Energy recovery from waste incineration: economic aspects

Abstract

If we consider the desirability of reducing fossil fuel consumption, together with the increasing production of combustible solid wastes, there is clearly a need for waste treatment systems which achieve both volume reduction and energy recovery. Direct incineration method is one such system. The aim of this work was to analyze the municipal solid waste incineration situated in the Province of Turin (Piedmont, North Italy), especially its economical effects in consequence of the energy recovery that can be achieved. In order to perform this analysis, two kinds of energy recovery have been studied: electric energy (electrical configuration) only, and both electric and thermal energy (cogenerative configuration). So after a reconstruction of the economic situation, by considering all the costs and revenues, for both the possible energetic configurations the correspondence between the environmental convenience (that was evaluated in a previously work) and the economic convenience has been defined. The main obtained results highlight that currently the environmental convenience corresponds to the cogenerative configuration; instead the economical convenience in the actual condition corresponds to the only electric configuration. Anyway, by working on the thermal energy price, it is possible to obtain at the same time an environmental and economic convenience.

Appendix

In order the defined the avoided CO₂ emissions due to by not sending the MSW to the landfill the procedure is here explained. In this case, we considered only the stoichiometry of conversion (Panepinto and Genon 2012): it can be established that for 100 g of MSW there is a generation of 29.78 g of CH₄ and 58.91 g of CO₂. These amounts, as well as the following ones, were obtained using the mass balance method.

The amounts of methane and carbon dioxide correspond to the following avoided emissions (owing to the elimination of the landfill option).

$$ {\text{C}}{{\text{H}}_4} = 421,000\;{\text{tonnes}}/{\text{year}}\;*\;0.2978\;{\text{tonnes}}/{\text{tonnes}} = 125,374\;{\text{tonnes}}/{\text{year}}, $$

(1)

$${\text{C}}{{\text{O}}_2} = 421,000\;{\text{tonnes}}/{\text{year}}\;*\;0.5891\;{\text{tonnes}}/{\text{tonnes}} = 248,011\;{\text{tonnes}}/{\text{year}} . $$

(2)

By considering (as indicated by Cossu and Pivato 2002) an efficiency of 0.55 in biogas collection we have:

• Emissions avoided by capturing biogas (burned in a torch)

$$ {\text{Avoided}}\;{\text{C}}{{\text{O}}_2}\;{\text{emissions}} = 0.55\;*\;248,011\;{\text{tonnes}}/{\text{year}} = 136,406\;{\text{tonnes}}/{\text{year}}, $$

(3)

$${\text{Avoided}}\;{\text{C}}{{\text{H}}_4}\;{\text{emissions}} = 0.55\;*\;125,374\;{\text{tonnes}}/{\text{year}} = 68,956\;{\text{tonnes}}/{\text{year}} . $$

(4)

The generated amount calculated by Eq. (3) is not considered climate relevant because it is a result of the degradation of biogenic substances (making it “climate neutral”). But the generated CH₄ calculated by Eq. (4) is subsequently oxidized to CO₂ (because it is burned) and it is considered climate relevant:

$$ {\text{Avoided}}\;{\text{C}}{{\text{O}}_2}\;{\text{emissions}}\;\left( {{\text{from}}\;{\text{C}}{{\text{H}}_4}} \right) = 68,956\;{\text{tonnes}}/{\text{year}}\;*\;(44/16) = 189,629\;{\text{tonnes}}/{\text{year}}, $$

(5)

where 44 is CO₂ molecular weight, 16 is CH₄ molecular weight, and 44/16 is oxidation factor.

• The biogas not captured (a ratio of about 0.45) is released as diffusive emissions:

$$ {\text{Avoided}}\;{\text{C}}{{\text{O}}_2}\;{\text{emissions}} = 0.45\;*\;248,011\;{\text{tonnes}}/{\text{year}} = 111,604\;{\text{tonnes}}/{\text{year}}, $$

(6)

$$ {\text{Avoided}}\;{\text{C}}{{\text{H}}_4}\;{\text{emissions}} = 0.45\;*\;125,374\;{\text{tonnes}}/{\text{year}} = 56,688\;{\text{tonnes}}/{\text{year}} . $$

(7)

The generated amount calculated by Eq. (6) is not considered climate relevant because it is a result of the degradation of biogenic substances (making it “climate neutral”), whereas the generated CH₄ calculated by Eq. (7) is considered climate relevant. Using the quantity derived in Eq. (7) and taking into account that the global warming potential of methane is approximately 21 times higher than that of carbon dioxide per unit of weight, this amount of methane generates an avoided amount of CO₂ of 1,190,448 tonnes/year.

Therefore, the total avoided amount of CO₂, taking into account the landfill elimination, can be calculated by:

$$ {\text{Total}}\;{\text{C}}{{\text{O}}_2}\;{\text{avoided}} = 189,629\;{\text{tonnes}}/{\text{year}} + 1,190,448\;{\text{tonnes}}/{\text{year}} = 1,380,077\;{\text{tonnes}}/{\text{year}} . $$

(8)