## 7 The Concept of Microalgae Biorefineries
The possibility of annexing microalgae biomass production to existing sugar mills is currently being intensively examined in the industrial environment and in the scientific community. In the unified sugarcane-microalgae biorefinery scheme, biodiesel can be produced from microalgae biomass, which will be used as fuel for the needs of sugar can cultivation. In this way, the total greenhouse gas emissions associated with bioethanol production are reduced by approximately 30%, as the microalgae for their growth capture 50% of the CO\({}_{2}\) produced in the fermentation process of ethanol production. This approach has been attempted in Brazil since 2014, at the level of factory production, to produce 100 thousand tons of biodiesel in a unified sugar industry production scheme with microalgae biomass production. The main available sources of the sugar-biofuel industry that could directly contribute to microalgae cultivation are the following:  
CO\({}_{2}\) produced during the fermentation process for ethanol production or produced during the operation of boilers producing heat and energy for the needs of the sugar mill. This ensures the supply of CO\({}_{2}\) for the cultivation of microalgae. The byproduct of bioethanol production after fermentation is vinasse, which can be used as a culture medium for the development of heterotrophic or mixotrophic crops. The surplus electricity produced in the sugar cane plant can be used directly in the various stages of microalgae growth and processing. In case that the production scheme is second-generation, carbohydrates produced from microalgae biomass can be fermented either together with sugar cane juice and molasses, by fermenting microorganisms that can ferment pentoses and hexoses, or in independent fermentors [58]. Vinasse is an acidic, dark brown liquid, rich in organic compounds (e.g., glycerol, lactic acid, sugars), nitrogen, phosphorus, and ions, such as potassium, calcium, and magnesium, and produced in huge quantities. For every liter of ethanol produced from sugar cane, 12-14 L of vinasse is produced as a byproduct after fermentation. Its main use is as a fertilizer on sugarcane plantations, but it has the disadvantages of altering soil composition and reducing sugar cane productivity when applied continuously [59]. For the production of first-generation ethanol, the production process includes purification and cutting of sugar cane, extraction to produce dilute juice, concentration into concentrated juice, juice purification, fermentation for ethanol production, and then distillation and dehumidification of the produced bioethanol. The residue of the extraction is bagasse (lignocellulosic synthesis), of which 50% is used for the production of second-generation ethanol, and the rest as fuel in the combined heat and power plant. For the production of second-generation ethanol, bagasse is subjected to physicochemical treatment to break the lignin-cellulose-hemicellulose bonds and increase the percentage of cellulose for the enzymatic hydrolysis that follows. Through hydrolysis, cellulose is converted into monomer sugars, and the resulting sucrose solution is concentrated. In the next step, it is mixed with the dense juice from the stream of the first generation, and all together sent to fermentation for ethanol production. Finally, for the production of value-added products from microalgae biomass such as lipids and carotenoids, supercritical extraction is preferred, which is an effective method. It is a \"clean\" technology applied in the context of a holistic approach of the biorefinery, for the recovery of substances from plant materials, and is preferable to chemical treatments [60]. Furthermore, palm oil production has contributed significantly to the economy of several tropical countries, such as Malaysia, which is the world's largest producer. Palm trees are important crops due to the high productivity and high oil yields from small areas of land.