Aanpak van industrieel afval Cementovens vergeleken met verbrandingsovens - Een milieuvergelijking

6. Which treatment option leads to lower environmental impacts?

The Life Cycle Assessment by TNO states:

4.1 Overall results

This report starts with the main conclusions on the question which thermal treatment (clinker kiln or waste incinerator) causes overall the lowest environmental impact, given the assumptions and limitations of this study.

Figure 6  and Figure 7  show the environmental comparison of the thermal treatment of 5 waste streams in a clinker kiln with that in a waste incinerator, calculated with two different impact assessment methods - CML with shadowprices and Eco-indicator 99.

For both methods a higher score represents more environmental impact. A negative score (below zero) represents an environmental bonus. More negative points reflect a better result from an environmental point of view. Figure 6 and Figure 7 show results for cement kilns when the input of waste substitutes petcoke and raw meal as all clinker producers have indicated that this change is the most realistic change in a marginal change calculation. In the following paragraph the results are also shown for the theoretical substitution of coal; those results point in the same direction as the results for the petcokes scenario. With regard to the alternative treatment options (waste incineration) the Figures 6 and 7 show the results for electricity and heat substitution only, because the substitution of supportive fuel cannot be a substantial one in practice. For the waste stream solvents and waste oil in the following paragraph the results are shown also for the marginal substitution of fossil fuel.

Figure 6 Environmental comparison of the thermal treatment of 5 waste streams in a clinker kiln or a waste incinerator, calculated with the CML method and integrated with shadowprices

From Figure 6 and figure 7 we can conclude that the environmental performance of thermal treatment of all 5 selected waste streams in cement kilns is better compared to treatment in waste incinerators. The main difference between clinker kilns and waste incineration is the avoided product – as will also become more obvious when we take a closer look at the origin of the results in the next paragraph. The bonus is larger for avoiding petcoke or coal compared to the bonus for avoided electricity and heat production.

This conclusion does not depend on the choice of impact assessment method; as two different methods point in the same direction for all waste streams.

The results calculated with CML and shadowprices are dominated by the impact of CO2, whereas the Eco-indicator results are dominated by depletion of fossil resources. In both methods emissions of heavy metals from waste and petcoke, coal or raw meal, contribute little to the overall weighted results.

4.2 Origin of results

In annex 5 tables with characterised and weighed results can be found for

  • Pre-treatment and transport, and emissions per ton waste, petcokes, coal and raw meal
  • The same for 1 ton of waste in waste incinerators, and for the avoided energy (per MJ caloric value in waste).

These tables and the substitution scenarios per ton waste can be used to recalculate the environmental impact scores on every level. The net results (after subtraction of the substitution bonus) are also presented in tables in Annex 5; both the characterised and weighed results per ton waste.

Source & ©: TNO  LCA of thermal treatment of waste streams in cement clinker kilns (2007),
4. Results, 4.1 Overall results, p. 21-22

 See also Annex 5 LCA results

 

6.1 Impacts of the treatment of solvents & waste oils

The Life Cycle Assessment by TNO states:

4.2.1 Solvents and waste oils

In Figure 8  you will find the characterised results for the comparison of thermal treatment of 1 ton solvents/waste oils, analysed with the CML method. Because the calculated results of the different environmental impact categories are expressed in different units, they cannot be visualised in one figure, in absolute numbers. Therefore these results have been normalised. Per impact category the highest score in the comparison of 4 scenarios (two for the cement kiln and two for the waste incinerator) has been put on 100% and the results of the other scenarios have been scaled relative to this maximum score. However, this does not mean that all impact categories have the same environmental significance. In Annex 5 you can find the absolute results expressed in equivalency factors per impact category.

The environmental bonus for decreased input or avoided energy generation results in lower impacts than caused by the incineration of the solvents/waste oils itself for some environmental impact categories. This lower impact results in net negative (below zero) impact results:

  • When petcokes are avoided, this has a positive effect (=negative scores) on 8 out of 10 environmental categories
  • When coal is avoided, this has a positive effect on 5 out of 10 categories.

For incineration of solvents in a rotary kiln, the bonus results in:

  • a positive effect on 6 out of 10 categories when electricity and heat is avoided
  • a positive effect on 9 out of 10 categories for the small amount of fossil fuel that might be substituted.

As the avoided petcoke scenario for thermal treatment in a cement kiln has a lower impact in 6 out of 10 categories, and the waste incinerator has the lowest impact in 1 category for each bonus scenario, no conclusion can be drawn on which one is the best without weighing of the categories.

