Processing Orchard Grass into Carbon Bio Pellets via Hydrothermal Carbonisation—A Case Study Analysis

Abstract

The presented case study evaluates the environmental and economic effects of carbon bio pellet production from orchard grass by Farmutil HS Inc. through the hydrothermal carbonisation method, using obtained biofuel as a substitute for natural gas for heat production. Hydrothermal carbonisation is a zero-waste technology that produces renewable bioenergy by substituting fossil fuels for non-renewable resources. Processing 60,000 t/y of orchard grass with this method resulted in a total amount of heat production of 456,780 GJ/y. This means that over 60% of the energy produced from natural gas could be substituted by renewable bioenergy. It is also very important that the estimated cost of heat produced from carbon bio pellets is 29% lower in comparison to the cost of heat produced from natural gas

1. Introduction

The total annual primary biomass production on the surface of the Earth is estimated to be about 1260 EJ/y, including biomass used for food, fodder, fibres, and other industrial products of 219 EJ/y. However, estimations considering sustainability constraints show a yearly potential of 200–500 EJ/y of the current global energy use. By 2050, the share of bioenergy is estimated to be in the range of 100–300 EJ/y, contributing between a quarter and a third of the future global energy mix [1]. World biomass quantities are assessed at ~280 trillion t/y. In 2017, 55.6 EJ of biomass was utilised for energy purposes—86% of this use was in the form of primary solid biofuels, including wood chips, wood pellets, fuel for cooking and heating, etc., while 7% of the biomass was used as liquid biofuel. Biogas, municipal waste, and industrial waste had an almost equal share at 2–3%. There are no determined limitations on the use of biomass due to the diverse types of organic materials that are found everywhere on the globe [2,3,4,5]. Biomass is a popular source of renewable energy that has been used for centuries. Several types of biologically obtained materials, for example forest, wood, and agriculture waste, energy crops, and others, are used as biomass feedstock.

Biomass, unlike fossil fuel, can be treated as carbon dioxide-neutral fuel due to its ability to absorb nearly an equivalent quantity of CO₂ from the air during photosynthesis as that emitted during biomass combustion. Therefore, biomass could be a bioenergy source that decreases emissions of greenhouse gases (GHGs) [6,7].

Biomass, used for obtaining energy, is a substance from recently existing organisms. Examples are wood, energy crops, maize, switchgrass, and agricultural waste [6,7,8]. Wood and wood residues are, at present, the greatest biomass energy sources. The processing of raw biomass into bio fuels is typically realised using thermal, chemical or biochemical technologies [9,10].

Agriculture waste has a great energetic potential for utilisation, and the energy contained in the global production of agriculture waste is estimated to be in the range of 18–82 EJ/y [4,8]. Key technologies include direct combustion, anaerobic digestion (for biogas), fermentation (for alcohols), oil extraction (for biodiesel), pyrolysis (for biochar, gas, and oils), and gasification (for syngas). The worldwide use of sustainable and economically viable agricultural wastes is estimated to increase to 37–66 EJ by 2030 [10,11,12].

Grass is a plant that is widespread all over the world. Terrestrial lignocellulosic biomass covers ~26% of the world’s land area [13]. The permanent grassland area is estimated at 3.27 billion ha. This area includes 75.7 million ha in the EU countries and Turkey [3]. Grasslands are an important element of the world’s cultivation, food sources, and habitats for wild and domestic animals. Grasslands are also carbon sinks and are useful for erosion and desertification prevention, habitats for small animals and birds, nitrogen sources, biodiversity havens, and water sources.

Previously, grass has frequently been used as feed for animals [14]; but, today, it is mainly used to produce grass pellets, which have now become one of the most popular fuel pellets. It is important to note that grass pellet production units are relatively cheap and very environmentally friendly compared to fossil fuels.

Grasses are monocotyledonous crops, which include the Pinaceae family [15]. Pastures and meadows include plant species in many families, including Pinaceae and Fabaceae. Different types of the Pinaceae family are the most important forage plants in agricultural lands. Forage grasses are herbaceous crops divided into annuals, biennials, and perennials, that can then be divided into cool- and warm-season forage grasses.

