Carbon Footprint Questionnaire for Your LLDPE Resin: What Data to Collect

A carbon footprint questionnaire for linear low-density polyethylene must be structured as a primary-data collection instrument rather than a compliance survey. The objective is to obtain site-specific, train-specific, and grade-specific mass and energy flows that can be aggregated into a cradle-to-gate product carbon footprint under ISO 14067:2018. The functional unit is set as 1 kg of unpacked LLDPE pellet at the production site gate. The questionnaire records resin grade identifiers measured by ASTM D1505 and ASTM D1238 so that density, melt index, and comonomer type can be linked to extrusion energy and upstream mass balance. A generic plant-wide emission factor is not acceptable because upstream ethylene from ethane and naphtha differ in cracker yield pattern and energy burden.

Feedstock Boundary and Cracker Allocation Data

Ethylene source data are collected at the steam cracker battery limit. The questionnaire requires monthly average mass flow of each cracker feed type—ethane, propane, butane, naphtha, gas oil, and any bio-naphtha or pyrolysis oil—reported in t/t ethylene. Total olefin output is not sufficient because the allocation of cracker energy among ethylene, propylene, mixed C4s, pyrolysis gasoline, and fuel gas changes the LLDPE monomer burden. Furnace data requested include coil outlet temperature, steam-to-hydrocarbon ratio, and ethylene feed conversion during the campaign. These variables are used to apply the allocation hierarchy of ISO 14044:2006, Section 4.3.4.2. If physical relationships cannot be established for individual furnace products, energy-based allocation using lower heating values of the co-products is preferred. The questionnaire asks for lower heating value of each co-product and its mass flow at the battery limit, with the allocation factor calculated separately for each campaign month. Transport of imported ethylene is recorded as distance, mode, and share of pipeline, ship, or rail in the supplier chain.

Comonomer selection introduces a second upstream boundary. LLDPE grades based on 1-butene, 1-hexene, or 1-octene have distinct cradle-to-gate burdens because 1-octene from an ethylene oligomerization unit is more energy-intensive per tonne than 1-butene separated from mixed C4 streams. The questionnaire requests the comonomer addition rate as a percentage of total feed and whether the comonomer is produced within the same site or purchased. For purchased comonomer, the data provider supplies the supplier product carbon footprint, but only when the underlying allocation method and database version match the LLDPE study. Otherwise the questionnaire asks for comonomer feedstock source and production route. If the site operates a purge gas recovery system and returns comonomer to the reactor, the recovered mass is netted out to avoid double counting.

Gas-phase LLDPE trains using Unipol or Innovene fluidized-bed reactors require process data that are not captured in total plant utility records. The questionnaire asks for reactor inlet and outlet temperatures, cycle gas compressor differential pressure, bed weight, and superficial gas velocity because these variables influence compression energy and monomer carryover. The cycle gas cooler and recycle gas compressor are normally the largest electricity users in the reaction section; therefore the compressor drive power is reported in kWh/t and separated from extruder power. Operational upset data are included because fouling of the cycle gas cooler or distributor plate reduces heat transfer and increases compressor pressure drop over the campaign. The questionnaire records the frequency and duration of condenser cleaning cycles and the associated nitrogen purge volume. Monomer recovery data include the flow of vent gas to the flare, the recovered monomer mass returned to the reactor, and the mass of heavy wax collected from the cooler knockout pot. For solution LLDPE lines using an adiabatic or stirred autoclave process, the questionnaire separately records solvent type, solvent recovery energy, and the quantity of solvent lost per tonne of resin because solvent losses above site-specific baseline values can change the upstream solvent manufacturing burden. Process emission data for the resin degassing unit are collected as residual volatile hydrocarbons in the pellet, measured by headspace gas chromatography according to the site test method. The data provider reports nitrogen use for degassing, purge, and pneumatic conveying in Nm³/t; this stream is often omitted from plant-level carbon accounting because it is purchased as a utility but its embodied carbon arises from air separation upstream.

Catalyst and additive data are kept categorical unless the addition rate exceeds the cut-off criterion of 1% of resin mass. Ziegler-Natta, metallocene, and post-metallocene catalysts are recorded by type because catalyst productivity influences reactor residence time and off-gas treatment. Antioxidant, slip, antiblock, and processing aid masterbatches are reported as supplier name, addition rate, and carrier resin type. The questionnaire requests the percentage of off-spec or transition material generated during grade changes because this material is recycled internally or downgraded, and its burden is allocated into prime resin under the site mass balance. Off-spec generation above 3% of campaign output should be flagged for a separate allocation calculation rather than absorbed into the production average.

Which Energy and Utility Streams Require Metering at the Pelletizing Stage?

