A Logistics Analysis for Advancing Carbon and Nutrient Recovery from Organic Waste

Introduction

Modem societies are characterized by the generation of large amounts of waste, arising from the production of goods and sendees to satisfy social demands. The traditional manufacturing pattern is oneway linear, following the raw material extraction from the environment, manufacturing, usage, disposal of goods, energy consumption, and discarding the generated residues along the linear path. Hence, when considering the definition of sustainable development formulated by the World Commission on Environment and Development (WCED) in 1987, as “a development that meets the needs of the present without compromising the ability of future generations to meet then own needs”, the linear production is an unsustainable production model, depleting the natural resources and degrading the environment.

Therefore, one of the main challenges of the current societies is to find processes and technologies that can transform the traditional linear production paradigm into a circular structure, where the materials are recovered to be reused, remanufactured, repaired, and finally closing the loop when these generated residues can be reused as raw material again. These circular production systems are aligned with the definition of a circular economy, which is formally defined by Korhonen et al. (2018) as follows:

“Circular economy is an economy constructed from societal production-consumption systems that maximizes the sen-ice produced from the linear nature-society-nature material and energy throughput flow. This is done by using cyclical materials flows, renewable energy sources, and cascade-type energy flows. Successful circular economy contributes to all the three dimensions of sustainable development. Circular economy limits the throughput flow to a level that nature tolerates and utilizes ecosystem cycles in economic cycles by respecting then natural reproduction rates”.

Therefore, the circular economy pursues both environmental and economic objectives. The environmental objectives are to maximize the generation of valuable products, minimize the needs of raw materials extracted from nature, and reduce releases to the environment by maximizing the reuse and recycling of materials. Regarding the economic goals, these consist in the reduction of raw materials and energy costs by applying material cycles and mitigating the costs associated with waste and emission management.

Within the purview of circular economy, the management of carbon and nutrients from organic waste can have a huge contribution due to the large amounts of organic residues generated from agricultural activities (e.g., livestock), food waste, biosolids, etc. In addition, the concept of waste to energy and value-added chemicals, which pursues the development of processes for the recovery of energy and production of high added-value chemicals fr om all kinds of residues, can be applied to organic waste in order to achieve a circular economy framework. To illustrate this, a diagram of a circular economy system built around organic waste is shown in Figure 1. Particularly, an anaerobic digestion process can be used to decompose the organic matter and produce biogas, constituted mainly of methane (CH4) and carbon dioxide (CO,). With proper treatment to recover the CO„ the resulting high CH, purity gas can be used as a renewable alternative to the fossil-based natural gas (NG) in the production of power and heat. In addition, the sub-product from the anaerobic digestion, so-called digestate, is rich in nutrients (particularly nitrogen and phosphorous) that can be recovered and used as fertilizer for crops, thus closing the nutrient cycle loop.

The livestock agricultural sector is one of the largest economic activities in the world, as it must meet the needs of a demanding society. Additionally, the increasing income-spending potential of the middle class in developing countries has increased the demand for dairy and beef products. This results in some areas where livestock activities generate more GHGs than the transportation sector,[1] nutrient pollution, and other environmental challenges.

In developed societies, waste valorization to energy and material recovery represents a business opportunity for circular economy. Furthermore, organic waste management provides a great opportunity towards the production of sustainable resources and energy (WEC, 2016) and the capability of replacing fossil-based fuels. For instance, capturing carbon through the production of bio-based chemicals derived from the biogas generated after the anaerobic digestion of the organic wastes offers an alternative for replacing the generation of electricity and heat from fossil fuels with renewable sources throughout the production of bio-methane. It is reported that about 250 operational projects in the U.S. (Nov. 2017)[2] have avoided the emissions of 3.2 x Ю6 tons CO,e (equivalent to annual emissions from 680.000 cars)[3] and generated 1.03 x Ю6 MWh of energy (enough to power 96,000 U.S. homes/yr).[4] [5] Other studies have evaluated the potential of some regions to meet all their NG needs by using biogas instead (Taifouris and Martin, 2018). Therefore, waste-to-energy initiatives har e gained support in the context of a circular economy philosophy to enhance the development of sustainable process alternatives (Korhonen et al., 2018). Among the organic waste treatment technologies, anaerobic digestion is deemed as an interesting and promising alternative that serves a double objective when processing residues, mitigating the potential environmental and human health issues and producing valuable products that are incorporated into the economic cycle in the form of energy and chemicals.

