Physicochemical conversion routes

The densification of residues in the form of briquettes and pellets contributes to high volumetric concentrations of energy, improving the handling and reducing the volume and resulting storage and transport costs (dos Santos et al., 2015). This solid fuel is widely used for any type of thermal application such as steam generation in boilers, heating, drying, food processing industries, brick making units and gasifier systems (Lubwama and Yiga, 2018). Currently, the high demand for briquettes and pellets is driven, significantly by governmental policies and incentives. The EU as the main consumer, accounted for 80% the global biomass briquette market and reported around 20 million tons of wood pellet production, since the solid fuel is used also in non-industrial environments, such as domestic heating and commercial boilers, on a small scale (FAO, 2020).

The briquette is one of the cleanest and most environmentally friendly solid fuel on the market today. Its emissions of nitrogen and volatile organic compounds are lower than the feedstock. Likewise, the moisture content and CO2released, when under thermochemi-cal conversion processes, are also significantly reduced, improving the efficiency of the transformation. Briquettes are generally produced in a piston (mechanical or hydraulic) press by applying load on a die to biomass particles. Additional binder and heat treatment can be applied depending on the properties of the feedstock. Generally, the densification is carried out at high pressures in the order of 1200 Psi, which causes an increase in the process temperature, usually in the range of 100-140^◦C, causing the “plastification” of lignin, that acts as an agglomerating element, which is strongly influenced by the humidity of the material, which is typically between 8-15% (Mendoza Martinez et al., 2019b).

Agro-forestry residues are the more commonly used for the formation of briquettes and are generally compacted in the order of 5 times. A notable advantage in the use of bri-quettes is the reduction of time in the production of fuels and the implementation of residual raw material, thus reducing the rate of deforestation (Pallavi et al., 2013). Sev-eral studies have evaluated the diversity of agro-forest waste as by-products for briquet-ting, such as, rice residues (Yank et al., 2016; Lubwama and Yiga, 2018); rice and coffee husks briquettes for domestic cooking applications (Amaya et al., 2007); production of activated carbon briquettes from rice husks and eucalyptus wood (Muazu and Stegemann, 2015), coffee-pine wood briquettes as an alternative fuel for local firing systems in Brazil (Mendoza Martinez et al., 2019b), explained inPublication IV. This rational use of bio-mass tends to promote the development of less economically favored regions, through the creation of jobs and reduction of external energy dependence, based on local availability (dos Santos et al., 2015).

In order to produce good quality briquettes/pellets, the pretreatment of the feedstock plays a fundamental role. The particle size, moisture content, apparent density, use of binders and process variables such as the pressure and temperature are parameters that need to be measured and controlled. The feedstock should be composed of a mixture of particles of various sizes, where the maximum particle size must not exceed 25% of the equipment

matrix, and the percentage of powder component (<4 mesh) must be in the range of 10 -20% (Grover and Mishra, 1996). The presence of particles of different sizes improves the mechanical behavior, contributing to the high strength of the product (Vidal et al., 2011).

The final density of the solid fuel is also affected by the particle size, higher density and less compaction forces can be reached with a small particle size. However, this can lead to low porosity in the solid fuel, as well as mass transfer during drying, devolatilization and combustion (Hern´andez et al., 2010).

The moisture content also highly influences the manufacture of briquettes/pellets. The presence of water favors heat transfer, promoting the softening of lignin and, conse-quently, the connection between particles (Grover and Mishra, 1996). Depending also on the type of feedstock and compaction process, an adequate moisture range is typically 8 - 12%. According to Grover and Mishra (1996), this moisture content, briquettes/pellets are generally resistant and free from cracks. On the other hand, very dry material affects the particles’ connections, which impairs the briquette’s stability and resistance. Mois-ture percentages that exceed 15% must go through a drying process, otherwise they may compromise the efficiency of combustion process of the briquettes due to possible explo-sions due to the formation of water vapor inside the interstices (Yaman et al., 2000). The high density of the feedstock is also not attractive due to little gains in densification, and low density biomasses demand greater energy for their adequate compaction. Therefore, a possible solution is a mixture of biomasses that increase the energetic and mechanical characteristics of the briquette, taking into account proportions that produce a lower per-centage of ash and pollutant gas emissions in the thermochemical conversion processes.

Pressure is another relevant factor in the final properties of the product which is respon-sible for the compaction and transfer of energy between the particles. The internal and external friction caused by the pressure, produces heating in the material which increases the process temperature.

