*3.3. Integration of Renewables*

Many companies have sought replacements for fossil fuels from renewable sources in steam production to reduce CO2 emissions. It has been shown that in powdered milk production, almost 100% of energy could be supplied from renewables [121]. In the first article on this theme, entitled "Renewable Energy Integration for Steam Supply of Industrial Processes—A Food Processing Case Study", Hechelmann et al. [122] presented in detail several alternative technologies for steam generation using renewable sources, e.g., biogas and biomethane boilers, solid biomass boilers, fuel cells, micro gas turbine, solar panels, and heat pumps. The research was conducted for a plant producing wet animal food. Most of the steam was used for sterilisation. Batch production with large fluctuations in steam consumption is typical for such plants, so a dynamic simulation was performed for each technology using the MATLAB/Simulink program. Steam production with two natural gas boilers was considered as a reference case. Authors designed alternative renewable technologies; assessed the capital cost, energy cost, and CO2 emissions; and compared them to a reference case. The results of the analysis showed that the highest reduction of CO2 emissions (approx. 64%) compared to the reference case would be achieved with a biomass boiler in combination with a biogas backup boiler to cover peak steam demands. Biomass has a low carbon footprint, and a low price; the increase in energy costs, in this case, was about 28%. The smallest increase in energy costs (6.6%) compared to the reference case was achieved with the use of solar collectors, but the reduction in emissions was very small. The use of fuel cells was associated with the largest increase in energy costs due to high investment and low thermal efficiency. The authors concluded that a biomass boiler, in combination with a steam storage tank, represents a reasonable compromise between the reduction of CO2 emissions and the increase in energy production cost.

The next article on this topic was a review paper, "Operational Management Implemented in Biofuel Upstream Supply Chain and Downstream International Trading: Current Issues in Southeast Asia", written by Hoo et al. [123]. The authors provided an overview of methods for strategic, tactical, and operational decision making in biofuel supply chain planning. On the upstream side, important decisions include biomass cultivation, availability, harvesting, the modes of transport, pretreatment and processing, product storage, distribution and inventories, and selection of locations. The methods considered in the literature use mathematical programming [124], heuristic approaches [125], and multi-objective optimisation [126] to optimise the biofuel supply chain. The downstream side includes, in particular, international trade in biomass and biofuels, in which there is considerable uncertainty, especially regarding prices and demand. Analyses of various scenarios, including pessimistic and

optimistic situations, are common. It is important to include the impact of different policy instruments and measures on the international flow of biofuels, in addition to the barriers and drivers, in the optimisation models of regional and global biomass and biofuel supply chains. The goal is usually to maximise economic efficiency, but it is also necessary to optimise the overall footprint, which takes into account not only economic but also environmental and social impacts. The authors analysed the bioenergy situation in Malaysia, Indonesia, and Thailand in more detail. These countries have various sources of biomass (palm oil, cassava, sugar cane), and their governments employ several measures to encourage the increased use of biofuels in transportation, industry, electricity, and commercial uses. The authors concluded that sustainable biofuel supply chain planning requires a comprehensive approach and interdisciplinary research that enable appropriate policy decisions for sustainable resource use, reduced environmental impact, improved energy security, and economic growth.

In the article entitled "Determination of Various Parameters during Thermal and Biological Pretreatment of Waste Materials", the authors Hren et al. [127] examined how the pretreatment of waste materials affects their further conversion into useful products, such as biofuels and biofertilisers. With a suitable pretreatment, it is possible to increase the efficiency of waste recycling and improve the circular use of resources [128]. Two waste materials—sewage sludge and riverbank grass—and their mixtures were examined by applying thermal pretreatment at lower and higher temperatures, and biological pretreatment with the addition of cattle rumen enzyme at a lower temperature. Various parameters in the liquid phase (e.g., the content of nutrients N, P, and K) before and after pretreatment were measured, in addition to the concentrations of CH4, CO2, and H2S in the gas phase after pretreatment. The results of the experiments showed that thermal and biological pretreatment were most favourable at a lower temperature of 38.6 ◦C. The potassium and phosphorus concentrations increased in all substrates after pretreatment. The pretreatment of the grass and sludge mixture showed the highest concentrations of potassium compounds. The highest concentration of phosphorus was found in the pre-treated sludge. The total nitrogen content also increased in most cases during pretreatment, with the highest total nitrogen content found in the samples of sludge and its mixtures with grass. Analyses of the obtained gas phase show that the biological pretreatment of the sludge was most favourable at a temperature of 38.6 ◦C because it results in the highest concentration of methane and the lowest concentration of H2S. The study suggested that the choice of the pretreatment process for waste material depends on the intended further use because the pretreatment influences the quality of the product made from waste material.

In the final paper on this topic, "Biowaste Treatment and Waste-To-Energy—Environmental Benefits", the authors Pavlas et al. [129] compared the environmental impacts of three well-developed biowaste processing technologies—composting [130], fermentation [131], and incineration of biowaste combined with residual municipal solid wastes [132]. Global Warming Potential (GWP) was used as a criterion for assessing the environmental impact [133]. The results showed that all of the studied technologies reduced greenhouse gas emissions, which means that the overall change in GWP for each technology was negative. The smallest reduction in GWP was shown for composting technology, which was the least costly and investment intensive. By fermenting biowaste, an almost four-fold reduction in greenhouse gas emissions was achieved compared to composting. The result of the incineration of bio-waste as a component of the residual municipal waste depends on the ratio of heat and electricity generated in the cogeneration unit because heat generation entails a greater reduction in GWP than electricity generation. If the incineration plant mainly produces heat for district heating, the reduction in GWP would be greater than for fermentation. If the primary production were electricity, the reduction in GWP would be smaller than for fermentation. The authors concluded that if heat utilisation is ensured, it is best to incinerate biowaste as a part of the residual municipal solid waste. Otherwise, from a greenhouse gas emissions perspective, it is better to collect biowaste separately and process it by fermentation.
