The oil palm industry generates a huge amount of biomass, as an example, worldwide production of oil in 2019/20 was 72.27 MMT, which generated over 200 MMT of waste from solid and liquid wastes. It is estimated that in Costa Rica, 0.52 MMT of waste was produced in that same year. As oil palm biomass is a readily available lignocellulosic biomass, it has the potential to be a low-cost feedstock for conversion into higher value products [
131]. Only about 10% is used to increase the value of the production chain by adding products with high value or by exploiting biomass to produce energy. Biodiesel is one of these valuable products that may be adopted by the oil palm industry. In the late years, demand for biodiesel has increased. Since the combustion of fossil-based fuel has become a concern due to carbon emissions and its relationship with global climate, biodiesel emerges is a notable alternative [
121]. On the other hand, agricultural or industrial wastes can be revalorized by converting them to energy; the conversion of biomass to renewable energy requires the breakdown of the main organic components. As mentioned before, these organic compounds range from their complexity and functional groups, which ultimately influence the structure and chemistry of the biomass. This knowledge is important to develop biological and chemical processes that add value to the biomass [
128]. Hereon, we describe different applications that can be adopted by the oil palm industry using a circular economy approach to increase the value of their products and biowaste.
3.3.1 Bioenergy
The ability to convert biowastes into energy and to incorporate it into the industrial process is a key point for integrated bioeconomy solutions. The development of sustainable processes that harvest, collect, store, and transform biowaste efficiently into energy is generally the determinant to establish the feasibility of the process. Biomass logistics is defined as the flow of material from the production site (e.g., agricultural field) to the point designated to the processing plant for material transformation [
110]. In Costa Rica, major palm oil producers are located in the central- and south-pacific region of the country, and so far, all transformation processes were always included within the production facilities.
Different schemes of biomass valorization have been conceived within the oil palm industry in Costa Rica. Biological conversions can transform biomass into renewable energy products; these involve biochemical reactions that degrade biowaste into sugars, starch, and cellulose, and further conversion into bioethanol or biobutanol through a series of biochemical reactions, or the transformation of these products to methane or hydrogen through anaerobic digestion [
121]. So far, not much work has been done in this direction in Costa Rica, besides the academic-industry research projects which currently are in the state of “proof of the concept” [
132,
133].
Biomass can also be converted to energy using different kinds of thermochemical processes. The end-user requirements in terms of power and heat influence the applied process, as well as the biomass source and its nature. Different thermochemical processes can be used to produce energy from biowaste: (1) combustion for the production of steam and power; (2) gasification for the production of gas, which is then used for power/heat generation; and (3) pyrolysis, which produces a liquid that can be used for transportation fuel [
109].
In Costa Rica, palm oil mills meet their electricity and steam requirements partially by burning biomass. Combustion is usually carried out in a furnace or a stove, where the fuel is burned directly to produce heat. The industry does not use all the wastes described in Table
5. Usually, palm kernel shells and the palm press fibers are used to produce energy. The main goal of a combustion process is to release the energy stored in the biomass while minimizing losses due to incomplete combustion. It requires an ignition of the biomass; therefore, low moisture is one of the key characteristics required for an efficient process. Hence, palm kernel shells are preferred over another biowaste such as the trunk and fronds, which are normally not used to produce energy. According to Sulaiman et al
. (2011) for each kilogram of palm oil, roughly another 4 kg of dry biomass are produced, which are derived from FFB and organic material. Only a small fraction is used within revalorization process for mulching and as fertilizer, the rest is left in the fields. For each kilogram of palm oil, electricity consumption is around 0.1 kWh and 2.5 kg of steam, this is met by burning around 0.4 kg of palm kernel shell [
105]. Therefore, little effort has been invested into optimizing the process or including other wastes. Furthermore, the extension of electric production is a major element of the economic viability and is entangled with tariffs and norms that regulate the cogeneration of electrical energy.
Gasification and pyrolysis are two kinds of technologies used to generate power from biomass. The gasification process converts the solid biomass to a gaseous fuel that may later be used for the power/heat generation, transportation, or chemicals. Oil palm residues have around double of volatile matter, between 70 and 90% (w/w), when compared to coal. This is ideal for gasification since it produces high volumes of gas and lower amounts of char upon heating. Nevertheless, disadvantages such as a low energy density and complexity of the system when compared to a stove limit its applications within the palm oil industry. On the other hand, pyrolysis converts biomass into liquids, solids, and a gaseous fraction by heat (with or without the aid of a catalyst) in the absence of air or oxygen. The liquid fraction can be used as a biofuel for transportation or a source for chemical feedstock. To our knowledge, so far, gasification and pyrolysis have not been used for the production of energy in Costa Rica, although these processes offer many technological advantages [
134].
3.3.2 Biodiesel
Energy consumption is growing more rapidly than population growth. Sustainable and consistent energy supplies are primordial for any economy [
135]. Gasoline and diesel are two of the most important fuels used worldwide. The latter one is largely applied for energy demanding sectors (i.e., transportation, agriculture, and industry) for the generation of power/mechanical energy. It is mainly supplied by petrochemical sources, which brings along problems associated with pollution and the limited quantity of petroleum. This has stimulated the interest in alternatives, and biodiesel fulfills the technical requirements needed to substitute or complement the petrochemical industry: biodiesel production is technically feasible at a large scale, it is competitive and environmentally acceptable [
136,
137].