Processing grass as a feedstock to produce bioenergy is a well-known technique, with interesting applications in biological transformation methods, especially in the conversion of grass to ethanol. It is expected that the development of genetic engineering technologies for grass and fermenting organisms has the potential to greatly improve the economic viability of ethanol fuel-based production systems. Other energy applications of grass include anaerobic digestion for biogas generation and pyrolysis for syngas production [16].

The physicochemical characteristic of grasses result in their thermal treatment being a potential problem in terms of their high moisture content, low bulk density, and limited frailness. Another problem is ash composition; due to elevated concentrations of alkali metals and chlorine, the thermal processing of grasses could cause slagging, fouling, and corrosion problems [17].

The goal of a circular economy (CE) strategy is sustainable development for integrating waste utilisation, with the manufacturing of value-added products in closed-loop production systems with zero-waste manufacturing [18,19]. The CE is a production model planned and designed to be restorative and regenerative [20]. It is focused on the utilisation of renewable, sustainable energy and the elimination of the use of hazardous materials, as well as actions eliminating waste through the better development of production systems and products within business models [21,22]. The circular economy is a global economic development model that promotes eco-innovative solutions and meets the following assumptions: the added value of raw materials/resources, materials, and goods is increased in the value chain, i.e., from design to consumption; the amount of waste produced is decreased and the waste generated is managed by the waste management hierarchy [23].

In this context, this paper presents the development of obtaining and utilising of biofuels from Farmutil HS Inc., the biggest, ultra-modern Polish agri-food consortium [24]. The strategic production systems developed and implemented by Farmutil are illustrations of the practical implementation of the basic rules of CE on a microeconomic scale, which are based on a cleaner production methodology. Farmutil’s strategic action program assumes the comprehensive use of biofuels obtained from waste and agri-food products. A particular area concern also relates to such agricultural biowaste as grass, containing over 80% of organic matter. The production of bioenergy and the recovery and reuse of matter from grass could be beneficial from economic and environmental points of view [24,25].

Different, cleaner technologies, such as torrefaction, gasification, and hydrothermal technologies, i.e., hydrothermal liquefaction (HTL), hydrothermal carbonisation (HTC), and hydrothermal gasification, result in the effective utilisation of biowaste to obtain value-added products like biofuels [25]. The implementation of renewable bioenergy production is a basic purpose that is needed to fulfil the United Nations Sustainable Development Goals (SDGs) [26,27], and biowaste stewardship is one of the most significant questions in the transformation towards a CE. The hydrothermal carbonisation HTC process is consistent with the circular economy concept of recycling and reusing carbon contained in biowaste, which can be used as a solid fuel, and is a solution containing important nutrients (N, P, and K) in the liquid fraction of biofertilisers [14,28,29].

This work presents a technical and economic case study of the production of carbon bio pellets from grass, focusing on the use of obtained biofuel as a substitute for natural gas, as used by Farmutil for heat (steam) production, in order to propose the cost-effective and sustainable utilisation of grass in a closed-loop production cycle.

Section 2 of the work presents the physicochemical properties of grass, the characteristics of the production processes of bio pellets from orchard grass, and the processing of hydrated biomass using the hydrothermal carbonisation (HTC) method. The most important elements of processing and the ability to realise the above-mentioned purposes are analysed. Section 3 describes the processing of orchard grass into carbon bio pellets by the HTC method and discusses the obtained results, illustrated by technical figures, and an economic evaluation of carbon bio pellet production from grass. The conclusions are contained in Section 4.

2. Materials and Methods

Plant Farmutil HS Ltd. (Śmiłowo, Poland) developed and implemented the production of grass pellets [24] from orchard grass cultivated on its meadows located in the Notec River Valley in the northern part of Poland. The Farmutil plant produces biofuel grass pellets from 40,000 t/y of raw grass cultivated on 5000 ha of meadows. The grass pellet biofuel containing 6% H₂O has a calorific value of 16 GJ/t. In poultry farming, grass pellets are also utilised as bedding (on-site recycling). Grass pellets have a selling price of about EUR 153.5. This production is not very profitable (~5%), but this allows utilising all the grass harvested by Farmutil HS.

