Biogas Cogeneration Research Articles

State Aid to Electricity Producers: National and EU Law Aspects

At European Union level, there is a detailed regulatory framework allowing Member States to introduce state aid. State aid is always granted for a specific purpose and helps Member States to achieve their common objectives. The energy sector is being moved towards reaching targets for both energy efficiency and climate neutrality, with a gradual withdrawal from fossil fuels. However, it is not always easy to align these targets at Member State level and to create the appropriate national regulatory framework. Therefore, this article in the light of the two most recent judgements of the Constitutional Court of the Republic of Latvia (hereinafter – Constitutional Court), analyses the importance of a fixed term of state aid for electricity production and some aspects of state aid for biogas cogeneration plants, including the development of EU law perspective. The article also examines the impact of the enforcement of the Constitutional Court’s judgments in the area of state aid in Latvia and explains the emerging concept of environmental constitutionalism.

ПРИМЕНЕНИЕ БИОГАЗОВОЙ КОГЕНЕРАЦИОННОЙ УСТАНОВКИ ДЛЯ ОБЕСПЕЧЕНИЯ ТЕПЛОСНАБЖЕНИЯ ТЕПЛИЧНОГО КОМПЛЕКСА

For the yearround population by vegetables supplying, it is necessary enclosed area’s crops growing. At the same time, vegetable production’s cost prevailing in the Irkutsk region is thermal energy to the greenhouse’s complex supplying. One of the ways of this problem solving is thermal energy by a biogas cogeneration’s installation producing. To provide heat supply due to such biogas cogeneration installation, biogas that consists of 55-75% methane is used, the rest are the impurities. Biogas in methane tanks due to the livestock farms’ organic waste anaerobic digestion is produced. Thus, at the same time, the problem of livestock by products in agricultural enterprises recycling is being solved. It is known that there are no greenhouses’ operating all year round in the Irkutsk region. The task is to find out if cattle farm will be able a greenhouse complex with an area of 500 m2 with by products’ processing the thermal energy’s necessary amount providing. An analytical dependence is presented for the thermal energy’s required amount calculate that by a biogas cogeneration installation for a greenhouse complex producing. The Irkutsk region greenhouse complex thermal processes calculation conditions’ was carried out. The temperature of the coldest five day period with a security of 0,92 was -33°C. Heat losses through structures enclosing, transoms and due to infiltration are calculated. A heat month losses’ graph is constructed. It was revealed that heat’s most losses through structures enclosing are lost. The calculations provide for soil heating as well. The heating devices required surface which will be 207 m2 was also calculated. Based on previous studies, it was revealed that the cattle farm’s theoretical energy potential a greenhouse complex with its own thermal energy can providing.

Electrification of oil refineries through multi-objective multi-period graph-theoretical planning: A crude distillation unit case study

Electrification using renewable energy sources is the key to paving a sustainable and cleaner future for the oil and gas sector, which is known to be a significant carbon dioxide emitter. Nevertheless, the suitability of the electrification designs heavily depends on the seasonal availability of renewable energy sources. This work proposes to use a multi-period graph-theoretical (P-graph) approach to determine the optimal retrofit strategy to achieve electrification with consideration of economic and environmental factors. Both single-period and multi-period models are considered via a graph-theoretical approach to rank and evaluate all the combinatorically feasible electrification pathways based on the overall performance. The effectiveness of the proposed method is developed using a crude distillation unit (CDU) case study adopted from a multinational company. The effectiveness of the proposed method is illustrated using a crude distillation unit (CDU) case study shown in three different scenarios that include prioritizing economic aspect (Scenario 1), prioritizing environmental aspect (Scenario 2), and considering equal importance of both aspects (Scenario 3). For single-period operation, the results showed a mix of natural gas and hydropower energy, exclusive use of onshore wind energy, and a mix of onshore wind energy and biogas cogeneration energy for Scenario 1, Scenario 2, and Scenario 3, respectively. In contrast, the multi-period model also utilized nuclear energy for Scenario 2 and Scenario 3 given the seasonal availability constraint. Following that, a sensitivity analysis is conducted to see the effect of the absence of the most influential energy sources on the optimal solution of each scenario and the top solutions under budget and CO2 emission constraints. Pareto analysis is outlined to offer an understanding of tradeoffs between differently prioritized solutions that decision-makers can select. The combination of the proposed analysis provides a systemic approach towards transforming traditional industries towards a cleaner future via electrification.