Circular economy generated around organic waste

Figure 1. Circular economy generated around organic waste.

Biogas Upgrading and Carbon Capture

Bio-methane as a renewable source of energy

The biogas generated through anaerobic digestion of waste is mainly composed of CH4, CO„ small amounts of impurities (ammonia and hydrogen sulfide), and moisture. After removing these impurities, biogas represents a renewable source of CH4 and CO,. Biogas can be employed either to produce energy, as it is currently implemented in many industrial facilities, or as raw material to produce chemicals through a syngas/dry reforming path.

As a source of energy, the usage of biogas can be classified according to its quality. Raw biogas can be used for the co-generation of heat and electricity in combined heat and power (CHP) units for onsite consumption, mainly to supply heat to the digestor unit and produce hot water. In addition, it is possible to use biogas in gas turbines and power generators if upgraded by reducing the concentration of CO, and obtaining high purity methane (Somehsaraei et al., 2014; Reddy et al., 2016; Leon and Martin, 2016). Additionally, the large infrastructure available for the transportation and distribution of NG in Europe (Entsog, 2015) or the USA (EIA, 2018), can support the use of biomethane as a renewable alternative to NG. Likewise, high-quality biomethane can be compressed or liquefied to be transported using trucks or railways if a pipeline grid is not available. Finally, a remarkable feature of compressed or liquefied biomethane is its use as transportation fuel, replacing compressed-NG (CNG) and liquified-NG (LNG).

There are several pathways for upgrading biogas. For example, it is possible to hydrogenate the CO, content of the biogas into CH4. This process is widely used as a purification step for ammonia production and has been evaluated for the production of biomethane from biogas at experimental level (Stangeland et al., 2017; Schidhauer and Biollaz, 2015) and lately full process engineering, including techno-economic analysis (Curto and Martin, 2019), have been reported. The main issue associated with the hydrogenation of CO, to obtain renewable biomethane is the high need for hydrogen from renewable energy sources instead of using fossil fuel sources since its production is energy intensive (Davis and Martin, 2014a. b). The high investment costs for renewable energy production using either photovoltaic panels and/or wind turbines limit the placement of these facilities to favorable locations where solar fields and wind farms can achieve competitive production costs of CH, from CO, (de la Cruz and Martin, 2016). Additionally, the recovered CO, has enough purity to be used as a raw material for other chemical processes.

Regarding the use of biogas as raw material to produce chemicals, some recent studies evaluate different routes to produce bio-syntliesis gas (Hernandez et al., 2017), methanol for biodiesel, and as a feedstock for algae-based oil production (Hernandez and Martin. 2017). However, the investment and production costs of these chemicals through these novel routes are high, resulting in using biogas as a primary energy source alone.

Technical alternatives for biogas-carbon management

The common CO, capture techniques are based on separation processes, such as gas-liquid chemical absorption, gas-solid physical adsorption, and permeation processes using membranes. Currently, the most mature technologies for CO, capture, based on the studies available in the literature, are amine scrubbing, pressure swing adsoiption (PSA), and membrane separation systems. Water scrubbing is another technology that is widely used for CO, capture, but it has some major disadvantages. Particularly, the air used for the degassing of water (Budziauowski et al., 2017) exits the unit with a high concentration of CO, that should be treated before being released to the atmosphere. Furthermore, this CO, can be used after being recaptured through additional processing steps.

Chemical absorption: Amines

For the CO, absorption using amines solution, the gas stream is passed couuter-currently to an aqueous alkali solution containing the amine in order to promote a chemical absorption of the CO,. As described in Figure 2, this process is carried out in a two-exit absorption column. The gas phase output stream is composed of the purified gas stream, while the liquid phase output stream consists of the aqueous solution with the absorbed CO,. The aqueous stream is sent to a regeneration column in order to perform the desorption process by heating the solution. Ел ен though the absorption process does not require external energy at ambient temperature, the regeneration process is energy intensive (Unveren et al., 2017).

Flowsheet describing the amine absorption process

Figure 2. Flowsheet describing the amine absorption process.

The CO, absorption systems using amines typically operate at a temperature range of 25-30 °C, partial pressures above 0.05 bar, and removal yields of 90%-95% (Zhang and Chen, 2013). However, the high concentration of CO, in the biogas, compared to post-combustion gases, results in lower operating pressures, reducing the compression costs.

