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Annex 2: Methodology and data collection for estimating emissions from fossil fuel combustion

New Zealand emission factors are based on GCV (gross calorific value) since energy use in NZ is conventionally reported in gross terms, with some minor exceptions. The convention adopted by New Zealand to convert GCV to NCV (net calorific value) is to follow the OECD/IEA assumptions that:

  • NCV = 0.95*GCV for coal and liquid fuels
  • NCV = 0.90*GCV for gas.

A2.1 Emissions from Liquid Fuels

Activity data:

Statistics New Zealand conducts the Delivery of Petroleum Fuels collection and is New Zealand's national statistical office and administers the Statistics Act 1975.

The Delivery of Petroleum Fuels collection is a quarterly census. The purpose of the census is to provide data on the amount of fuel delivered by all oil companies to end-users and other distribution outlets. Each oil company in New Zealand supplies Statistics New Zealand with the volume of petroleum fuels delivered to resellers and industry groups. It is assumed there is a 5 percent uncertainty associated with the sectoral energy allocation although the annual totals are likely to be more certain (MED, 2003).

This is a census which means there is no sampling error. The main sources of non-sample error are:

  • Respondent error: Statistics New Zealand makes every effort to confirm figures supplied by the respondents, and given assurances of accuracy, Statistics New Zealand are bound to accept them. If a discrepancy is discovered at a later date, revised figures are supplied at the earliest possible opportunity.
  • Processing error: there is always the possibility of error, however, Statistics New Zealand has thorough checking procedures to ensure that the risk of processing errors is minimised.

Emission factors:

CO2 emission factors are described in Table A2.1. The CO2 emission factors for oil products are from the New Zealand Refining Company (NZRC) data, import data from industry and from a New Zealand source (Baines, 1993 [The LPG CO2emissions factor was confirmed by checks of 2002 gas data.] ). The same values are used for each year of inventory. There is a direct relationship between fuels' carbon content and the corresponding CO2 emissions during combustion. However, the carbon composition of oil products is not closely monitored and there will be variation over time, depending on the crude oil used in production. The NZRC estimates the uncertainty in emission factors to be within 5 percent (MED, 2003).

New Zealand's review of emission factors (Hale and Twomey, 2003) identified a number of non-CO2 emission factors (Tables A2.2 and A2.3) where the supporting information (Bone et al, 1993; Waring et al, 1991) was assessed to be insufficient to retain the country-specific emission factors used in previous inventories. The changes mainly affected the mobile combustion category. Where a country-specific value was not available, the emission factor used by New Zealand is either the IPCC value that was closest match to New Zealand's conditions or the mid-point value from the IPCC range.

Many of the sources in the stationary combustion category already used the IPCC (1996) emission factors, however there were minor changes to the 2002 inventory related to the re-interpretation of the IPCC tables resulting from the Hale and Twomey review. The changes were made to obtain the closest match to New Zealand conditions. The decision to change from the country-specific emission factors to IPCC values was confirmed by an expert review panel prior to including the emission factors in the inventory. There was no further review of emission factors for the subsequent inventory reports. All emission factors from the IPCC guidelines are converted from NCV to GCV.

Table A2.1 CO2 emission factors used in the energy sector category

View CO2 emission factors used in the energy sector category (large table).

Table A2.2 CH4 emission factors used in the energy sector category

View CH4 emission factors used in the energy sector category (large table).

Table A2.2 N2O emission factors used in the energy sector category

View N2O emission factors used in the energy sector category (large table)

A2.2 Emissions from solid fuels

Activity data:

Statistics New Zealand "Coal Sales" survey is an ongoing quarterly survey which commenced on 1 March 1981. The purpose of this survey is to measure the amount of coal which is sold and available to users. The target population is all coal mines and major resellers of coal in New Zealand. Completion of the survey has been approved by the Minister of Statistics and returning the completed questionnaire, duly signed, is a compulsory requirement under the Statistics Act 1975.

The survey is a full coverage of the sector and therefore there are no sampling errors. Non-sampling errors in the survey data may result from errors in the sample frame (ie, units with the wrong New Zealand Standard Industrial Classification), respondent error (ie, wrong values supplied), mistakes made during processing survey results or non-response imputation. Statistics New Zealand adopts procedures to detect and minimise these types of errors but they may still occur and they are not quantifiable.

