ME515 Life Cycle Assessment (LCA) The refined model for inventory analysis

Transcription

1 ME515 Life Cycle Assessment (LCA) The refined model for inventory analysis Winter quarter 2018 Joyce Cooper Design for Environment Lab University of Washington

2 Limitations of the basic model: making A invertible The technology matrix A must have an equal number of rows and columns. This means that there must be a production process for every economic flow The basic model must be altered to deal with Cut off economic flows Process alternatives Multifunctional processes Closed loop recycling

3 Limitations of the basic model: making A invertible Cut off economic flows Production processes are not included for any economic flows (flows have been cutoff): a cleaning solvent is used in a process but the upstream processes are not included in the system boundaries Process alternatives A choice exists between alternative processes electricity may be produced from coal or uranium a solvent might be disposed of or recycled Multifunctional processes A single process produces more than one valuable product or material (the process is multifunctional): a refinery produces kerosene and diesel fuel Closed loop recycling (as a special case of multifunctionality) A secondary material is fed back into a unit processes in the same system (closed loop recycling) aluminum scrap us used to make an aluminum part

4 Cut-off of economic flows Cut offrefers to incomplete systems (e.g., a certain material or component is used in a process but the upstream processes are not included in the system boundaries) For the example Solid Fuels presented Processing the Technologies notes for chapter 2, suppose we consider the use of cleaning solvent and chromic acidin anodizing as a part of part production (arguably it should have been included before this ) but we do not want to include the life cycles of the cleaning solvent and chromic acid in our system boundaries.

5 Cut-off of economic flows The components of the demand and scaling vectors and the rows of the technology matrix would capture: kg aluminum L fuel kwh electricity kg aluminum part kg cleaning solvent kg chromic acid part production aluminum production electricity production A= and f= So that we cannot * invert A * find a scaling factor for part production to create 1kg aluminum part with no demand for cleaning solvent or chromic acid fuel production

6 Cut-off of economic flows The text notes 3 ways to solve this problem estimate the missing information add a "hollow process" remove the cut-off flows from the technology matrix Also consider putting them in the intervention matrix (the B matrix) This allows you to keep track of what you are cutting off, and can be easier than the use of hollow processes

7 Such that Cut-off of economic flows A hollow process simply adds a column (a process) to the process matrix for every cut-off economic flow. The additional processes produce one unit of each economic flow. part production aluminum production electricity production fuel production hollow process for solvent production hollow process for chromic acid production E E A= and A -1 = E E E E E E E This assumes waste solvent and chromic acid are environmental flows. part production aluminum production electricity production fuel production hollow process for solvent production hollow process for chromic acid production and s is therefore: 1 which uses Excel's matrix multiplication function s= A -1 f= MMULT 7.9 combined with the index function, as before Same as original solution

8 Cut-off of economic flows Removing the cut-off flows from the technology matrix (essentially ignoring them as a part of the economic system) is accomplished by further partitioning of the technology matrix.

9 Removing the cut-off flows from the technology matrix (ignoring them) is completed by solving A s=f which is essentially the original problem (as presented in the notes for chapter 2) such that Cut-off of economic flows

10 Cut-off of economic flows Removing the cut-off flows from the technology matrix Again, the discrepancy vector is intended to give us information about the magnitude of what we are leaving out Computationally, you will get the same information by putting cut-off flows in the intervention matrix So, it would be scaled in the same way as an air emission

11 Limitations of the basic model: making A invertible The technology matrix A must have an equal number of rows and columns. This means that there must be a production process for every economic flow The basic model must be altered to deal with Cut off economic flows Process alternatives Multifunctional processes Closed loop recycling

12 A choice between alternative processes A choice between process alternativesrefers to alternatives within (as opposed to between) a given product system. For example, Choice of suppliers What if one economic flow is produced by more than one unit process with different Solid characteristics Fuels Processing (electricity Technologies from fuel, coal, hydro, and nuclear power) or in different locations (supplier 1 is in Seattle and supplier 2 is in West Virginia)? THISISNOTTHESAMEASLCAscomparingproductionine.g. Seattle and West Virginia such a comparison is represented by 2 separate LCAs Choice of deposition What if one economic flow can be disposed of in different manners (land filled, energy recovery, recycled)?