An Assessment of the Development of a Mobile Agricultural Biogas Plant in the Context of a Cogeneration System

This article presents examples of cogeneration systems, which are standard equipment for biogas installations, based on the production of heat and electricity. It has been shown that in the case of microgeneration, ease of servicing and low installation costs are crucial. Characteristic aspects of developing concepts for mobile installations (small scale) that produce biogas, often with a simple container structure that is ready to be located in the economic infrastructure of the agricultural industry, were indicated. Recommendations for the operation of micro-biogas models are presented, which have the greatest impact on the advisability of using agricultural waste for energy purposes. A characteristic farm was selected, which has a substrate necessary for the process of methane fermentation of slurry from pig farming. The cogenerator, which constitutes a potential energy demand from the point of view of Polish agriculture in the context of renewable energy production, was analyzed. The research goal was to adapt the cogenerator to the conditions existing on a farm, which should meet the technical and technological expectations for the process of managing the produced methane with a value of 80% in agricultural biogas. The assessment of the impact of the amount of biogas on the level of CO, NO, NO2 and PM emissions was carried out at a constant engine speed for various load levels; the percentage of biogas was changed from 40 to approximately 70–80%, i.e., until significant knocking combustion was detected in the tested engines. As a result, the existing control and control system for the operation of the cogeneration unit prevents the most effective mode of operation of the research installation as a prosumer micro-installation. When the AG20P biogas unit operated in parallel with the grid with an active power of up to 11.7 kW, the electricity produced by the unit met the adopted assumptions and requirements. What is new in this article is the use of a cogeneration unit that has been adapted to its functionality, taking into account the assessment of the prospects for optimizing the cogeneration system in the context of the use of renewable energy sources as agricultural biogas. The best method was to attempt to determine the operating conditions of the cogenerator to develop the optimization of a biogas cogeneration unit producing electricity and heat in a micro-installation for the needs of an individual farm.

Conventional vs. alternative biogas utilizations: An LCA-AHP based comparative study

This study compares the performance and the environmental impacts of three biogas utilization processes: combined heat and power cycle, dimethyl ether production, and methanol production. The processes are evaluated based on key performance indicators and life cycle assessments. Results show that molar methane conversion in the reforming unit is 93% for both dimethyl-ether (DME) and methanol processes. Methanol reactor reports a per-pass hydrogen conversion of 21%. DME production achieves an 89% conversion rate. The chemical pathways exhibit a 33% molar conversion of COx, contributing to the conversion of CO2 into advanced chemicals. The environmental footprint is evaluated through a life cycle assessment. Biogas cogeneration has the lowest global warming potential since DME and methanol production processes are influenced by steam production and electricity intake from the national grid, which relies on fossil fuels. An analytic hierarchy process is employed to assess the overall performance of the processes. DME production performs best with a score of 39%, followed by cogeneration with 31%, and methanol production with 30%. Different case studies are examined by modifying the weight criteria for each impact category. The findings provide that cogeneration and DME technologies perform better in all the iterations.

Life Cycle Environmental and Economic Assessment of Different Biogas and Biogas Residue Operation Models

The utilization of biogas and biogas residues is an important objective of animal manure resource management. Regarding the environmental friendliness and economic suitability of utilization modes, previous studies have evaluated and compared biogas or biogas residue individually, which may lead to incomplete environmental and economic assessments. In this study, the environmental and economic aspects of six integrated biogas and digestate operation modes, i.e., biogas utilization (direct combustion, cogeneration, and purification) and biogas residue utilization (field return and composting), were analyzed via life cycle assessment (LCA) and the net present value (NPV) method, with an animal manure biogas plant in Tangshan City as the study object. The results of LCA showed that biogas cogeneration + biogas residue composting disposal was superior to other models in most environmental indicators. NPV analysis showed that direct biogas combustion + biogas residue composting was the best, breaking even in the 3rd year with a cumulative profit of 250 million CNY. With increased environmental costs and benefits, the biogas cogeneration + biogas residue composting model had the best performance in terms of efficiency ratio at 2.58; the NPV of both operating models of biogas cogeneration increased, while the NPV of the other operating models decreased about 20%.