Figure 9  shows the weighed results for the thermal treatment of solvents/waste oils, with a split up in the contribution of transport/pre-treatment and the emissions resulting from incineration. Left from the black line you can find the scenarios for the cement kiln, right for incineration in a rotary kiln. The net impact results form subtracting the impact of input or energy to be avoided. The dotted lines split the incineration of 1 ton solvents/waste oils and the substitution scenarios.

The impact from incineration of solvents/waste oils is comparable between the clinker kiln and the rotary kiln (see Figure 9 ). Therefore the net avoided impact is determined by the impact of the avoided processes. It appears that for the clinker kiln the avoided impact by petcoke or by coal exceeds the avoided impact of energy generation for the rotary kiln. In case the use of fossil fuel oil is substituted by the incineration of solvents/waste oils in the rotary kiln, more impact is avoided than in the case of energy generation. But even in this not representative case it is still less than is the case of the clinker kiln.

The net avoided burden of the thermal treatment of solvents and waste oils in cement kilns is due to the avoidance of incineration emissions of petcoke/coal and by the avoidance of the impact of the pre-treatment stage (see Figure 9). The impact of the use of additional raw meal is insignificant. In the case of petcoke this latter is beneficial as it avoids the emission of SO2 from the flare of crude oil production, which is the source of petcoke. In case of the avoided use of coal the avoided SO2 of the transoceanic transport of coal is important.

For the incineration of solvents and waste oils in a rotary kiln the main burden is the emission of CO2, which makes up 90% of the shadow cost. The emissions of the avoided energy production cannot counteract the impact of the solvents/waste oils incineration fully and a net burden remains. In case the solvents/waste oils are seen as to avoid the use of supportive light fuel oil in the waste incinerator a net avoided burden remains. This is the result of (heat from) light fuel oil having an almost 3 times higher CO2 emission per MJ. Other avoided emissions like that of SO2 play a minor role.

When applying the alternative impact assessment (EI99) the avoidance of fossil fuels dominates the results. Results can be found in the Annex 5.

Source & ©: TNO  Global Biodiversity Outlook 2 (2007),
4. Results, 4.2 Origin of results, p. 22-25

 See also Annex 5 LCA results

 

6.2 Impacts of the treatment of sludge

The Life Cycle Assessment by TNO states:

4.2.2 Sludge

Figure 11  shows the characterised results for the comparison of thermal treatment of 1 ton sludge, analysed with the CML method. Incineration in fluidized bed has the lowest impact in 1 out of 10 categories. Incineration in a clinker kiln avoiding petcoke has the lowest impact in 8 out of 10 categories. Again weighing is needed to draw conclusions on which scenario has the lowest environmental impact.

The weighed results for sludge are shown in Figure 12 and Figure 13.

Figure 13 : Shadowprice weighed CML results comparing thermal treatment of sludge in a clinker kiln or in a waste incinerator, with a split up in the contribution environmental categories.

The drying of sludge with heat from natural gas incineration leads to emissions of CO2 and this is the main cause of the burden of sludge pre-treatment for cement kilns. Sludge is not dried before it is incinerated in a fluidised bed resulting in a smaller impact of pre-treatment and transport compared to pre-treatment for cement kilns.

As the carbon in sludge is non-fossil it does not contribute to global warming. Therefore the emissions from co-incineration of sludge contribute little to the environmental profile of both the cement kilns and the fluidised bed.

The incineration of petcoke does lead to fossil CO2 emissions and these emissions are avoided by the incineration of sludge. Furthermore the avoided production of petcoke avoids the emission of SO2 (flare). This eventually results in a net avoided environmental impact of the thermal treatment of sludge in a cement kiln.

The emissions of mercury, cadmium and nickel through the fluidised bed flue gasses determine the impact of the ‘Emissions’ in Figure 12. The benefit of the avoided energy is largely due to the avoided emission of CO2; avoided emissions of SO2 and vanadium are of lesser importance.

Source & ©: TNO  LCA of thermal treatment of waste streams in cement clinker kilns (2007),
4. Results, 4.2 Origin of results, 4.2.2 Sludge, p. 25-27

 See also Annex 5 LCA results

 

6.3 Impacts of the treatment of filter cake

The Life Cycle Assessment by TNO states:

4.2.5. Filter cake

Figure 20  shows the characterised results for the thermal treatment of filter cake. The avoided petcoke scenario of cement kilns results in the lowest environmental impact in all categories. Even without weighing it can be concluded that treatment of filter cake in cement kilns results in a lower environmental impact compared to treatment in a waste incinerator. In the contribution analysis we will see that this is mainly due to more impact of emissions from the rotary kiln and a larger substitution bonus in cement kilns.

Filter cake treated in a rotary kiln causes emissions to water and NOx emissions and this results in the impact result for toxicity, acidification and eutrophication. Cement kilns have no emission to water and the NOx emissions are substituted by the avoided fuel, and supposed to be equally high in both cases (as is explained in chapter 3.3). In addition there is a large difference in the substitution bonus.

The environmental impact of this type of waste in a cement kiln is dominated by the release of CO2. The use pre-treatment and transport gives small contribution only.

The thermal treatment emissions of filter cake clearly differ in the rotary kiln from those of the cement kilns. The amount of CO2 produced is the same. However, the rotary kiln shows larger emissions of process related nitrous oxides and substance related hydrogen fluoride. Incineration of filter cake in a rotary kiln thus shows a net impact and the bonus for avoided energy is limited.

Source & ©: TNO  LCA of thermal treatment of waste streams in cement clinker kilns (2007),
4. Results, 4.2 Origin of results, 4.2.5 Filter cake, p. 31-32

 See also Annex 5 LCA results

 

6.4 Impacts of the treatment of paint and ink

The Life Cycle Assessment by TNO states:

4.2.3 Paint and ink

Figure 14  shows the characterised results for the comparison of thermal treatment of 1 ton paint/ink residue. In most toxicity categories the substitution bonus cannot compensate the impact caused by the thermal treatment of 1 ton paint/ink residue. The avoided petcoke scenario for cement kilns results in the lowest environmental impact in 9 out of 10 categories. In all environmental categories, the incineration in a rotary kiln causes more environmental impact compared to cement kilns for at least one of the two cement substitution scenarios. In the contribution analysis (Figure 15 and Figure 16) we will see that cement kilns have a lower impact in most categories as a result from a larger substitution bonus, compared to the substitution bonus for avoided electricity/ heat.

Due to the difference in pre-treatment the impact of the thermal treatment of paint and ink in the clinker kiln has a higher impact compared to the incineration of paint/ink residue in the rotary kiln (see Figure 15). For thermal treatment in cement kilns, paint and ink residues are absorbed using wood. The electricity used at the pre-treatment site is contributing most to the impact of pre-treatment of paint and ink residues.

The environmental burden of the thermal treatment of paint/ink impregnated sawdust in cement kilns is due to the emission of CO2 and SO2. It must be noted that only the fossil carbon accounts for the CO2 emissions, and not the non-fossil CO2 from the sawdust that is used for impregnation. In the pre-treatment it is mainly the CO2 related to the use of electricity and the emissions of VOCs at the pre-treatment sites that contribute to the impact.

The emissions of CO2 and hydrogen fluoride from the rotary kiln determine the impact of the incineration of paint and ink residues. Due to the relatively lower heating value of the paint/ink residues (7.3 MJ/kg) the amount of avoided energy is limited. The avoided energy emissions can therefore not fully compensate the incineration emissions and a net burden remains.

Source & ©: TNO  LCA of thermal treatment of waste streams in cement clinker kilns (2007),
4. Results, 4.2 Origin of results, 4.2.3 Paint and Ink, p. 27-29

 See also Annex 5 LCA results

 

6.5 Impacts of the treatment of fluff

The Life Cycle Assessment by TNO states:

4.2.4 Fluff

The characterised results for the comparison of thermal treatment of 1 ton fluff are shown in Figure 17. The avoided petcokes scenario of cement kilns results in the lowest environmental impact in 7out of 10 categories. Treatment of fluff in a fluidised bed results in the lowest impact in 2 out of 10 categories.

The contribution to the results is shown in Figure 18 and Figure 19.

Similar to analysis of wastes in this study, the impact of the emissions of the clinker kiln and the fluidised bed incinerator do not differ significantly (see Figure 18 ). It is the substitution that determines the balance between the clinker kiln and the fluidised bed incinerator. The clinker kiln is therefore in advantage.

The thermal treatment of the fluff in a cement kiln and in a fluidised bed incinerator is dominated by the emission of CO2. For both processes the impact of the pre-treatment is limited and almost related to the use of electricity for fluff handling.

The avoided use of either petcoke or coal at the cement kiln results in a net avoided environmental impact of the thermal treatment of fluff in cement kilns. The CO2-emission is more or less the same for fluff and for either petcoke or coal, but the other emissions (especially of SO2) are less in the case of fluff.

Due to the limited energy efficiency of the rotary kiln the avoided energy does not results in a net avoided environmental impact.

Source & ©: TNO  LCA of thermal treatment of waste streams in cement clinker kilns (2007),
4. Results, 4.2 Origin of results, 4.2.4 Fluff, p. 29-31

 See also Annex 5 LCA results


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