Introduction
Thermal Hydrolysis is a thermal process and its goal is to prepare biomass for a more efficient conversion to bioethanol. It is a process necessary for second generation biofuels where the micro structure of lignocellulosic biomass needs to be broken up in order to make efficient bioethanol processing possible.
A research group of the University of Cartagena, Spain, headed by professor Antonio Lopez, has successfully applied thermal hydrolysis to citrus peel waste and have obtained very high yields to bioethanol. For the research a thermal hydrolysis pilot plant has been used which has been developed by HRS, a company which specializes in heat transfer equipment for biomass applications.
Citrus peel as feedstock for bioethanol
Citrus peel is a waste product from juice processing with a low value. Approximately 85 percent of citrus peel is water. The remaining 15 percent is biomass, mainly cellulose, essential oils, proteins and sugars. In order to convert citrus peel into a product with value, such as animal feed, it needs to be dried. After pelletization of the dried citrus peel it can be sold as animal nutrition.
Recently the market for animal nutrition has shown a decrease in price per ton of product. This makes it interesting to look at converting citrus peel waste into something with more value. It has been known that citrus peel is a feedstock for bioethanol. An efficient process for extracting bioethanol with high yields from citrus waste would increase the economic balance of the juice producing plant and would convert citrus waste in a renewable energy source. In total, from the citrus peel, 4 main products can be obtained, including ethanol, animal feed, a small fraction of limonene (essential oils) and water recovered from the peel which can be used for irrigation.
High ethanol yields through thermal hydrolysis with steam explosion
In order to obtain high yields of ethanol from citrus waste, a pretreatment step is necessary before fermentation. Many types of pretreatment are already applied in second generation biofuels processing. In the study done by the Cartagena University a thermal process called thermal hydrolysis has been chosen for two main reasons. The first reason is that successful results for biomass pretreatment with thermal hydrolysis had already been obtained by HRS in a biogas application. The second reason is that thermal hydrolysis is a clean process. It does not require any additional chemicals such as acids or others. This means that this process does generate any undesired effluent.
In the setup of the pilot plant a similar way of working as in the biogas application has been used. In a reactor, crushed citrus peel mixed with water was heated to temperatures around 320 ºF and 88-102 psi of pressure and was held at this temperature for a certain period of time. The high temperature weakens the links and opens the microstructure of the biomass. In addition to heating, a steam explosion is applied. During steam explosion, the pressure in the reactor is suddenly lowered from 6-7 bars to atmospheric pressure (1 bar). This causes the water contained within the microstructure to evaporate. This is combined with a big increase in volume of the water (converting from liquid to gas). This is a violent process in which the expanding water breaks up the micro structure even further.
The net result of this thermal process is citrus waste with an opened microstructure. The cellulosic materials and the sugars have become more easily available for the fermentation agents that have to convert these materials into ethanol.
The next step has been to add enzymes to the thermally pretreated citrus waste and measure the time needed for fermentation and the amount of ethanol obtained. These results have been compared with bioethanol production from citrus waste without thermal pretreatment. The results obtained have been very promising; up to 46 percent of all solids contained in citrus peel could be converted into ethanol. Also it has been shown that the thermal hydrolysis process has significantly reduced the amount of enzymes needed for fermentation.