Energy, exergy and economic analysis of an integrated ground source heat pump and anaerobic digestion system for Co-generation of heating, cooling and biogas

This study designed and simulated a feasible polygeneration system, which integrated with a ground source heat pump and anaerobic digestion units to co-generate cooling, heating, and biogas for buildings as well as to utilize biomass residues in rural areas. The quantitative investigation of the energy, economics, and exergy of such a system was conducted in a village in Chongqing, China. The results indicated that by incorporating an anaerobic digestion unit, the integrated system could save 21.6% and 32.2% of primary energy, as well as 35.6% and 34.4% of annual costs, when compared to the separated and current systems, respectively. A Sankey diagram of exergy flow showed that exergy efficiency was 32.2%, and the anaerobic digester and heat pump units were the main exergy destruction components. Increasing the operating supplied temperature of the heat pump for the anaerobic digester from 45 °C to 60 °C improved exergy efficiency from 32.2% to 40.7% but decreased the overall energy efficiency ratio from 4.09 to 3.83. In addition, increasing the coefficient of performance (COP) of heat pump units was an effective way to enhance energy and exergy efficiency. The study may aid designers and decision-makers in optimizing the system and its components.

Integrated Energy Flexibility Management at Wastewater Treatment Facilities.

On-site batteries, low-pressure biogas storage, and wastewater storage could position wastewater resource recovery facilities as a widespread source of industrial energy demand flexibility. This work introduces a digital twin method that simulates the coordinated operation of current and future energy flexibility resources. We combine process models and statistical learning on 15 min resolution sensor data to construct a facility's energy and water flows. We then value energy flexibility interventions and use an iterative search algorithm to optimize energy flexibility upgrades. Results from a California facility with anaerobic sludge digestion and biogas cogeneration predict a 17% reduction in electricity bills and an annualized 3% return on investment. A national analysis suggests substantial benefit from using existing flexibility resources, such as wet-weather storage, to reduce electricity bills but finds that new energy flexibility investments are much less profitable in electricity markets without time-of-use incentives and plants without existing cogeneration facilities. Profitability of a range of energy flexibility interventions may increase as a larger number of utilities place a premium on energy flexibility, and cogeneration is more widely adopted. Our findings suggest that policies are needed to incentivize the sector's energy flexibility and provide subsidized lending to finance it.

Contribution of a photovoltaic solar system to the energetic sustainability of a Portuguese WWTP

The concerning growth of energy and water demand worldwide presents two major issues that can both be tackled by using renewable energy sources to oppose the energy consumption of wastewater treatment plants (WWTP), while taking advantage of their energy production potentialities. This work deals with the sizing of a photovoltaic system of a mediumsized Portuguese WWTP designed to meet the energy consumption needs, benefitting from the higher levels of irradiance in the country. The goal is to act as a model for future similar projects. The potentiality of producing electricity through cogeneration of biogas, in WWTP’s of activated sludge, needs to be taken into account. In the study case, 70% of the energy consumption needs were covered via cogeneration. This led to the conception of two different scenarios concerning the PV System: the first one covers the total electric needs of the WWTP and the other covers 30%, taking into account that the cogeneration system ensures 70%. During the 25 year-life of the PV System, an average annual performance ratio of 0.805 for scenario 1 and 0.789 for scenario 2 was achieved. Furthermore, the average energy contribution of scenario 1 and 2 was 36.5% and 32.8%, respectively, ensuring 100% of self-sufficiency, when adding the cogeneration contribution.

