A PROPOSAL FOR A SMALL-SCALE BIOETHANOL FACTORY IN TUSCANY (ITALY) BASED ON SWEET SORGHUM

by   Eugenio Macchia

December 09, 2010

On the basis of the model proposed by CETA, Green Engineering developed a proposal for a small scale biorefinery for the production of bioethanol starting from sweet sorghum for Tuscany (ITALY).

INTRODUCTION

From about 2 years we are developing a proposal for the realisation of a small-scale bioethanol factory in Tuscany (ITALY) , based on the sweet-sorghum model hypothesized by C.E.T.A. .
Our idea is that the sustainability of bioethanol production from sweet sorghum in small decentralized plants can be the base for future upgrades like the development of cellulosic ethanol (2nd generation) and waste to ethanol processes (3rd generation) .
Beside this, a sharp design of the whole production chain (from the landowners to the fuel stations) can maximize the environmental benefits and also the social benefits of this kind of production .
For all these reasons we have developed a multi-phase project in which the first phase already includes an experimental line to treat the whey discharged by the local dairy industry.
The general diagram of the biorefinery (first phase) is the following :

The green blocks represent the main line for the conversion of the sweet sorghum into ethanol. The light blue blocks represent the experimental line for the “whey to ethanol” process. The red block is the biomass boiler and the blue blocks are electrical power generators .

There are two possible strategies to design the production model. The above scheme is a logical scheme that applies to both these strategies, but specific technologies and design apply to each logical block of the process accordingly to the chosen production strategy .

First let’s take a look to the general process represented in the above scheme, then we will investigate the two strategies mentioned.


THE MAIN PROCESS

The “green line” is composed by the following process units :
GE/UNIT100 – This unit is simply a storage unit for the sweet-sorghum coming from the crop.
GE/UNIT200 – In this unit the juice contained in the stalks of the sweet sorghum is extracted. The juice has a sugar content of about 15 % and it is the sugar that is going to be converted into ethanol in the successive units. The residuals of the extraction process is the bagasse, mainly lignocellulosic materials . This by-products can be burnt for energy production in a biomass boiler, and in future could be used for 2nd generation bioethanol production. The standard technology foresees a battery of mills, in which sweet sorghum is crashed and the juice (containing sugar) is recovered and sent to the next process unit.
GE/UNIT300 Fermentation – In this unit the juice recovered from the GE/UNIT200 is fermented by the use of standard yeasts. During fermentation the sugars contained in the juice are converted into ethanol from yeast, producing a mash with an alcohol content of about 8-14 %. This mash is also called “wine”.
GE/UNIT400 Distillation – The wine produced in fermentation is then distilled with a system of distillation columns in a multi-effect configuration. The distillation allows to separate the alcohol from water, producing two different streams: an alcoholic stream with a proof of 96% (azeotropic point) and a stream of water, containing the organic compounds contained in the sweet sorghum juice.
GE/UNIT900 Dehydration – The alcoholic stream at 96% obtained in distillation needs an other step in order to comply with the ASTM specification for the fuel-ethanol. This step is called dehydration and rise up the ethanol concentration up to 99.7%. The dehydration unit works with a special technology called “dehydration by molecular sieves” in which special materials (the molecular sieves) are able to trap water molecule and to flow out the ethanol molecule.
The product of this unit is an extremely purified ethanol stream complying with the ASTM specification for fuel ethanol and commonly called “bioethanol” .
GE/UNIT500 Storage – The bioethanol produced is sent to the storage system, waiting to be sold to a petrol company that will use it for the blending with gasoline. This unit closes the main production line (conversion of the sugar into bioethanol).
GE/UNIT800 Biogas recovery – The water extracted from the bottom of the distillation columns are called “spent wash” or “slops”. This water has an high COD content (35.000 – 70.000 ppm) due to the organic compounds contained in the “wine”. For this reason this water is sent to a Biogas reactor in which by means of the anaerobic digestion process organic compounds are converted in Biogas (a mixture of CH4, CO2 and other gases) and stabilised sludge. In this way water is purified and the recovered Biogas can be used to produce electricity with a gas-turbine generator .
BIOMASS POWER STATION – The other main byproduct of the process is the bagasse. Bagasse can be sent to a biomass boiler in order to produce steam by combustion. The steam produced is used as energy source for the factory (mainly distillation and dehydration ), but the large amount of it is available for energy production by means of a steam turbine.

