Technology and Environment

The chosen technology is a step grate boiler producing medium pressure steam used to drive a steam turbine and alternator for the generation of 8.8 MW of electricity. Biomass is introduced into the boiler that utilizes the heat of combustion to generate steam. This is not the most efficient or technologically advanced method of converting energy to electricity but has been chosen on the basis that it is proven, tried and tested and within budget.

Combustion is the process used by over 90% of the world’s bioenergy plants. Boilers are designed to utilise heat transfer from the processes of radiation, conduction and convection. In general, the hot gases derived from burning the biomass flow over a set of tube banks, heat the water in the tube banks to produce steam, with the steam driving a turbine to generate electricity.

Biomass is introduced onto a step grate. The step grate provided is of a pin-hole cast iron type, made in sections for ease of transportation, installation and replacement of parts. The grate is arranged in such a position for fuel spreading and to achieve maximum combustion caused by optimum air/fuel mixing. The fixed pin-hole cast iron grate sections are arranged and supported by the cast iron bars and cooled by the flow of combustion air from under grate. The reciprocating grate or moving cast iron grates are driven by hydraulic cylinders, and are mounted between the fixed step grate sections. These reciprocating grates assure even fuel spread and control the progression of the fuel over the entire length of the step grate for complete combustion.

This is the technology used by many mills where wood waste is used to produce heat and steam for drying or to operate fibreboard presses. By adding a steam turbine to the direct combustion processes, these mills can also produce electricity to be used on-site or sold into the grid.

The process of choice, the step grate boiler, is a tried and tested process and has been operated worldwide for decades, with particular technical refinement having been achieved in the last 20 years. The term step grate is used to describe the spreading and movement of biomass as it burns on the grate and releases energy. Air to the grate is supplied through the grate from a forced draft fan. Once the combustion chamber is heated to temperatures in excess of 600 C, organic waste such as wood waste fed to the boiler will burn and maintain a controlled temperature. The hot grate and burning biomass act as a flywheel where different types of organic waste can be incinerated. The energy in the hot gases is used to generate mainly steam or hot air. There are many such facilities operated by sugar mills in South Africa, designed and built locally. These types of boilers are used for the incineration of carbon sludge, biomass and wood by-products such as bagasse, wood chips, sawdust etc. The energy generated from the combustion of this organic waste is used in waste heat boilers to generate steam or for the generation of hot air for drying purposes. The current project proposes the use of a step grate boiler for incineration of wood waste and sawdust from saw mills.

The biomass-to-electricity project falls within regulation 387 of the Environmental Impact Assessment (EIA) regulations (July 2006) and the following activities are identified which require the issue of a Scoping Report to environmental authorities.

In terms of the EIA Regulations (R385, April 2006), the following activities are applicable to the project:

Regulation 386:1l: The transmission and distribution of electricity above ground with a capacity of more than 33KV and less than 120KV.

Regulation 386:1O: The recycling, re-use, handling, temporary storage or treatment of general waste with a throughput capacity of 20m3 or more daily average measured over a period of 30 days, but less than 50 tonnes daily average measured over a period of 30 days.

Regulation 386:1S: The treatment of effluent, wastewater or sewage with an annual throughput capacity of more than 2 m3 but less than 15 000m3.

Regulation 387:1E: Any process or activity which requires a permit or license in terms of legislation governing the generation or release of emissions, pollution, effluent or waste and which is not identified in Government Notice No. R. 386 of 2006.

CCE Solutions is also aligned with the Kyoto Protocol’s Clean Development Mechanism (CDM) and the executive board approved methodology ACM0006 / Version 04 will be used. This falls under Sectoral Scope: 01 and is entitled "Consolidated methodology for grid-connected electricity generation from biomass residues".

The methodology is applicable under the following conditions:

• No other biomass types than biomass residues, as defined are used in the project plant and these biomass residues are the predominant fuel used in the project plant (some fossil fuels may be co-fired); biomass residues are defined as biomass that is a by-product, residue or waste stream from agriculture, forestry and related industries. This shall not include municipal waste or other wastes that contain fossilized and/or non-biodegradable material (small fractions of inert inorganic material like soil or sands may be included). Note that in case of solid biomass residue for all the calculations in this methodology, quantity of biomass residue refers to the dry weight of biomass residue;
• For projects that use biomass residues from a production process (e.g. production of sugar or wood panel boards), the implementation of the project shall not result in an increase of the processing capacity of raw input (e.g. sugar, rice, logs, etc.) or in other substantial changes (e.g. product change) in this process;
• The biomass residues used by the project facility should not be stored for more than one year;
• No significant energy quantities, except from transportation or mechanical treatment of the biomass residues, are required to prepare the biomass residues for fuel combustion, i.e. projects that process the biomass residues prior to combustion (e.g. esterification of waste oils) are not eligible under this methodology. This methodology is applicable to grid-connected and biomass residue fired electricity generation project activities, including cogeneration plants.

The project activity may include:

• The installation of a new biomass residue fired power generation plant at a site where currently no power generation occurs (greenfield power projects); or
• The installation of a new biomass residue fired power generation unit, which replaces or is operated next to existing power generation capacity fired with either fossil fuels or the same type of biomass residue as in the project plant (power capacity expansion projects); or the improvement of energy efficiency of an existing power generation plant (energy efficiency improvement projects), e.g. by retrofitting the existing plant or by installing a new plant that replaces the existing plant; or
• The replacement of fossil fuels by biomass residues in an existing power plant (fuel switch projects). The project activity may be based on the operation of a power generation unit located in an agro-industrial plant generating the biomass residues or as an independent plant supplied by biomass residues coming from the nearby area or a market. This method is valid from 2 November 2006 onwards and falls under sector 1: Energy industries: renewable - / non-renewable sources (UNFCCC, 2006).