Comparative anaerobic digestion of sewage sludge at different temperatures with and without heat pre-treatment

Anaerobic digestion of sewage sludge is generally conducted under mesophilic (around 35 °C) or thermophilic (around 55 °C) conditions, whereas it is conducted at lower temperatures in some wastewater treatment plants without heating. In this study, we compared the anaerobic digestion of sewage sludge at 15, 25, 30, 35, 45, and 55 °C following hyperthermophilic pre-treatment at 80 °C for 24 h. Laboratory-scale reactors were operated continuously for more than 1000 days, and batch experiments were performed to evaluate the reaction kinetics. Biogas production rates at 15 °C with and without pre-treatment divided by that at 35 °C without pre-treatment were 0.73 and 0.78, respectively. The dewaterability of the digested sludge was evaluated by the capillary suction time (CST). The CST was approximately 50 s at 15 °C with and without pre-treatment and was slower than the CST at 35 °C. Compared to the shear rate, viscosity was higher at lower temperatures; however, it decreased with pre-treatment, which reduced the energy required for mixing in the reactors. Pre-treatment eliminated Escherichia coli from the sludge; however, E. coli (approximately 105 colony forming unit/g-total solids) was detected after digestion at temperatures ≤30 °C. Pre-treatment was also useful to replace a part of heat treatment required for digested sludge before it was used as fertilizer. Gene sequencing analyses indicated the effects of pre-treatment and digestion temperature on the microbial community in the digested sludge. Co-generation of biogas is useful to obtain both electricity and heat; however, heat from co-generation is sometimes limited. To maximize electricity recovery, the use of low temperature digesters has the potential to reduce fuel costs. The results indicate that anaerobic digestion at low temperatures with or without heat pre-treatment can be an efficient and cost-effective method of treating sewage sludge.

Renewable hydrogen production perspective in Serbia via biogas generated from food processing wastewaters

Renewable hydrogen produced from the waste streams can be a good option for clean energy production and reduction of greenhouse gases emissions into the environment. In this study, the potential for hydrogen production from the food industry wastewaters in Serbia and the further use of the calculated potentials were investigated. This was performed through two technological routes including the biogas steam reforming process and the electrolysis after biogas cogeneration process. Furthermore, the environmental impacts of these processes were considered. The total hydrogen potential via the biogas steam reforming process was 4094.70 tons, while the total hydrogen potential via the electrolysis after biogas cogeneration process was 1751.08 tons. These potentials can be used for direct energy production in the food industry, electricity production for the national electric grid, transport, injection in the natural gas grid and/or as a resource in the chemical industry. Regarding the Greenhouse Gas emissions reduction, hydrogen production by both routes represents a promising pathway. The results of this work can serve the decision makers to plan hydrogen application in the energy sector of the Republic of Serbia defining supporting measures for its application. Also, the presented analysis is enforceable in the Western Balkan countries for the application of hydrogen as an energy source in their systems for energy production, transport and economy.

Project of a cogeneration system using biogas

Brazil has been among the countries with the cleanest energy matrix in the world. The country showed high growth rates on the economy during the first years of the 21st, which brought to a rapid demand on energy supply. The situation got worser with long dry seasons, resulting on the commissioning of several thermoelectric plants. In the sugar-alcohol sector, the cogeneration became essential. In this context, the use of biomass for the generation and burning of biogas shows great potential in the production of electricity and heat. The discussed study involves the design of a biogas cogeneration system for the Federal University of São Carlos. The considered biomass originates from the campus itself and is composed of organic waste. The system consists of a modified diesel engine, accoupled to an electric generator and three heat exchangers. The monthly production of electricity is 7,7 MWh with a power of 30 kW. The monthly production of thermal energy is 9,8 MWh with a power of 37,6 kW. The costs are quoted adding up to R$ 162,877.00 for the complete system. The annual savings with the system is estimated at R$ 91,126.40, reaching an amortization time of one year and ten months.

Water resource recovery facilities as potential energy generation units and their dynamic economic dispatch

The dynamic nature of wastewater treatment and the significant number of energy conversion and recovery technologies applicable to the process, require a modelling framework that can be used for optimization and planning. This paper introduces a dynamic energy dispatch optimization framework with a combination of energy recovery and reserve options for water resource recovery facilities (WRRFs). Energy potentials, including biogas, electricity, and heat, were assessed for 15 large-scale WRRFs, which serve approximately 80% of the Irish population. The framework was applied by simulation of the national electrical system on an hourly basis for 2030 using mixed-integer programming (MIP) for eight scenarios. The simulation enabled automatic operational command scheduling for the production, conversion, storage, and supply over a year at an hourly time resolution. Meanwhile, the significance of several energy resources was assessed, and the impact of dynamic energy dispatch examined. A total potential cost saving of €46.35 M was possible through implementation of dynamic economic dispatch optimization (54% from biogas cogeneration, 40 %from thermal energy recovery, and 6% from micro-hydropower electricity). In addition, reductions in carbon emissions (2%), fossil fuel consumption (3%), startup costs (18%) and outsourcing dependency (5%) were possible. Up to 0.5% of whole system power demand and 90% of considered heat demand at peak periods could be provided by the proposed framework through dynamic economic dispatch. Therefore, this study suggests that WRRFs can participate into energy system through the dynamic economic dispatch of recovered energy, and provide potential energy flexibility, as well as adding value to overall sustainability of energy system.

Carbon neutral and techno-economic analysis for sewage treatment plants

Sewage treatment plants consume great lots of power and result in a large amount of indirect carbon dioxide emissions. Four typical energy conservation and emission reduction technologies were selected for sewage treatment plants, included reclaimed water source heat pump for heating, reclaimed water source heat pump for sludge drying, sludge anaerobic digestion technology with a biogas cogeneration system and photovoltaic power generation. The carbon neutral, energy and economic analysis model were established for the four typical technologies. The results show that reclaimed water source heat pump can achieve a higher carbon neutrality effect than sludge anaerobic digestion technology with a biogas cogeneration system. The reclaimed water source heat pump for sludge drying can recover a large amount of waste heat. The payback period of photovoltaic power generation is more sensitive to the electricity price changes, while reclaimed water source heat pump for heating is more sensitive to the heat price changes. From the discussion, Z-value have a greater effect on the reclaimed water source heat pump for heating. With the treatment load of the sewage plants increases, the reclaimed water source heat pump can achieve a higher carbon neutrality effect. The application of photovoltaic power generation needs to consider the geographical location, and it can achieve higher carbon neutral efficiency in rich solar energy resources areas. A combined district heating method with reclaimed water source heat pump is proposed to expand the district heating radius of the reclaimed water source heat pump, and improve waste heat recovery efficiency in sewage. • Green technology for sewage treatment plants are studied by carbon neutral, energy and economic analysis. • Multi-factor and sensitivity analysis of technical methods are established. • Sludge drying is a better technical route in sewage treatment plants.

