Scope 1 emissions are the direct emissions from sources owned or controlled by the reporting organisation. For manufacturers, the dominant Scope 1 category is stationary combustion: burning fuel on-site to produce heat, steam, or power for production processes. Every combustion source has an associated purchase document, because fuel has to be acquired before it can be burned. This is why Scope 1 stationary combustion is both the most auditable and the most commonly mis-stated part of a manufacturer's GHG inventory.
The mis-statements almost always stem from one of two gaps: a combustion source that is in scope but was not included in the inventory because no one assigned it to the sustainability team's collection process, or a source that was included but with the wrong unit or the wrong emission factor. This article describes the common combustion sources in manufacturing, the invoice or record type that documents each, and the specific fields that matter for the GHG calculation.
Natural Gas: Piped Supply and Invoiced Separately
Natural gas is the most common combustion fuel for process heat and steam generation in Singaporean manufacturing. It arrives via the City Gas distribution network and is billed monthly per meter account. The invoice states consumption in therms or GJ depending on the account class, a billing period, a meter identifier, and a total charge.
For the GHG calculation, the relevant fields are: consumption quantity, unit (therms vs GJ matters because the conversion is not trivial), and billing period. The emission factor for natural gas combustion per GJ is drawn from the IPCC AR6 table for stationary combustion, which gives a CO2 emission factor and a separate CH4 and N2O factor that are added as CO2 equivalents using the applicable GWP values.
The common error with natural gas is treating all gas as equivalent. A facility with a process gas heater and a separate boiler on different meter accounts may find that one account is metered in therms and the other in GJ, producing a systematic unit mismatch if the person collecting the data applies the same conversion factor to both without checking the invoice header.
Diesel: Tanker Deliveries and Internal Fuel Logs
Diesel fuel in a manufacturing setting appears in two forms. The first is delivered fuel: a tanker delivers diesel to an on-site bulk storage tank, and the delivery is documented by a fuel delivery receipt from the supplier. This receipt states volume in litres, delivery date, and typically the product specification (typically Grade D or automotive diesel for non-power generation uses, or gas oil for stationary engines). The second is dispensed fuel: diesel drawn from the bulk tank for use in on-site vehicles, forklifts, or emergency generators, tracked in an internal fuel log.
For stationary combustion purposes, what matters is the fuel consumed in on-site stationary sources: boilers, generators, and fixed heating equipment. On-site vehicles may fall under Scope 1 mobile combustion (if they are owned and operated within the facility boundary) or may be excluded if they are contractor vehicles. The delivery receipt documents how much diesel entered the tank. The internal fuel log, if maintained, documents how much was used in each category. For facilities without internal fuel logs, the convention is to treat all delivered diesel as consumed in the accounting period, with an adjustment for beginning and ending inventory if the tank level is measured. Most facilities do not measure tank levels at period close, creating a timing mismatch that introduces a small but real inconsistency between delivery-based and consumption-based accounting.
LPG: Cylinder Deliveries and Bulk Reticulated Supply
Liquefied petroleum gas is used in manufacturing for process heating, particularly in industries where natural gas piped supply is not available or where portable heat sources are needed for specific operations. LPG arrives either in bulk tanker deliveries (documented by a delivery docket in tonnes or cubic meters of liquid) or in cylinder exchanges (documented by a cylinder exchange record showing the number of 47kg or other standard cylinders exchanged).
The unit conversion challenge with LPG is that delivered volumes are stated as liquid volume, but emission factors are typically expressed per kilogram or per GJ. Converting from litres of liquid LPG to kilograms requires the density of the specific LPG blend, which varies slightly by propane-butane ratio. For reporting purposes, using a standard density value (approximately 0.51 kg per litre for a standard commercial LPG mix) introduces a small uncertainty that should be noted in the methodology documentation.
