Some energy conversion processes transform an energy source into a second
source with different characteristics.
Two examples are combustion (fuel - > hot gases) and gasification (biomass - > combustible gas). Naturally, the products of these processes have to be transformed, with the use of other technologies, into their final form: hot water, steam or electricity (for example, with the use of boilers, engines, generators, etc., in the cases mentioned above).
Given the basic characteristics of small and medium-sized milk processing centers, the following technologies may reasonably be proposed: combustion, gasification and anaerobic fermentation.
These processes can be carried out by small plants and do not require
On the other hand, alcoholic fermentation and the production of oils and charcoal should usually only be undertaken by companies or specialized organizations.
Section 6 considers the use of alcohol and oils for the operation of engines, but the assumption is that these products are found on the market.
As in the case of energy sources, a summary table is presented for each of
the proposed conversion processes (with the exception of combustion, for
which the reader is referred to section 6.2.1).
A standard outline is not included here, since the specific examples are considered to be sufficiently clear.
Combustion consists of the oxidation of a substance (the fuel) through a
comburent (oxygen or air). The combustion process has been completed when
the related products (high-temperature gas) are incombustible (e.g., CO2,
The process is exothermal and takes place at temperatures on the order of 1,000-2, 000°C; it requires initial energy input. For example, solid and liquid fuels require that part of the material be heated to 200–300°C in order to start the phase of distillation of volatile substances.
The stoichiometric ratio Va is the weight of air theoretically necessary, per unit of weight or volume of fuel, to obtain complete combustion.
This value is calculated by knowing the content, in weight Pi, of the various elements (C, H2, S).
By way of approximation:
Va = (8C/3 + 8H + S - O)/0.23
In practice, an excess of air E is supplied to assure complete oxidation of all fuels. E is determined by the equation:
E = (Weight air used-Va)/Va
In the case of gases (good miscibility between fuel and air), low values of E are employed; the opposite is true for coarse fuels (e.g., large pieces of wood).
Combustion involves the use of burners, which, when they exist, are an integral part of boilers (see section 6.1.1).
Gasification is the transformation of a liquid or solid fuel into a gaseous
This process may be desirable when the operative characteristics of a gas are required (feeding endothermal engines, combustion with low excesses of air, etc.), despite the fact that low-quality fuels (biomass, coal, etc.) are available.
In the case of solids, high-temperature (900–1500°C), incomplete oxidation is employed to generate combustible gases. In this situation, typical reactions are:
|C+O2||→||CO2||C+CO2||- (for t>1000°C) →||2CO|
|H2+O2/2||→||H2O||C+H2O||- (for t>800°C) -→||CO+H2|
This is obtained by using a quantity of comburent air equal to 10–15% of
the stoichiometric air.
The moisture content of the original material may be beneficial if it is < 20% (wet basis).
The process normally involves fixed- or fluid-bed gasifiers. The former,
which are easy to set up and are adapted to small plants, can either be
equi- or counterflow; the distinction is based on the movement of the
material and the gas. The latter are only used in medium-sized and large
plants and require substances broken down into small pieces (which can be
kept in suspension).
In the case of fixed-bed gasifiers, the gas produced should contain approximately 60–70% of the chemical energy of the original material (assuming the moisture, content is 10–15%).
Typical production levels in terms of Sm3 of gas (LHV: 3.6–4.7 MJ/m3) per kg of transformed material are:
|- wood residues:||2.9;|
|- rice straw and corn cobs:||2.5;|
|- wheat straw and maize stalks:||2.7.|
References: , , , , , , .
|Summary Technology Table:||GASIFIERS|
|b)||operation: relatively problem-free with the simplest versions (fixedbed gasifiers), as long as fuels with consistent piece size and physical characteristics are utilized.|
|c)||most obvious limitations: i) automatic gasifier feeding may be complicated; ii) use in milk processing centers would led to frequent start-ups and shut-downs (presumably once or twice a day) with consequent increase in labor requirements; iii) engine feeding requires purification of the gas and greater care in terms of management; iv) the fuel has to be prepared.|
|d)||auxiliary machinery needed: saw for preparation of the fuel.|
|e)||recommended technical solution and its technological level:|
|- type:||fixed-bed gasifiers;|
|- suggested fuel:||small, prism-shaped pieces of wood (size of a pack of cigarettes);|
|- production:||2.5–3 m3 of gas/kg of wood. 10–11 m3 is needed to obtain the same energy potential of 1 kg of Diesel fuel;|
|- labor required:||approximately 1 man-hour for start-up; operation must then be checked on (not constantly, however) and fuel must be added (depending on consumption). Total: 3–6 man-hours/day;|
|- workshop required||for construction: any place in which solid-fuel stoves with power over 100 kW are produced. The plate must be bent and welded, and simple kinematic motions and gas circuits are necessary.|
|f)||final conversion technologies to be added: i) boilers for the production of hot water and steam; ii) Otto engines. In case i), the gas may be used as is; in case ii), it has to be cooled (to eliminate condensable substances) and filtered (to eliminate particles). The engines' feed systems should be completely modified, and use of the original fuel will no longer be possible.|
This is a biochemical process resulting in the breakdown of organic
substances (proteins, lipids, glucides and their polymers, such as
cellulose), starting from biomass with a high moisture content (>60%) and
utilizing various groups of anaerobic bacteria.