2.1. Analytical Methods

The chemical composition of the biocarbon sample was determined by quantitative analysis using the ICP-OES method (Perkin Elmer Avio 500, Waltham, MA, USA). The determination of total carbon, nitrogen, and sulphur content in tested materials was carried out using the CN628 and S628 elemental analysers (LECO Corporation, Stonebridge, WI, USA). The certified standards (LECO Corporation, US) containing 41.04 ± 0.09 wt.% of carbon, 9.56 ± 0.03 wt.% of nitrogen, and 7.46 ± 0.08 wt.% of sulphur were used for the preparation of a calibration curve.

Ash content was also checked by calcination at 550 °C and very similar values and ash composition were obtained. The alkali content (Na + K) was 5% or lower in all cases. For elemental analysis (CHNS), samples were milled (<0.2 mm) and dried. Samples were analysed on a Fisons EA 1108 CHNS-O apparatus. The values were stated on a dry and ash-free basis. The composition of the ashes was determined by the ICP OES method. Therefore, a sample (20–30 mg) was disaggregated in a HNO₃/HF/HCl mixture (1:1:3) and the solution analysed on a Varian 715-ES apparatus.

The humidity was determined by the weight loss after heating to 100 °C for several hours. Fixed carbon content was calculated using the following formula: fixed carbon/% = 100 − volatile content/% − ash content/%. Calorific value was determined using a Calorimeter KL-12Mn2 (produced by Precyzja Bit Co., Bydgoszcz, Poland). Electrical conductivity meter apparatus used was produced by Spray Quick Bradbury, Stockport, UK. Volatile substances VOCs erre measured with gas chromatography–mass spectrometry (GC-MS) produced by Yokogawa, Poland (Warszawa, Poland). A 5E-HA0711 Hardgrove Grindability Index Tester LC-100 (Gilson Company, Inc., Lewis Center, OH, USA) measured the grindability of carbon bio pellets.

2.2. Grass Production Pellets from Orchard Grass

The grass family is important for people. Domestic animals breed on diets based on grasses. Grasses also form a substantial part of the urban and countryside landscape in the world. The grass family is environmentally dominant, covering about 20% of the earth’s land area [30]. The grass family contains over 10,000 species classified into 600 to 700 genera [31,32]. Grasses are incorporated into the group known as the monocotyledons, including all flowering crops with a single-seed leaf.

Grass species contain low protein (8–22%). Grass may contain the following [%]: cellulose 25–40; hemicellulose 25–50; lignin 10–30; ash 2–8. The digestibility of grass is negatively correlated with lignin occurrence within the cell wall. Mineral and protein utility in forage crops depends on soil traits and the available content of nutrients in it, fertilisation and other tillage actions, climatic conditions, plant growing stages, and various morphological features of plants. Orchard grass contains the following in dry mass [%]: crude proteins (CP), 14.0; neutral detergent fibres (NDF), 54.23; acid detergent fibres (ADF), 30.38; Ca, 3.0; P, 2.85; Mg, 0.31 [33]. Grass (Figure 1) is now widely processed into grass bio pellets [34,35].

Farmutil’s grass pellet production unit processes 40,000 t/y of grass into pellets. The flow sheet of the production of grass pellets is presented in Figure 2. The first operation is cutting the grass into little scraps using a hammer mill in order to increase process yield. The water content in the grass affects its density. If the water content is too high, the grass is dried in a rotary dryer. Next, the dried grass is extruded by die and rollers, and then shaped into pellets. After pelletising, the grass pellets are cooled to room temperature from high processing temperatures.

In comparison to other typical fuel-producing processes, grass pellet manufacturing is rather cheap and quite environmentally friendly with fossil fuels. By using grass as a feedstock, the global warming potential of combined heat and power (CHP) plants can be significantly reduced compared to coal- and natural gas-fired plants. Grass grows year after year without replanting. This makes it a sustainable resource for biomass production. It can be grown on marginal lands that are not suitable for other crops. It does not require intensive fertilisation, making it an excellent choice for utilising less productive farmland. Growing grass helps sequester carbon dioxide from the atmosphere, contributing to climate change mitigation [35,36,37].

The water content of the pellets was 9.3% and the dry matter density was 2265 kg/m³ [38]. The net calorific value (NCV) of raw grass pellets typically falls within the range of 16.3–17.0 MJ/kg of dry matter. This value classifies raw grass pellets as a high-quality biofuel according to the international norms for solid biofuels [39], and as an approved solid recovered fuel.

The grass pellets have a heat value of nearly 8000 British thermal units, BTUs (0.844 MJ), comparable to hardwood and nearing that of coal [33,37]. Its energy conversion factor is 20:1, in comparison to 10:1 for wood and 5:1 for biodiesel. The obtained pellets could be a good heat source. Grasses are typical residues on farms and in forestry. They are inexpensive and easily available. It only takes 70 days to grow grass, which is much shorter than the growth of wood and crops. Thanks to the low cost of raw materials and production, grass pellet fuel has price advantages over traditional fuels. Pellets could be used also as litter and soil conditioners in gardens and farmland. They can increase to four times their original size when wet. Additionally, their ash after combustion can be used as fertiliser [33,37].

Grass pellets can be compared to other biomass fuels using standards. For instance, the EN ISO 17225 standard “Solid biofuels–Fuel Specifications and Classes” [37,39] provides thresholds for various requirements. Grass pellets and their processed forms can meet these standards, making them suitable for energy production. In summary, grass pellets have a significant calorific value, especially after processing, and can serve as an environmentally friendly energy source.

Produced grass fuel pellets have a pellet diameter of 8 mm, a calorific value of 15–16 kJ/kg, and a maximum humidity of 12.5%; they are 7.5% ash, 0.22% sulphur, and 0.39% chlorine. Grass biomass is often enriched with chlorine, which, combined with potassium, poses a risk of chloride corrosion at elevated temperatures of about 400 °C. In the case of small biomass-fired steam boilers, it is not recommended to take special anti-corrosion measures, as the hazard of chloride corrosion is small due to the low temperature of steam and flue gas [40]. For medium and large steam boilers, in which the share of biomass in the fuel stream does not exceed 10%, the risk of chloride corrosion is rather small.

Carbonisation processes can enhance the fuel properties of grass pellets and hydrothermal carbonisation (HTC) development is proposed for this case. HTC processes increase the heating value of grass pellets by improving their carbon content by up to 29.8%. Additionally, the sulphur, chlorine, and ash content are reduced during the HTC process. From this viewpoint, it was very interesting to analyse the possibility of the production of carbon bio pellets from grass for use by Farmutil HS.

2.3. Processing of Hydrated Biomass by the HTC Hydrothermal Carbonisation Method

HTC is a physicochemical conversion method for organic carbon compounds contained in hydrated biomass under anaerobic conditions with water as the reaction medium [25]. HTC technology imitates the method of creating carbon from biomass, which normally occurs in nature over 50,000 to 50 million years [41]. The HTC technology is an exemplification of industrial ecology, creating an analogy of a natural ecological system that, in the natural world, functions in proper networks of connections, forming interactive models. The goal of industrial ecology is to reduce the amount of waste produced by large systems and to completely restructure these systems [42].

The idea of industrial HTC technology remains like the naturally occurring process. Still, the temperature (180–220 °C) and pressure (20–25 bar) of HTC process are intensified to decrease the reaction time (ranging from 1 to 72 h), depending on the kind of biomass used. These parameters are key factors influencing the process, while the type of biomass processed influences the quality of the products obtained. The basic product of hydrothermal carbonisation is a product like coal, named hydro char, while the by-products are the water phase (rich in fertilising ingredients) and the gas phase (low quantity of CO₂). Water contained in wet biomass is an excellent solvent [43].

Toxic organic chemical wastes, acutely poisonous, carcinogenic, mutagenic, teratogenic, and medical wastes, spanning the range from tissues and fluids capable of harbouring infectious disease-causing organisms, pesticides, and residual micropollutants (industrial chemicals, pharmaceutically active residues, steroid hormones, personal care products originated mostly from domestic, agricultural, hospital, and industrial activities detected in aqueous systems at a concentration range of a few ng/L to several μg/L), are also broken down during the HTC reaction.

The carbon content increases after carbonisation, the oxygen and mineral content is reduced, and there is little gaseous product. Hot compressed water, the reaction medium, is ecologically safe, inexpensive, and easily available. The HTC reaction takes place in an aqueous medium; hence, the raw material does not require any drying. Therefore, the energy-consuming and expensive pre-treatment steps used in conventional thermal processes (such as combustion or pyrolysis) are unnecessary. The HTC method allows the use of feedstock with a high moisture content (over 80%), e.g., sewage sludge. Hydrogen chains of water slack off upon compression, leading to the modification of the dielectric constant and allowing water to catalyse the process, in which water can function as both a base and an acid at temperatures from 200 °C to 280 °C due to its degree of ionisation being maximised. At these temperature parameters, the dielectric constant of water is decreased, so it behaves like a non-polar solvent. In hydrothermal carbonisation, hemicellulose and cellulose are hydrolysed into oligomers and monomers, while lignin remains largely unchanged [44,45,46,47].

HTC mainly includes decarboxylation, dehydration, and polymerisation [44,46]. The removal of the carboxyl and hydroxyl groups considerably reduces the oxygen/carbon O/C proportion and the end-product has an increased energy density. Hydrothermal carbonisation technology does not use any supplementary catalysts. Also, the temperature, 180–220 °C, and pressure, 20–25 bars, are relatively moderate, due to the cost of production being rather low (Figure 3). The hydrocarbons obtained in the HTC method have a high carbon content, because the dehydration and decarboxylation remove hydrogen and oxygen from the feedstock in the form of water and carbon dioxide, respectively, reducing nitrogen and sulphur content in comparison to the inputted raw material. In addition, they have lower ash content compared to other types of coal, because inorganic compounds that form ash after combustion are extracted into the liquid phase during the HTC process, and additional equipment for the separation of impurities and hydro char quality improvement is installed next to the HTC reactors for different market applications [48,49]. The HTC method was first investigated by Bergius in 1913 [50].

The greatest benefit of the HTC method is that it is realised in a water solution, so the biomass humidity is not important. Excess HTC process water contains soluble nutrients (nitrogen, potassium, iron) which serve as a fertiliser for plant growth. HTC is an exothermic reaction with very low thermal energy consumption and no water evaporation of the biomass feedstock is required [51].

Hydro char concentrates most of the carbon content present in the processed biomass. Hydro char contains low water-soluble chemicals such as sulphur, chloride, and potassium. Hydro chars obtained by the HTC method even have over 60% carbon content and a lower ash content compared to types of coal because the inorganic compounds that form ash after combustion are extracted into the liquid phase during the HTC process [52,53].

HTC is a superior solution for handling biomass waste. It is realised in a water solution, so the water content in biomass is not a problem. It is universal, as the chemical reaction works with any type of hemicellulose biomass. Hydrothermal carbonisation is a waste-free method, with no odour emissions and no toxic waste products. Excess process water contains the fertilising soluble nutrients of nitrogen and potassium, which can be used as a bio-fertiliser [54,55].

The energy consumption in the HTC process and all HTC process costs are relatively low. The technological and equipment solutions are not very complicated, and the implemented installations are fully automated and easy to use. The typical investment expenditures for HTC installation are relatively low, due to the estimated payback with gross profit being from 3 to 4 years for this type of installation. The HTC method allows for the processing of all types of biomass [25,56].

3. Results and Discussion

Processing of Orchard Grass into Carbon Bio Pellets by HTC Method

The proposed HTC method is based on the Ingelia biorefinery business model, which is a sustainable and attractive solution for the industry in terms of the newest circular biorefinery concept [57,58].

The thermal carbonisation process of the aqueous suspension of grass is accomplished inside a continuous reactor with an inverted flow. The carbonisation reaction time is about 5 h, the temperature inside the reactor is about 210–230 °C, and the pressure is about 20 bar. The reactor is heated diaphragmatically with process steam. The steam used to heat the reactor is recovered as condensate from the process. The mixture of biocarbon and process water slurry is evacuated from the HTC reactor through a system of outlet pipes, creating biocarbon slurry. This slurry is cooled and expanded before it is processed in the post-treatment stage. In the analysed case, there is no contamination with inert sand-type materials, and there is no need for gravity separation in the hydraulic classifier to remove impurities from the slurry. To be pumpable, the biochar slurry is crushed and then routed to a separation system. The separation of the biocarbon solids from the fertiliser solution (after hydro-carbonisation liquid AHL) is carried out using a filter press. The filter sediment contains 45–55% of water. It is dried in a fluidised bed dryer to obtain a moisture content below 10% in biocarbon (biochar). Carbon bio pellets are produced by compressing the biochar through a die with holes forming cylinders, usually 6–8 mm in diameter [53,59,60,61].

Ingelia’s vertical reactor size is the standard (14 m long) and the total plant capacity is determined by the number of reactors and annual operating hours. The scale of production can be expanded by adding additional reactors. The minimum installation capacity of one reactor is estimated to be around 700 kg/h (biomass with 50% humidity) or 1.2 t/h (sludge with 80% humidity), with yearly production rates of 5500–10,000 tons (7800 h yearly operation time).

The HTC process (Figure 4) starts with dosing feedstock, grass slurry containing 20% DM of grass, and 80% recycled filtrate liquid; using a piston pressure pump through heated pipelines into the reactor, a pressure of 20 bar is obtained. The continuous carbonisation process in the reactor lasts about 5 h and the temperature inside the reactor is about 200 °C. The medium in the reactor is heated diaphragmatically by process steam. No additional catalyst is used in the process. The biochar sludge obtained after the HTC process is then cooled and expanded through an exhaust piping system before treatment in the post-processing stage, where the separation system removes inert impurities. In the case of biochar slurry produced from grass not contaminated with inert sand-type materials, there is no need for separation systems to remove impurities from the slurry. Next, the solid phase of biochar is separated from the filtrate using a frame filter press. The filter cake, having a moisture content of 45–50%, is next shredded and dried to reduce the water content in the biochar to below 10%. The dried biochar is pelletised using an extruder, without the use of any binding agent [61].

The hydrocarbon produced by the hydrothermal method of biomass is a substance with high carbon content that has better energy density in comparison to raw biomass, e.g., energy density, and improved general physicochemical properties. Hydro char is characterised by an increased hydrophobicity, resulting in an improvement in the stages of storage and transport. Additionally, the chemical composition and higher energy content of biocarbon are like that of natural coal, making it possible to use them as a solid fuel in typical incineration processes. The characteristics of hydro char are mostly dependent on the properties of the biomass, especially water (optimum 20%), and carbon content and the treatment parameters, such as temperature (typically about 200 °C), reaction time (5–20 h), and the feedstock-to-water ratio (minimum 20% of dry mass) [51,52,61,62]. Hydrothermal carbonisation has several advantages in carbonising biomass.

The advantages of the HTC method include the fact that no toxic waste is produced; the excess process water contains a soluble compound of nitrogen, potassium, and iron, which have a fertilising quality on plant growth; the exothermic reaction keeps thermal energy consumption very low; and no evaporation of water in the biomass feedstock is required. Hydro char concentrates most of the carbon content present in the processed biomass. Hydro char is very low in water-soluble chemicals like sulphur, chloride, and potassium because their compounds pass into the liquid phase during processing [48,61,62].

The characteristics of carbon bio pellets obtained from the processing of orchard grass are presented in

Table 1. Characteristics of carbon bio pellets from the HTC method.

Parameters	Unit	Value *
Calorific value	MJ/kg	>23
Bulk density	kg/m3	700
Ash content (SiO2, CaO)	%	4–6
Water		<10
C		>58
C solid		25
H		5.8–6.4
N		0.6
S		<0.2
Cl		<0.2
Volatile substances		65
Durability		95–98
The melting point of ashes in an oxidising atmosphere	°C	>1200
Grindability (Hardgrove index ISO 5074:2015 [63])	-	44–52

* Values refer to dry weight, without ash (daf).

.

The second product obtained from the HTC processing of grass is after hydrothermal carbonisation liquid AHL. Its characteristics are presented in

Table 2. Characteristics of fertiliser solution AHL.

Parameters	Unit	Result
Water content	%	96.5
Dry mass at 105 °C		3.50
Total organic matter		2.40
C organic total		1.39
Humid extract		1.49
pH	-	5.80
C/N	-	10.6
N total	g/kg	1.307
N ammonium (N-NH4+)		0.277
N nitrate (N-NO3−)		0.007
P2O5		0.043
K2O		2.4
CaO		1.8
MgO		0.827
Na2O		1.109
B	mg/kg	3.20
Fe		24.5
Cu		0.070
Mn		2.79
Zn		1.54
Ni		0.26
Pb		0.045
Cd		0.0040
Cr		0.086
Cl		1.523
Electrical conductivity	dS/m in 25 °C	15.5

.

AHL fertiliser solution contains macro-fertilising ingredients (N, P, K, Mg, Ca, and S) and trace elements extracted from biomass. The advantage of these fertilisers is the significant content of nitrogen and potassium, the basic fertiliser biocomponents.

AHL was also analysed for heavy metal contents. The basic feedstock for the HTC method was orchard grass and waste contamination was expected to be lower. This was confirmed by the results in Table 2.

An analysis of the possibility of producing carbon bio pellets from grass shows that carbonisation processes can enhance the fuel properties of grass pellets and hydrothermal carbonisation development. The existing Farmutil infrastructure in all potential HTC installation locations would allow investments to be carried out without additional costs for this purpose.

Currently, Farmutil processes approximately 40,000 t/y of grass (variant 1). The plan to increase its processing capacity to 50,000 t/y (variant 2), and ultimately even to 60,000 t/y (variant 3), results from increasing grass yields. Therefore,

Table 3. The material balance of producing carbon bio pellets from orchard grass using the HTC method.

Specification	Capacity Variants (t)
	1000	40,000 *	50,000 **	60,000 ***
I. Preparation of feedstock (grass slurry)
Input:
1. Feedstock (grass 50% H2O)	1000	40,000	50,000	60,000
2. Recycled filtrate from III	1500	60,000	75,000	90,000
Total	2500	100,000	125,000	150,000
Output:
1. Feedstock (grass slurry 20% H2O)	2500	100,000	125,000	150,000
II. Hydrothermal reaction
Input:
1. Feedstock (grass slurry 20% H2O)	2500	100,000	125,000	150,000
Output:
1. Hydro char slurry	2500	100,000	125,000	150,000
III. Filtration of hydro char slurry
Input:
1. Hydro char slurry from II	2500	100,000	125,000	150,000
Output:
1. Filtration sediment hydro char (50% H2O)	447	17,880	22,350	26,820
2. Filtrate (fertiliser solution AHL)—recycled to I	1500	60,000	75,000	90,000
3. Filtrate (fertiliser solution AHL)–product	503	20,120	25,150	30,180
Total:	2500	100,000	125,000	150,000
IV. Drying of filtration sediment (50% H2O)
Input:
1. Filtration sediment hydro char (50% H2O)	447	17,880	22,350	26,820
Output:
1. Dried hydro char (10% H2O)	331	13,240	16,550	19,860
2. Vapours	116	4640	5800	6960
Total:	447	17,880	22,350	26,820
V. Production of carbon bio pellets
Input:
1. Dried hydro char (10% H2O)	331	13,240	16,550	19,860
Output:
1. Carbon bio pellets (10% H2O) product	331	13,240	16,550	19,860

* Actual Variant 1, ** developed Variant 2, *** final Variant 3—capacities.

presents a material balance of HTC for capacities for processing 40,000, 50,0000, and 60,000 t/y of raw grass.

Another potential major economic benefit would be the possibility of producing cheap liquid organic mineral fertilisers (~30,000 t/y) and using them to fertilise Farmutil’s fields.

Table 4. Annual production incomes and revenues of orchard grass sewage treatment variants by HTC method.

Items	Unit	Variant of Capacity
		1	2	3
Amount of orchard grass (50% DM)	t/y	40,000	50,000	60,000
Amount of feedstock grass sludge (20% DM)		100,000	125,000	150,000
Carbon bio pellets production		13,240	16,550	19,860
Bio pellets price	EUR/t	465	465	465
Carbon bio pellets production value	EUR million/y	6.2	7.7	9.2
Estimated operation cost		2.0	2.4	3.0
Gross profit		4.2	5.3	6.2
Quantity of Ingelia reactors	pieces	8	10	12
Investment expenditures	EUR million	19.5	24.4	29.3
Revenue	year	4.6	4.6	4.6
Amount of heat after combustion of carbon bio pellets (LHV = 23 GJ/t)	GJ	304,520	380,650	456,780
Decreasing fee for CO2 emission (60 €/t of CO2)	EUR million/y	2.9	3.9	4.4

presents the annual production incomes and revenues of orchard grass sewage treatment variants by the HTC method.