Different amines, like monoethanolamine (MEA), diethanolamine (DEA), andmethyl diethanolamine (MDEA) are among the common amines used for CO, capture, because of their high CO, affinity. The selection of the optimal amine to minimize the overall operating costs should be made considering several important aspects, such as the CO, to amine molar ratio, the heat of reaction (associated to the reactor refrigeration cost), and the absoiptiou solution concentration and its price (Nuchitprasittichai and Cremaschi, 2013).

Amine absoiptiou is a relatively simple CO, capture technology, which does not require the development of complex or novel equipment. The requirement of energy involved in the regeneration column is the largest share of the operating cost.

Pressure swing adsorption

A pressure swing adsorption (PSA) operation is based on physical adsoiptiou at moderate pressure and low-pressure regeneration at a constant temperature. As shown in Figure 3, in order to perform the CO, adsoiption, the raw gas is compressed prior to the adsoiptiou stage. Then, the compressed gas is introduced into a column filled with the solid adsorbent material, forming a fixed bed. The gas is flowing across the bed, interacting with the adsorbent, and carrying out the CO, adsoiption on the fixed bed through reaching a certain partial pressure for CO,. After the bed reaches the saturation point, the operation stops, and the unit undergoes a desorption step. The desorption step consists of a concurrent flow of a gas stream with low CO, partial pressure which drags the CO,. Commonly, PSA systems are composed of twice the amount of design-required adsoiption columns, so while one-half of the units are in use. the other half are being regenerated.

Some of the most common adsorbent materials include zeolites, such as zeolites 13X and 4A. Hie CO, removal before the breakpoint using zeolites 13X and 4A is almost total, leaving only traces of CO, in the exit gas (Hauchhum and Maliauta, 2014). Additionally, some experimental results show that CO, adsoiption onto zeolites can be modeled through theoretical isotherms, like the Langmuir isotherm (Hauchhum and Maliauta, 2014).

The adsoiptiou capacity of the zeolites is directly related to the CO, partial pressure. Therefore, a system of compressors with an intermediate cooling system should be implemented. However, more detailed studies should be carried out in order to determine the optimal trade-off between compression cost and carbon capture efficiency.

Flowsheet descnbing the pressure swing adsoiption process

Figure 3. Flowsheet descnbing the pressure swing adsoiption process.

Membrane separation systems

Membrane separation technologies are based on the permeability of different gases through a membrane. The species with higher permeability can cross the membrane, forming an outlet stream, i.e., permeated, while the species with lower permeability cannot go through the membrane barrier, being evacuated from the unit through an outlet stream, i.e., retentate.

As the din ing force of the process is the gradient in the species concentration on both sides of the membrane, certain partial pressure values for the permeated compounds should be adher ed before introducing the gas stream in the membrane unit. As shown in Figure 4, membrane systems consist of compressor units followed by a set of membrane modules. For the design of membrane systems, two variables should be considered: The membrane material and the arrangement of the units, since the membrane units can be arranged in different configurations such as single-stage or multi-stage

Multi-stage membrane system arrangements arrangements

Figure 4. Multi-stage membrane system arrangements arrangements. A multi-stage configuration results in larger methane recovery and lower operational costs (Scliolz et al., 2013). Among the multiple membrane stage systems, one can find configurations with one compression stage (Makaruk et al., 2010; Scholz et al., 2013) or multiple compression stages (Molino et al., 2013; Scholz et al., 2013). Some studies suggest that dual-stage membrane systems with a single compression stage can be the most flexible option, obtaining between 97% and 80% CO, recovery yields for CO, contents in the feed of 10-40% (mass) (Kim et al., 2017).

Since membrane materials are characterized by their gas permeability, many of these materials have been evaluated for CO, recovery, from traditional materials, including polymers, to the latest developed membrane materials, such as carbon molecular sieves. Among the common materials with larger CO, selectivity values, one can identify cellulose acetate, polyimide, and polycarbonate. Other membrane materials for the separation of CO, and CH, can be found in several reviews (Zhang et al., 2013; Clieu et al., 2015; Vrbova and Ciahotny 2017).