Pelletizing energy inventories are invalid when the questionnaire requests only total plant electricity. The extrusion section is divided into main drive power, gear pump oil heating, pellet dryer air blower, vibrating screen and classifier, and underwater pelletizer water circulation. Main drive power is taken from the extruder variable-speed drive panel in kWh/t rather than from the plant transformer total because the transformer may serve multiple units. The questionnaire records water temperature and flow in the pelletizer system because chilled water electrical demand at the chiller compressor is the correct greenhouse gas input, not the thermal energy removed from pellets. Steam is reported as saturated steam flow in MJ/t at the measured pressure. Drying air may be heated by electricity or steam; the energy source is stated separately. Nitrogen for pellet conveying and silo blanketing is reported in Nm³/t with the upstream production source, usually an air separation unit or purchased merchant nitrogen.

The questionnaire includes start-up and shut-down inventories because these periods are not reflected in steady-state utility meters. For each transition, the data provider reports the mass of off-spec pellets, extruder purge compound, and scrap produced. Extruder screen pack changes cause melt temperature deviations and require purge polymer; these events are recorded with their duration and material loss. The burden of transition scrap is allocated to prime resin according to the site's mass balance method, following ISO 14044:2006, Section 4.3.4.2. The distinction between closed-loop recycling and downcycling is required because internal scrap returned to the same extruder is treated differently from downgraded material sold as lower-grade compound. If the site uses an underwater pelletizer, die plate freeze-off events during start-up are recorded as a separate production loss because they consume additional polymer purge and cooling water.

Logistics and packaging are appended as optional modules for cradle-to-gate-plus-distribution studies. The questionnaire asks for mode, distance, average payload, and empty return rate for finished pellet shipments. Bulk hopper trucks often use diesel, railcars may use grid electricity or diesel, and marine container shipments require terminal handling emissions. The data provider reports fuel consumption in L/100 km for truck shipments and kg/t km for rail or marine, with the return leg allocated to the product if the vehicle returns empty. LLDPE sold in 25 kg bags on wooden pallets has an additional packaging burden from the bag film, pallet wood, stretch wrap, and label. The questionnaire requests the mass of each packaging component per tonne of resin and whether the pallet is reused or one-way. Packaging waste treatment is included only if the study boundary is cradle-to-gate plus end-of-life; otherwise it is documented as an excluded stage with a justification. Published data for this specific configuration is limited when customer-specific unloading equipment or local distribution hubs vary, so the questionnaire marks these fields as optional and requires a sensitivity check only if packaging exceeds 2% of total product carbon footprint.

When Recycled or Bio-Based Ethylene Enters the Mass Balance

Bio-circular and chemically recycled feedstocks introduce allocation and biogenic carbon flows that differ from fossil ethylene. If the LLDPE contains bio-based ethylene from renewable naphtha, the questionnaire asks for the bio-feedstock input rate, the conversion factor from feedstock to ethylene, and the accounting period for the mass balance certificate. A claim of 20% bio-circular content does not translate automatically into a 20% reduction in fossil emissions; the questionnaire requires the mass balance document from ISCC PLUS or REDcert and the underlying allocation method. Biogenic carbon uptake is reported as a separate negative emission flow only when the carbon remains in the product at the system boundary. ISO 14067:2018 requires separate reporting of fossil and biogenic carbon flows; the questionnaire therefore has two columns for CO₂ emissions and CO₂ removals.

For chemically recycled pyrolysis oil from mixed plastic waste, the data provider supplies waste feedstock origin, pyrolysis oil mass rate, process energy, and whether the oil is allocated to ethylene or to fuel gas. The questionnaire records the chlorine and moisture content of the pyrolysis oil because hydrotreating energy increases if the oil contains higher halogen and oxygen levels. The burden of waste sorting and washing is included upstream if the waste is attributed to the LLDPE system. Mass balance credits from a separate chemical site are not transferred automatically; each receiving site must have a unique batch certificate or credit transfer document. If recycled content is physically segregated, the questionnaire requires the segregation plan and the analytical method used to verify the recycled content in the finished resin.

Foreground data quality is assessed for every data field using the data quality matrix of ISO 14044:2006, Section 4.2.3.6. The questionnaire captures age, geography, technology, and precision for the reactor, extruder, and cracker datasets. Electricity grid mix emission factors are selected from the same database version for foreground processes and background processes; mixing database versions changes the carbon footprint and complicates verification. Purchased steam from a combined heat and power plant is allocated by the power bonus method only when the supplier reports total fuel input, net electricity export, and boiler efficiency; otherwise the enthalpy allocation method is applied. Flare emissions are calculated from measured flare gas flow, composition, and the global warming potential values of the selected IPCC assessment report. The questionnaire records the IPCC report and time horizon because methane GWP changes between IPCC AR4, AR5, and AR6. Emission factors for purchased electricity from the public grid are selected from the most recent national inventory or a recognized life cycle database. Mixing grid emission factors from different years within the same product system is prohibited unless the data quality section justifies the temporal alignment. For purchased nitrogen, the air separation unit electricity intensity or supplier product footprint is required; a default value is accepted only with an explicit data quality score and sensitivity range.