Methanol, formed during the pulping of wood and is contaminated with reduced sulfur compounds and terpenes, is the largest single source of VOC emissions from kraft pulp mills, accounting for 70–80% of total emissions. The Cluster Rule (Cluster Rule Regulation: 40CFR Part 63 1998) limits methanol emissions for all pulp mills in the United States. Canada faces similar legislation (Das 1997, 1999a, b; Das and Houtman 2004; Das and Jain 2001).
Methanol emissions can be reduced by collecting condensate streams from the digesters, evaporators, and other sources in the mill. The collected condensate streams can then be steam‐stripped to concentrate the methanol for incineration. A few mills are “hard‐piped” to send this methanol‐laden stream to bio‐treatment plant, thus avoiding incineration.
Stripper overhead gas (SOG) contains roughly equal part of methanol and water (40–50 wt%) and roughly equal parts of TRS and terpenes (1–5 wt%).
In the United States, the SOG is likely to be burned in an incinerator, kiln, or boiler. Recently, some mills have found it advantageous to rectify the SOG to about 80% methanol and collect it as a liquid, which has a higher fuel value. With this concentrated (70–80%) methanol stream available, plants can cut down on the amount of natural gas that must be bought.
A process has been developed at Georgia‐Pacific that converts the methanol and TRS (mostly mercaptans) in the rectified SOG into formaldehyde. Based on work by Wachs (1999a, b), this patented catalytic process (Burgess et al. 2002) has achieved commercially viable yields of formaldehyde (70–80%) from a typical pulp mill SOG feedstock containing methanol, water, and TRS compounds.
The conversion of methanol and TRS to formaldehyde presents kraft mills with a more profitable alternative for SOG than incinerating the gas as a fuel. The formaldehyde produced can be used by resin manufacturers to produce thermosetting resins commonly used in plywood and other structural panels. A typical pulp mill of 2000 air‐dried tons per day (ADTPD) output may achieve a payout of 2–4 years, depending on the price of methanol and local economics.
Table 7.4 Annualized income, costs, and earnings 2000 ADTPD mill, 14 lb methanol/T SOG.
Source: From Burgess et al. (2002).
| Income 15 000 000 lb/year formaldehyde (50%) at $0.06/lb, FOB mill | $900 000 |
| Operating cost | |
| Direct labor | $110 000 |
| Utilities | $25 000 |
| Methanol in feed at $2.25/MMBTU fuel value | $260 000 |
| Misc., catalyst, supplies, etc. | $70 000 |
| Total operating cost | $465 000 |
| Gross margin | $435 000 |
| Depreciation | $210 000 |
| Earning before tax | $225 000 |
| Credit for steam generated (depending on the mill’s steam balance) | $95 000 |
| Credit for terpene recovered and sold | $110 000 |
| Credit for SO2 recycled | $45 000 |
| Total by‐product credits | $250 000 |
| Total earnings, including by‐product credits | $475 000 |
The process also produces two levels of low‐pressure stream, 60 and 70 psig, usable within a paper mill, by reducing most of the methanol to formaldehyde, rather than CO2. The reduction in CO2 emissions is about 80–85% of the amount otherwise generated by incineration or by the bio‐treatment plant. For a 2000 APTDP mill, this equates to 28 T/Y of CO2 emissions that are avoided.
A typical itemization of income, costs, and earnings is given in Table 7.4. For a 2000 ADTPD mill, a payout of 3–4 years is calculated based on formaldehyde prices of $0.06/lb (50%) basis. This assumes a customer‐shipping radius of 500 miles from the mill producing the formaldehyde. A heat value credit to the mill is included (equivalent of natural gas fuel) for the methanol that would otherwise have been incinerated and used to generate steam via heat recovery exchangers.
Leave a Reply