Before looking to the experimental line of whey to ethanol production it is important to say something about the two different strategies to manage the bioethanol production from sweet sorghum. To understand these strategies we have to discover a problem of the sweet sorghum: its ability to start fermentation immediately after harvesting. The problem is related to the high sugar content in the stalks: common yeasts and bacteria present in the environment start to metabolize the sugars which are contained in the biomass, decreasing the content of polysaccharide material to produce ethanol. So a system to keep the sugars safe for the GE/UNIT300,where the conversion into ethanol occurs, must be fixed.
The first strategy is to ensile all the harvested sweet sorghum, with a proper “sugar saving” technology – see the specific topic in the forum section), and to feed the biorefinery day by day only with the feedstock necessary for the daily production. With the ensilage, the sweet sorghum can be sent continuously to the plant during its operation, for all the 300-330 working days: the biorefinery receives only the daily feedstock required to produce ethanol, so the process is very straight and easy. We will call this strategy as the “ensilage strategy” .
The second possible strategy is to send all the harvested sweet sorghum to the second unit, the juice extraction unit, and then to store all the juice extracted in order to feed the biorefinery for the whole year of production. In this case the problem is that we cannot store the juice itself, because also in this case it will start to ferment immediately. To store the juice we have to concentrate it up to a sugar content of about 50-80 %; at this level of concentration the fermentation cannot start. The concentrated juice is called “syrup”. To produce syrup we need an evaporation plant. The syrup can be stored without problems for many months, and can be used to feed day by day the fermentation unit after proper dilution with water.

Of course, as already mentioned, the choice of the strategy (ensilage vs evaporation) has a strong influence on the design and sizing of the first two units, so an iterative optimisation procedure has to be implemented in order to choose the best strategy accordingly to local econometric and logistic parameters. This work of optimisation must be done side by side with the landowners, because they know the soils, the lands available, the facilities that can be used for the feedstock storage and the consequent logistics .

MAIN FIGURES

The project for Tuscany has been designed around these main figures :

Feedstock : about 100,000 tons/year of sweet sorghum
Bioetanol production : about 5,000 tons per year
Electrical power production : about 1,5-1,8 MW
Land requirements : from 1,200 to 2,000 ha

EXPERIMENTAL LINES AND FUTURE UPGRADES

As anticipated, our proposal for Tuscany includes in the first phase also an experimental line to produce ethanol from the whey discharged by the dairy industry. This line is represented in light blue in the scheme above and it is composed by: a stabilization and storage unit for the whey (GE/UNIT2100) , a UF/NF ultrafiltration/nanofiltration unit (GE/UNIT2200) and a fermentation unit (GE/UNIT2300) . Whey stabilisation is necessary because whey is a perfect environment for the life of many bacteria, some of which are also dangerous . Then the stabilized whey is sent to a Ultrafiltration and Nanofiltration unit that concentrates the whey and allows to recover the WPC (Whey Protein Concentrate); the WPC can be sold to the animal feed industry. The concentrated whey is sent to a special fermentation unit, in which the sugars contained in whey (mainly lactose) are fermented into bioethanol. The alcoholic mash obtained from this unit is then sent to the main distillation/dehydration plant .

This experimental line is only one of the experimental lines that can be realised inside a biorefinery. The successive phases of the Tuscany project foresee the implementation of cellulosic fermentation, waste to ethanol fermentation and the exploitation of high-value chemical compounds from the solids by-products of the factory to produce green chemistry compounds, like: biodyes & biopolymers .

Finally, another interesting line that could be developed in this kind of biorefinery is the production of biodiesel using the process of vegetable oil trans-esterification by ethanol (instead of methanol). This will be the subject of the next article from me.