Carbon recovery by chemical production

Several chemicals can be produced from the gases generated in the anaerobic digestion process. In particular, Hernandez et al. (2017) developed a platform approach to produce dimethyl ether (DME), methanol, ethanol, and synthetic fuels based on the Fischer-Tropsch process. The production of hydrocarbons is based on the diy reforming of biogas to produce renewable syngas, which is then- precursor. Due to the paramount importance of methanol, it was the first to be addressed (Hernandez and Martin. 2016). In addition, DME, ethanol, and synthetic fuels can also be produced using the biosyngas as raw material by controlling the H, to CO ratio. Diy reforming is based on the equilibrium of the set of reactions shown in equation (1), which is dependent on the operating pressure and temperature. Diy reforming is an endothermic process, which can be heated by combusting a fraction of the generated raw biogas.

After the syngas production, methanol is produced following the set of reactions described in equation (2), catalyzed by a solid catalyst formed by a mixture of CuO-ZnO-AlO.

The production of methanol is favored at high H,/CO ratios, between 1.75 and 3, and it is considered that a CO, concentration between 2% and 8% is necessaiy due to the reaction mechanisms, see equation (2). Since the reaction equilibrium process is exothermic, and the concentration of species is lower at the right side of the reactions, the optimal operating conditions are defined by low temperatures and high pressures (Hernandez and Martin, 2016). However, high pressure implies higher compression costs. Therefore, the trade-off between yield and operation cost is sought. The current industrial processes operate at pressures of 50-100 bar and temperatures of 473-573 К to activate the catalyst and boost the reaction (Hernandez and Martin, 2016).

Furthermore, the production of ethanol and DME from biosyngas requires H,/CO ratios of 1. Regarding the DME production, this is produced via methanol as an intermediate product using copper catalysts, followed by a dehydration process catalyzed by acidic catalysts. As it is shown in the set of equations described by equation (3), together with the methanol and DME production, the water gas shift reaction takes place as well.

Similar to methanol production, DME production is governed by chemical equilibrium and is usually earned out at temperatures between 508 К and 553 K. A more detailed description of DME production from biosyngas can be found on Peral and Martin (2015).

The production of ethanol from biosyngas can be achieved following two paths. One is the chemical synthesis of alcohols using catalysts, producing ethanol and other alcohols as byproducts. This is an energy intensive process since the biosyngas must be heated up to 573 К and it operates at high pressure. As several alcohols are synthesized, a further purification process must be implemented. Another alternative for ethanol production is the fermentation of the biosyugas in a bioreactor. This biological process operates at low temperature, 311 K, and low pressure. However, the water-etlianol separation, which is energy intensive, must be carried out. Additionally, to ensure the survival of the biota of the bioreactor, the ethanol concentration must be kept below 5% by feeding water in order to maintain the ethanol concentration under this threshold, thus limiting the yield of the process. More details of these processes can be found in Martin and Grossmann (2011).

Finally, synthetic fuels can be obtained from biosyngas based on Fischer-Tropsch (FT) processes. The FT processes are based on reactions equivalent to polymerization reactions where the chains of hydrocarbons grow due to the dissociation of CO over a catalyst, as shown in equation (4), where nc denotes the number of carbon atoms and nh the number of hydrogen atoms. Usually, the catalysts used in FT processes are based on cobalt or iron.

Other reactions that take place in the process are the methanation of the CO, and the water gas shift reaction, as described in equation (5).

FT synthesis can be carried out following two paths: High and low temperature processes. Both processes require H,/CO ratios between 1 and 2. Each path is associated with a different distribution of products. On the FT high-temperature process, the fuel production is achieved at temperatures of 590-630 К and pressures between 10 and 40 bar, using iron-based catalysts. In this process, only traces of heavy fractions are produced, making further separation processes easier (Hernandez and Martin, 2018). This separation process consists of a first separation of the light hydrocarbon compounds as gases, the removal of water generated in the process, and a distillation column to separate the different liquid alkanes. The low-temperature FT process is performed at 440-530 К with cobalt or iron-based catalysts. This process produces larger amounts of heavy fractions than the high-temperature FT process. Therefore, a hydrocracking step must be included in order to break the long chain alkanes and enhance the conversion to gasoline and diesel. The rest of the separation step is similar to separation steps in the high-temperature FT process.

  • [1] http://www.fao.org/newsroom/en/news/2006/1000448/mdex.html.
  • [2] https://www.epa.gov/agstar/agstar-data-and-trends.
  • [3] 5 A typical passenger vehicle emits about 4.7 metric tons of carbon dioxide per year, https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-typical-passenger-vehicle.
  • [4] In 2016, the average annual electricity consumption for a U.S. residential utility customer was 10,766 kWh, https://www.
  • [5] eia.gov/tools/faqs/faq ,php?id=97&t=3.
 
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