Organic waste as a biomass resource - Biomass as Energy Source: Resources, Systems and Applications - page 118

6.4.3 Development considerations
The development of combustion of waste is from treatment of waste to a competitive energy
conversion process. This requires limiting the costs and improving environmental performance.
Important factors concerning the costs are pre-treatment costs and maintenance costs. A study
comparing mixed MSW and RDF as fuel show that using RDF might be more competitive if the
energy prices increase when pre-treatment cost is not included. A disadvantage with RDF plants
can be that they are less flexible concerning what fuels can be used compared to plants designed
for using mixed waste. On the other hand, less pre-treatment can cause higher maintenance costs.
Generally, waste combustion plants have higher operation and maintenance costs compared to
combustion plants using other types of fuels due to more advanced handling but not many studies
have been done and knowledge about possible improvements is low (European Commission,
2006; Fendel and Firege, 2010; Johansson et al. , 2009).
Another important issue is the risk for overcapacity due to several competing waste-to-energy
concepts, for example:
• the combustion of mixed municipal waste;
• co-combustion of waste with other fuels;
• production of RDF followed by combustion;
• the use of waste fractions in cement kilns.
This can be caused because there are different actors on the market driven by different aims
with the public sector increasing the municipal waste combustion capacity to decrease the lack
of waste treatment capacities and the private sector increasing the RDF combustion capacity and
waste use in cement kilns with the aim to reduce energy costs and using the possibility to sell
electricity to the grid. (Fendel and Firege, 2010).
6.5 EXAMPLES OF USE OF ORGANIC WASTE IN OTHER CONVERSION PROCESSES
6.5.1 Ethanol and butanol from organic waste
Another biological conversion process that can use waste as feedstock is fermentation to ethanol
or butanol. Raw materials that can be used for fermentation are materials containing sugars
or substances that can be converted into sugars. The raw materials can be grouped as directly
fermentable sugary materials, starchy materials and lignocellulosic materials. Wastes of interest
for fermentation are industrial waste material and waste waters, for example by-products of the
sugar industry, wastes from the food industry, waste water from breweries and lignocellulosic
wastes such as straw, wood wastes and waste from the pulp and paper industry. Also part of
MSW could be used for ethanol production. There are studies pointing out that the long-term
sustainability of ethanol production will ultimately depend on the use of lignocellulosic wastes.
Fermentation of sugary wastes to ethanol is a mature and well-proven technology but fer-
mentation of lignocellulosic waste and fermentation to butanol is still at the research stage. The
development areas include increasing the yield, process design, and increasing the energy effi-
ciency including process integration (Adler et al. , 2007; Farrell et al. , 2006; Flavell, 2007; Kim
and Dale, 2005a,b; Kszos et al. , 2001; Prasad, 2007). In addition, the selection of the production
organism could be an option for improved processes. For example, in the ethanol production the
yeasts could sometimes be replaced by such anaerobic bacteria as Clostridium thermocellum or
Zymomonas spp. (Righelato, 1980). Their involvement could increase such biocatalyst parame-
ters as thermal resistance, or ethanol tolerance, which then could have profound effects on the
overall process economics.
Biofuels produced with conventional technology from easily convertible rawmaterials, such as
direct fermentable sugarymaterial, are often referred to as first generation biofuels, while biofuels
produced in more advanced processes from lignocellulosic material are called second-generation
biofuels (UN-Energy, 2007).