13 A choice between alternative processes It has become common to fold alternatives into a single process representing multiple technologies. This provides a representation of market shares or the electricity grid Some call these mixer processes

14 A choice between alternative processes So this can be represented as a mix process in your inventory, producing a kwh of electricity, with inputs of fractions of a kwh from each generation option. With x1 = 0

15 A choice between alternative processes Instead of combining technologies, another option is to keep things separate by being more specific in the definition of processes AND economic flows. part production aluminum production fuel-based electricity production fuel production kg aluminum A'= L fuel A= kwh electricity from fuel kg aluminum part kg coal kwh electricity from coal coal-based electricity production coal mining

16 A choice between alternative processes If you are going to keep your process options disaggregated, you may also be able to automatically analyze scenarios Although it can be a bit tricky, scenarios can be assessed using multiple demand and scaling vectors are disaggregated (denoted as f1, f2, s1, and s2). To handle alternatives throughout the life cycle, disaggregation of the demand vector requires careful scenario tracking efforts.

17 You can make the decision You cannot make the decision If your LCA is disaggregated, things get complicated quickly

18 Production of Material A using P sa Production of Material B from P sb1 Production of Material B from P sb2 Production of Material C through P sb1 Production of Material C through P sb2 Technology Matrix Material A Material B from P sb Material B from P sb Material C through P sb Material C through P sb Material F through P sb Material F through P sb Material D Material E from P E Material E from P se End Product with D and F through P sb1 1 End Product with E from P E1 and F through P sb1 1 End Product with E from P se2 and F through P sb1 1 End Product with D and F through P sb2 1 End Product E from P E1 and F through P sb2 1 End Product with E from P se2 and F through P sb2 1 Production of Material F through P sb1 Production of Material F through P sb2 Production of Material D Production of Material E from P E1 Production of Material E from P se2 End Product with D and F through P sb1 End Product with E from P E1 and F through P sb1 End Product with E from P se2 and F through P sb1 End Product with D and F through P sb2 End Product E from P E1 and F through P sb2 End Product with E from P se2 and F through P sb2 Disaggregated Demand Matrix Scen. 1 Scen. 2 Scen. 3 Scen. 4 Material A Material B from P sb1 Scenario 1: End product with B from P sb1 and E from P E1 Scenario 2: End product with B from P sb2 and E from P E1 Sceanrio 3: End product with B from P sb1 and E from P se2 Scenario 4: End product with B from P sb2 and E from P se2 A bit of a data management mess! Itisdoable,but it can be challenging! Material B from P sb2 Material C through P sb1 Material C through P sb2 Material F through P sb1 Material F through P sb2 Material D Material E from P E1 Material E from P se2 End Product with D and F through P sb End Product with E from P E1 and F through P sb1 20 End Product with E from P se2 and F through P sb1 20 End Product with D and F through P sb End Product E from P E1 and F through P sb2 20 End Product with E from P se2 and F through P sb2 20 Resulting Scaling Matrix Production of Material A using P sa Production of Material B from P sb Production of Material B from P sb Production of Material C through P sb Production of Material C through P sb Production of Material F through P sb Production of Material F through P sb Production of Material D Production of Material E from P E Production of Material E from P se End Product with D and F through P sb End Product with E from P E1 and F through P sb1 20 End Product with E from P se2 and F through P sb1 20 End Product with D and F through P sb End Product E from P E1 and F through P sb2 20 End Product with E from P se2 and F through P sb2 20

19 Limitations of the basic model: making A invertible The technology matrix A must have an equal number of rows and columns. This means that there must be a production process for every economic flow The basic model must be altered to deal with Cut off economic flows Process alternatives Multifunctional processes Closed loop recycling

20 Multifunctional unit processes Often in industry, single processes produce more than one valuable product or material Refineries produce petrol, kerosene, heavy oil, diesel, etc. Platinum mining also yields palladium Cows produce milk, meat, hides, calves, etc. Additional VALUABLE products are called "coproducts"

21 Multifunctional unit processes Categories of multifunctional processes are 2 or more valuable outputs are produced (platinum mines, and cows) 2 or more wastes Solid Fuels are Processing managed Technologies (incineration of municipal waste streams) a waste is converted to a valuable product (metal recycling) three or more valuable functions are provided (combinations of the above)

