Climate effects of MBP waste gas treatment

Waste raw gas from the biological treatment process of a MBP facility contains a broad variety of climate relevant substances i) from constituents of the waste as well as ii) from substances which are caused by the biological process. The total emission potential of organic substances (TOC, total organic content) in the raw gas is about 1 kg per ton of waste, whereas nitrogen compounds amount about 0.1 kg/t wet waste. To comply with the law emissions must be reduced by gas cleaning. There are several techniques to reduce the emissions such as biofilters and Regenerative Thermal Oxidation (RTO) which both are broadly applied in state-of-the-art MBPs. Only is the RTO technology able to reduce the residual organic load in waste gas to the level of the allocation values (see table 12.12).

In the RTO technology the waste gas is oxidized by co-combustion of natural gas or liquid gas. As a result, most of the organic components are destroyed. The residual specific load of organics in the waste gas is far less than 55 g per ton of waste product. However the combustion of the gas is combined with emissions of CO2 as well as with methane emissions in the gas processing chain, which have to be considered in the total budget. Incineration in the RTO technology furthermore results is N2O emissions.

Evaluating the net effect under climate respect the benefits as well as the drawbacks have to be considered. An example is given in table 12.13 for two technological options applied in two RTO facilities. In one facility natural gas is used, in the other case, LFG from a nearby landfill.

Table 12.13 Comparison of GHG effects of RTO variants (Zeschmar-Lahl, 2000)

RTO process step

GHG effect (kg CO2-eq./t)

Facility 1

Facility 2

Natural gas pre-processing

0.21

0.0015

CO2 from natural gas incineration

6.17

0.44

Power consumption

3.4

3.4

CO2 from mineralization of organics

0.62

0.62

Effects of N2O deliberation

1.9

1.9

Mineralization of HFCs

-8.67

-8.67

Oxidation of methane

-4.45

-4.45

Benefits of landfill gas oxidation

0

-69.2

Total

-0.82

-75.95

By application of landfill gas instead of natural gas huge benefits result for the process. About 76 kg CO2-eq. are saved per ton of waste treated compared to facility 1, where the net benefit is less than one kg. This is close to the breakeven-point where no positive climate effect is achieved by application of the RTO process. Such a case may occur, for example, if the amount of HFCs is by about 10 percent less, e.g. as a long term result of phasing-out of these substances under the provisions of the Copenhagen Amendments to the Montreal Protocol (see chaper 10).

As a message from this it can be drawn that a technology able to reduce emissions of certain components must be totally budgeted to be sure that a net benefit can be achieved even under changing process conditions in the long term.

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