3 Energy potential of residual biomass

3.1 The effect of biomass for energy applications

In order to achieve efficient utilization of biomass, the most appropriate characteristics according to the final application, need to be selected. This section of the thesis evaluates the main parameters which influence the residual biomass in the coffee production chain for energy purposes. The concern for the increase of coffee bean productivity has ignored the variation of the residues provided by the coffee production chain. Additionally, little or only partial information currently exist in the literature. The obtained results reported inPublication IandPublication IIcan be used for the evaluation of the applicability of coffee residues for various energy production pathways.

Based on a chemical and thermal characterization analysis and the available data on dif-ferent treatment technologies, the characterization parameters obtained for coffee solid residues, the effects of the properties of biomasses evaluated in diverse conversion routes, as well as their impact on operational and economic parameters are summarized as fol-lows (Mendoza Martinez et al., 2019a):

• The high moisture content of coffee pulp would be a drawback for most of the thermo-chemical conversion routes except for HTC, since a large amount of energy would have to be spent on water evaporation. The high ash content is also a drawback that may cause sintering at high temperatures. The high fractions of cellulose, hemicellulose and phenolic compounds in the chemical composition of the pulp are a favorable for obtaining bioethanol, proteins, enzymes, vitamins, amino acids, lactic acid and acetic acid through hydrolysis and biodegradation (Mussatto et al., 2011).

• For combustion, biomass with a high basic density (> 500 kg·m⁻³) results in more concentrated fuel energy, due to the greater mass of the fuel contained in the same unit volume (Obernberger and Thek, 2004). Thus, coffee wood is a desirable material for charcoal production. Moreover, a high charcoal density can be achieved, resulting in greater strength and higher energy per volume.

• The coffee wood shrub holds a high economic potential for energy applications. The low moisture content means low transport and storage costs (Hamelinck et al., 2005).

Additionally, the energy consumption in the drying stage when subjected to processes such as gasification, combustion and pyrolysis, is low.

• For the production of bio-oil in a fast pyrolysis process, the biomass is subjected to high temperatures and short vapor residence time. High ash content generates secondary vapor cracking, reducing the bio-oil quality and liquid yield. However, the presence of inorganic components such as potassium favors the formation of charcoal (Rocha et al., 2015). As a result, leaves from the coffee shrub and parchment of the coffee cherry are desirable biomasses for the fast pyrolysis process.

• During combustion, sulfur is converted into H₂S and SO₂, which are strong sources of contamination. The leaves of the coffee shrub are poor in the respect the high content of sulfur. Moreover, high nitrogen contents have also been reported for coffee leaves, resulting in the formation of nitrous oxides.

• A drawback of coffee parchment as a biochar feedstock is the ash content. The ash not only reduces the heating value, but alkali metals and chlorine can be problematic due to the formation of alkali chlorides. These can cause agglomeration, fouling, and corrosion in high-temperature processes such as gasification and combustion.

• Hydrothermal carbonization can be an attractive solution to turn raw coffee parchment into a valuable product, i.e. soil conditioner, activated carbon or an adsorbent agent.

Further analysis of coffee cherry parchment HTC products should be performed.

• Some characteristics of the husks obtained in the coffee dry treatment route are suitable for thermochemical and physicochemical conversion. The high content of polysaccha-rides, proteins and minerals can also make them suitable as a substrate support in the fermentation process for the extraction of compounds for the food and pharmaceutical industries, in addition as a material for composting applications. Value-added applica-tions of coffee husks are already commercialized, mainly in bioprocessing, detoxifica-tion and vermicomposting.

• Spent coffee grounds include large amounts of carbon with a low ash content, which are factors that contribute to the production of fuels with a high calorific value through thermochemical conversions. Additional advantages of SCG for energetic conversion are the high heating value and low sulfur and nitrogen content. The high content of volatile matter in SCG involves a large number of compounds that can be devolatilized, which increases the ignition speed of the feedstock. Moreover, their high lignin and low extractive content indicate a potential application in physical processes such as the production of briquettes/pellets.

• The phenolic and organic compounds in the chemical composition of the SCG, such as lipids improves extraction and hydrolysis processes, which is an advantage for bio-chemical and physical conversion routes. Among the applications, SCG is used as raw material for the production of bio-oil and biochar by slow pyrolysis, biogas through anaerobic co-digestion, solid fuel for direct combustion in boilers, bio-alcohol by fer-mentation, biodiesel production through transesterification, biofuels, biopolymers, an-tioxidants and biocomposites. Additionally, SCG also has other applications such as fertilizer in the agro-industry, production of activated carbon, animal feed and as an adsorbent.

• The residues in the coffee production chain consists of desirable materials for densifi-cation processes, especially coffee parchment, wood and spent coffee grounds due to the relative high lignin content, that promotes agglomeration (Obernberger and Thek, 2010). Briquettes increase the energy density of waste materials and reduce storage and transportation, which are desirable factors for the energy industry.