22 Multifunctional unit processes Again assume we have the square technology matrix, but assume we get 18MJ of heat with our electricity This time, the components of the demand and scaling vectors and the rows of the technology matrix would capture: * kg aluminum * L fuel * kwh electricity * kg aluminum part * MJ heat Such that part production aluminum production 2 L fuel electricity production fuel production A= And once again we cannot * invert A Electricity Production 10 kwh electricity 18 MJ heat The heatflow makes A non-square! * solve the resulting overdetermined system of equations (we cannot produce only what is demandedwe cannot produce 1kg aluminum part without creating 18MJ of heat for which there is no demand (at least in this system)

23 Multifunctional unit processes There are several ways to solve systems with multifunctional unit processes including The substitution method/ system expansion (aka adding avoided processes in a consequential LCA) The partitioning method (aka allocation in an attributional LCA) The surplus method (aka ignoring the coproducts)

24 Multifunctional unit processes: partitioning in an attributional LCA On the other hand, many use partitioning, a.k.a. allocation Essentially allocation is dividing the upstream environmental interventions of a unit process among products/co-products Allocation can be based on mass, energy, economic value, or process attribution/stoichiometry

25 Multifunctional unit processes: partitioning in an attributional LCA Allocation factors can be based on economic value 50 kwh electricity Solid Fuels Processing machining Technologies 1.5 kg aluminum 1 kg aluminum part at $5/kg or $5/part 0.5 kg scrap at $1/kg or $0.5/part (5*50/5.5) kwh electricity (5*1.5/5.5) kg aluminum (0.5*50/5.5) kwh electricity (0.5*1.5/5.5) kg aluminum machining the part machining for scrap 1 kg aluminum part 0.5 kg scrap

26 Multifunctional unit processes: partitioning in an attributional LCA Allocation factors can be based on mass 50 kwh electricity 1 kg aluminum part Solid Fuels Processing machining Technologies 1.5 kg aluminum 0.5 kg scrap (1*50/1.5 = 33) kwh electricity (1*1.5/1.5 =1) kg aluminum (0.5*50/1.5 = 16) kwh electricity (0.5*1.5/1.5 = 0.5) kg aluminum machining the part machining for scrap 1 kg aluminum part 0.5 kg scrap

27 Multifunctional unit processes: partitioning in an attributional LCA In the partitioning method, multifunctional processes are split into monofunctional processes: 1 for the product and 1 for each co-product Consider again the electricity production process producing both electricity and heat. The components of the process vector would be 0 kg aluminum L fuel -2*λ (2a) -2*λ (2b) 10 kwh electricity kg aluminum part 0 0 p 3 = 18 for MJ heat could be split into p 3a = 0 p 3b = 18 (electricity 0 kg waste aluminum (electricity 0 (heat 0 production) 0 L water production) 0 production) 0 0 L wastewater kg carbon dioxide 1*µ (4a) 1*µ (4b) 0.1 kg sulfur dioxide 0.1*µ (5a) 0.1*µ (5b) 0 kg bauxite kg residues kg crude oil 0 0 such that p 3a produces kwh electricity and p 3b produces MJ heat and λ and µ are allocation factors between zero and 1

28 Multifunctional unit processes: Also, the "100% rule" tells us p 3a +p 3b =p 3 or the sum of the partitioned monomfunctional processes are equal to the unpartitioned process from which they were derived Which allows the technology matrix to be solved using the basic model for a given allocation factor. partitioning in an attributional LCA 0 0-2*λ (a) -2*(1-λ (a) ) p 3a = 0 p 3b = *λ (a) 1*(1-λ (a) ) 0.1*λ (a) 0.1*(1-λ (a) )

29 Multifunctional unit processes: partitioning in an attributional LCA Which for an allocation factor λ a = 0.7 (or 70% of the burdens are allocated to electricity production and 30% to heat production) tells us part production aluminum production electricity production fuel production A= and gives UWME But since Design you for are Environment not using the Lab heat, you just leave the column out, and then A is square. part production aluminum production ELECTRICITY production (p 1a ) kg aluminum A'= L fuel kwh electricity kg aluminum part MJ heat fuel production HEAT production (p 1b )

30 Multifunctional unit processes: partitioning in an attributional LCA Mass based allocation has its problems (Boustead, 1999) The chloralkaliprocessyields chlorine, sodium hydroxide, and hydrogen: 2NaCl+2H 2 O->2NaOH+Cl 2 +H 2 Suppose a plant has the following inputs and outputs Inputs Sodium chloride 117 kg Water excess Outputs Chlorine 71 kg Sodium hydroxide 80 kg Hydrogen 2 kg The total output of usable products is 153kg and using mass allocation, the input of sodium chloride is allocated to chlorine, sodium hydroxide, and hydrogen in ratios of 71:80:2. These ratios imply that: (117*0.464)=54.3 kg sodium chloride produces 71 kg chlorine (117*0.523)=61.2 kg sodium chloride produces 80 kg sodium hydroxide (117*0.013)=1.5 kg sodium chloride produces 2 kg hydrogen a.k.a kg SODIUM CHLORIDE PRODUCES 71 kg CHLORINE--NOT

31 Multifunctional unit processes: partitioning in an attributional LCA Allocation factors can be based on stochastic partitioning (Boustead, 1999) For the chloralkali process Inputs Sodium chloride 117 kg Water excess Outputs Chlorine 71 kg Sodium hydroxide 80 kg Hydrogen 2 kg The reaction leading to the production of chlorine from sodium chloride is strictly an ionic reaction and the ionic species attributable to the different products can be identified and related as follows: Allocation factor Inputs Sodium chloride 117 kg = 46 kg Na+ and 71 kg Cl- Water excess Outputs Chlorine 71 kg 71/117 = 61% Sodium 80 kg 46/117 = 39% hydroxide Hydrogen 2 kg 0%

32 The standards says allocation should be avoided ISO indicates that whenever possible, allocation should be avoided by 1. Collecting data for sub-processes 2. Expanding the product system to include avoided processes. In practice, this has been replaced by Using allocation in attributional LCAs Using system expansion in consequential LCAs

33 Multifunctional unit processes: substitution/ system expansion in a consequential LCA In the substitution/ system expansion method, a separate avoided process for the production of the co-product is added to the system. For example, Suppose a stand-alone process for producing heat burns 5 L fuel to produce 90MJ of heat and 3 kg carbon dioxide The components of the process vector would be * kg aluminum 0 * L fuel -5 5 L fuel avoided 90 MJ heat Heat 3 kg CO 2 Production * kwh electricity giving us 0 * kg aluminum part 0 * MJ heat p 6 = 90 * kg waste aluminum 0 * L water 0 * L wastewater 0 * kg carbon dioxide 3 * kg sulfur dioxide 0 * kg bauxite 0 * kg residues 0 * kg crude oil 0

34 Multifunctional unit processes: substitution/ system expansion in a consequential LCA Such that the new technology matrix is part production aluminum production electricity production fuel production "avoided" heat production E A'= A' -1 = which can be inverted and the scaling vector can be found: for we get 0 1 part production f'= 0 s'= aluminum production electricity production fuel production "avoided" heat production part production aluminum production electricity production fuel production "avoided" heat production and remember we used to have 1 part production Noting that s= aluminum production * The scaling factor (7.9) for the production of electricity is unaffected electricity production * The scaling factor for fuel production has decreased (the need for fuel was avoided) fuel production * The scaling factor for heat production is negative

35 Multifunctional unit processes: substitution/ system expansion in a consequential LCA Problems with the substitution method include Which process do you use as the "avoided process" Avoided processes might be multifunctional (needs another avoided process - sort of a snowball effect) Assumptions about the quality of products (is the heat of the same quality?) If demand for a co-product dips below the amount produced, the co-product becomes a waste

36 Multifunctional unit processes: the surplus method used by no-one? In the surplus method, all burdens are allocated to the main flow (co-products are ignored) Most LCA practitioners do not use the surplus method You mightseeitused,ifapractitioneriscomparing methods for dealing with coproducts

37 Limitations of the basic model: making A invertible The technology matrix A must have an equal number of rows and columns. This means that there must be a production process for every economic flow The basic model must be altered to deal with Cut off economic flows Process alternatives Multifunctional processes Closed loop recycling (a special case of multi-functionality)

38 Closed-loop recycling In general, recycling is just another unit process. In closed loop recycling, however, secondary material is fed back into one of the unit processes in the same system. When the secondary material displaces the use of virgin materials, flows for the secondary material are a part of the product system and there is no need to allocate Solid their Fuels burden Processing elsewhere Technologies nor expand the system to avoid e.g., virgin production Aluminum scrap Aluminum Production Part Production Aluminum part Fuel Production Electricity Production Aluminum scrap is added as an economic flow (used to be an environmental flow)

39 Closed-loop recycling To accommodate this, we assume waste aluminum is produced by part production (scrap) and is used by aluminum production such that 1000 kg aluminum is produced using 200kg scrap and about 60% of the electricity as before. part production aluminum production using recycled material electricity production fuel production kg aluminum kg fuel A = and f = 0 kwh electricity kg aluminum part kg waste aluminum Energy consumption is less Waste aluminum is consumed A is not square Used to be

40 Closed-loop recycling Instead of treatment as a co-product, a pseudo-inverse can be used in place of the matrix inverse: s=a -1 f can be represented as s=(a T A) -1 A T f where the pseudo-inverse (or the Moore-Penrose inverse) is A + =(A T A) -1 A T such that s=a + f Or s= (A T A) -1 A T f

41 Closed-loop recycling: s = (A T A) -1 A T f Since the pseudo-inverse essentially performs a least-squares regression to minimize the discrepancy vector (d), A*s will not exactly equal f unless all of the material (and no more) is used by the process that recycles it. part production aluminum production using recycled material electricity production fuel production kg aluminum kg fuel A = and f = 0 kwh electricity kg aluminum part kg waste aluminum ratio alum to waste alum =-1.5/0.5 =1000/-200 Here, because the ratio of aluminum to waste aluminum is different for part production and aluminum production, you get the wrong scaling factor (as shown by inspection of the discrepancy vector, telling us the magnitude of what we are leaving out). A T (A T A) , , ,365, , , ,000 (A T A) -1 A T f s discrepancy vector d=a*s-f part production alumium production electricity production fuel production

42 aluminum production using recycled material open loop aluminum recycling part production electricity production fuel production kg aluminum kg fuel A = f = 0 kwh electricity kg aluminum part kg waste aluminum: closed loop kg waste aluminum: open loop ratio alum to waste alum -5-5 Closed-loop recycling: s = (A T A) -1 A T f The text recommends allocating exactly enough material to the recycling process so that it can essentially be ignored (the pseudo-inverse will equal the true inverse), and send the rest to an open-loop recycling or waste treatment process. Now the ratios are both -5 and the A matrix has 5 columns and 6 rows A T (A T A) , ,365, (A T A) -1 A T f s discrepancy vector d=a*s-f part production alumium production electricity production fuel production

43 Closed-loop recycling Alternatively, if more recycled material is needed than is produced, you would have to supplement the supply with an additional process that produces the required material (though I would use less than if there were no closed loop recycling). Otherwise the system will be over-defined and even though you can get a scaling vector from the pseudo-inverse, the A*s vector will not equal exactly the demand vector. In fact, it will yield less than the demanded item, and will produce technology flows that were not demanded at all.

44 Limitations of the basic model: making A invertible The technology matrix A must have an equal number of rows and columns. This means that there must be a production process for every economic flow The basic model must be altered to deal with Cut off economic flows Process alternatives Multifunctional processes Closed loop recycling (a special case of multifunctionality)

45 In summary Situations in which the basic model must be altered 1. Cut off economic flows 2. Process alternatives 3. Multi-functional processes 4. Closed loop recycling Ways to address the situation Estimate the missing information Add a "hollow process" Remove the cut-off flows from the technology matrix Put the cut off flows in the intervention matrix Fold alternatives into a single process representing multiple technologies (as in the use of market shares or the electricity grid ) Keep things separate by being more specific in the definition of processes AND economic flows. The substitution method/ system expansion (a separate avoided process for the production of the co-product is added to the system; credit is taken; offsets are considered) The surplus method (co-products are ignored) The partitioning method (allocation by mass, energy, or economic value) A pseudo-inverse can be used in place of the matrix inverse