**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.

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.


**Table 2.**

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

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 fuelvinasse 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 boilertype 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.

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 *Challenging Biomass Feedstocks for Energy and Chemicals DOI: http://dx.doi.org/10.5772/intechopen.103936*

#### **Figure 3.**

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

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

#### **Figure 4.**

*The number of yearly publications on slow and fast pyrolysis. The data was retrieved from SciFinder<sup>n</sup> 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.*

(e.g., activated carbon). Moreover, returning biochars to the soil as fertilizers serves as CO2 sequestration, thereby contributing to greenhouse gas emission reduction [26].

Technologies for fast pyrolysis that utilize woody biomass as a feedstock have been introduced as the first demonstration plants. For instance, a 30 MWth 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.



#### **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., [31–33], 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

#### *Challenging Biomass Feedstocks for Energy and Chemicals DOI: http://dx.doi.org/10.5772/intechopen.103936*

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.
