises a series of steps to purify, dry, mix and compress the WCG. The WCG may or may not include IWCG.
The WCG are purified by removing any plastic and other rubbish by feeding the material into a trommel. The material is sieved through a rotating cylindrical screen to separate the coffee grounds from large contaminants. The undesired contaminants are collected and removed.
Coffee grounds exiting the trommel are sent to a rotary kiln drum dryer via a conveyor belt. The grounds are dried to the desired moisture content, whilst being mixed to ensure even drying.
The coffee grounds are then fed into a mixing screw, in which they are combined with other components to be included in the final composition.
When the solid biofuel is to be made in the form of pellets, the WCG mixture is transferred to a pellet press. It may be agitated before or during transfer to ensure it is evenly mixed and to help prevent any blockages or accumulation of material.
The pellet press consists of a die ring that runs around fixed rollers. The material is fed to the rollers sideways and pressed through the boreholes of the die from the inside to the outside. The conditioned material is fed into the pellet press and distributed evenly. It then forms a layer of material on top of the running surface of the die. The rollers overrun this layer and press the material that is in the channels of the die through the die forming a string of pellets. The long string of pellets that comes out of the die is cut into the desired length by a knife.
Pellets exiting the pellet press are at a temperature of about 80-120° C. and so are transferred to a cooler. Cooling enhances the mechanical durability of the pellet.
Following cooling, the pellets are sieved to screen out pellets that do not match the desired size.
Where the solid biofuel is to be in the form of a briquette or puck, the mixture is transferred from the mixing screw into a compressor, where it is compressed into briquettes or pucks. Again, the briquettes or pucks may be cooled.
Analysis of the WCG Pellets
NOx Emissions
The inventors have recognised the need to limit the emissions of NOx produced when liquid, solid and gaseous biofuels are burned. In some countries, solid biofuels are required to meet standards on emissions in order to be sold and/or to receive government incentives and subsidies. Whilst it is generally understood that a biofuel that contains components with an increased level of nitrogen, such as WCG, is likely to produce higher NOx emissions than a biofuel with low nitrogen levels, the inventors have identified that the relationship between biofuel nitrogen content and NOx emissions is not linear. The mechanism of NOx emissions is poorly understood. The inventors have identified that NOx emissions are not solely dependent on biofuel nitrogen content and are affected by whether the nitrogen is volatilised or whether it remains in the char after burning.
WCG contain more nitrogen than some other forms of biomass, such as wood. However, the inventors have found that it is not simply a matter of combining WCG with a lower nitrogen biomass in order to reduce NOx emissions. NOx emissions may also be controlled by altering the oxygen available during combustion. The compositions prepared by the inventors meet the NOx requirements set out in the ISO standards for pellets for use in biomass boilers and for briquettes.
Mechanical Durability
Durability is important as the pellets and briquettes must remain intact during storage and transport. Durability is particularly important for the pellets as pellets with poor durability can be broken up during use in biomass boilers. The mechanical durability can also affect the emission release. If the durability is poor, then particulate matter emissions will be high because of the dust and fine emissions generated.
The inventors prepared WCG pellets and briquettes according to the invention and tested their durability.
The desired final durability of pellets for use in domestic applications is at least 85% and in industrial settings is at least 75%. The durability of the pellets was tested on pellets that had just been produced and that had not been sieved to remove less durable pellets. The inventors found that allowing the pellets to sit for at least 24 hours after production and sieving the pellets to remove loose material or poorly formed pellets increased the overall average durability by around 10%. Conditioning steps have also been found to improve overall durability. Based on this, the inventors considered durability of in excess of 70% for domestic use pellets produced on the main plant and 65% for pellets produced on the mini-pelleter, at this stage of testing, to be indicative of a pellet that would meet their final durability target. A durability in excess of 60% at this stage would be acceptable for industrial use pellets.
Ash
Upon complete thermochemical conversion (such as combustion in a biomass boiler) of WCG, an inorganic solid residue remains; this is referred to as ash. The ash content in biomass poses many problems in regards to its behaviour during combustion, in particular how it relates to slagging and fouling. Pure WCG have a very low ash content of 1.8%.
Ash fusion tests were carried out on ash from the pellets to measure the initial deformation temperature, the softening temperature, the hemisphere temperature and the flow temperature. These temperatures indicate the behavior of the ashes in the boiler. An indicative figure for the ash fusion temperatures of wood biofuels is that the initial deformation is in the region of 1200° C.
Calorific Value
One of the most important properties of biofuels in regard to their use in energy production is their energy content. There are various ways of expressing the energy content of biofuels, heating values such as the higher heating value (“HHV”) or gross calorific value and the lower heating value (“LHV”) or net calorific value (“CV”). HHV is defined as the amount of heat produced/released from the complete combustion (of a unit quantity or volume) of biofuel once the combustion products cool down and reach the same temperature as prior to combustion (this is usually 25° C.).
Combustion of most biofuels releases water, which is then evaporated in the combustion chamber. The process of evaporating the water requires energy and this is known as the latent heat of vapourisation. In most boilers/combustion chambers the water vapour released by combustion exits the system via the exhaust stream and is lost. There are advanced boilers that undergo a secondary condensation process, which condenses the water vapour and recovers the majority of the latent heat with it, which can be used to do work. HHV includes the latent heat of vapourisation and the difference between that and the LHV is equal to the amount of latent heat of vapourisation that can be recovered.