PROSPECTS OF BIOGAS OBTAINING FROM SWEET SORGHUM IN UKRAINE

The development of energy has a decisive influence on the state of the economy in the country and the standard of living of the population. The production of biogas from renewable sources meets the goals of sustainable development for the countries of the world, approved at the meetings of the UN General Assembly in September 2015. With the help of biogas technologies, it is possible to increase the share of energy from renewable sources in the national energy balance, reduce the volume of waste generation and increase the volume of their processing and reuse based on innovative technologies and productions, as well as limit greenhouse gas emissions. Biogas is a flammable natural mixture of gases, which is formed as a result of the decomposition of organic substances under anaerobic conditions, is a mixture of 50-70% methane and 30-50% carbon dioxide, as well as a small amount of hydrogen sulfide, nitrogen, hydrogen and other impurities. The ratio of methane to carbon dioxide depends on the nature of the substrate and the pH during the process. It is possible to regulate the performance of anaerobic fermentation using macronutrients and micronutrients. They are stimulants, as well as more economically and environmentally safe catalysts compared to chemical reagents, which often require significant energy costs. When using such by-products of agro-industrial production as straw or bagasse and bard, in addition to the main product (alcohol with an energy potential of 22.56 MW•h/ha•year), almost twice as much energy can be obtained – 45.06 MW•h/ha• year from by-products. Sweet sorghum is a grassy plant of the cereal family, the stems of which contain juice with 12-20% sugars, it is characterized by good adaptability to arid conditions, temperature fluctuations, salty and acidic soils, and therefore it is grown as an energy crop for the production of bioethanol and biogas, in including on marginal lands. Sweet sorghum juice is a raw material for the production of bioethanol, and bagasse, as a by-product, can be used at the same enterprise to generate biogas. Thus, the enterprise produces a commercial product, solves the problem of disposal of production waste and provides itself with electricity and heat through the operation of a biogas cogeneration plant. Sorghum bagasse as a raw material for biogas has a high biological value, rich in trace elements, so it is a good candidate for anaerobic fermentation. Sweet sorghum bagasse (Botanical variety) has a high content of organic dry matter (87.85%), is rich in trace elements, therefore it is definitely a promising raw material for anaerobic fermentation. The theoretical yield of biogas from sorghum bagasse is 476.38 l/kg of dry matter, with a methane content of 52.90%. The expected energy value of methane is 9.02 MJ/kg dry matter of sorghum bagasse. As part of the study, a biochemical analysis of sweet sorghum bagasse was carried out. The moisture content of dried sorghum bagasse was less than 10%, which ensured its long-term storage as a substrate for anaerobic fermentation. The energy potential of sorghum bagasse was calculated. Since carbohydrates make up more than half of the dry matter of sorghum bagasse, they mainly provide the potential for biogas formation. At the same time, the content of proteins and fats increases the level of the target product methane in biogas. For further methane fermentation, the macro- and microelement composition of sorghum bagasse is of great importance. Ni stimulates biogas production and methane content in biogas. Addition of Ca and Mg salts as energy additives can increase CH4 production and prevent foaming. The optimal C:N:P ratio for increasing methane yield is 200:5:1. Analyzing the obtained results, we can see that such a macroelement as calcium was found to be the most in sorghum bagasse, which indicates the potential for obtaining a high yield of methane. An important indicator is also that the content of heavy metals cadmium and lead is very small. This indicates that the soil on which the sweet sorghum was grown is not contaminated with heavy metals and that the digestate after fermentation can be used as a fertilizer because it is safe. Sorghum is one of the crops with the highest C/N ratio, which determines the prospect of co-fermentation with a substrate that has a low value of this parameter (sugar beet vinasse). Sweet sorghum is a promising candidate for biogas production due to its good chemical composition, relatively high methane yield and the possibility of exploiting marginal lands in Ukraine for its cultivation.

Biogas from source separated organic waste within a circular and life cycle perspective. A case study in Ontario, Canada

The appropriate transformation and valorisation of biogas offers environmental and economic opportunities in a future with restrictions upon fossil-based fuels and materials. The LCA method was used to quantify and compare the potential environmental impacts of an AD plant incorporating biogas co-generation and upgrading options, namely AD-CHP and AD-RNG. Using an average Anaerobic Digestion facility in Ontario, Canada, modelled after real facilities, as a case study, electricity and steel were identified as potential hotspot input materials carrying a disproportionate environmental burden for biogas production. With a system expansion approach, the biogas was subsequently utilized to produce (1) both heat and electricity using a Combined Heat and Power (CHP) system, or (2) upgraded to renewable natural gas (also called biomethane) through chemical amine scrubbing, respectively.

A comprehensive review of energy scenario and sustainable energy in Kenya

The review examined the potential of renewable and non-renewable energy resources and the current state of exploitation in Kenya. Energy demand is on the rise, coupled with the rapid increase in population. As a result, the state spends a large amount of money on the importation of oil. Oil and electricity are the primary determiners of the economy. The production mix include 52.1% of hydro, 32.5% of fossil fuels, 13.2% of geothermal energy, 1. 8% of biogas cogeneration, and 0.4% of wind. There is a projection of an increase in current energy demand from 600 MW to 2,600–3600 MW by 2020. The national energy review indicates that there is a significant reliance on wood for fuel and the other biomass accounting for 68% of overall energy usage. It's used especially among the rural areas, urban, and informal market which has considerably affected its supply and demand. Consequently, there is the destruction of most of the vegetation and the forests and existing ones are on the verge of diminishing. The Ministry of Energy and Petroleum manages its overall strategy and provides advice on the production and growth of energy sub-sector, including power, petroleum and renewable energy. The major problem in the energy sector is limitation in accessibility to the modern energy, high pressure on biomass supplies, rising energy prices, high demand for electricity than the ability to build new generation, inability of Kenyan Power and Lighting Company to link every consumer willing to purchase and consider cost of rural electrification by grid development due to dispersion.