Cylinder exchanges are particularly prone to inventory timing issues. If 20 cylinders are exchanged in the last week of December but returned empty in early January, the exchange record documents a delivery in December. Whether this belongs to the December or January accounting period depends on when the cylinders were actually consumed. For large cylinder users, this creates a year-end cut-off question. For small users, the materiality is low enough that any consistent convention is acceptable provided it is documented and applied the same way each year.
Fuel Oil: Heavy Industrial Use
Fuel oil (sometimes called HFO, heavy fuel oil, or by its grade designation such as IFO 180 or IFO 380) appears in manufacturers with high-temperature process requirements: ceramics, glass, metal processing, and some chemical production. It is delivered by tanker and documented by a delivery docket stating volume in kilolitres or tonnes and the product specification.
Fuel oil has a higher carbon content per GJ than natural gas or diesel, and the emission factor is more sensitive to the specific grade than lighter fuels. The IPCC AR6 tables provide emission factors for heavy fuel oil as a category, but for facilities with high fuel oil consumption and variable grade procurement, using the precise grade specification for factor selection produces a more accurate calculation. Whether this level of precision is warranted depends on the materiality of fuel oil in the overall Scope 1 inventory.
Process Gases: The Category Most Often Missed
Manufacturers in electronics, chemicals, and metal fabrication often use industrial process gases that are themselves greenhouse gases or that produce greenhouse gases when thermally processed. Common examples: perfluorocarbon (PFC) gases used as etch and cleaning agents in semiconductor fabs, SF6 used as a dielectric medium in high-voltage electrical switching equipment, and nitrous oxide (N2O) from chemical processes.
These sources are technically Scope 1 but are frequently missed by sustainability teams focused on combustion fuels, because they do not appear on utility bills or fuel delivery receipts. They appear on industrial gas procurement records: purchase orders or delivery records from industrial gas suppliers, stating quantity in tonnes or cylinders of specific gas. The global warming potential of SF6 is approximately 23,500 times that of CO2 (using IPCC AR4 values, still widely used for regulatory reporting), which means a small quantity omission produces a large CO2e error.
Not every manufacturer has process gas emissions. But identifying whether the question applies requires a walkthrough of the production process, not just a review of energy invoices.
Refrigerants: Scope 1 Fugitive Emissions
Air conditioning systems, cold storage, and process cooling equipment use refrigerant gases that can leak from the system over the equipment's operating life. Refrigerant top-ups are documented by the service record from an air conditioning maintenance contractor, which typically states the refrigerant type (R-32, R-410A, R-134a, or older refrigerants still in service) and the quantity added in kilograms.
Refrigerant emissions are classified as Scope 1 fugitive emissions under the GHG Protocol, not as combustion, but they belong in the same inventory calculation. The GWP values for common refrigerants range from near zero for some natural refrigerants to over 2,000 for HFC blends. The service record quantity, multiplied by the applicable GWP, gives the CO2e for each maintenance event.
The coverage gap here is systematic: air conditioning maintenance records are typically held by the facilities management team or the contracted maintenance company, not by the accounts payable function that processes the invoice. If the sustainability team's data collection only covers AP invoices, refrigerant service records may not surface at all. Identifying this gap in the first reporting year and building a process to collect it is preferable to discovering it during verification when prior periods cannot easily be corrected.
What Invoice Data Cannot Fully Cover
Invoice-based data collection is the most practical starting point for Scope 1 stationary combustion, but it has a specific limitation: invoices document purchases, not consumption. Fuel purchased in one period may be consumed in the next. For most manufacturing operations with relatively consistent production levels, this timing difference is small enough to be immaterial on an annual basis. For operations with significant seasonal variation or for periods where a large bulk purchase was made ahead of expected price increases, the timing gap may be worth explicitly noting and quantifying.
Additionally, invoices are reliable only for externally procured fuels. A facility that generates biogas from on-site waste treatment and burns it in a boiler has a Scope 1 combustion source with no purchase invoice. The activity data for this source comes from internal meters or operational logs, not from AP records. Identifying all combustion sources, not just the ones that generate invoices, is a process step that requires someone who knows the physical production footprint.