The CV of pure WCG is in the region of 22 MJ/kg, which is considerably higher than wood (wood chip has a CV in the region of 12.5 MJ/kg and wood pellets have a CV in the region of 17 MJ/kg. A mixture of 75% WCG, 20% sawdust and 5% binder gives a net CV of 18.72 MJ/kg which, despite the dilution of WCG with sawdust, is higher than wood pellets.
The calorific value of the biofuel is tested using standard techniques, generally using dry samples. Net CV is calorific value of a sample without it having been dried. Since the moisture content of samples varies, the Net CV is not comparable between samples. To allow a comparison, the Net CV is normalised for a moisture content of 10%. This is calculated using the Gross CV and hydrogen content, measured in a dry sample.
Durability
Pellets containing 95% WCG and 5% glycerol were produced and tested for durability. Initial tests with a small-scale pelleting machine produced pellets with mechanical durability of 90% using 5% glycerol and 95% WCG.
Durability
Pellets produced from feedstock mixtures containing WCG, sawdust and lignosulphonate and WCG, sawdust and glycerol were tested. The results were as follows:
The results show that the use of glycerol in place of lignosulphonate lowers the sulphur content from 0.31% to 0.12%. The initial deformation (ash melting temperature) of the 5% glycerol, 10% sawdust, 85% WCG mixture was 1270° C., which is lower than pure WCG but greater than the minimum of 1200° C. required for use in biomass boilers.
Tests Completed with ‘Retail’ Coffee.
Compositions Produced in Accordance with the Invention, Using Different Coffees and Sawdust as the Filler.
Compositions Produced in Accordance with the Invention, Using Lignobond (Lignosulphonate) as the Binder and Sawdust as the Filler.
Lab Analysis of Wet IWCG Vs Wet RWCG.
Moisture content and Net CV are measured on the sample as received. All other measurements are made after the sample has dried.
Effect of Increasing Filler.
Example biofuels according to the invention were prepared. The biofuels contained IWCG, a filler, specifically sawdust, and approximately 5% lignobond (lignosulphonate) as a binding agent. The durability of the biofuels was tested, the results are shown in the table below and in the graph in
Durability Distribution Over a Production Run.
The durability distribution of a production run of biofuel comprising approximately 5% lignobond (lignosulphonate) and about 20% sawdust. Samples were taken at 30-second intervals and tested.
Again, the production run did not involve conditioning of the feedstock before compression. Conditioning the feedstock would allow for further improvements in durability. The results are shown in the table below and in
Production of Various Compositions.
Examples of compositions that may be produced in accordance with the invention are as follows.
Further compositions according to the invention were prepared and tested for durability. The experiments were carried out on a pilot scale mini-pelletiser and the full scale plant. The results are detailed in tables 9 to 12 and
Mini-Pelletiser
A set mass of WCG was dispensed into a flexi-container and was mixed by hand with a set amount of binder. This mixture was passed through the prewarmed pelletiser to further homogenise and add heat to the mixture. The remaining binder material was added to the mixture and this feedstock was passed through the pelleter. Samples collected mid-stream whilst discharging from the pelletiser.
Full Scale Pelletiser
400 kg of WCG was measured into a hopper and an appropriate amount of sawdust was added to obtain the desired composition. For example, 21 kg of sawdust was added to make up the batch with 95% WCG and 5% sawdust. The WCG and sawdust were then conveyed via a mixing auger screw up to a live bin above the pellet head. The mixture was left in the live bin for 10 minutes to allow the components to mix thoroughly. The mixture was then passed through the preconditioner into the pellet press at a fixed feed rate. Lignobond was added to the mixture at a fixed rate through a nozzle at the feed end of the preconditioner. Samples were taken midway through the run at the outlet of the cooler.
Results
A. WCGs/Filler Ratio
Table 9 shows the variation of pellet durability with WCG/Filler ratio. This demonstrates that, surprisingly, good pellet durability can be obtained across a range of coffee:filler ratios.
B. Retail WCGs Instant WCGs Ratio
The table below shows the blend of Urban WCGs and Instant WCGs. In these experiments the total proportion of coffee grounds in the mixture was set at 75%.
C. Binder Level
In these experiments the amount of binder used in the pellet composition was investigated. The results are tabulated below. These results suggest that filler has more impact on the pellet durability than binder.
D. Alternate Filler
Pellets were prepared using milled bean husk as an alternative filler, instead of sawdust. The pellets with the milled bean husk had a durability of 55.5%. Although the pellets with the milled bean husk did not achieve the durability target of >75%, it is believed that this could be attained. The results are shown in Table 11.
78%
E. Alternate Binder
Pellets were prepared using various types of glycerol binder. The durability was tested as before. The results are shown in Table 12.
High durability values from experiment 1 are probably attributed more to the higher sawdust content than the action of any particular glycerol binder. This is further evidenced as when the sawdust concentration decreases the durability also decreases. When sawdust concentration reaches a “typical” value (c15-25%) durability is somewhat reduced but values between 70% and 85% is still achievable.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1521464.6 | Dec 2015 | GB | national |
| 1607149.0 | Apr 2016 | GB | national |
| 1617690.1 | Oct 2016 | GB | national |
| 1617692.7 | Oct 2016 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2016/053810 | 12/2/2016 | WO | 00 |