The products obtained include a gas with useful energy properties (biogas or biological gas) and a biomass whose volatile solid (VS) content is lower than that of the original material.
In the absence of O2, the process takes place in three phases. For example, the following reactions occur with the use of cellulose (simplified illustration):
|(C6H10O5)n + nH2O||→ (hydrolysis) →||nC6H12O6|
|nC6H10O6 →||(acidification) →||3nCH3CO2H|
|3nCH3CO2H →||(methanization) →||3nCH4 + 3nCO2|
In practice, complex organic compounds are transformed by hydrolytic bacteria into soluble compounds with a simpler molecular structure (hydrolysis). These compounds in their turn are transformed into volatile carboxyl acids by the acetic and homo-acetic bacteria (acidification), and subsequently into CH4 and CO2 by the methane bacteria (methanization).
Below, the influence of various chemical and physical parameters is described:
Temperature: determines the predominant bacterial group and hence the rate of the chemical reactions. The process is difficult to start below 10 or above 80°C. Temperatures of 25 to 35°C are considered to be an adequate compromise between desired gas production and energy requirements. It is important to keep the temperature constant; variations of 5°C have an impact on microbial activity.
pH: values of 5.6-6 are optimal for hydrolysis and acidification, while neutral values (6.8–7.2) are better for methanization. Neutrality is also preferred when the entire process takes place in a single environment.
Type of biomass: the volatile solid (VS) and total solid (TS) content and the ratio of the two values (VS/TS) are related to the production of biological gas.
C/N ratio: to encourage the development of bacteria, the chemical composition of the substrate must meet the following condition: 20<C/N<30.
Oxidation-reduction potential: -300<rH←350 mV to encourage microbial activity.
Retention time (RT): this is the amount of time the biomass remains in the processing environment (reactor). When the biomass is not homogeneous (as frequently happens), RT is distinguished from the solid fraction (RTs ; that is, from the part that tends to be deposited in the reactor) and the liquid fraction (RT1). The ratio between reactor volume and the volume of biomass added daily is defined as the hydraulic retention time (RTi; sometimes also abbreviated as HRT). Depending on the type of plant: 10<RTi<30 days.
Specific load (Ls): is the quantity of VS/day added to the reactor per unit volume. Under mesophylic environment (20<process temperature<40°C): 1<Ls<5–6 kg/day of VS per m3 of reactor (depending on the type of plant). Higher values favor the acidification phase, with inhibition of methanization (the pH is lowered).
Plants are classified according to the type of feeding involved and the organization of the process. There are two basic types:
continuous loading plants, in which loading (and generally unloading as well) is a continuous process.
The following plant models (with indications as to the complexity of their
construction and management) may be classified as type b):
plug-flow: there is only one reactor, and motion is obtained through the introduction of waste (construction and management: relatively simple).
variable volume digesters: loading is continuous throughout the RT, but unloading is not (construction and management: simple).
References: , , , , , , , .
|Summary Technology Table:||DIGESTERS|
|a)||operative flexibility:||average to poor|
|b)||operation: not simple when a fairly consistent level of gas production is desired.|
|c)||most obvious limitations: i) the process must be kept going at all times, and when it stops, it takes several days to start up again; ii) high-level production can only obtained by heating the waste; iii) the gas has to be stored.|
|d)||auxiliary machinery needed: all the equipment required for loading and unloading the waste.|
|e)||recommended technical solutions and their technological level:|
|- type:||batch (intermittent loading and unloading);|
|single-stage (plug-flow or the like);|
|- suggested substrates:||animal waste with total solid content between 8 and 15%;|
|- quantity of waste:||the addition of 100 kg/day of undiluted material produces 1–2 m3 of gas/day. 2m3 of gas is needed to obtain the same energy potential as 1 kg of Diesel fuel;|
|- labor required:||variable for loading and unloading (depending on the level of mechanization);constant attention is required, especially in the case of continuous digesters;|
|- workshop required for construction:||facility capable of doing construction work, (masonry, tanks, etc.);(the plate must be bent and welded; hydraulic circuits must be set up and boilers have to be installed for the production of hot water.|
|f)||final conversion technologies to be added: i) boilers for the production of hot water and steam; ii) Otto engines.|
|In the latter case, the gas has to be dehumidified, and the machinery cannot contain any copper parts whatsoever. The engines' feed systems should be modified slightly.|