**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]. Nonutilized 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 sidestreams. 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.

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

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. [5–8]. 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,

**Figure 1.** *Concentration of impurities in the woody and low-grade biomass feedstocks.*

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 (NOx and SOx). These problems are discussed in Section 5 of this Chapter.

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.
