Environmental Engineering Reference
In-Depth Information
The current fixed crushing line consists of a Lindner Jupiter 3200 fixed crusher, magnetic
separator, a Doppstadt SM - 718 drum screen and a windscreen, all operated with electricity. All
incoming mixed municipal waste is pre-sorted with an excavator, after which the waste is fed into
the crusher. The drum screen has a diameter of 1.8m, and a screen net with 40 mm holes. The
permeate, with particle size is of 40-200mm, is transferred to the wind screen and the retentate,
with particle size of 0-40mm, drops to a silo under the drum. The retentate fraction contains e.g.
biowaste, sand and glass. The windscreen sorts the waste into a lighter fraction, consisting of e.g.
paper, carton board, plastic and wood. The heavier fraction from the windscreen contains mostly
food waste such as potatoes, carrots, oranges, apples and aluminum cans. The latter, containing
a lot of biowaste, is mixed with the retentate from the drum screen and composted. The permeate
drops to a silo, from which it is transferred with a wheel loader to a storage silo. Jätekukko Oy
uses the standard SFS 5875 and Technical Specification CEN/TS 15359 for their quality ratings
of the produced RDF. Representative samples from mixed, construction and energy waste are
analyzed in the laboratory. Based on the results the classification according to SFS 5875 and
CEN/TS 15359 are defined. Mass balances for the mixed and crushed construction waste have
been studied by weighing different mass quantities produced in the crushing equipment. Hence,
the company has been able to draw up product declarations and grades for the recycled fuels that
will facilitate the marketing of the fuel. Based on the results the contaminant concentrations of
the RDF produced from waste products are known. The results can be compared to the quality
requirements set by combustion of RDF (Jätekukko Oy, 2009; Laitinen, 2011; Juusola, 2011).
6.3 BIOGAS PRODUCTION
6.3.1 Basics of the biogas process
The production of biogas is a natural process, occurring for example in swamps, soils or cattle
paunches. Biogas plants are technical solutions for simulation of the natural degradation processes.
In principle, most kinds of organic materials are suitable as substrate for the biogas process.
Examples of substrates are (Ahrens and Weiland, 2007):
liquid and solid wastes from livestock husbandries (like manure);
ensiled energy crops;
harvesting residues;
residues from private and industrial waste material market.
According to latest developments in Europe, different and more complex waste fractions (like
household wastes) also become substrates of interest for biogas applications. Hand-in-hand with
waste utilization comes the demand for development of suitable process applications in terms of
effectiveness and reliability. A focus on these aspects will be given in section 6.3.2. The organic
load of a fermenter is catabolized by anaerobic bacteria (hydrolysis, acidogenesis, acetogenesis
and methanogenesis) over carbonic acids as intermediate products to methane within a retention
time of 20-30 days, see Figure 6.2
Methane concentration in biogas mixtures varies between 55 and 65% depending on substrate
and fermentation conditions (Ahrens andWeiland, 2007). The biogas can be combusted in energy
conversion systems for producing electricity and heat; depending on local conditions a gas upgrad-
ing to natural gas quality can be done, followed by utilization in natural gas grids or as vehicle
fuel (Ahrens and Weiland, 2007).
General important aspects for implementing biogas technology are:
electrical and thermal efficiency of the process;
economic aspects;
technical aspects concerning the fermentation process;
process requirements (gas quality/gas production);
substrate aspects.
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