Determination of Calorific Value in Solid Biofuels with Sundy SDAC1200 Bomb Calorimeter According to ISO 18125

Application Note: SD-AC-1200-001

Introduction

Solid biofuels, originating from non-fossil organic feedstock such as plant biomass, animal waste, wood residues and by-products, are widely applied for heat production and electricity generation. Calorific value is one of the most critical indicators for evaluating biofuel performance, since it defines energy content, guides combustion control and supports fuel fiscal billing. Diverse biomass feedstocks lead to large property fluctuations, making accurate calorific value testing essential for industrial deployment under the growing bio-based economy. This paper focuses on the practical testing of calorific value for solid biofuels to support its industrial application.

Method Reference

ISO 18125 — Solid biofuels — Determination of calorific value

Principle

A weighed portion of the analysis sample of the solid biofuel is burned in high-pressure oxygen in a bomb calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined in calibration experiments by combustion of certified benzoic acid under similar conditions, accounted for in the certificate. The gross calorific value is calculated from the corrected temperature rise and the effective heat capacity of the calorimeter, with allowances made for contributions from ignition energy, combustion of the fuse(s) and for thermal effects from side reactions such as the formation of nitric acid.

Sample Preparation

The biofuel sample used for the determination of calorific value shall be the general analysis sample (ground to pass a test sieve with an aperture of 1.0 mm) prepared according to the procedure given in ISO 14780.

Sieve with an aperture less than 1.0 mm (0.5 mm or 0.25 mm) might be necessary for some solid biofuels to ensure the requisite repeatability and a complete combustion.

Samples shall be kept in effectively closed containers or packages.

Reagents and Materials

Benzoic acid, of calorimetric-standard quality, certified by (or with certification unambiguously traceable to) a recognized standardizing authority.

Oxygen, at a pressure high enough to fill the bomb to 3.0 MPa, pure with an assay of at least a volume fraction of 99.5 %, and free from combustible matter.

Ignition wire, of nickel-chromium approx. 0.1 mm in diameter.

Combustion aids of known gross calorific value, composition and purity, like n-dodecane, paraffin oil, combustion bags or capsules may be used.

NOTE 1: A homogeneous mixture of sample and benzoic acid might be difficult to achieve. The use of benzoic acid with combustion bags or capsules is recommended.

Testing Parameters

Internal volume of bomb: 300 mL

Distilled water: 1.0 mL

Ignition heat: 50 J

Oxygen pressure: 3.0 MPa

Oxygen filling time: 15 s

Ambient temperature: 22 °C

Jacket temperature: 28 °C

Bucket temperature: 26 °C

Tank temperature: 24 °C

Procedure

Instrument Preparation

Prepare the instrument and bomb as outlined in the operator manual.

Calibration of Calorimeter Heat Capacity

a. Select “Heat capacity” test mode in the software.

b. Weigh approx. 1.0 g of benzoic acid and perform heat capacity test.

c. Repeat the heat capacity test five times.

d. The software automatically calculates the effective heat capacity.

Determination of Gross Calorific Value

Select “Calorific value” test mode in the software.

Weigh approx. 1.0 g of the sample into a polyethylene bag of known weight whose calorific value has been determined, and then place the bag into a crucible for testing. The mass and calorific value of the polyethylene bag must be entered into the software so that the heat contributed by the polyethylene bag can be subtracted.

Refer to the operator manual for the determination of gross calorific value.

NOTE 2: Certain biofuels may persistently burn incompletely, “exploding” and/or leaving residues that contain significant amounts of unburned sample or soot. By adding known amounts of an auxiliary material (e.g. benzoic acid, n-dodecane or paraffin oil), by using bags or capsules, by omitting the distilled water from the bomb, or by using a lower oxygen filling pressure, a clean combustion can, in most instances, be achieved.

NOTE 3: In this application, all experiments were conducted by directly testing samples enclosed in combustion bags.

Typical Results

Summary

The experimental results confirm that the test repeatability of four solid biofuels falls within the repeatability limits specified by the standard, demonstrating that the Sundy SDAC1200 calorimeter is fully compliant with ISO 18125 for the determination of calorific value in solid biofuels.