Biodiesel is defined as mono-alkyl esters derived from renewable feedstocks. There is a great structural similarity between biodiesel and diesel derived from petroleum. Like conventional diesel, biodiesel can be produced from vegetable oils, animal fat, and used cooking oils and obtained by transesterification with alcohol. It is a non-toxic and biodegradable fuel that does not contain sulfur, aromatic hydrocarbons, metals, or any crude oil residue. Also, when compared with conventional diesel, biodiesel reduces the emission of CO
2 by 78%; this is due to biofuel closes the carbon cycle [
121].
Sustainability of the production process depends on several factors: the source of the feedstock (edible crops, wasted oils, others), the pretreatment method of the feedstock, the amount of free fatty acids, type of catalysis employed during the production process, type of alcohol used, and the source of energy used during the process [
137]. Out of this, the type of feedstock represents around 75–90% of the total production costs; therefore, special attention should be paid to the input resources. Thence, palm oil represents a comparatively suitable source of feedstock for biodiesel production since it fulfills several criteria such as high productivity, efficiency, and competitive prices.
In Costa Rica, few companies have integrated a whole process to produce of biofuel and several businesses have emerged in the last years: Central Biodiesel HTP, Biodiesel H&M, Energías Biodegradables, Dieselloverde Derivel, and Cia Coto 54. Some of these are currently not dedicated to the production of biodiesel [
138]. A common factor that limits their production is access to the feedstock at an affordable price. Also, Costa Rica has a National Biofuel Plan in which the Costa Rican Petroleum Refinery (
Refinadora Costarricense de Petróleo—RECOPE) strives for a mass production and commercialization of blended biofuels, but, for now, it has been limited to an experimental production [
139].
3.3.3 Bio-glycerol
Crude glycerol is the major by-product of palm oil biodiesel production, representing 10% of the products from the biodiesel process [
140]. A glycerol biorefinery is more likely to be seen as a promoter for a circular economy, where glycerol becomes an attractive, cheap, and local resource and inter-industry sharing of resources becomes an integrated part of modern and green business models [
141]. As a result, palm oil biorefineries, especially in Malaysia and Indonesia, generate high amounts of crude glycerol, which endanger the environment, if not managed properly. However, there is a high demand for crude glycerol; its world production for 2020 is estimated at 4.0 billion liters [
142]. There is an enormous potential for the usage of crude glycerol in high-value products within the food, oleochemical, and cosmetic industry.
Depending on the source of palm oil and the production process, the recovered crude glycerol contains impurities, which affects the purification costs greatly [
143]. Herein, we summarize the main production and purification methods according to the palm oil sources.
The traditional method for the production of crude glycerol is the chemically catalyzed transesterification, due to its low cost. The reaction is characterized by the transesterification of triglycerides from PO with an alcohol (usually methanol), in presence of a catalyst (e.g., acid, base), generating fatty acid methyl esters (FAME), and crude glycerol as side product [
144,
145]. Generally, the transesterification reaction yields approximately 90% biodiesel, 9–9.6% crude glycerol, and 0.4–1% of impurities [
146].
After transesterification, crude glycerol is separated from biodiesel by centrifugation or decantation, whereas the impurities remain mostly in the crude glycerol. These impurities comprise catalysts, soaps, alcohols, metals, salts, and acylglycerides [
147]. Removal of impurities can be performed by different strategies depending on the catalyst used. When a homogeneous catalyst is used, a neutralization stage is performed, whether with an acid (H
2SO
4) or a base (NaOH). This generates inorganic salts (e.g., fatty acid salts), which are removed by decantation/filtration [
146]. With heterogeneous catalysts, the soaps are converted to salts and fatty acids by an acid treatment. Further distillation removes the alcohol and water impurities. Afterward, the mixture is neutralized with a base (e.g., caustic soda) and the salts are removed by filtration, resulting in an 80% crude glycerol [
146,
148].
Production of crude glycerol from PKO is performed with homogeneous alkaline catalysts (NaOH), which are removed by an acid wash with HCl. After splitting the soaps, the mixture is neutralized with NaOH, and salts are filtered and further evaporated, yielding crude glycerol [
149,
150]. Similarly, production from CPO is also performed by the industry with homogeneous catalysts (NaOH). However, Isahak et al
. (2010) demonstrated a process with heterogeneous catalysts (KOH and Al
2O
3). After removal of the catalyst by filtration, the mixture was distilled to remove methanol. Crude glycerol was then recovered after neutralization with 85% phosphoric acid and microfiltration for salt removal.
The recovered crude glycerol from the distinct production process requires further purification to eliminate residual contaminants such as alcohol (e.g., methanol), soaps, and salts. The most common method is vacuum distillation, which can eliminate methanol. The process operates at 120–126 °C and high purity can be achieved at low costs. Eventually, vacuum distillation can generate side products, if not well performed, including oxidation, polymerization, and dehydration [
146].
During the crude glycerol production, a small fraction of catalysts is present as free ions. For their removal, adsorption by ionic exchange resins is a successful method. The use of two resins for binding free anions and cations is a promising strategy, thus hydrogen and hydroxyl ions are exchanged, yielding water. As an example, Isahak et al
. (2010) used Amberlite 200 °C and IRN-78 resins to recover glycerol from glycerol residue from distillation [
148]. As for the commercial pure glycerol, the number of free acids was less than 0.08–0.1%. Purified glycerol from palm oil is processed until commercial or pure grade and used for the production of bulk chemicals, food supplements, additives, cosmetics, and medicaments. Alternatively, it is used for the production of solvents, surfactants, and polymers [
151]. These derivatives can be produced via different pathways, which include dehydration, condensation, transesterification, hydrogenolysis, esterification, oxidation, and halogenation, among others.