Retrofit solution for natural gas steam boiler with a wood chip burner

I. Retrofit Background and Objectives

For natural gas-fired steam boilers, key challenges include significant natural gas price fluctuations, high fuel costs (up to RMB 280 per ton of steam), and strong dependence on fossil energy.

By leveraging the advantages of wood chip fuel—stable pricing, abundant supply (from agricultural and forestry residues), and renewability—this retrofit using a wood chip burner aims to achieve the following objectives:

1. Replace natural gas and reduce fuel costs by over 60%, while avoiding price fluctuation risks;
2. Maintain the original steam output and pressure of the boiler without affecting production;
3. Optimize the energy structure, align with renewable energy policies, and enhance the company’s green competitiveness;
4. Simplify the retrofit process by retaining existing equipment as much as possible, reducing investment and shortening the retrofit cycle.

II. Core Retrofit Scope

(1) Combustion System Adaptation

1. The original natural gas boiler body and furnace structure are retained (no major modifications required). The existing gas burner is removed and replaced with a modular wood chip burner suitable for the boiler capacity.
   The modular design enables quick integration with existing systems, minimizing installation time and cost, and is suitable for steam boilers ranging from 4 to 15 tons/hour.
2. The interface between the burner and furnace is optimized, and a fuel pyrolysis device is added to decompose volatile components in wood chips at high temperatures, ensuring complete combustion and compensating for the lower calorific value compared to natural gas.
   The air supply system is also upgraded with staged air distribution to match biomass combustion characteristics, ensuring stable steam output and pressure equivalent to pre-retrofit performance.
   Combustion efficiency can exceed 90%.
3. Different grate types are available depending on fuel conditions, including moving grate, chain grate, and reciprocating grate systems.
   Combined with automatic ash removal and slag discharge functions, the system ensures continuous operation even with uneven moisture content in wood chips, solving issues such as slagging and incomplete combustion.

(2) Fuel Feeding and Storage System

1. An automated wood chip feeding system is adopted, including screw conveyors and dual-screw hoppers, reducing labor requirements.
2. A wood chip crushing unit is added to process fuel into optimal sizes, ensuring smooth feeding, preventing blockage, and improving combustion efficiency.

(3) Flue Gas Treatment and Control System Optimization

1. The existing flue gas treatment system is retained, with an additional simple bag filter installed to remove particulate matter from biomass combustion, ensuring emission compliance without major system modifications.
2. The control system is upgraded by integrating the wood chip burner and feeding system with the existing boiler control system.
   Original steam pressure and temperature controls are retained, while adding automatic adjustment of fuel supply and air volume to ensure precise matching between combustion and steam output.
   A natural gas backup function is also maintained for emergency use (e.g., insufficient biomass supply), ensuring production continuity.
   The system supports one-button start/stop and automatic fault alarms for improved operational convenience.

III. Advantages and Benefits

1. Cost Benefits

• Wood chip fuel costs are only about one-third of natural gas
• Steam cost can be reduced from RMB 280/ton to around RMB 100/ton
• For a plant consuming 500 tons of steam per day: