Challenging Biomass Feedstocks for Energy and Chemicals

Abstract

The Nordic countries have a long tradition of utilizing agro-industrial sidestreams for heat and power production and recovery of chemicals. A typical example is black liquor from pulp mills. Here, the woody biomass undergoes a digestion process where the fibers are separated to produce pulp for paper production. The liquid by-product from the digester, black liquor, contains wood lignin and the spent cooking chemicals. Through the chemical recovery cycle, the black liquor is utilized for heat and power production and recovery of cooking chemicals. Worldwide, there are several challenging biomass sidestreams that can be utilized in a similar fashion as with black liquor. Some examples of these are vinasse from the integrated sugar-ethanol production process; straw and manure from agriculture sources; forest residues; by-products from the food industry; etc. This book chapter will review the availability of these types of feedstocks and discuss their applicability and challenges to be used for energy and chemicals. Pyrolysis, gasification, and combustion are the potential thermal conversion options considered for the utilization of these types of challenging biomass feedstocks.

1. Introduction

This chapter deals with the worldwide and local resources of challenging (or low-grade) biomass feedstocks available for the production of energy and chemicals. It aims to highlight the potential of different sources of low-grade biomass and make aware of their content of impurities, mainly ash-forming elements (e.g., Na, K, Ca, Si, P, S, and Cl) and nitrogen (N). It also suggests what kind of utilization techniques, such as combustion, pyrolysis, and gasification render the best yields and the least problems in industrial conversion systems.

1.1 Challenging biomass feedstocks as a renewable source for energy and chemicals

Biomass is a renewable resource and a short-term carbon sink [1]. The carbon cycle explains how carbon atoms continuously travel from the atmosphere to the ground and then back again as carbon dioxide or methane into the atmosphere. The biomass on earth both binds and emits greenhouse gases in the atmosphere. When dead biomass degrades, it becomes humus (soil), water, and carbon dioxide. In anaerobic conditions, every second of carbon dioxide will form methane, a 28 times more potent greenhouse gas than carbon dioxide [1].

Carbon dioxide and methane are the two most dominant greenhouse gases from anthropogenic emissions [1]. Emissions lead to increasing concentrations in the atmosphere and cause the Earth’s temperature to rise. Therefore, the focus must be on all unutilized and low-quality biomass feedstocks available worldwide. These low-grade biomass feedstocks are present in different forms and have the potential to replace fossil-based feedstocks for energy and chemicals production. If they are not utilized for energy or chemicals production, they will inevitably contribute to greenhouse gas emissions. This chapter aims to shed light on different low-grade biomass feedstocks for energy and chemicals and discusses the concentrations and roles of impurities in the thermal conversion of the feedstocks.

1.2 Low-grade biomass of different sources

Agriculture and forestry give rise to a large amount of non-used biomass [2]. Only a small fraction of the field crop ends up as food or some other product, maybe as little as 10–20% [2] of the total above-ground mass of biomass. Some of it finds use in farming or as a soil fertilizer. In forestry, only the trunk of a tree is of industrial value. The rest 30% of the above-ground biomass of a tree neither becomes timber nor fiber [2]. Non-utilized sidestreams from agriculture and forestry are important biomass feedstocks for energy and chemicals. Some of these are challenging due to their content of impurities.

Industrial processes also render large amounts of biomass feedstocks as side-streams. The largest sidestreams come from industries producing food and beverage, textile and fibers, liquid biomass fuels and wood, pulp, and paper. Sometimes, an industrial biomass feedstock contains large concentrations of impurities, but just as often, it is only slightly processed and quite low in biomass impurities.

The last sector to produce large amounts of low-grade biomass feedstocks is the household consumers and the service sector businesses. These produce biomass feedstocks in several aspects such as gardening and park managing waste, construction demolition of wood and furniture waste, packaging and food waste, and municipal solid waste. Finally, via the sewage system, it produces biomass sludge from the wastewater treatment plants.

1.3 The waste hierarchy

In a circular economy [3], the waste hierarchy model states that the first goal is to prevent or minimize the formation of waste. The most significant potential to do so for biomass waste is within the households and in the service industry. In the forestry and agricultural sectors and the manufacturing industry, the prevention or minimization of biomass waste is not possible without compromising the volume of food and materials production from biomass.

The next step in the waste hierarchy is to reuse or recycle the waste material. Reusing means, for instance, renovating an old sofa or using milk cartons as plant pots, whereas recycling constitutes the conversion of waste streams into new products and chemicals. Returning the biomass back to the soil as a fertilizer is also a way of recycling the biomass; the only problem is that half of the biomass will form carbon dioxide, and some of this also forms methane if it is done in an uncontrolled way.

The last option in the waste hierarchy is the recovery of the organic fraction of waste streams as energy, and the very last option is landfilling the waste material. The European Union has banned the landfilling of any organic material in all its member states since 2018 [4]. The best option for waste biomass feedstocks is to utilize them for energy and chemical production and return the final residues as fertilizer to the soil.

The final residues, mainly the impurities, from utilizing the low-grade biomass feedstocks for energy and chemicals should be returned to the soil to complete the nutrient cycle and in part for carbon storage. One interesting way of long-time storage of the carbon bound in biomass feedstocks is to produce biochar for use as a soil fertilizer for growing crops. The biochar also serves as a way of carbon sequestering and storage.

2. Challenging biomass feedstocks

The biomass feedstocks considered challenging for thermal conversion can be categorized as agricultural residues, e.g., wheat and rice straws and husks; industrial by-products such as rapeseed oil cake, molasses, vinasse, and black liquor; herbaceous energy crops, e.g., miscanthus, switchgrass, and reed; forestry by-products including forest residues and wood barks; and municipal solid wastes. Compared to wood, these feedstocks are of low quality and several challenges are associated with them for utilizing in thermal conversion processes. The challenges are primarily due to the high levels of impurities in the feedstocks. The impurities are mainly ash-forming elements (e.g., Na, K, Ca, Si, P, S, and Cl) and nitrogen (N). Figure 1 shows the concentrations of the impurities in these feedstocks and woody biomasses from refs. [5, 6, 7, 8]. As seen from the figure, most of the industrial side streams and agricultural residues contain the highest total concentration of impurities, followed by herbaceous energy crops, forest residues, and wood barks. However, the woody biomasses contain the least, indicating that they are less problematic for thermal conversion systems. The main thermal conversion problems caused by the impurities are ash-related problems (e.g., corrosion and ash-deposit formation) and air emissions (NO_x and SO_x). These problems are discussed in Section 5 of this Chapter.

Figure 1.

Concentration of impurities in the woody and low-grade biomass feedstocks.

The causes for the high levels of impurities in the low-grade feedstocks are very variable and versatile. The main ones are (1) type of feedstock, (2) application of chemical fertilizer(s) to the soil, (3) contamination during collection and handling of the feedstock, (4) environmental factors including soil type, water quality, and climatic conditions, and (5) type of the industrial process generating the feedstock and chemical additives used during the industrial process. The influences of these factors on the concentration of impurities in the feedstocks are briefly described below, one after the other.

2.1 Feedstock type

The majority of the low-grade feedstocks described above originated, one way or another, from plants (woody or herbaceous). These plants require impurities as nutrients for their growth, and the degree of nutrient uptake from the soil depends, among others, on the type of the plant. For example, high concentrations of Si in rice plants, and thus in the rice straw and husk given in Figure 1, are ascribed to the presence of a gene, Ls1 [9], specific to rice-plant roots. This gene is reported to be the primary transporter of Si from the soil to rice plant roots. Similarly, reports, e.g. [10], indicate that the concentration of impurities in a plant is directly related to the water uptake capacity of the plant. This is due to the increased amounts of the water-soluble fractions of impurities with increased water uptake by the plant. For instance, the cause for the higher concentration of the impurities, such as Si, K, and Cl shown in Figure 1, in the reed than in the switchgrass and miscanthus may be attributed to the higher water uptake ability of reed than the latter plants. Bakker and Elbersen [10] mentioned that the reed has a higher water uptake capacity than switchgrass and miscanthus.

Apart from feedstock type, it is well established that the different parts of a plant have different levels of impurities. For example, in the woody biomass samples used in the work by Werkelin et al. [11], the concentrations of the impurities are mostly higher in the leaves, shoots, needles, and twigs of the biomasses than in their stems (woods). This is likely one of the reasons why forest residues, which are mainly composed of tree branches, leaves, and tops, have higher levels of impurities than the woods shown in Figure 1.

2.2 Chemical fertilizers

Another factor for the high levels of impurities in the low-grade feedstocks is the type and amount of chemical fertilizers applied to the soil. This is especially true for agricultural residues and industrial byproducts, e.g., wheat and rice straws and husks, rapeseed cake, and molasses originating from the production of food crops where chemical fertilizers are used to enhance soil fertility (or productivity). The chemical fertilizers are mostly applied to the soil in the form of nitrates (for N), phosphates (for P), and potassium salts, mainly potassium sulfate and chloride (for K and S). The extent to which these minerals are taken up by the crops partly depends on the amount of chemical fertilizers applied to the soil.

2.3 Feedstock contamination

Contamination is a typical cause for the high level of impurities in a low-grade feedstock. It occurs primarily when the feedstock comes in contact with soil during harvesting and transporting. Feedstock contamination with soil is often the case with agricultural and forest residues and herbaceous energy crops, where mechanical harvesting techniques by swathing or raking [12] are used. However, according to Bakker and Elbersen [10], feedstock contamination during storage is less common.

2.4 Environmental factors

The environmental conditions in which plants grow play a dominant role in determining the mineral (impurity) contents of the feedstocks. These environmental factors include soil type, warm or cold weather conditions, rain or irrigation water supply, and location (latitude, longitude, or altitude). For example, switchgrass grown on clay soil has shown higher Si contents than that grown on sandy soils.

2.5 Type of industrial process

The high level of impurities in the vinasse, given in Figure 1, is mainly due to the influence of the industrial process used. Figure 2 shows a simplified schematic diagram of the integrated sugar-ethanol production process. The concentrations of impurities in the crushed sugarcane and byproducts, i.e., bagasse, filter cake, molasses, and vinasse, from the integrated process are as shown in Figure 1. Detailed descriptions of the integrated process and the fate of impurities in the process are available from Dirbeba et al. [8]. Here, the influence of the process steps on the concentrations of the major impurities in the various byproducts is described.

Figure 2.

Simplified schematic diagram of the integrated sugar-ethanol process.

As seen from Figure 1, vinasse > filter cake > molasses > crushed cane > bagasse in terms of the total concentration of impurities. These variations in the concentration of impurities in the cane and byproducts from the integrated process arise mainly from the sugar and ethanol production process steps shown in Figure 2. The influences of the process units are summarized as follows: (1) The low total concentration of impurities in the bagasse is due to the leaching of most of the impurities from the crushed cane by the imbibition water added during the milling and extraction process step. (2) The raw juice treatment involves first heating the juice, then liming and sulfiting it with quick lime (CaO) and SO₂ gas, respectively, and finally sedimenting and filtering the treated juice to remove soils, sediments, and other suspended solids in it as a filter cake. This and the removal of most of the CaO and SO₂ added for the juice treatment with the filter cake as sulfites and phosphates of calcium make the concentration of impurities in the filter cake considerably high. (3) Vinasse, the final byproduct from the integrated process, has the highest concentration of impurities. There are two main process-related causes for the high level of impurities in the vinasse: First, most of the impurities (ash-forming elements) left in the treated juice end up in the vinasse while the organic fractions in the juice are removed as products, sugar, and ethanol, leaving the vinasse concentrated with the impurities. Second, other chemicals such as H₂SO₄, UREA, and DAP are added during the molasses fermentation stage. These chemicals contain some impurities, S, N, and P, that are partly removed with the vinasse.

Another example of an industrial process where the process steps influence the concentration of impurities in the by-product is a pulp mill. In a pulp mill, the pulp is the main product and black liquor is the sidestream. As seen in Figure 1, the total concentration of impurities in the black liquor is very high, whereas the levels of the impurities in the input feedstocks, woody biomasses, for pulp mills are low. The high concentration of impurities in the black liquor is due to the addition of wood pulping chemicals, NaOH and Na₂S, during the wood digestion process.

3. Availability of different types of challenging biomass feedstock

Low-grade biomass feedstocks from primary production like agricultural residues, clearing and thinning residues, and energy crops are largely available in many parts of the world [2]. The utilization of these feedstocks for energy and chemicals involves large-scale industrial processes. Despite their availability in large quantities and at low prices, their low energy densities and high moisture contents decrease their availability in practice. This is because of the high transportation costs and long distances involved with the centralized processing of these feedstocks.

The biomass feedstocks from industrial sidestreams are food or material industry by-products. The forest industry produces sawdust and bark or more processed feedstocks like black liquor or tall oil. These types of feedstocks are already inside the fence of a production site, and they pose severe waste-related problems if not utilized. However, there are still some challenges. Heat, steam, or electricity production is not always in the interest of the industry where the feedstocks are generated, or there might be a lack of competence or experience to utilize them for energy and chemicals production.

Table 1 shows the 2019 world production of the top 10 crops and estimates of the crop residues they generated in the field during harvesting and after they were processed on industrial sites. The estimates for the crop residues were made based on the data obtained from The Food and Agriculture Organization of the United Nations (FAO) [2]. As seen from the table, close to 15,000 million metric tons of agricultural residues were generated, indicating the high availability of these feedstocks for energy and chemicals production.

	Crop type	World annual production		Harvest residues from each crop (by residue to crop ratio, RPR)			Process residues from each crop (by residue to crop ratio, RPR)
		Million tons	% of total	Residue	RPR kg/kg	Million tons	Residue	RPR kg/kg	Million tons
Top 10 crops in the world	Sugarcane	1955	20	Tops	0.3	587	Bagasse	0.33	645
	Maize	1141	12	Stalks	2	2283	Cob/Husks	0.473	540
	Wheat	765	8	Straw/husk	1.75	1339
	Rice, paddy	749	8	Straw	1.757	1316	Husk	0.267	200
	Oil palm fruit	416	4				Fibre/Shell	0.435	181
	Potatoes	355	4
	Soybeans	336	3	Straw	2.5	841	Pods	1	336
	Cassava	299	3	Stalks	0.062	19	Peelings	0.015	4
	Sugar beet	281	3				Pulp	0.075	21
	Cotton	250	3	Stalks	2.755	689
	Sum	6548	66			7072			1927
Total production		9862	100	Estimate:		10651	Estimate:		2902

Table 1.

World production of the top 10 crops and their residues generated in the field and on industrial processing sites [2, 13].

3.1 Availability and energy density

In the Nordic countries, residues from primary production like wheat straw and logging residues find their way to the local heat and power plants for conversion by combustion to district heating and electricity. Only a certain distance from the heat and power plant is suitable for collecting these biomasses; too long distances make the transportation costs too high [14].

It is mainly the demand for heat in the winter that makes this feasible. The Nordic countries have systems for district heating in all cities and even in smaller municipalities. In most countries globally, there is no market or infrastructure for district heating. Instead, converting the biomass into high-grade chemicals for materials and fuels for transportation is a sustainable alternative to replace fossil-based raw materials for these commodities.

Low energy density is the result of low bulk densities (100 kg/m³), high moisture contents (typically 40%), and low heating values (about 20 MJ/kg dry solids) of the feedstocks. This results in low energy densities, around 1000 MJ/m³. Compared to crude oil, 35,000 MJ/m³, and bituminous coal, 25,000 MJ/m³, biomass feedstocks should not be transported long distances without first upgrading.

The oxygen content of the biomass feedstocks is high; a typical empirical formula for biomass is CH₂O. The maximum available thermal energy per kilogram from these feedstocks is 20 MJ, which is obtained when they are fully oxidized to CO₂ and H₂O [15]. This is relatively low compared to other primary energy sources, like natural gas, crude oil, and bituminous coal, which are 55, 45, and 30 MJ/kg, respectively.

High porosity is due to the vascular structure of biomass. It leads to the overall low density: maximum 900 kg/m³ for dry solid hardwood, but typically much lower bulk densities: 30–170 kg dry solids per cubic meter, kg_D.S./m³ [13]. Higher bulk densities usually include moisture, which is of no value for its utilization for energy and chemicals.

Biomass feedstocks are naturally hygroscopic and contain some 20% moisture in equilibrium with air humidity. It dries further by applying heat and may obtain close to zero moisture content before utilization. Fresh biomass from newly harvested feedstock contains over 50% of moisture. Each percent of moisture content lowers the effective heating value by 1.125%; i.e., moisture content of 40% reduces the effective heating value of the biomass by 45%, expressed as energy released per kilogram biomass combusted.

3.2 Availability of typical industrial by-products

Biomass feedstocks from industrial by-products such as rapeseed oil cake, molasses, vinasse, and black liquor are much easier to access since these biomass feedstocks are already inside the industry gate. The challenge is if the industries where these feedstocks are generated do not find use(s) for them and if there are no proven techniques to have them processed.

The pulp and paper industry have a long tradition of utilizing all the sidestreams it generates. The largest sidestream is the spent liquor from wood pulping: black liquor. Its utilization has a long history in Scandinavian countries, but the primary purpose of processing it further within the pulp mill is to recover the inorganic cooking chemicals, which are contained in the black liquor. For every ton of pulp, seven tons of black liquor are produced. After concentrating the black liquor to a dry solid content of about 75%, it is burned in a chemical recovery boiler to produce heat and electricity for the pulp mill, which is often self-sufficient in energy. Roughly 195 million metric tons of black liquor are produced annually as a by-product from all the Kraft pulping processes in the world [16].

The integrated sugar-ethanol industry produces molasses and vinasse as by-products. The amount of molasses produced annually is about 70 million metric tons with a thermal energy value of approximately 225 TWh [17]. For every liter of ethanol produced, 10–15 liters of vinasse form as a by-product. In Brazil alone, 370 million cubic meters of vinasse is generated annually [18]. There is not yet a sustainable use for these by-product streams. They are both potential biomass feedstock for the production of energy and chemicals.

4. Thermal conversion processes as options for utilizing challenging biomass feedstocks

There are two main energy production routes from biomass: biochemical and thermochemical (herein referred to as thermal) conversion routes. Biochemical conversion of biomass involves the production of liquid (ethanol) or gaseous (methane) fuel using microorganisms, which break down the organic fraction of biomass into ethanol or methane depending on whether fermentation or anaerobic biodigestion process is used. According to Christofoletti et al. [19], biochemical conversion processes are mentioned to be slow and expensive and produce other harmful gases, such as hydrogen sulfide. Moreover, these processes generate effluents, such as vinasse, that are difficult to treat and have potential environmental pollution. However, a review of the applicability of low-grade biomass feedstocks to biochemical conversion is beyond the scope of this chapter.

On the other hand, the second option, the thermal conversion route, uses heat to disintegrate biomass into often solid, liquid, and/or gaseous fractions depending on the type of thermal conversion used. The main biomass thermal conversion processes are combustion, gasification, pyrolysis, and hydrothermal liquefaction. These processes are described briefly and discussed more in detail below in connection with the utilization of the challenging biomass feedstocks.

4.1 Combustion

Combustion is a well-established and most commonly used thermal conversion technology for the production of heat and power from biomass. It is also a proven means for waste disposal. In the latter case, combustion is often referred to as incineration. In addition to eliminating wastes that otherwise would cause environmental damage(s), incineration also produces heat utilized, for example, for district heating. Among the available technologies for biomass combustion, the fluidized bed (FB) combustion is the most advanced and efficient technology for heat and electricity production from biomass feedstocks containing low levels of impurities. There are two types of FB combustion technologies: bubbling fluidized bed (BFB) and circulating fluidized bed (CFB).

BFB and CFB technologies that utilize some low-grade biomass feedstocks have been developed and operating in some European countries. Table 2 lists some examples of these technologies, designed and supplied by Valmet [20], along with their capacities and locations as well as the type of challenging feedstocks used.

Company name	Type	Capacity	Location	Challenging feedstock used
Termomeccanica Ecologia	BFB	2x30 MWth	Calabria, Italy	RDF
Borås Energi och Miljö	BFB	2x20 MW	Borås, Sweden	RDF, SRF
Stora Enso Langerbrugge nv	CFB	125 MWth	Gent, Belgium	RDF
Mälarenergi AB	CFB	155 MWth	Västerås, Sweden	RDF, SRF
Lahti Energia Oy	CFB	2x80 MW	Lahti, Finland	RDF, REF, SRF

Table 2.

Some examples of BFB and CFB boilers using low-grade feedstocks for heat and power production [20].

As seen from the table, solid recovered fuel (SRF), refuse-derived fuel (RDF), and recycled fuel (REF) are the main problematic feedstocks used in the FB boilers. These feedstocks are obtained after crushing and pretreating their sources—recycled wood, MSW, and industrial and commercial wastes. The pretreatments, e.g., removing mechanical or coarse impurities such as plastic wastes, minimize the ash-related problems (discussed in the next section) that would have been caused by utilizing these feedstocks in FB boilers. In addition to the pretreatments, special design features are incorporated in these boilers to partly offset the adverse impacts of the impurities contained in the feedstocks. For example, the use of highly alloyed steels for the construction of superheater tubes of the boilers and placement of the superheater tubes in the flue gas path where the flue gas temperature is lower are some technological options for alleviating the adverse impacts of the impurities.

Another well-developed combustion technology to burn a specific challenging biomass feedstock is the Kraft recovery boiler. As discussed in the previous section, this boiler is solely designed for burning black liquor from pulp mills. It generates not only heat and electricity from the black liquor for the pulp mills but also recovers the wood pulping chemicals. However, efforts to combust a similar challenging fuel-vinasse in a recovery boiler have not been successful so far. This is in part due to the higher levels of K and Cl in the vinasse than in the black liquor as shown in Figure 1. Nevertheless, a recent Ph.D. thesis work by Dirbeba [21] suggests a recovery boiler-type system with a simpler lower furnace than that of the black liquor for vinasse combustion. Such a system could decrease the ash-related problems (see next section) while at the same time allowing most of the ash in the vinasse to be recovered for use as fertilizer. However, this system will inevitably produce steam with low temperatures and pressures, ultimately leading to low electrical power efficiency.

4.2 Gasification

Gasification is one of the promising routes for biomass thermal conversion due to its potential for providing high energy efficiency cycles [22]. The primary product from biomass gasification is syngas, which is composed of mainly CO and H₂. The syngas can be combusted to produce heat and power, or it serves as a feedstock for the production of liquid fuels and other value-added chemicals via, for instance, the Fischer-Tropsch process.

A novel gasification technology that has been demonstrated for low-grade biomass feedstocks is a CFB gasifier coupled with a syngas cleaning system and subsequently combustion of the clean syngas in a boiler as shown in Figure 3. Here, the syngas cleaning system removes impurities released during the gasification of the biomass feedstock in the CFB gasifier. As a result, combusting the clean syngas in the boiler enables to obtain higher steam parameters (and thus higher energy efficiencies) than the steam parameters that would be obtained from direct combustion of the feedstock [23]. A commercial-scale plant of such a process has been installed in Lahti, Finland, by Valmet [20]. The plant uses SRF as a feedstock, and it has a capacity to gasify 250,000 tons of SRF per year, which is equivalent to 150 MW of combined heat and power supply.

Figure 3.

Schematic of low-grade biomass feedstock gasifier coupled with syngas cleaning and combustion systems. Adapted with permission from [23].

Another promising gasification technology for low-grade biomass feedstocks is the high temperatures (up to 1500°C) and high pressures (up to 3 MPa) entrained flow gasifier. One of the advantages of this technology is its feedstock flexibility-dried and ground biomass feedstocks or biomass feedstocks with high moisture contents, such as black liquor and vinasse, can be injected into the gasifier with the gasifying agent. Moreover, the smelt (composed of mostly molten ash) from this process can be recovered as an aqueous phase bottom product. A pilot-scale plant of this technology has been demonstrated for black liquor to be economically feasible.

Another form of the gasification process, supercritical water gasification (SCWG), converts wet biomass feedstocks into gaseous fuels, composed of mainly methane (CH₄), hydrogen (H₂), and carbon monoxide (CO), at temperatures and pressures above the critical point of water. Several studies, e.g., [24, 25], have reported that SCWG is more suitable for biomass feedstocks with very high (≥80 wt.%) moisture contents. This makes challenging biomass feedstocks such as black liquor and vinasse potential feedstocks for SCWG processes. However, SCWG technologies have not yet found their way into commercialization due to the challenges discussed later in Section 5.

4.3 Pyrolysis

Pyrolysis is a thermal conversion process where a feedstock is heated under inert gas conditions, i.e., in the absence of oxygen, to produce solid, liquid, and gaseous products often referred to as biochar, bio-oil, and non-condensable pyrolysis gases, respectively. Based on how fast the feedstock is heated to the pyrolysis reaction temperature and on the type of the final product sought to be maximized, there are mainly two types of pyrolysis processes: slow and fast pyrolysis. Slow pyrolysis, also known as conventional pyrolysis, is characterized by slow heating rates, long residence times of the pyrolysis products in the pyrolysis reactor, and biochar is the target product. In the fast pyrolysis processes, however, the feedstock is rapidly heated to the reaction temperature, at heating rates of as high as 10⁴°C/s, and the pyrolysis vapors are rapidly withdrawn from the pyrolyzer and cooled to maximize the bio-oil yield.

In recent years, more emphasis is given to research in fast pyrolysis compared to that of the slow, as shown in Figure 4. This is because the fast pyrolysis bio-oil has attracted interest for use as a renewable fuel and as a feedstock for the production of chemicals. In addition, reduced storage and transportation costs and higher energy density make the bio-oil more advantageous than the original biomass feedstock from which the bio-oil is produced. Besides the bio-oil, there are growing market interests in the biochars from the fast pyrolysis: biochars can be used as a soil conditioner/fertilizer, and they have the potential to substitute fossil-based industrial carbons (e.g., activated carbon). Moreover, returning biochars to the soil as fertilizers serves as CO₂ sequestration, thereby contributing to greenhouse gas emission reduction [26].

Figure 4.

The number of yearly publications on slow and fast pyrolysis. The data was retrieved from SciFinderⁿ with the key search words “slow” and “pyrolysis” and “fast” and “pyrolysis”. The types of publications considered for the data were journal articles, review papers, conference papers, books, and dissertations.

Technologies for fast pyrolysis that utilize woody biomass as a feedstock have been introduced as the first demonstration plants. For instance, a 30 MW_th Savon Voima’s (formerly Fortum’s) fast pyrolysis process has been built integrated with a CHP plant in Joensuu, Finland [23], and other industrial-scale plants are under construction [27]. However, commercial-scale fast pyrolysis technologies for low-grade biomass feedstocks have not been developed so far.

4.4 Hydrothermal liquefaction

Hydrothermal liquefaction (HTL), also known as direct liquefaction, is similar to SCWG: water is used as a solvent (or reaction medium) in both processes, and wet biomass feedstocks do not require drying for liquefaction. However, liquefaction is distinct from SCWG in the following aspects. (1) Unlike SCWG processes whose products are gaseous fuels, bio-oil is the main product from liquefaction systems. (2) HTL processes are carried out under subcritical water conditions and at moderate temperatures, 250–350°C. Although there are some pilot-scale HTL processes, as listed in [28], for low-grade biomass feedstocks, none of them have been developed into a commercial-scale technology so far.

5. Challenges and opportunities in thermal conversion of the feedstocks

As discussed in the previous section, the utilization of low-grade biomass feedstocks in thermal conversion processes has started to show a green light in some cases. However, there are still versatile challenges for marketizing these feedstocks-based thermal conversion technologies. Table 3 lists these challenges. The challenges can be categorized into process- and product quality-related challenges. Both types of challenges are primarily due to the type and level of impurities (ash-forming elements) present in the feedstocks.

Thermal conversion process	Challenges	Opportunities/remedies
Fluidized bed (BFB and CFB) combustion	Corrosion Bed agglomeration	Well-developed technology Development of additives, such as kaolin, to minimize bed agglomeration Use of corrosion-resistant metal alloys
Fluidized bed (BFB and CFB) gasification	Same as in (1) Syngas cleaning	Same as in (1) High energy efficiency cycles Development of syngas cleaning technologies Syngas conversion to other high value-added chemicals through processes such as Fischer Tropsch
Supercritical water gasification (SCWG)	Corrosion Extreme process conditions, i.e., high pressures Handling the aqueous phase product Less-developed and no large-scale technology Expensive	Potential technology for feedstocks with very high moisture content Syngas production and conversion to other high value-added chemicals through processes such as Fischer Tropsch Development of SCWG reactors with corrosion-resistant materials
Hydrothermal liquefaction (HTL)	Same as in (3)	Potential technology for feedstocks with very high moisture content Bio-oil production and upgrading bio-oils to transportation fuels and high value-added chemicals Development of HTL reactors with corrosion-resistant materials
High-temperature entrained flow gasification	Corrosion Extreme process conditions, i.e., high pressures and temperatures Syngas cleaning Risk of explosion Handling the aqueous phase bottom product Less-developed technology No commercial-scale process Expensive	High flexibility in biomass feedstocks as a feed Potential technology for biomass feedstocks with very high moisture content Syngas production and conversion to other high value-added chemicals through processes such as Fischer-Tropsch Recovery of inorganic chemicals with the aqueous bottom product
Low-temperature fast pyrolysis	Low bio-oil yield Unfavorable bio-oil physicochemical properties Low heating value of the non-condensable pyrolysis gases (NCG) Market problems	Upgradation of pyrolysis oils to transportation fuel through catalytic fast pyrolysis and hydrodeoxygenation Production of high value-added chemicals from the bio-oils Production of bio-chars for fertilizer and carbon sequestration Combustion of pyrolysis oils and gases for heat and power production Less risk of ash-related problems from the combustion of the bio-oils and NCG Demonstration-scale commercial plants are available

Table 3.

Challenges and opportunities/remedies for thermal conversion of low-grade biomass feedstocks.

The process-related thermal conversion problems associated with the type and concentration of impurities in the feedstocks include ash-deposit formation, corrosion, bed agglomeration (hence bed defluidization), fouling, and slagging [29, 30]. These problems often limit heat transfer in thermal converters, lower electrical power efficiency, decrease plant availability due to unplanned shutdowns for maintenance, and even cause irreversible damage(s) to the installations, leading to permanent loss of capital investment.

Moreover, several reports, e.g., [31, 32, 33], show that the impurities in the biomass feedstocks considerably influence the quantity and quality of products from thermal conversion systems. For example, alkali and alkaline earth metals in the feedstocks decrease the yield and quality of bio-oils from fast pyrolysis and HTL processes. These metals are known to render the bio-oils unfavorable physicochemical characteristics such as acidity, corrosivity, inhomogeneity, phase separation, instability, low heating value, and high solids, water, and oxygen contents. These unfavorable physicochemical properties make the direct utilization of the bio-oils as a transport fuel and a feedstock for the production of high-quality value-added chemicals unsuitable.

Nevertheless, Table 3 also lists some opportunities or remedies for the challenges arising from the thermal conversion of low-grade biomass feedstocks. The solutions involve both technological innovations and political (regulatory) commitments. Some examples of the former case include extensive efforts in designing and developing technologies for syngas cleaning, bio-oil upgrading, and minimizing corrosion and bed-agglomeration-related problems via the use of high-quality metal alloys and bed additives, such as kaolin. The latter (or policy) option requires commitments from governments and incentives for businesses to utilize low-grade biomass feedstocks. A typical example is the adoption of circular economy, at least by most European countries, where companies implementing the policy are incentivized, climate change challenges are tackled, and dependence of economies on depleting natural resources is minimized. In this regard, the fast pyrolysis of low-grade biomass feedstocks seems promising. According to a recent review paper by Oasmaa et al. [27], using low-grade biomass and waste feedstocks including waste plastics as input for fast pyrolysis processes is environmentally and economically sustainable. Moreover, the EU renewable energy directive (EU RED II) [34] lists and promotes low-grade biomass and waste feedstocks for bio-oil production. Upgrading of the fast pyrolysis oils to transportation fuels through, for example, catalytic fast pyrolysis and hydrodeoxygenation are increasingly gaining attention and attracting interest from the industry. Moreover, the biochars from the fast pyrolysis processes have not only the potential to replace the non-renewable chemical fertilizers but also minimize greenhouse gas emissions via carbon sequestration. Thus, the future of utilizing challenging biomass feedstocks in thermal conversion systems appears encouraging.

6. Conclusions

This book chapter sheds some light on the use of low-grade biomass feedstocks as a sustainable and renewable source for the production of energy and chemicals through thermal conversion processes. The chapter assesses the availability of the feedstocks for thermal conversion and provides the sources of impurities (ash-forming elements) contained in them. Also, it emphasizes that the main challenges associated with the utilization of the feedstocks in thermal conversion systems are related to the type and concentration of the impurities in the feedstocks. Furthermore, it details the challenges of low-grade biomass combustion, gasification, pyrolysis, and liquefaction and the potential alternative options to address them. Evaluation of the existing information on the thermal conversion of these feedstocks reveals that the fast pyrolysis process is a promising thermal conversion route for these feedstocks. The pyrolysis oils from this process can be upgraded to transportation fuels or can be used for the production of high value-added chemicals. Moreover, the fast pyrolysis biochars can be returned to the soil for use as fertilizer and at the same time minimize CO₂ emission through carbon sequestration. Overall, this work provides useful information for the design and development of state-of-the-art thermal conversion technologies for challenging biomass feedstocks.

Acknowledgments

This work has been carried out at the Laboratory of Molecular Science and Engineering, Åbo Akademi University, as part of the activities at the Johan Gadolin Process Chemistry Centre (PCC). The authors are grateful to Svenska Kulturfonden and PoDoCo (Post Docs in Companies) for the grant for Dr. Meheretu Jaleta Dirbeba under the project “Role of feedstock impurities in novel pyrolysis based technologies for low-grade biomass and waste feedstocks” (PoDoCo application no. 173788). Support from Valmet Technologies Oy is also gratefully acknowledged.

Conflict of interest

The authors declare no conflict of interest.