Table 3 shows that processing 60,000 t/y of grass sludge (20% DM) allows for obtaining 19,860 t/y carbon bio pellets with a maximum value of EUR 9.2 million/y (Table 4). Investment expenditures were estimated to be EUR 29.3 million.

The calculated return on such an investment would take 4.6 years (at the carbon bio pellets price of 2000 PLN/t). A payback period below 5 years is good, so, in this case, the proposed investment is profitable. The analysis also shows environmental benefits in terms of saving fees for carbon dioxide emission trading systems with a maximum value of EUR 4.4 million/y.

The balances of the amount of heat used for obtaining meat bone meal (MBM), meat, and other Farmutil production products were shown in earlier works [55,64,65]. The highest meat waste treatment capacity of Farmutil plants is 700,000 t/y (2000 t/d). The heat needs of all Farmutil production units are assessed to be 11.29 GJ/h (which includes the steam consumption for MBM production being 6.55 GJ/t) on average and the MBM production capacity is 100,000 t/y. The total heat consumption by Farmutil was estimated to be 753,900 GJ/y.

When processing 40,000–60,000 t/y of grass, the HTC method results in the producing of 13,240–19,860 t/y carbon bio pellets (having an LHV = 23 GJ/t). The total amount of heat potentially produced from these quantities of carbon bio pellets is estimated to be 304,520–456,780 GJ, i.e., 40.2–60.6% of the heat used in Farmutil HS. In this way, over 40–60% of energy from natural gas could be replaced by renewable bioenergy, depending on the variant capacity implemented.

In the case of carbon bio pellets, the cost of heat produced is EUR 20.2/GJ, 29% lower in comparison to the cost of heat produced from natural gas at EUR 28.3/GJ [66].

The second product of sewage sludge treatment by the HTC method is the fertiliser solution AHL, as presented in

Table 5. Production of liquid fertiliser AHL from HTC method.

Items	Unit	Capacity
Amount of orchard grass (50% DM)	t/y	40,000	50,000	60,000
Production of fertiliser solution AHL	t/y	16,000	20,000	24,000
(N + K) **		66.4	83.0	99.6
AHL price *	€/t	7.7	7.7	7.7
AHL production value	€/y	122,791	153,488	184,186
Area of lands fertilised	ha/y	332	415	498

* N and K price = 10 PLN/kg; contents in AHL: N = 0.13%, K = 0.2%; ** dose of N + K—200 kg/ha.

. At the assumed price level, the produced AHL solution may be worth approximately 3% of the value of biochar production. The cost-effective delivery distance to fertilised fields was assumed to be approximately 50 km.

The amount used as AHL solution is lower due to the in-process recycling of 60% of the produced AHL solution into HTC reactors (see Table 4) to dilute feedstock to 20% DM. Table 5 shows that the maximum AHL solution quantity could fertilise only 498 ha of land, delivering valuable nitrogen and potassium nutrients to the soil. AHL can also be used without restrictions for the irrigation of forest crops or lawns.

4. Conclusions

The presented case study analysis showed a technical figure and an economic evaluation of carbon bio pellet production from orchard grass by Farmutil HS Inc. using the HTC method, considering that the utilisation of obtained biofuel as a substitute for natural gas used in Farmutil for heat (steam) production allows for the beneficial and sustainable management of grass in a closed-loop model. The HTC process has a lot of benefits, such as increasing the efficiency of biowaste recycling, reducing greenhouse gas emissions associated with its disposal, and reducing dependence on raw fossil materials.

Processing 60,000 t/y of orchard grass with the HTC method results in the production of 19,860 t/y of carbon bio pellets. The total amount of heat potentially produced from these quantities of carbon bio pellets is estimated to be 456,780 GJ, i.e., 60.6% of the heat used by Farmutil HS. In this way, over 60% of energy from natural gas could be replaced by renewable bioenergy, depending on the variant implemented. In the analysed case, the cost of heat contained in carbon bio pellets produced is 29% lower in comparison to the cost of heat produced from natural gas.

Carbon bio pellets, when used as biofuel, are chemically stable and storable, contributing to reducing CO₂ emissions and improving air quality. The growing interest in the hydrothermal carbonisation method may contribute to reducing the amount of organic waste and producing renewable bioenergy by substituting natural gas with non-renewable resources.