Life Cycle and Economic Analyses of the Removal of Pesticides and Pharmaceuticals from Municipal Wastewater by Anodic Oxidation

Several pesticides and pharmaceuticals (PP) have been detected in the effluent of a full-scale Portuguese Wastewater Treatment Plant (WWTP). Their presence contributed to the environmental burdens associated with the existing treatment of the Municipal Wastewater (MWW) in the impact categories of Human Carcinogenicity, Non-Carcinogenicity, and Freshwater toxicities on average by 85%, 60%, and 90%, respectively (ReciPe2016 and USEtox methods). The environmental and economic assessment of the installation of an Anodic Oxidation (AO) unit for PPs’ removal was performed through Life Cycle and Economic Analysis, considering two types of anodes, the Boron-Doped Diamond (BDD) and the Mixed Metal Oxides (MMO). The operation of the AO unit increased the environmental burdens of the system by 95% on average (USEtox), but these impacts can be partially compensated by the avoided the production of non-renewable energy in the Portuguese electricity mix by biogas cogeneration at the WWTP. If the construction of the AO unit and the manufacturing of the electrodes are considered, the Human and Freshwater Toxicities are often higher than the environmental benefits derived from the PPs’ removal. On the economic side, the MMO configuration is clearly more advantageous, whereas BDD is environmentally more favorable. The issue of the presence of PP in MWW effluents has to be addressed as an integrated solution both improving upstream PP’s management and adopting PP’s removal technologies strongly supported by renewable energies. Further insights are needed for the assessment of fate and of the environmental effects of PP in the sludge.

Thermo-economic optimization of a Rankine cycle used for waste-heat recovery in biogas cogeneration plants

In a typical biogas cogeneration plant, a part of the exhaust gas energy is used for heating buildings within the plant. However, a certain amount of this energy remains unused. This study examined the utilization of this waste-heat through the Rankine cycle and organic Rankine cycle. A multiobjective thermo-economic optimization procedure of a waste-heat recovery unit installed at the exit of an engine before the engine cooling fluid–exhaust gas heat exchanger is proposed herein. The optimization procedure includes sizing all heat exchangers in the waste-heat recovery unit based on the measured Colburn and friction factors. The thermo-economic optimization was performed to investigate the maximal power output and the minimal payback period while preserving the function of the heating system. The procedure was applied to a biogas power plant with two engines (mass flow rate of the exhaust gases from each engine was 1.77 kg/s at a temperature of 410 °C). The electrical efficiency of this system was 42.1% and the measured overall yearly energetic efficiency was 66.7%. Optimization shows that the waste-heat recovery unit based on Rankine cycle is more economical than the unit based on the organic Rankine cycle using toluene. The electrical efficiency of the entire power plant increased by 2.97% and the payback period of the investment was 6.8 years, while the Levelized Cost of Electricity was 0.0419 $/kWh. The proposed method could be used to analyze the investment profitability of waste-heat utilization in other cogeneration plants.

Is the Anaerobic Digestion (AD) sustainable from the energy point of view?

This article aims to apply the Energy Sustainability Analysis (ESA) to the process of Anaerobic Digestion (AD), considering five different feedstocks: the organic fraction of municipal solid waste (OFMSW), agro-industrial products (energy crops) such as maize and sorghum silages, and agro-zootechnical wastes (slurry and manure) from cattle and pig livestock. The case study focuses on an AD bioreactor which is fed with a constant flow (50 ton/d) at a fixed total solids (TS) concentration (15% TS w/w). The processing chain is divided into three phases: pre-digestion operations, AD and the cogeneration of biogas, and post-digestion operations. The ESA methodology uses dedicated metrics and considers the entire technology chain from the energy resources up to useful energy. Energy crops present important energy footprints that must be spent for their production, while in the AD process, the pre-digestion and post-digestion phases present different energy costs for each analyzed feedstock. The results indicate that although the produced energy depends on the energy potential of the biomasses, the pre-digestion and post-digestion phase have an important weight in the overall energy sustainability of the system. In addition, in the case of the use of agro-zootechnical wastes and energy crops as feed, the availability of sufficient land to spread digestate on the field is essential to ensure the sustainability of AD technology, in its absence the energy sustainability can be critical as shown for the case of maize silage.