The three ranks of coal measured are bituminous, sub-bituminous and lignite coal. From 1988 onwards the coal sales questionnaire broke coal sales into seven end use sectors, however these do not match the IPCC sectors. The sectoral shares of coal use that can be used for the UNFCCC inventory are based on Coal Research Limited's (CRL) survey of sectoral coal use for 1990 and 1995. Data is interpolated between 1990 and 1995 and extrapolated for all years beyond 1995. The exceptions are for the coal used for steel manufacture, electricity production and the residential household sector where the MED use figures from Statistics New Zealand's "Coal Sales" survey. Sectoral shares are calculated by:

  • summing the four calendar year quarters of coal sales data from Statistics New Zealand's "Coal Sales" survey
  • subtracting coal exports and coal used by the Residential sector (from Statistics New Zealand's "Coal Sales" survey) and coal used for steel and electricity (both known accurately) dividing CRL annual coal tonnage for each sector by the total (excluding exports, steel, electricity and residential coal use) to give sectoral shares of coal use for 1990 and 1995
  • interpolating sectoral shares between 1990 and 1995 and extrapolating for beyond 1995
  • applying the sectoral share estimates to the "Coal Sales" survey's total coal sold (excluding exports, electricity, steel and residential coal use).

The process of dividing coal use between different sectors will introduce uncertainty larger than the uncertainty in total coal sales. Uncertainty is also introduced from the assumption that coal used by sector is an average of the different ranks. An uncertainty of ± 5 percent is assumed to cover these issues (MED, 2003).

The sectoral partitioning used for coal was examined in 2003 by the Ministry for the Environment officials. There was concern in extrapolating sectoral allocations from 1995 to 2002 given some probable changes in sectoral coal usage. However, coal industry experts (Hennessy, Personal Communication) did not consider a survey could be justified because of the difficulty and expense in collating and verifying data from a number of sectors. Furthermore the major categories of coal exports, coal used by the residential sector and coal used for steel and electricity are all known accurately and are not affected by the sectoral partitioning.

Emission factors:

The value for sub-bituminous coal (91.2 kt CO2/PJ) is used to calculate New Zealand's emissions from coal burning (Table A2.1). Using only the sub-bituminous value for all ranks of coal is a reasonable assumption for New Zealand as the bulk of the high-quality bituminous coal is exported and all coal used in public electricity generation is of sub-bituminous rank (MED, 2003). The range in emission factors across all grades of coal is 5.5 percent. Therefore the estimated uncertainty in coal emission factors is taken as ± 3 percent (MED, 2003). An uncertainty of ± 2 percent is used for the sub-bituminous coal used in public electricity generation. All New Zealand values are within 2 percent of the IPCC defaults (1996). The non-CO2 emission factors are shown in Tables A2.2 and A2.3.

A2.3 Emissions from gaseous fuels

Activity data:

The Natural Gas Corporation (NGC) has contracts with large users that allow metering errors of ± 2 percent. Whenever the error between the meter reading and actual gas supplied exceeds 2 percent, adjustments are made to the reported quantities of gas supplied. The uncertainty is therefore assumed to have an upper limit of ± 2 percent (MED, 2003).

Emission factors:

The emission factors for natural gas used in distribution and sold to large users are shown in Table A2.1. The values are calculated by averaging daily gas composition data supplied by industry. The composition, hence the emissions factor varies slightly between daily measurements. Taking annual bounds, it is estimated that the uncertainty in the natural gas emission factors is less than 1.7 percent (MED, 2003).

It has been assumed that half of the gas in the system is from the Maui gas field with an average CO2 emissions factor of 51.9 kt CO2/PJ for this 2004 inventory and the other half is treated gas (53.7 kt CO2/PJ). The average value of 52.8 kt CO2/PJ is used for this 2004 inventory (Table A2.4).

Table A2.4 Variation in CO2 emission factors for natural gas

Year Maui (kt CO2 / PJ) Treated (kt CO2 / PJ) Average (kt CO2 / PJ)

1990

53.2

52.4

52.8

1991

52.9

52.8

52.8

1992

52.9

52.7

52.8

1993

52.6

52.5

52.5

1994

52.4

52.2

52.3

1995

52.1

52.9

52.5

1996

52.2

52.9

52.6

1997

52.3

52.4

52.4

1998

52.1

52.2

52.1

1999

51.8

52.4

52.1

2000

52.1

52.1

52.1

2001

51.9

52.6

52.3

2002

52.3

52.5

52.4

2003

52.0

52.6

52.3

2004

51.9

53.7

52.8

A2.4 Energy balance for year ended December 2004

Table A2.5 New Zealand energy balance for year ended December 2004

New Zealand energy balance for year ended December 2004 [PDF, 289 KB]